Hidden stealth aircraft antenna

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OKDonn

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Don
I'm new to the forum, so let me know if I'm stepping out of bounds ...

I'm building an experiment Tailwind with the Wittman steel rod gear legs. I'm also a general class ham who knows just enough about antennas to get into trouble, so here goes.
I've read that an antenna will radiate just fine whether it's resonant or not, the matching to it matters to the transmitter, reflected power in specific. That having been said, I look at those gear legs and a j-pole comes to mind, though they are too short for the classic j-pole, too long to be 1/4 wave elements, plus I suspect that the wheels on the end will act as capacitive loads.
I really don't want an antenna sticking out in the airstream, so keep looking for other options.
Does anyone think it would be possible to match a comm radio to these 28" legs that are physically grounded to the tubular airframe?

Anyone know of an auto-tumer that works in the aviation comm band?

Clear as mud? - Thanks for putting up with my crazy idea.
 
OKDonn said:
Thanks for putting up with my crazy idea.
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WeirdJim is definitely your guy. But in my very limited experience in the aviation AM/FM freq spec --length is important. Plus it would be interesting how much wattage would be required to push out a signal from some thick steel tubes.

But on a different note... isn't the Tailwind fabric covered? If so you could mount a conventional antenna in the interior tail area and with the appropriate ground plane still xmit vhf with some range restriction. But Jim is the expert in this.
 
Bell206 said:
WeirdJim is definitely your guy. But in my very limited experience in the aviation AM/FM freq spec --length is important. Plus it would be interesting how much wattage would be required to push out a signal from some thick steel tubes.

But on a different note... isn't the Tailwind fabric covered? If so you could mount a conventional antenna in the interior tail area and with the appropriate ground plane still xmit vhf with some range restriction. But Jim is the expert in this.
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That has been done - but the firewall blocks the signal in the forward direction, not always good when approaching a controlled airport. All those grounded steel tubes less than a 1/4 wave from the antenna can't help either.

I've also toyed with loading up the leading edge tube of the vertical stabilizer, figuring that what's behind me is of less interest than what's in front of me.
 
The first statement that an "antenna will radiate just fine regardless of length" is bunk. The answer is most aircraft designs are pretty broadbanded, but that's not to say length is unimportant. If you have a mismatch, your modern (transistor final) radio will put out much less power than it would in a properly designed system.

Inside a non-metal skinned aircraft (even with metal tubes), you have a lot of options. You can do anything from a simple dipole on up. You just need to design something to hold it away from the metal parts. You want to at least try for vertical polarization for comm antennas.
 
flyingron said:
The first statement that an "antenna will radiate just fine regardless of length" is bunk. The answer is most aircraft designs are pretty broadbanded, but that's not to say length is unimportant. If you have a mismatch, your modern (transistor final) radio will put out much less power than it would in a properly designed system.
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Actually, it's the impedance that the transmitter "sees" at it's output connection, and the impedance that the transmission line "sees" where it connects to the antenna that count. As long as those two connections are happy, the system will work. It's just that the simplest way to accomplish that is when all elements are 50 ohms. When some of the elements are not 50 ohms, you use matching elements or transformers, etc. to make them look like 50 ohms to the other part.
 
You could look at the Archer designs for in wing antennae. The Nav versions work very well (I've got on in my right wing) but the Comm versions get less respect. I use standard whips on the belly for comm.
 
Ravioli said:
You could look at the Archer designs for in wing antennae. The Nav versions work very well (I've got on in my right wing) but the Comm versions get less respect. I use standard whips on the belly for comm.
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Yes, the horizontally polarized antennas are good in a non-metal wing. My comm and transponder antennas are already in the wood wings. I have not found a physically horizontal design that is vertically polarized yet ...
 
Aside from impedance matching, which can be handled in various ways as has been mentioned, another way that length matters is that for efficiency you want the radiation resistance to be larger than the loss resistance. My understanding is that this becomes harder to achieve when the length is a lot shorter than 1/4 wavelength.
 
