| Author | Message | ||
     John Harlow |
Need help on using nav antenna for glide slope antenna. I realize you can purchase a splitter for approx $70 that will allow use of the nav antenna for dual purpose. I don't need the extra antenna and so I want to buy/build a unit that will just convert directly and therefore cut down on splitter losses (I understand its approx 30%). Any help out there? | ||
| Tom R. Hutchison |
Correct me if I misunderstand your question! You want to use an existing NAV(VOR) antenna just for glide slope? Here is what Bob Nuckolls has to say in the AeroElectric Connection. "Compared to VOR or COM antenna performance, glide slope antennas can be pretty sloppy. G.S. signals are seldom utilized more than 5 or 6 miles out from the end of the runway and G.S. transmitters are strong." He goes on to say "A VOR/LOC antenna with a 108-118 Mhz operating range could be expected to be a fair performer at 324-354 Mhz as well. G.S. receivers operate at 329-335 Mhz." Therefore I don't think you need any kind of converter. If you want to fabricate a dedicated glide slope antenna, Jim Wier of RST Engineering has a simple design in the RST-802 Antenna Reference Text. It is a shallow V dipole 16" in total length. Tom | ||
| John Harlow |
Tom, Thanks for the info. I will hook it up direct because if it doesn't work well I can still purchase/build a splitter in the future. I installed a nav antenna in both wings and have installed a Garmin GNS 430 as my only radio with a ICOM handheld antenna disconnect($60) for my backup, so it leaves the other nav antenna as a spare. | ||
| LOUIE LACY (Llacy) |
I am having trouble getting Center radar to pick up my transponder. When I am near large cities I get a reply light on approach control but center doesn't get my transponder. I have the antenna mounted behind bulkhead 162 the antenna pointed down. Any suggestions Lou | ||
| Gary Markwardt (Gmark) |
Louie: Express folks told me they had not found a transponder antenna that worked okay when mounted inside. So, mine is mounted outside approximately under the passenger seat. Reinhard Metz said at Sun-N-Fun this year that he had a PC board mounted transponder antenna behind bulkhead 162 that worked good after trying other internally mounted ones without success. Suggest you give him a shout. Gary | ||
| Reinhard Metz |
Re: Transponder antennas:I made my own antenna out of printed circuit board material. It is a dipole that is mounted vertically, and works much better than my previous version, which was the classic 2 1/2" quarter wave type with the ball on the end, mounted facing down on a ground plane behind the 162 bulkhead. A picture is attached. \image | ||
| Reinhard Metz |
One more try with the picture.  | ||
| Tom R. Hutchison (Tomhutch) |
Bill and/or John (Copeland), I read about your about your antenna woes in the latest Express Link. I too was planning on installing a Bob Archer "folded dipole" comm antenna on the right inside of the fuselage just behind bulkhead 162. In fact I have it taped in place ready for me to commit to its location. I installed a cooper strip antenna in the vertical stab (see: http://www.express-builder.com/gallery/album01/abd for a picture of the installation). My intention was to use the Bob Archer antenna as my comm 2 (or comm 1 whichever worked better). Do you have any suggestions re: the location of the folded dipole antenna based upon your experience? Tom | ||
| Wayne norris |
You might be able to fish the coax through the V tail, bond copper strip to the inside of the rudder cove as a retro fit. An acro buddy of mine has his in this location. Wayne Norris | ||
| Tom R. Hutchison (Tomhutch) |
After my posting above regarding the installation of a Bob Archer antenna, Bill Copeland was kind enough to forward an e-mail from Bob Archer with some comments. I am posting Bob's comments below. The attached articles he mentions can be viewed in the , Express Builder's Technical Documents Section of the web site at: http://www.express-builder.com/docs. If anyone has information of a technical nature of interest to aircraft builders please submit it to me for possible inclusion in the Technical Documents Section Tom ------------------------------------------------ As per our telephone conversation I am including attachments of the articles I wrote on the subject of antennas for composite aircraft. I just read your article again and I will comment on some of the problems I see. 1. Marker Beacon: This is the simplest one on the airplane. A fancy antenna from some manufacturer is not required. A 40 inch conductor of any type bonded to the inside bottom skin with the center conductor of the coax connected to one end and you are there. The braid of the coax need not even be connected. You are so close to the transmitter and the signal is being beamed straight up at you that nothing fancy is required. 