Antennas and wooden airplanes

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fly3g

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fly3g
I am currently building a primarily wood aircraft of my own design and had a general RF question regarding antenna placement. The fuselage is wood frame and plywood skins with an overlay of thin fiberglass topped off with probably some automotive paint finish system (similiar to a Falco). My question is if I place the antennas (gps, comm, transponder, APRS tracker) inside the tailcone area is the fuselage sufficiently "RF transparent" for good RF performance? I am planning to use J-poles for the comm and APRS antennas, and as in Jim Weir's Kitplanes article this month (6/2011) a thin aluminum plate for the transponder ground plane. The J-poles will be oriented primarily verticle along the inside wall of the fuselage or up through the vertical stabilizer . Is there any suggested rule of thumb on minimum spacing requirements for the various antennas and coax's that feed them? Thanks in advance for any info on these questions.
 
Wood frame, plywood skin, glass overlay, automotive paint are all transparent to RF from VLF to about 3 gigahertz. Above this, water in the wood and skin will start to play a part.

General rule of thumb says that antennas need to be about a quarter wave separation from each other. At VHF com this is about two feet.

J-poles are a lousy design for the com band. The bandwidth is WAY too narrow for any good use. I have no idea what you are using for APRS so can't comment.

There is no rule of thumb for coax spacing other than to keep it the aforementioned quarter wave away from any other antenna.

Jim
 
fly3g said:
I am currently building a primarily wood aircraft of my own design and had a general RF question regarding antenna placement. The fuselage is wood frame and plywood skins with an overlay of thin fiberglass topped off with probably some automotive paint finish system (similiar to a Falco). My question is if I place the antennas (gps, comm, transponder, APRS tracker) inside the tailcone area is the fuselage sufficiently "RF transparent" for good RF performance? I am planning to use J-poles for the comm and APRS antennas, and as in Jim Weir's Kitplanes article this month (6/2011) a thin aluminum plate for the transponder ground plane. The J-poles will be oriented primarily verticle along the inside wall of the fuselage or up through the vertical stabilizer . Is there any suggested rule of thumb on minimum spacing requirements for the various antennas and coax's that feed them? Thanks in advance for any info on these questions.
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- Wood won't block your signal at all at VHF. Metallic paint might, though, if you go for that look. Be careful which paint you choose.

- J-poles aren't the best for being broad-banded and their pattern is going to be toward their "horizon" which may not be what you're really shooting for.

Stuff to think about:

- Once you seal it up in the tail, how do you trim it to tune it? A dipole would work better even than the J-Pole and be more broadbanded.

- Think about polarization... you shooting for horizontal or vertical?

- Avoid paint with metallic flakes in it in the antenna area if your antennas are going to be buried in the fuselage.

I tried to help a builder out who'd built a glass ship and buried some goofy antenna horizontally in the aft section of the wing root where he couldn't get at it, change it, anything. It was way out of whack VSWR-wise and the Icom radio was unhappy with it on transmit, lowering his power considerably.

He also had multiple BNC female-to-female connectors and lots of super-cheap Radio Shack 70% shield coax running to it. At those distances, it probably wasn't a significant amount of loss, and at least they were BNC and not UHF connectors... but what a mess.

I'm an old "purist"... you want quarter-wave verticals outside the aircraft for best performance.

Anything else is going to be a trade-off and will probably work as long as the VSWR isn't too high for the radio, just because of altitude, but at least get it tuned right before you make it totally inaccessible.

Where the non-performing antennas will really show up is on the ground... buildings and other objects blocking your signal to the FAA antenna or in the case of your APRS antenna... to the local digipeater.

For ham stuff in the past, I've taken aloft a standard dual-band quarter wave Larsen mounted on a flat plate of steel strapped down in the cargo area of a 182. It did fine on FM (of course) and because of altitude, it also did just fine on 2m and 70cm SSB, where everyone is typically horizontally-polarized.

So since "altitude trumps all" in RF propagation -- I think you can get away with a whole bunch of "sins" that wouldn't work well for terrestrial weak-signal work.

As far as "minimum spacing requirements"... I don't know. Perhaps an avionics person will be along to answer if there's any specific rules. But I do know that putting one antenna in the near-field of another is going to make it seriously difficult for the front-end filtering on the receiver to keep the massive signal getting shoved down its throat from getting through to the other stages of the receiver, and front-end overload/clipping is likely.

