Hidden stealth aircraft antenna

Discussion in 'Maintenance Bay' started by OKDonn, Aug 24, 2019.

  1. Cap'n Jack

    Cap'n Jack Final Approach

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    It seems that way, trying to get a piece of equipment to pass the CE RF emissions standards. It's amazing how much gets through a tiny hole or slot.
     
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  2. Stephen Poole

    Stephen Poole Pre-takeoff checklist

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    THIS. :yeahthat:
     
  3. wsuffa

    wsuffa Touchdown! Greaser!

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    Then there are slot antennas.... ;)
     
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  4. OKDonn

    OKDonn Pre-Flight

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    The OP here - I want to operate the aviation comm frequencies (2M and 10M will be later, after the aircraft is flying). I'm not trying to QRP DX here, just adequate performance to talk to center, etc.
    I'll likely do some experimenting and see what I can come up with - antennas in several different locations and field strength measuring. It will be a while yet, I'm welding the engine mount now.
     
  5. Ghery

    Ghery Touchdown! Greaser!

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    Where did Lambda / 10 come from? A good rule of thumb, used for decades in the EMC world. Openings smaller than that have little (but not "none") impact of the shielding effectiveness of a chassis. Larger than that, and then multiples, and you have significant impact on the shielding effectiveness.

    Now, the OP needs to build a comm antenna that he would like to hide inside the body of his plane to minimize wind resistance. I suspect that experimentation should work well for him. He'll find something that works, and learn more about antenna design. Not a bad combination. He's building a rag and tube airplane, so he shouldn't have too much trouble with the fuselage shielding the antenna too much. Distort the pattern? Probably. Detune the antenna if it is located near metal structure? Probably.
     
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  6. weirdjim

    weirdjim Ejection Handle Pulled

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    Ghery, I thought I said as much ten or so posts ago. We did the tests with a typical rag and tube airplane for a manufacturer. It cut the usable range of the com radio down somewhere between 75 and 90%. Not zero. Not half. But perhaps we didn't pick exactly the right spot for half. Or zero. He's got a toothpicks and tissue paper wing. Nearly perfectly transparent. Put a dipole with as much slant verticality as you can get in the wing. Or a ground plane with the radiator bent over at the top.

    As you said, you've got "some" radiation pattern effects, "some" detuning if you get "too" close to the steel tubes. Why not eliminate the variables and go with a relatively simple wing antenna?

    Jim
     
  7. OKDonn

    OKDonn Pre-Flight

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    I think that the 5" of slant available in the wing would be as in-effective as a ground plane inside the tube fuselage with a solid firewall in the front.
    I'll either figure out how to load the gear legs, or build a wood/fiberglass vertical stabilizer with an embedded antenna.
    The wings already have the nav and transponder antennas.
     
  8. weirdjim

    weirdjim Ejection Handle Pulled

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    Don'cha just love people coming on here asking for advice and then telling us their opinion on what will and will not work. If I hadn't done it successfully I sure as hell would not have suggested it.

    Jim
     
  9. OKDonn

    OKDonn Pre-Flight

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    Well, if I didn't have to cut open the completed plywood skinned wings to try it, I'd add it to the list of experiments. As it is, I'll explore the other choices.
    I really hate leaving those gear legs just hanging there creating drag in between landings though ...
     
  10. bluerooster

    bluerooster Pattern Altitude

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    Is there room to install a 1/2 wave, center fed, vertically polarized dipole, inside the fuselage? If so, there ya go. The antenna system will present a 50 ohm load with no tuner, and will be a good radiator. But the metal framework may (will) interfere with radiated power.

    To use the landing gear for antenna, won't you have to isolate it from the rest of the fuselage? And you'd have to use a tuner.

    The wings are plywood skinned? Will you be putting fabric over the plywood, or just paint? I would think a wire antenna could be installed on the wing exterior with some pinked tape. Surely there are inspection holes where you can gain access to run coax.
     
  11. flyingbrit

    flyingbrit Pre-takeoff checklist

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    It is quite possible to use a grounded pole for an antenna, and such is sometimes used for MW broadcasting. It is called a shunt fed or slant wire antenna. There are plenty of articles on line regarding this, for example:

    http://www.vias.org/radioanteng/radio_antenna_engineering_02_03_04.html

    A tuner is definitely required. Not a problem for a broadcaster operating on a single frequency, but a major complication for the aircraft COM band. That's one reason I did not suggest it to the OP; the other reason being that adding a slant wire goes against his desire for "hidden-stealth".
     