2. Com: The vertical stab is the best location in these aircraft for the com antenna. Being high above all the rest of the airplane the airframe can't cause much interference and if it does you will be so close it won't matter. But it should be a GOOD antenna. Not two copper strips without a balun. And ferrite beads do not a balun make. I have tested them and they do NOTHING! Think of it this way, If an eight foot water wave were coming at you would a 1/2" lifesaver save you? A wave length at VHF frequencies is about eight feet. And the ferrites? The copper strip antennas with a little black box in the middle test very good because the box contains a ferrite transformer. These transformers test well because they are lossy. I have tested these and the best of them lose about 2dB (36%). That is throwing away more than 1/3 of your energy. When measuring VSWR you are measuring the two way loss so the VSWR looks extra special good. With com antennas installed on the side wall of the fuselage there should be good coverage to the sides and rear but to the front there will be a reduction of signal due to the engine, firewall, instrument panel, wiring and don't forget those lossy saltwater bags called people. Anything else installed in the fuselage that is in the vertical plane will effect the antenna as well as another antenna on the other side wall. If you have only one GOOD antenna second radio could be connected through a coaxial switch so you could switch the antenna to either radio at will. 3. Nav/VOR: Basically the same as Com except horizontally polarized so must be installed in horizontal plane. Or mostly. Not as critical as Com but is nice if it works well. Still mostly needed for ILS. Should have a balun as for com. For VSWR testing a model MFJ-259 SWR Analyzer is available from Ham Radio supply stores for about $200. It has a built in signal generator, freq. counter and a swr meter. Very simple to use. I use a cable with connector on a stick that I can plug into the back of a case when the radio is removed. I can then test the entire antenna /cable system at the same time. I guess that should be enough for now. Don't hesitate to call with questions if you have them. I am usually in the office in the mornings running errands in the afternoon. I would rather answer your questions early than have you with problems later. Regards Bob | ||
| wayne |
ref Xponder ant. for Garmin 327 ,I mounted my 64$ little post ant, to a 6x6 alum plate. drilled a 1/8" hole in the floor under the rt front seat so it polks out the bottom and screwed the plate down inside. I have only checked it out to about 50 miles at 4000'but seems to be working. now to finish up the wiring of my com and audio panel. | ||
| LOUIE LACY (Llacy) |
I just purchased a Narco 122D and it has connections for both nav and gs antenna on the back of the unit. Do I need a splitter for this connection and where can I get one and will it work Lou new email llacy311@tds.net | ||
| Tom R. Hutchison (Tomhutch) |
Are you using both your wing VOR antenna's? If not, you could use one for NAV and the other for GS. Check out: http://www.aircraftspruce.com/catalog/avpages/splitters.php For splitters from ACS. Tom | ||
| Eric Holmberg (Erich) |
On another note, does anybody know of a part for joining / splitting the signal for attaching two COM radios to one antenna? How has everyone else handled this issue -- maybe by installing two antennas? I'm not very willing to install a physical switch to choose which radio is connected to the single antenna since I would like to use both radios at the same time and be able to transmit from both. Does anyone have experience with the Bob Archer antennas that will fit in the wing tips (yes, that's a COM antenna)? -Eric | ||
| Reinhard Metz |
Chief Aircraft also has many splitters. p.22 of the current catalog. Yes, they work, but not as well as having a separate GS antenna. I have two nav antennas, and one GS. The first radio uses a nav and dedicated GS, the other a splitter off the second nav. The one with the splitter is not quite as sensitive, but well within the specs of how far out the GS should work. As far as the com antennas go, it is advisable to have separate ones. Besides the location in the tail, I have two additional, one on each side wall, just behind the door. There is just enough vertical space there. Requires working the balun and cable into the interior, One of my additional antennas is on comm 2, the other on a panel BNC for the handheld, if ever needed. Both these antennas work good, but not quite as well as the tail one, in terms of tested receive sensitivity. Reinhard Metz | ||
| Eric Holmberg (Erich) |
Thanks Reinhard, I also talked with Jerry and he mentioned using a 45 degree external antenna which is mounted upside down on the belly with a ground plane on the inside. That sounds like it may be better than the foil antenna inside, although I may put one inside for my handheld and one underneath for the second COM. Or maybe two underneath inline like you normally see on aluminum aircraft. GEEESH, looks like I need to get a girlfriend to make sure I can use all of the COM radios at one time ;) Any advice here before I get carried away and turn my plane into an acupuncture experiment? From what I've heard, the folded dipole Bob Archer antennas get mixed reviews. Has anybody actually used one? Tom Hutchison, did you actually the one you had or is it sitting in a box somewhere? -Eric | ||
| Bill Copeland |
Eric: We have a Bob Archer folded dipole installed in our Express on the right side of the fuse, about 30 inches behind blkhd 162. We originally used it for our Comm 1 but were very disappointed with it's performance. It seemed like the mass of stuff in front of it proved to be a serious problem. We have since mounted a "bent" antenna out the bottom of the fuse in the area of the baggage compartment. We are using a 24X24 mylar coated plastic film for a ground plane. Works much better. Bill C | ||
| Eric Holmberg (Erich) |
Bill, Great, that's the info I wanted  I'll go that route instead. I don't mind sticking something out of the fuselage if it gives me superior performance. Nothing irritated me more with my old plane than the fact that the controllers couldn't hear me when I was more than 20 miles from the field. -Eric | ||
| Kevin Dennes (Kdennes) |