True, short antennas are not usually very effective. In my case, the ¼ wavelength is 22" and the gear leg is 28", plus a wheel on the end which effectively lengthens the antenna via capacitive loading (I think). I'm going to experiment with variuos matching stubs and see what I can accomplish, but was hoping that someone had BTDT ...
 
flyingron said:
The first statement that an "antenna will radiate just fine regardless of length" is bunk. The answer is most aircraft designs are pretty broadbanded, but that's not to say length is unimportant. If you have a mismatch, your modern (transistor final) radio will put out much less power than it would in a properly designed system.

Amen to that. jw

Inside a non-metal skinned aircraft (even with metal tubes), you have a lot of options. You can do anything from a simple dipole on up. You just need to design something to hold it away from the metal parts. You want to at least try for vertical polarization for comm antennas.
Click to expand...

Inside of a steel tube fuselage, you are in what is called a "Faraday Cage". Same effect as driving your car through a long concrete tunnel. The concrete has nothing to do with the fact that your AM/FM radio signal goes away, but the steel rebar holding the concrete together puts you effectively inside of a metal screened enclosure.

I can't comment on the Tailwind because I've never seen an x-ray view of the internal structural components and I try not to guess at stuff I've never seen. Point me to such a picture and i'll take my best rectorandom estimate.

Jim
 
OKDonn said:
Yes, the horizontally polarized antennas are good in a non-metal wing. My comm and transponder antennas are already in the wood wings. I have not found a physically horizontal design that is vertically polarized yet ...
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If your com antenna is already in your wood wings, why are we having this discussion?

Jim
 
VNA is your friend. An antenna needs to present a decent impudence match at the radio, and preferably at the feed line connection to the antenna.... That's why antenna tuners are used in ham radio. And in commercial broadcast, too - for medium wave (AM radio) there are tuning units at the base of each tower.... For FM and TV, the antenna is designed to provide a good impedance match, and is tuned by a matching section, adjustment of the element lengths, or the feed point design. While you CAN match at the output of the radio because the feed line merely rotates the impudence, but can present high voltage points that can jump through the insulation.
 
Ah - I mis-typed - it is the NAV antenna that's in the wing, not the comm. What I'm interested in is loading the steel rod landing gear for comm - it's 28" long.
 
OKDonn said:
Ah - I mis-typed - it is the NAV antenna that's in the wing, not the comm. What I'm interested in is loading the steel rod landing gear for comm - it's 28" long.
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It isn't going to happen. HOWEVER, the antenna is quite forgiving of perfect horizontal legs. Me? I find the fattest spot in the wing and run a dipole made out of copper tape on a dowel from the lowest spot to the highest spot -- maximum verticality --- and try it. Best shot you've got.

JIm
 
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wsuffa said:
VNA is your friend. An antenna needs to present a decent impudence match at the radio, and preferably at the feed line connection to the antenna.... That's why antenna tuners are used in ham radio. And in commercial broadcast, too - for medium wave (AM radio) there are tuning units at the base of each tower.... For FM and TV, the antenna is designed to provide a good impedance match, and is tuned by a matching section, adjustment of the element lengths, or the feed point design. While you CAN match at the output of the radio because the feed line merely rotates the impudence, but can present high voltage points that can jump through the insulation.
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And you can use such devices where the instantaneous bandwidth is in a couple of percent, and even at that, you wind up retuning from one place in the band to another. For example, on 15 meters, the band is from 21.0-21.450. .450/21 = 2.1%. However, the aircraft VHF COM band is from 118-137, 19 MHz. Band center is around 127 MHz, or an instantaneous bandwidth of 19/127 or 15%. That tuner dog just ain't gonna hunt.

Jim
 
OK - that's why I asked ..... I know just enough to be dangerous.
 
weirdjim said:
And you can use such devices where the instantaneous bandwidth is in a couple of percent, and even at that, you wind up retuning from one place in the band to another. For example, on 15 meters, the band is from 21.0-21.450. .450/21 = 2.1%. However, the aircraft VHF COM band is from 118-137, 19 MHz. Band center is around 127 MHz, or an instantaneous bandwidth of 19/127 or 15%. That tuner dog just ain't gonna hunt.