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  12. flyingbrit

    flyingbrit Pre-takeoff checklist

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    With all due respect, forums like this are not a classroom where the instructor's words are treated as gospel and if you don't believe them you won't pass the test! More like a discussion among peers, where some dissension is to be expected and occasionally can be enlightening.
     
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  13. wsuffa

    wsuffa Touchdown! Greaser!

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    Yeah and no. At MW a shunt feed works (though slant-wire shunt feeds are really not used anymore in favor of folded unipoles, which can be modeled as transmission line). At VHF, the stray capacitance and other considerations work against a shunt feed, though they might be used in certain specialize applications. (think military, who might also use autotuners). I spent a lot of time working with antennas for the broadcast industry and designed a number of MW systems - you really don't want to use a shunt feed in a non-directional aviation system (shunt feeds are more often used in directional yagi feedpoints, but those are directional).

    For regular folks, though, a quarter-wave antenna over a ground plane (or dipole in situations without ground) generally have enough bandwidth and "close enough" match to the transmitter to work well. I can't find it at the moment, but I've got a couple of pictures off a VNA of a 1/4 wave antenna at 2 meters compared to a 5/8 wave antenna with matching section at the bottom. The 1/4 wave is pretty flat match-wise over the whole band and then some - the 5/8 is much narrower.
     
  14. OKDonn

    OKDonn Pre-Flight

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    I was thinking of the feed like on a 2 meter j-pole antenna, however my radiating element (gear leg) is much too short. I'm now thinking that building a ¼ wave ground plane into the vertical stabilizer is the better approach, though I'll try several different locations and see what I find. Note that a single element ¼ wave whip will not cover the entire com band - wide and parallel different length elements are required, best I can tell (an old article from weirdjim).
    I have what I call the pipe organ antenna that is 6 1½" diameter aluminum tubes around a piece of PVC sewer pipe fed by a single coax that covers the ham bands from 14MHz to 50MHz - the ground plane version of the fan dipole. I works great (on top of the metal hangar roof).
     
  15. weirdjim

    weirdjim Ejection Handle Pulled

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    Not to mention hanging more ironmongery out in the slipstream to an airplane that has a hard time getting out of its own way to begin with.

    JIm
     
  16. weirdjim

    weirdjim Ejection Handle Pulled

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    Hey, Brit ...

    It is one thing for me to say, "I think this might work." and have somebody else (even the OP) say, "I don't think that will work.". It is quite another for me to say, " I did this and it worked like this." and have another person say, "I don't think that will work." That says that I really didn't do what I said I did, or that I'm intentionally sending out false information.

    As for the classroom, in my 52 years in the college system, I have said innumerable times that I probably didn't say it as well as I might or that I didn't write the test question well enough. Gospel? Not hardly. Only a damned fool sets themselves up as infallible.

    Jim
     
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  17. wsuffa

    wsuffa Touchdown! Greaser!

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    VNA.JPG
    If you're going to bother with a jpole, just do a straight 1/4 wave antenna. The ground under a whip is key. You may well find that a 1/4 wave whip with a good ground plane works perfectly well - most commercial antennas on metal GA planes are 1/4 wave antennas. They're bent back, but work acceptably with tranceivers in common use. No, it won't be perfect, but it should be acceptable.

    The attached photo shows a VNA VSWR plot for a quarter-wave whip at 2 meters on the roof of my metal truck. I'd have no hesitation using something with a plot like this in the aviation COM band on a plane. Of course, you have a rag/fabric plane, so the ground plane will be different, and therefore the radiation characteristics will be different.

    I will certainly defer to Jim and his tests (especially the absorption and blocking tests - radiation efficiency is the other part of this) - all I'm trying to point out is that a single element whip can work and have the bandwidth from an impedance match standpoint under the right circumstances. YMMV, of course, and the only way to learn is to do it. If you can borrow a VNA or antenna analyzer from a ham, it'll help. And well worth buying an inexpensive one if you're going to do much antenna work. Decent field strength measurement equipment is more expensive (even though I have a spectrum analyzer, I don't have a calibrated antenna).
     