This is valuable discussion indeed. I am so impressed with the knowledge that the members of this group seem to have. I hope this is not a silly question but . . . . Is there any advantage in installing an external type of fixed antenna inside the fuselage behind 162 instead of a ribbon type of antenna? I have put one ribbon nav antenna in each wing and one in the vertical fin but I do need to install another antenna for com 2. Would a fixed one work as well inside the fuselage compared with outside? Any ideas would be appreciated. Regards. Kevin (from Downunder) | ||
| Jerry Sjostrand (Jerry) |
Kevin and builders: That is exactly what I did when one of my copper vertical tape com antennas didn't work well. I installed a Comant # CI 122 "bent" antenna inside on the shelf behind 162. I also I have my ELT antenna there and they are about 12 inches apart. The com antenna points to the rear. It is necessary to provide as large a ground plane as possible (you can use aluminum foil) and to connect the bases of both antennas to a common ground. Mine work great. I had made 2 suggestions to Eric but he only referred to the one under the belly which is what Copelands did and is another alternative which works very well. I ended up mounting normal blade antennas under the foot area of the rear seats for my transponder and DME with a 15" round aluminum ground plane inside on the floor. | ||
| Eric Holmberg (Erich) |
Somehow, I misunderstood the second suggestion from you, Jerry :P Sorry! I didn't realize that you installed yours on the rear shelf so it was completely inside the airplane :P I was thinking your installed it underneath the rear shelf and had it sticking out of the bottom of the airplane. I'm surprised that that provides an acceptable solution since I thought the only reason why the internal foil dipoles didn't work well when inside of the fuselage was because of the engine and other items blocking the signal to the front of the aircraft (which is also normally where you want to direct your communications). Maybe it has to do with the foil dipoles directing most of the radiation out to the sides (since the large foil sides are to the side) instead of forward? The 45 degree antenna that you used is round, so it should presumably radiate all energy in an omni-direction manner. If mounting it inside provides acceptable communication, I will gladly do that instead. It gives me a reason to use the composit 02 tank instead of the aluminum one, too ;) Jerry, you mentioned that the COM2 antenna is still not as good as the one in the vertical stab, how much worse is it? | ||
| Jerry Sjostrand (Jerry) |
Hi Eric; I don't remember saying that the Com2 antenna was not better. I use the Comant bent antenna on top of the rear shelf (it can be pounted underneath) as Com1 and the copper tape on the vertical stabilizer shear web as Com2. They work equally well as far as I can tell. There are times, however, when I switch from one radio to the other to get better reception. You are correct that you need to be concerned with "shadowing" from any metalic items and a metal oxy tank would be a problem. Regarding the other items on the airplane, engine, instrument panel, and control rods, etc, all present a problem to consider. However, with all the flying I do, I have no problem with my current setup. I do like the Transponder and DME antennas (Narco blade antennas) on the bottom of the fuselage as they need to look down as is true with the glideslope. I added a splitter in the Com1 coaxial cable to allow my handheld to connect and it does very well for the extra range when needed. I also have a 12 volt connection next to it , a head set connection and push-to-talk button so I can use my handheld that same as an installed com radio if needed. Hope this helps you make up your mind. Jerry | ||
| wayne norris |
My 2cw, I put in the foil di pole that came with the kit in the V tail before I closed it. I am just using a hand held com for now and it is reaching out 50+ miles. Not bad. My xponder is a cheapy mounted to a 6x6 plate under the pax seat and sticking out the bottom, it works well also. | ||
| Reinhard Metz |
I keep being surprised by peoples' abandonment of foil antennas and sticking conventional ones outside or adding the other types inside. My foil antennas EASILY reach towers >50 miles out, from either the KX-155 or the GNS530, when above 5K or 6K feet. What I would suggest is if you are having trouble with a foil antenna, put a VSWR meter and/or a vector impedance meter on it, and if the transmit VSWR is greater than 1.5, (or the impedance in the 118-136mhz band is way off 50 ohms, replace the ferite balun donuts with a better balun (if you can still get to them), from an appropriate avionics catalog. This will likely require some help from an avionics shop to get the right equipment. These antennas are definitely capable of doing the job. When vertically mounted, they will exhibit a horizontal planar donut sensitivity pattern, which is ideal. It seems a shame to unnecessarily cut holes in a glass plane. Reinhard Metz | ||
| Eric Holmberg (Erich) |
Reinhard, Can you tell us more about the location of your antennas? That would help greatly as I think the majority of the problems are location, and as you said, the impedance. Anytime you bend the dipole elements, you change the impedance of them. | ||
| Reinhard Metz |
My tail comm antenna is exactly as per the building instructions. The other two comm antennas are the same length as specified for the tail, and are exactly vertical, one on each side of the fuselage, right behind the two front windows, behind the vertical uni re-inforcing tape. The right one is therefore right behind the door. They are vertically placed such that the bottom end is just above where the sidewall starts to curl into the floor. The top follows a slight curve of the side wall. The tail antenna on comm1 will sometimes show some shadowing from the engine when headed right at the field. The receive squelch may not trigger till about 20 miles out. Under those circumstances, comm2 will be ok, its antenna being one of the side ones. I'll look for a picture and put it up if I find one. Reinhard | ||