Jim
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You would be surprised at what's done in certain applications...

That said, I don't disagree that it's a challenge. Besides at VHF frequencies, a quarter wave is short enough to not be a real issue space-wise.

I agree that I wouldn't try to load the legs - one thing to use them as passive directional elements, but not some thing to try and load.
 
wsuffa said:
VNA is your friend. An antenna needs to present a decent impudence match at the radio, and preferably at the feed line connection to the antenna.... That's why antenna tuners are used in ham radio. And in commercial broadcast, too - for medium wave (AM radio) there are tuning units at the base of each tower.... For FM and TV, the antenna is designed to provide a good impedance match, and is tuned by a matching section, adjustment of the element lengths, or the feed point design. While you CAN match at the output of the radio because the feed line merely rotates the impudence, but can present high voltage points that can jump through the insulation.
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Also, putting the matching network between the radio and the feed line has the disadvantage that the losses in the feed line go up as the SWR goes up. For an open wire line or a gas dielectric coax, no problem, but for solid dielectric lines, it can get to be significant. especially at VHF and above.
 
I'm going to have to take issue with the esteemed Mr. W. The metal frame of an aircraft is NOT going to be a Faraday Cage at the aircraft frequencies. In order to be reasonable at blocking RF in that manner, no hole can be bigger than 1/10 wavelength, in this case about 9". While there may be some pattern distortion, it's not going to block the signal.

A PP points out, that while power is maximally transferred with a good impedance match, that doesn't mean that you have an effective radiator.
 
flyingron said:
I'm going to have to take issue with the esteemed Mr. W. The metal frame of an aircraft is NOT going to be a Faraday Cage at the aircraft frequencies.
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True, but that's a bit pedantic. I knew what @weirdjim meant: the metal fuselage would shield or reflect most of the signal. Maybe it should be a called a "resonant cavity with random slot aperatures and a spirographic radiation pattern." I'll let you figure out the resonant frequency(ies). :)

To the OP: theoretically, you could use the landing gear legs as a radiator, but you'd get better performance with a coat hanger sticking out a window. Seriously. As Jim and others have said, the issue isn't just to tune the transmitter to the load, it's to make sure that the antenna actually radiates the desired amount of power.

You did seem to get one thing backward: antenna length and even tuning aren't nearly as critical to receive. Transmit, on the other hand, wants a good match -- transmitter, through coax, into the antenna.
 
Slight thread drift but what about ELT antennae? I've seen a number of applications where the ELT antenna is inside the tail section of a super cub. The main concerns I hear are about: 1) making sure there is an appropriate ground plane, and 2) antenna orientation (although the latter usually devolves into a discussion of "which way are you going to oriented when you crash?")
 
the metal fuselage would shield or reflect most of the signal.
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There's no metal fuselage. The thing has a tubular frame with fabric covering that wouldn't "shield or reflect most of the signal." It's not being pedantic, Jim is just wrong (or seriously exaggerating).
 
flyingron said:
There's no metal fuselage. The thing has a tubular frame with fabric covering that wouldn't "shield or reflect most of the signal." It's not being pedantic, Jim is just wrong (or seriously exaggerating).
Click to expand...

The tubular frame can still act as a faraday cage even given the gaps.
 
Flybuddy said:
The tubular frame can still act as a faraday cage even given the gaps.
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You have no clue what you're talking about. It's not going to "act" as a Farraday cage nor is it going to stop the RF. As I said, it may provide some odd interactions with the antennas, but the holes are way too big to limit the RF. You open up a hole several nearly a wavelength long, you're not getting any attenuation to speak of. The firewall is a bigger influence.
 
Sam D said:
Slight thread drift but what about ELT antennae? I've seen a number of applications where the ELT antenna is inside the tail section of a super cub. The main concerns I hear are about: 1) making sure there is an appropriate ground plane, and 2) antenna orientation (although the latter usually devolves into a discussion of "which way are you going to oriented when you crash?")
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Which ELT frequency are we talking about? The 406MHz is even less of an issue than the 121.5 signal (which is not much). Polarization is less of an issue than for com. None at all really for the SARSAT receivers and if it gets down to the CAP chasing you down on the ground with their little yagi, they can cope with either orientation as well.
 