  18. OKDonn

    OKDonn Pre-Flight

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    What? A Tailwind has a hard time getting out of it's own way? Red Hamilton averages 230+ mph in the SARL races in his ---
    Everything will be in fairings, inside the fuselage, etc. - that's the whole point of this exercise ...
     
  19. OKDonn

    OKDonn Pre-Flight

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    Yes, I understand the importance of a good ground plane - and I will have one if I go that route. I've never gotten that flat a response from a ¼ wave 2M whip in the roof of my car, at least according to the MFJ analyzer.

    I will figure a way to test the dipole just a few degrees off horizontal for signal strength into a vertical antenna - doesn't seem feasable, but if Jim has done it successfully, it's worth considering.
     
  20. Cooter

    Cooter Ejection Handle Pulled

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    Maybe you forgot this one.
    It seems you want this forum to operate on your terms. Who are you to tell someone to not participate in a public forum? You think that because you have knowledge and experience in one area you get to dictate who contributes? POA is not your classroom. Hopefully your students were treated with more respect.
     
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  21. Ghery

    Ghery Touchdown! Greaser!

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    So the question remains (and not answered in your post above) - where was the antenna located? Close to, and more or less parallel to, a structural element of the aircraft? Or was it somewhere farther away? "Close" to the structure and all bets are off, as you note. You also have the issue of cross polarization. A significant loss there, as well.

    Mounting an antenna in the wing sounds good, as long as the cross-polarization loss is acceptable. Heck, if there was space and the wing was metal a slot antenna would be a good idea to try. Plenty of room. No aerodynamic drag, either. We used them on MX decades ago for the flight safety system.

    Whatever, it will be a good series of experiments and a learning opportunity for the OP.
     
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  22. weirdjim

    weirdjim Ejection Handle Pulled

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    Just for fits and giggles, this fellow had a carbon fiber fuselage but a glass vertical stab. Never did get a report back on how well it worked.



    Jim
     

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  23. Palmpilot

    Palmpilot Touchdown! Greaser!

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    Wow, a 22.5 foot radiating element - I wonder how he managed that! ;)
     
  24. weirdjim

    weirdjim Ejection Handle Pulled

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    'scuse me, Cooter, the OP clearly said in his original post that he wanted to try to load the gear legs in the aviation com band. This fellows reply was asking which ham band he wanted to use. If the subject for the day is donuts and the student asks how it applies to road graders I'm likely to ask what the connection is.

    Jim
     
  25. weirdjim

    weirdjim Ejection Handle Pulled

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    What's an apostrophe between friends? :cheerswine:

    Jim
     
  26. weirdjim

    weirdjim Ejection Handle Pulled

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    I pitched Cessna on the slot antenna a lot of years ago and didn't get the courtesy of a reply. I think it was the NIH factor that got them.

    Jim
     
  27. OKDonn

    OKDonn Pre-Flight

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    I love that VNA and it's display! I need one that has a lower frequency range, at least including the aviation band, but even better all the HF ham bands!
    I wonder if this one is any good, and will do the same thing as the AAI?
    https://www.amazon.com/AURSINC-Analyzer-Measuring-Parameters-Standing/dp/B07T6LXNTV
     
  28. wsuffa

    wsuffa Touchdown! Greaser!

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    I also have one of those. I like what it can do, but it is considerably less intuitive to use than the N1201SA. I have not put it fully through its paces (part of my HF antenna is on the ground), but I did use it to pretty quickly adjust a UHF duplexer - faster than using the spectrum analyzer. Do some checking on the various sites - there is not a lot of documentation out there. But it appears to work and gives results similar to the 1201.

    The 1201 is quick and easy to use. But only has one port and a limited frequency range. The Nano is 2 port and a wider frequency range.... It needs a good case. I like that the Nano will do a Smith chart.
     