| Eric Holmberg (Erich) |
That sounds like a good location as compared to the rear of the fuselage behind the cabin compartment since the antennas are further towards out towards the edge of the engine. I had thought about the placement further forward, but figured that the torque tube for the flaps would cause problems and forward of the doors you have people, avionics, and the control sticks. I'll go ahead and tape the dipoles up in the different locations and get some VSWR readings to see where I'm at. I'll also try to pull in some ATIS stations around the area. It will be a month or two until I get around to this point, though. | ||
| LOUIE LACY (Llacy) |
I ordered the splitter from Aircraft Spruce to connect my VOR and GS and it doesn't work I checked the connections on the coax and they appear to be good .What else could be wrong??? LOU | ||
| reinhard Metz |
Try the antenna directly on the GS and Nav inputs, one at a time and see if they work then. If they do, the problem is the splitter. If not, the antenna/coax run are the problem. Reinhard | ||
| LOUIE LACY (Llacy) |
Reinhard My problem is as some aviation writer put it(BIG NASTY CONNECTORS) BNC I followed the instructions for putting the connectors on the coax and messed it up. After checking with a VOM I found the problem .You can never be too careful putting these connectors on coax Lou | ||
| wayne norris Unregistered guest |
Here is how to make a $2 marker beacon antenna.You need +- 36" rg58 coax, 40" #20 wire,10" x1/4 wood dowel,10"x 3/8 heat shrink tube. Drill a hole near the end of the dowel to hold the wire, wrap the wire around the dowel, spacing nicely, slip the HST over the coil and shrink, solder the wire end to the coax center, hook up to your audio panel and you are there. The ant. end of the coax sheild is open. I placed mine under the carpet next to the nose/FW box. You can see a pix in my gallery of the final product. | ||
| Roy Howard Davis New member Username: Royhdavis Post Number: 7 Registered: 01-2003 |
I've just put some antenna experiences on our Web page that might be of interest. Specifically, tuning the straight dipole copper tape antenna commonly used in the Express, and designing and installing a copper tape meander line antenna that gave better results than the straight dipole. There are designs for COM, NAV, GS, ELT and even a ham radio 2M/70cm antenna. There are lots of pictures and commentary, and of course it's not done until the plane is flying, but there is a good start. You can find this at http://www.nancymoon.com/antennas.htm | ||
| Eric Holmberg New member Username: Erich Post Number: 64 Registered: 05-2002 |
Great job Roy! Where did you get the balun from and do you have a part number? -Eric | ||
| Roy Howard Davis New member Username: Royhdavis Post Number: 8 Registered: 01-2003 |
Hi Eric, Thanks! Writing up the pages for the Web site has been a way to focus design effort for the plane. I bought the copper tape and toroids from RST Engineering at: http://www.rst-engr.com/ Look under RST Catalog/Antenna Parts. I bought RST-2802, which combines the toroids, the tape and a compilation of articles by Jim Weir on how to build antennas. There is also an archive of other articles Jim has written on the RST site. He also sells the materials in larger quanity. | ||
| Jim Oyler New member Username: Midniteoyl Post Number: 25 Registered: 07-2003 |
Oh man! Thanks Roy. I read your earlier meanderline page and was just about to shoot you an e-mail with about 50 questions on it. But, this really clears things up. Except one. Where were the toriods place farther down the cable? I see that one is placed at the bend, what about the other? Thanks Again Jim | ||
| Roy Howard Davis New member Username: Royhdavis Post Number: 9 Registered: 01-2003 |
Jim, The placement of the toroids along the cable is not critical. What you don't want is quarter wave long sections of cable parallel to the antenna. I placed one at the bend because that's where the parallel section starts. Then I placed one about a foot further along the cable because it was about in the middle of a quarter wave. It probably wouldn't hurt to have a few more spaced a foot or so apart. This is very similar to what AM radio stations have to do to nearby towers that mess up their radiation pattern. They install insulators to break up the towers into smaller pieces to keep them from resonating. It also adds to the choke balun concept of keeping the currents from running back along the outside of the cable. Roy | ||
| Eric Holmberg New member Username: Erich Post Number: 65 Registered: 05-2002 |
Nothing like multiple low-pass filters in series I'm assuming that the ferrites on the outside of the coax don't do much to the signal on the inside of the coax (i.e. the ones we want), right? RF stuff is always a mystery to me. -Eric | ||
| Roy Howard Davis New member Username: Royhdavis Post Number: 10 Registered: 01-2003 |
Eric, That's exactly right. The signal going out to the antenna travels between the center conductor and the INSIDE of the shield. The outside of the shield should carry no currents that help the antenna transmit. All currents on the OUTSIDE of the coax should be choked off to keep them from getting back into your avionics. Roy | ||
| Jim Weir New member Username: Weirdjim Post Number: 1 Registered: 09-2003 |