Ron, I respectfully disagree, and I especially disagree with opening up holes a wavelength long. Lambda is roughly 90", or just a bit under 8'. That's one hell of a gap on any airplane, not to mention a Tailwind.

Hidden antennas have been my specialty since we put them on Burt's original EZ. Probably since we put them on Apollo. But more than that, I was asked by a prominent manufacturer of metal tube/fabric aircraft to design exactly what you are asking for. I did so with the caveat that I was not sure it would work well. It did not. Relative to an outside commercial whip the receive/transmit distance was cut by 75 to 90% relative to the longitudinal axis of the aircraft. We did it with both a dipole-on-a-stick and using a vertical with the junction of four tubes of the stringers the ground plane. In all directions.

I don't know where you got that 10% of wavelength number, but experience shows that it may be very conservative.

Jim
 
The original poster should clarify about what he is interesting in doing. Operate 2M, 10M , or 160M

Just because you can use a tuner to match something does not mean it is an efficient radiator, an oilcan dummy load can present a perfect 50 ohms. One also must be cautious about voltages, I have seen high voltages at the end of a wire start a fire.
 
Adventure Aviator said:
The original poster should clarify about what he is interesting in doing. Operate 2M, 10M , or 160M

Just because you can use a tuner to match something does not mean it is an efficient radiator, an oilcan dummy load can present a perfect 50 ohms. One also must be cautious about voltages, I have seen high voltages at the end of a wire start a fire.
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Adventure, just go away right now, or read only. You are not helping.
Jim
 
weirdjim said:
Ron, I respectfully disagree, and I especially disagree with opening up holes a wavelength long. Lambda is roughly 90", or just a bit under 8'. That's one hell of a gap on any airplane, not to mention a Tailwind.
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It doesn't take a whole wavelength to be pretty wide open for RF. Certainly you don't get the "farrady cage" effect until you're down to 1/10. Did a lot of RF leakage testing in the days.
I didn't say you wouldn't have problems, but your assertion that
1. It's a farraday cage
2. It would completely block RF
is patently absurd (and I think you're admitting that now.

I don't know where you got that 10% of wavelength number, but experience shows that it may be very conservative.
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It's an accepted number.
 
Ron ... it is NOT an accepted number, and that is my field (you should pardon the word). Can we be professionals and gentlemen about this?

The term is "Faraday cage". And I've built my fair number of screen rooms. You?

Argue with my results, not your prejudices.

Jim
 
weirdjim said:
Ron ... it is NOT an accepted number, and that is my field (you should pardon the word). Can we be professionals and gentlemen about this?

The term is "Faraday cage". And I've built my fair number of screen rooms. You?

Argue with my results, not your prejudices.

Jim
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Jim-

How are you defining a "Faraday cage"?

I have also seen that 0.1 lambda number bandied about, however I haven't seen a credible source for why it has been chosen. There has to be some sort if wavelength dependence on the mesh size of a Faraday cage.

I'm not disagreeing with your results- A welded tube fuselage messes up with the signal badly. but I don't feel it is a real Faraday cage, either, at 121.5 MHz- there will be a lot of leakage in and out of that frame, but it depends on how you define a Faraday cage.

weirdjim said:
Ron, I respectfully disagree, and I especially disagree with opening up holes a wavelength long. Lambda is roughly 90", or just a bit under 8'. That's one hell of a gap on any airplane, not to mention a Tailwind.
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Which wavelength did you mean here?
406 MHz has a wavelength of ~0.74 meters, about 29 inches.
It must have been the 121.5 value which is about 96 inches (2.47 meters).
 
My experience with an existing metal structure being a shield or RF transparent is basically a corollary of Murphy's Law:

If you want it to be a shield, it will leak like a sieve. If you want it be transparent, it will block RF signals.
 
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