  29. weirdjim

    weirdjim Ejection Handle Pulled

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    I can see right now that my HP 608 and slotted line are anachronistic :cheers:

    Jim
     
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  30. bluerooster

    bluerooster Pattern Altitude

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    Jim knows his antennae. And has probably forgotten more than I will ever learn. But anyway;
    I just went to the shop to test a hunch. What I found is this; a "down and dirty" 1/2 wave dipole cut for the center of the aircraft com band, is 44" long. 22" each leg. (when hanging about 6' off the wooden floor of the shop, fed with ~30' of RG58-u [I didn't want to cut the coax for an experiment]) Total time spent was about 20 min. in construction. (15 of that was waiting on the soldering iron to heat up) And about 15 min. cutting it to frequency.
    Is there anywhere that you can hang a 44" antenna, vertical, in your Tailwind, that it won't come into contact with any metal surface?
    If you build the vertical stabilizer out of something invisible to RF, there ya go. It won't be as good as a 22" whip in the slipstream with the tubing as a counterpoise, but it should work.

    Bear in mind that the antenna is an "antenna system", not just the radiating element, but also the counterpoise, and feedline.
     
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  31. jonvcaples

    jonvcaples Pre-takeoff checklist

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    It all boils down to how hard you want to make things for yourself. Imagine, you own a Ferrari but have decided to save money on gas. What is the best way to do this? Hmm watch my driving habits avoid going for the need for speed or install a volkswagen engine? Yes, you could do the engine swap but most folks would probably choose to change their driving...or go buy an econobox.
     
  32. donjohnston

    donjohnston Line Up and Wait

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    There is information from a few people which I accept without hesitation. George Braly or Walter Atkinson on engines, Burt Rutan on aerodynamics, and Jim Weir on antennas. :D
     
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  33. flyingbrit

    flyingbrit Pre-takeoff checklist

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    I agree, at least in theory. If you slanted the whole 44" dipole by 5" this would put the antenna 84 degrees away from the desired vertical. This would cause a polarization loss of about 24dB...not good. See the calculations/measurements here:

    https://interferencetechnology.com/antenna-polarization-measurements/

    Even if you slanted each 22" element by 5" (a la TV rabbit ears) you would still have a 77 degree mismatch or about 15dB loss.

    I suspect that Jim's success with this arrangement was due more to external factors than the small slant. Even a wood wing airplane is likely to have other metal inside for example nav light wiring, fuel tanks, aileron and flap control cables/rods. Conceivably this could change the antenna characteristics to one's advantage. How to predict that is way over my head.
     
  34. OKDonn

    OKDonn Pre-Flight

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    Thanks for the calculations and the link to the source - I'll use it!
    Yes. my wood wing has an almost full span flap/aileron torque tube, nav light wiring, coax for a VOR antenna in one wing and a transponder antenna in the other, plus the fittings for the strut attachment about halfway on the main spar. Not an ideal situation, unless the dipole loads the strut, which rotates the polarity a bit?
    I'll try loading the gear leg first, then if I can't do that, I'll move back to the vertical stabilizer, using aluminum screen under it and the fabric for a ground plane. There is a way!
     
  35. OKDonn

    OKDonn Pre-Flight

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    I do have a history of making things hard for myself - but in this case you got the analogy backwards - I'm trying to make my econobox perform like the Ferrari.
     
  36. jonvcaples

    jonvcaples Pre-takeoff checklist

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    I resemble that! :rolleyes:
     
  37. Ghery

    Ghery Touchdown! Greaser!

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    Somehow that doesn't surprise me. Of course, with all the other "stuff" hanging out in the breeze in a 172 or 182 the addition of a couple whips probably isn't much. But to not even reply?

    Yup. And I remember when the HP 141T was state of the art for spectrum analyzers. :p
     
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  38. flyingbrit

    flyingbrit Pre-takeoff checklist

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    Same here. I got so used to twiddling the multiple knobs of the 141T that the modern instruments with push buttons and a single knob slowed me down. Can't say I miss the flood gun CRT "storage" feature though.
     
  39. wsuffa

    wsuffa Touchdown! Greaser!

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    Same here. And when the Tek 7L12 & 7L13 were the "standard" that even the FCC used.
     
  40. weirdjim

    weirdjim Ejection Handle Pulled

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    The storage "feature" on the 141T was the biggest mistake since they let Bill and Dave go into retirement.

    Although I must say, struggling with students trying to make sense of a dual-trace triggered delayed sweep scope (for HOURS and HOURS of class time) became somewhat of a joy with the Tek digital two button PHD scopes. (PHD, Push Here Dummy).

    Jim