It requires a 10 cent capacitor, not a $70 splitter. If you do the measurement, you find that a 10 picofarad disk ceramic capacitor (NPO or COG) with half-inch leads on both sides is a self-resonant circuit at the Glideslope frequencies, but less than a 1% drag on the VOR frequencies. Short answer...use the current VOR antenna for your VOR/LOC reception and couple off with the capacitor for your glideslope. Jim | ||
| Tom R. Hutchison New member Username: Tomhutch Post Number: 144 Registered: 01-2003 |
Today I finished installing a second comm antenna behind the rear window on the starboard side of the fuselage. I Also installed a marker beacon antenna on the port side of the baggage compartment. I had read Roy Davis's article on his designs for various antennas for the Express. http://www.nancymoon.com/antennas.htm and decided to follow his example. I duplicated his "meander line" design for both antennas. After installing, I employed an antenna analyzer to the new comm antenna to "tune" it for the comm band. I have a graph of the analyzer results below. Out of curiousity I measured the comm antenna Express installed in my tail during my builder's assist. The red line represents SWR (standing wave ratio), the lower the number, the better, over the comm band between 118-137 MHz. According to Roy, anything below 2.1 is good. The red line represents the impedence which should be around 50 ohms. The blue line is reactance and should be as low as possible. As you can see, the "Davis design" measures better than the "old" Express design that wasn't tuned and can't be now. Now I wish I could redo my tail antenna to what Express is installing now. However I expect my tail antenna to perform acceptably but it could be better according to the analysis.   I envy all the new Express builders who will benefit from all the great improvements that have been made to the Express. Tom | ||
| Tom R. Hutchison New member Username: Tomhutch Post Number: 145 Registered: 01-2003 |
I should clarify my posting above. The tail comm antenna on my project was installed about 5 years ago when I participated in the bulder's assist program. The new Express's will have the new antenna designs installed as standard. See: http://www.express-aircraft.com/News.htm#3%20Antennas%20Added%20to%20Kit%20at%20 No%20Charge | ||
| Reinhard Metz New member Username: Reinhard_metz Post Number: 20 Registered: 03-2003 |
Just in case some are headed for major despair with their comitted old Express antennas, mine have been tested transmit and receive at ranges in excess of 70 miles from about 6K ft, and they work fine. Only thing I get, though rarely, is ocasional poor tower reception if pointed exactly the right direction wrt the field antennas. Turn a few degrees and the problem is gone. I suspect some kind of engine shadowing. | ||
| Roy Howard Davis New member Username: Royhdavis Post Number: 23 Registered: 01-2003 |
The performance of even a poorly tuned antenna is still pretty good. You probably wouldn't notice the difference. The big deal about tuning and matching is to reduce radio interference issues by keeping the RF energy off the outside of the cable, which is the path back to the sensitive panel. If you don't have an RFI problem don't worry about it. But you got me thinking about the problem of antenna pattern distortion due to other objects in the aircraft. If you take a vertical dipole and look at the azimuth pattern you get a perfect circle, even power in all directions. On the computer I modeled a dipole with a 30 by 15 by 20 inch box a little over 20 feet away to simulate the engine blockage. That's the first plot. On these plot close to the center of the circle is bad, the edge is good.  The wavefront from the antenna is pretty well estabished by that distance so an object that far away, even as big as an engine has little effect. I then modeled the feedline and the cable that goes up to the strobe light in the tail. The second plot is what that does.  Keep in mind these models are rather gross, but it's a good exercise to figure out what is distorting the pattern of the antenna. I then modeled placing toroids about every foot along the feedline portion that is parallel to the antenna and the cable to the strobe light. It was only about a half dozen toroids, about a bucks worth.  You can see that the last pattern is almost back to normal with the application of only a few cheap ferrite toroids. Now the only problem is figuring out how to put the toroids on the cable that goes through the tube in the rudder. | ||
| Reinhard Metz New member Username: Reinhard_metz Post Number: 21 Registered: 03-2003 |
Roy, you're right, the engine shouldn't have an impact - since it's size is less than the wavelengths in use, and several wavelengths away, that should have been obvious to begin with. Still, there is a small arc through which the tower sometimes loses me, relatively close (even though I don't lose them - i.e. transmit problem only). And otherwise, there is no problem, even at great distances. Perhaps there is a frequency dependance also. I already have several toroids on the coax. | ||
| Roy Howard Davis New member Username: Royhdavis Post Number: 24 Registered: 01-2003 |
Reinhard, it probably isn't the coax, since it starts in the center and drops away from the antenna. When I was modeling I started with only the feedline and it didn't have that much effect. The cable to the strobe in the rudder did the most damage. If you fiddle with the spacing and length you can get a pretty deep null in one direction. I'm looking for some more tubular ferrite beads that can slide over the strobe cable but still fit inside the plastic tube that is the conduit. | ||