On these two bands, as close as Aviation and Amateur VHF, I'd realistically say, "As far apart as practical." I never heard my dad operating on the low end of Amateur VHF in any of the Aviation band radios with that antenna in the cargo area, and him doing 100W SSB, but my Aviation Comm antennas are on top of the wing, partially blocked from directly being able to even "see" the cargo area, although it's still visible since they're raked backward. My aircraft is also aluminum, so there's some minor attenuation at those short distances there, too.

We also kept him off the aircraft's power system. He was operating from a (very large) sealed lead-acid UPS battery, that was also on-board for that event and strapped down good and tight.

Amateur receiver gear typically has very "hot" receiver front-ends with very wide frequency spectrum allowed in. Especially modern mobile rigs that are also set up to receive AM Aircraft.

Blowing away the front-end on receive probably isn't a huge deal to you on that rig if it's ONLY going to be used for APRS, unless you're going to try to track others from the aircraft or also use it for AM/FM/SSB voice. Since it's unlikely that by yourself, you'd be transmitting on both radios at the same time in that scenario, so you'd just have to watch out for "crashing" into your headset every time you transmitted on one and it was picked up by the other.

You could also fight with some common-mode currents getting into your intercom system, but we didn't experience that. Certainly could happen, though.

If you're going to try to receive APRS also, and it's dedicated to APRS only, you might consider looking for a higher quality older commercial FM rig with a much tighter front-end for APRS.

In fact, if all you're doing is transmitting ONLY for APRS tracking and not receiving, a number of transmit-only solutions are out there that'd probably work just fine without even putting an antenna in the aircraft for them.

Things like the Byonics self-contained transmitter/APRS combos that could just be suction-cupped to a window... no built-in antenna required at all.

Rambling thoughts... time for dinner. If that brings up questions, holler...
 
I appreciate all the responses. Some good things to consider. Will try routing my coaxes down opposite sides of the fuselage and keep the antennas spaced out as much as possible. As far as using the J-poles my experience so far has been with the one I use with a byonics 300mw APRS tracker. I have it mounted in a fiberglass wingtip in a non-optimal slightly folded horizontal position (I think a vertical placement is supposed to be better) and this thing has tracked me across the country and in the mountains and even low altitudes with barely a dropout. Since they don't require a ground plane it seemed like maybe a good idea for the comm as well. I have been using another J-pole attached to an ICOM handheld in some tests and it seems to have as good a range as my 1/4 wave whip although that is just a qualitative impression. Sounds like that may not be a good idea due to bandwidth issue mentioned above. I will have fair access in the tailcone area so I may still try it, otherwise I have some locations back there I can place ground planes and mount more traditional whips. It would be nice to keep the antennas all tucked away one way or another. It will be a few months before installation and flight test so stay tuned. Thanks again for the inputs.
 
Don't forget the good old dipole for hiding antennas too.

Can be made out of wire and kept ultra-light and exhibits less odd behavior and tuning touchiness vs a J-pole.

Can be "bent" too for routing around other things with very little effect on tuning.
 
i'd be cautious using automotive paint on a wood airplane. cars don't expand and contract and flex the way that wood airplanes do and the paint for them is very hard and stiff, not soft and flexible like airplane paint. We have a glider in our club that was painted on the cheap with car paint and within 10 years the finish is cracked all over. One of my gliders was re-painted with a very thick layer of Imron. It added something like 40 lbs to the glider. It does look pretty but there are a few areas that are now starting to crack
 
weirdjim said:
Only if bandwidth means nothing to you. WIth an instantaneous bandwidth of 15% anything narrower than 1/2" diameter will show up as lousy VSWR at the band edges.
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True. Copper strip works too. Very light.

I think I covered my preference. This was just another idea to try to avoid the J-Pole.

Put real verticals on the airframe, and call it good. :)
 
tonycondon said:
i'd be cautious using automotive paint on a wood airplane. cars don't expand and contract and flex the way that wood airplanes do and the paint for them is very hard and stiff, not soft and flexible like airplane paint.
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Actually, they do flex...lots of plastic parts on cars nowadays (like bumpers). The paint stores sell flex additives.

Ron Wanttaja
 
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