| Tom R. Hutchison New member Username: Tomhutch Post Number: 146 Registered: 01-2003 |
I have finished my antenna installations (except for the GPS) and put up pictures in the gallery at: http://www.express-builder.com/gallery/Antenna-Installation In case you haven't visited the Gallery lately, it now "sports" a fancy skin that gives it a little more finished look. Tom | ||
| Roy Davis New member Username: Royhdavis Post Number: 26 Registered: 01-2003 |
Hi Tom, Looks really professional! Both the aircraft work and the Web site. I didn't log in and noticed that the number of times viewed did not increment. Probably a lot more hits than registered. BTW, how did they tune up? Roy | ||
| wayne Unregistered guest |
I am happy to report that you can mount the GPS ant. on a 18" square of cheap alum flashing and fasten it up under the glair sheild on the right side and it will work just fine. As long as you didn't use alum or CF for the dash, I used plastic sheet. Same $ ,time and ease of install is my way of thinking. | ||
| Roger Spencer New member Username: Roger Post Number: 1 Registered: 05-2001 |
I have been reading the info on antenna design and placement with great interest since I'm currently at the point of installing my com antenna in the tail. The apparent affectivness shown in Roy's modeling of using the ferrite toroids in minimizing the impact of the rear position light wiring makes me wonder if a dipole could be installed near the leading edge of the vertical tail with the meander line antenna at the rear as shown in Roy's photos. The coax from the meander line antenna could run forward and that from the dipole aft and then down the front of the spar and finally forward. If ferrite toroids were placed every foot or so along the length of the coax it would seem only the effect of one antenna on the other would be significant. In measuring mine the upper part of the dipole could be layed in with a minimum separation of around 8-9 inches and would be separated by about 18-19 inches at the center increasing in the lower arm. I read somewhere that antenna should be separated by around 1/2 wavelength or more for good performance but there must be a trade-off between the affect of antenna placement on each other vs the affect of "field of view" of antenna located forward, i.e. people, other electrical lines and metal in engines and instrument panel on the antenna's performance. Does anyone have an opinion on this or has anyone tried to model it to see the effect on pattern? What happens to the pattern of an antenna located in the baggage or rear seat area when you throw in your umbrella or golf clubs and people? Is it very sensitive to position or placement of conductive materials such as this? | ||
| Roy Davis New member Username: Royhdavis Post Number: 29 Registered: 01-2003 |
The reason I put the second COM antenna farther forward was to separate the two COM antennas as much as possible. When you transmit on one antenna you want to try to keep as much energy out of the second receiver as possible. With one antenna in the tail and one in the baggage compartment you should be able to still hear strong signals on the second receiver while transmitting on the first. Useful if one pilot is getting weather while the other is talking to ATC. Another aspect is the distortion of the antenna pattern when you put two resonant elements close together. There will be peaks and nulls introduced that will make it seem like one antenna is far better than the other at times. I'd rather not have to switch radios looking for the good one all the time. Most bodies and baggage will have little effect on the antenna in the side wall, however you have a good point about something like a golf club that is metalic and about a quarter wavelength long will distort the pattern. I would take my chances on the occasional golf clubs instead of putting the two antennas right next to each other and having the problem worse and all the time. BTW, even though radio travels in waves like light, because the wavelength is so long even large objects like the engine have little blockage effect as long as they are separated from the antenna by some distance. It is conductive objects that resonate at the frequency of operation placed within about a quarter wavelength that mess up the radiation pattern. | ||
| Jim Weir New member Username: Weirdjim Post Number: 3 Registered: 09-2003 |
1. Separation of antennas is a good thing. The more separator the gooder. 2. Human bodies WILL have a detrimental effect if placed near an antenna. Humans are a 95% saltwater sack, and I guarantee you saltwater is a good conductor. Don't believe me? Stick one finger of each hand into the pins on a wall outlet. 3. Engines (and all other massive blocks of metal) WILL shadow radio waves. To get an idea of the shadow, put your eye at the geometric center of the antenna and sight into the metal mass. Note the angle that the metal mass subtends. With a slight correction factor for diffraction, this will be the shadow area for that particular antenna at that particular location. Jim | ||
| Roy Davis New member Username: Royhdavis Post Number: 30 Registered: 01-2003 |
Hi Jim, We like to think of radio waves acting like light waves, but that old wavelength factor and what it does to diffraction makes for non-intuitive results. For instance, an engine, as large as it is, is too small to completely block VHF radio signals. It does slightly distort the pattern, however the result may be more gain in that particular direction or less. Take a look at the first antenna pattern in my January 18th post above. There signal in the direction of the engine (to the left) is actually improved. If you think this is impossible then think about how a Yagi antenna array works. By purposly putting metalic objects in the way the antenna gain can be greatly increased. We worry about the large conductive objects, engines and ourselves, but the wires and cables that are routed close to the antenna have a much larger effect. | ||
| Charles M. Robinson New member Username: F15epilot Post Number: 5 Registered: 02-2004 |
Fellow Express Enthsuiasts, I spent the last week getting Auriga kit 15 back on the road to completion. The previous owner had a lot of work done, but there are some changes I want to make. As he lost interest in completing this project about 2 1/2 years ago (he had a CT and an FT to fly--he built both of those, first), a lot of 'fresh' info is not available and I'm still backtracking to re-engineer things. One of those is the avionics setup. The plane has a dipole antenna on the starbord side, inclined upward from a point about a foot behind the rear cargo bulkhead near the bottom of the tail boom, and proceeding on a straight line up to the top of the tail near the bulkhead where the elevator/rudder mechanisms are attached. There are two BNC connectors comming out of the rear of the tail--one obviously the tail comm antenna; the other labeled "#2 Exx (where the x's resemble a Y and L). I'm guessing the other BNC is for the ELT--he can't remember. He has a separate transponder antenna on the bottom of the fuselage. All the avionics (NARCO MK-12D and a COM-111TSO) are installed. His first thought was that the dipole was connected to the second COMM--I'm guessing that wasn't the intention, as it would be a poor comm antenna. My guess is that it was supposed to be the marker beacon antenna. He has the radios set with a single switch to toggle between the two--switches everything (8-poles) from speaker, phones, TX-ring, mic, etc. I plan to put in a standard COMM pannel where I can use both COMMs requiring two COMM antennas. I've gone over the posts above and plan to add whatever I need to get to a dual NAV and dual COMM setup. Any suggestions on where to get a second comm antenna to mount inside the fuselage (aft of the rear window)? Can anyone confirm that early Aurigas had the ELT in the rudder opposite the main COMM? I've got the original builder's pics(the guy before me was #3 in the train) of the wings, and they show the VOR antennas...but I'll check those, too. Looking for info on the comm antenna and elt. Thanks. Chuck | ||
| amoghadd Unregistered guest |
Charles, I just installed my comm antenna across from the baggage door. Although, I have not installed the torroids yet. Look at http://www.nancymoon.com/antennas.htm for instructions/ coordinates. Roy (or other knowledgeable folks), I have been having a hard time getting torroids from RST (multiple calls & email have not yielded any success in receiving parts). Digikey has several torroids (although, I have to buy 1000 of them for $60-$80)!I'll have plenty to share with other folks when I get them). The torroids are 5 mm IDs, 100 MHz but there is a choice for impedance of 33, 37, & 62. Which one should I get? Please advice. Thanks, Ali | ||
| Jim Ward New member Username: Jehward Post Number: 11 Registered: 02-2000 |
Chuck: In the CT version, two antennas were placed in the tail. 1) a marker beacon antenna, made of copper foil, was run along the leading edge of the horiz. stabilizer. 2) a comm antenna, also of foil, was run up and down the forward shear web of the vertical stabilizer. Transponder and ELT antennas were typically added in the area behind the baggage area (bulkhead#162). For a second comm, a lot of people have added that as Ali mentioned above. A Nav antenna was typically put in each wing. Hope that helps some. Jim | ||
| Roy Davis New member Username: Royhdavis Post Number: 31 Registered: 01-2003 |
Try Palomar Engineers. They sell to amateur radio people who are cheap. I am one. http://www.palomar-engineers.com/Ferrite_Beads/ferrite_beads.html Their FB-24-61 fits RG-58 or RG-55 cable and works from 100 MHz to 2 GHz. $4.85/dozen. The shipping will cost you more than the product. | ||
| ali moghaddas New member Username: Amoghadd Post Number: 4 Registered: 09-2003 |
Roy, Thanks for your recommendation. I got the torroids from Palomar Engineers in 3 days (Excellent, dependable people)! They did not have the -61, after talking to Palomar folks, they recommended FB24-43s. I installed them, and tested the antenna using my portable radio. It works fine. | ||
| Jim Weir New member Username: Weirdjim Post Number: 5 Registered: 09-2003 |
A couple of comments: 1. Toroid mix -61 is "good" (a purely subjective term) from 200 kilohertz to 100 MHz., neither of which is within the aircraft 108-137 MHz. frequency band. 2. Toroid mix -43 is good from about 10 kHz. to 10 MHz., and is even worse for aircraft use than mix -61. 3. I used to do my engineering across the street from Palomar Engineers back in the 1970s, and actually did a little engineering on the side for them. They are nice folks and know a lot about CB radio. 4. To say that you "tested" your antenna by hooking a portable radio to it and it "worked" is like saying you "tested" your water supply by drinking a glass of the water and it "tasted OK". In neither case have you "tested" anything but the grossest function test. Jim | ||
| Roy Davis New member Username: Royhdavis Post Number: 34 Registered: 01-2003 |
Jim, The frequency ranges you quote are for use as a transformer where the radio energy is being coupled through the ferrite core. In the configuration used here the ferrite makes up the choke balun that has to present a high impedance to the radio energy (block it) as opposed to couple it so the "RFI" frequency range is appropriate. The -43 is recommended for 1 to 1000 MHz and the -61 for 100 to 2000 MHz. The mixes recommended for higher frequency transformer use are not recommended for RFI use at all. | ||
| Reinhard Metz New member Username: Reinhard_metz Post Number: 29 Registered: 03-2003 |
OK, I can't resist to jump into this fray - The way I understand the use of ferrite beads on a coax feed to an antenna is indead as a balun, and the way a balun like this works is that it is a one turn transformer for both the inner and outer conductor. Therefore, imbalanced currents, such as those picked up on the shield, induce a balancing current into the other conductor, i.e. the inner conductor. While it is true that you can use a lossy toroid, it will induce less balancing current in the inner conductor in exchange for dissipating the power from the shield current in the toroid, which is not a good idea, both from a point of view of efficiency (you'd like to re-couple the picked-up power to the antenna), and the heat is not good for the toroid, in fact they may get too hot in some situations. Therefore, I think it is desireable to use toroids that act well in the transformer mode at the frequency of operation. | ||
| Jim Weir New member Username: Weirdjim Post Number: 6 Registered: 09-2003 |
Reinhard Metz... What he said. Jim | ||
| LOUIE LACY New member Username: Llacy Post Number: 74 Registered: 10-1999 |
If you push the microphone button and say Hello Center and he says back Radar Contact what else do you need?"much ado about nothing" | ||
| - JD Unregistered guest |
Agreed with Jim & Reinhard... I think the difference is whether you are flying smack dab in the middle of Class B airspace, or IFR over mountainous terrain far away from civilization and a RCO. Antenna efficiency can be critical. Press the button and nobody responds, then your challenges (and heart rate) just multiplied. Been there... done that. No thanks. | ||
| Roy Davis New member Username: Royhdavis Post Number: 36 Registered: 01-2003 |
The balun is not going to effect the efficiency of the antenna that much. The real payback for the choke balun is to provide a high impedance for the path back along the coax, which keeps the radio energy from being coupled into other intruments and other antennas in the aircraft. These meander line antennas are designed to closely match the coaxial cable impedance of 50 Ohms instead of the natural impedance of a straight dipole of 72 Ohm. You often see straight dipoles bent back toward the feedline in a V shape, which lowers the impedance but also increases the coupling to the outer shield. I'm struggling with the concept of the coaxial one turn transformer. The shield forms a Faraday enclosure around the center conductor so the fields coupled by the ferrite cannot be coupled to the center conductor. I think the operation of the choke balun is a simple Voltage divider, and that by presenting a very high impedance on the path along the shield the current are forced back to the opposite arm of the antenna. Since it is a high impedance (the RFI mode if you will) very little current flows that way and the losses in the ferrite itself don't amount to much. If the antenna were not matched well to the feedline then this simple choke balun mode wouldn't work very well. Bottom line, good antenna with cheap balun is better than a bad antenna with a good balun. | ||
| Jay Villalva New member Username: Jay Post Number: 14 Registered: 11-1999 |
Does anyone have copies of the files on meandering antennas that used to be posted on Roy and Nancy's website? The link above no longer seems to be working. Specifcally, I'd like to get the dimensions for the marker beacon antenna. | ||
| Jerry Sjostrand New member Username: Jerry Post Number: 82 Registered: 08-1999 |
I did a search under "utilities" and found this link. http://www.express-builder.com/old/forum/messages/4/42.html Jerry | ||
| Jay Villalva New member Username: Jay Post Number: 15 Registered: 11-1999 |
Thanks Jerry – I’ve tried all the links (including www.nancymoon/antennas.htm), I was not successful in retrieving the antenna information. I downloaded the information two computers ago and can’t find the files. | ||
| wayne norris New member Username: Wayne_n Post Number: 9 Registered: 09-2003 |
for my marker ant. I took a 36" long 20g wire, wrapped it around a 12" long 1/4" dowel, shrink wrap tube over it. hook it u to the audio panel MB local. Mine just lays on the floor under the carpet next to the side of the nose gear box. works fine. and cheap | ||
| Jay Villalva New member Username: Jay Post Number: 16 Registered: 11-1999 |
Thanks Wayne, I'm working on making an antenna for a class that I'm taking at ERAU. We have to make an antenna for a specific frequency (75 Mhz in this case), then tune it to some pretty tight specifications. Since I haven't made a marker beacon antenna for my Express, I figured this would be a good time to get it done. I'm using copper tape like the rest of the antennas in my Express. If I have time, it would interesting to put an antenna together as you described and test it, too. Roy Davis had information (including layout dimensions) for the meandering antennas. If any one has copies of Roy's data or knows how to get in contact with him - that would be great. | ||
| wayne norris New member Username: Wayne_n Post Number: 10 Registered: 09-2003 |
well then you know there is a particular length for a MB. it is long , that is why I wrapped it around a dowel, now the length is only about a foot. ie. like the canoe type you see on the bellys. cheap low tech and works is all I care about. cheers,w |