Suggestions on Comm Antenna for All Wood Aircraft?

[Mike Hongisto]

Does anyone have any experience with ground plane-less Comm antenna from "RST Engineering" or "Bob Archer"? I'd like to bury a comm antenna in back of my fuselage (all wood aircraft) and have considered this type of design. I don't know if this would be a good move or not.

I'd also construct my own antenna if anyone can suggest where to obtain plans on a design that they've tried and that works.

Thanks,

Mike

[Jim Hann]

Ron Wanttaja has an article about whipping up (no pun intended) his own antenna on his Fly Baby, an all wood airplane. Check it out:
http://www.bowersflybaby.com/stories/antenna.htm

Jim

[spungey]

I was about to recommend going down to your local HamRadioOutlet and talking to them, but the copper tape idea in Ron's article probably can't be beat for simplicity, reliability, and effectiveness. Even if you're really dirt-cheap you can probably make a workable antenna and ground plane out of coat hangars. (I've done -- don't recommend it.) :-) The real secret to a great antenna is to get someone who really knows antennas and has fun tools like field strength meters and reflected power meters (your local ham club will have a handful) to help you out.

Please let us know what you finally do.

[Mike Hongisto]

Thanks for the ideas an a comm antenna. Jim, your recommendation looks like it would work but I'd have an installation problem (difficult access) with the large foil area. I stumbled upon a post on another web site that describes how to build an antenna without a ground plane. The antenna build instructions can be found at: http://chrusion.com/BJ7/InvVeeAntenna4ULs.pdf. I'm going to give this a try. If it works I'll install it in the airplane.

I'll post the results here.

Thanks,

Mike

[Jim Hann]

Mike,

I've seen the exact same theory used for the antennas in composite airplanes (a la Rutan) and Jim Weir (http://www.rst-engr.com/) sells the supplies for that and does a lot of home made antenna stuff for Kitplanes. He also has a really good text on plastic airplane antennas that I've heard of people adapting to wood: http://www.cozybuilders.org/ref_info/RST_82704.pdf. I've heard of people using this info on wood planes, just bonding the copper tape to the wooden side of a fuselage, but I don't know performance gains/losses that might create.

Good luck with the antenna info. I'm going to save the article you found also, amazing how many ways you can skin a cat!

Jim

[rwanttaja]

I've been out of the country and unable to chime in on this, but when I did my research for the antenna for my Fly Baby, I had an email exchange with Jim Weir regarding the copper-tape antenna. Jim sent me some copper tape, toroids, and his instruction manual, and I built a temporary one using a long strip of plastic corner protector.

I didn't use the copper tape solution since it was too difficult to install on a completed airplane (the total run of the tape is about four feet long, and I couldn't reach the end where it needed to be).

Otherwise, the copper tape antenna would be an excellent solution for a wood or com, and does not require the flat-surface ground plane that I installed for the whip antenna in my Fly Baby. It doesn't have to be copper tape...in fact, the previous antennas in my airplane were just a piece of coax stripped so that the braid went one way and the center conductor went the other. But the self-stick tape is easier.

However, keep in mind that the antenna is MORE than just the tape. It requires the toroids (little metal donuts) on the incoming coax line.

Jim Hann posted a link to copy of a Weir article explaining the antenna... you can see the original article for Sport Aviation at:

http://www.oshkosh365.org/saarchive/...1981_01_19.pdf

Ron Wanttaja

[Mike Hongisto]

Here's what I've done and the results....

I built the Comm Antenna according to the instructions given here: http://chrusion.com/BJ7/InvVeeAntenna4ULs.pdf. I made the base out of plywood, rather than plastic. I also didn't bother with bending the tips of the rods for a screw attachment point. Instead I cut, sanded smooth (320 grit) and painted 23 5/8" brass rods (brazing rod) and epoxied these to the plywood base. I then used "Gorilla Glue" and some scrap 1/16" plywood to cover the saw slot & rods. I then wired and soldered the antenna as instructed. No filters or toroids - just built as instructed with the pawsey stub.

Results: This worked great! Reception is clear from airplanes landing at airports 80 miles away. I asked Duluth Approach for a radio check at KGPZ @ 4,500' AGL (about 60 miles) and they reported "loud and clear". This is with a handheld Vertex radio connected to the above antenna. Previous reception range with the stock "rubber ducky" antenna was probably about half. The transmission range improved from about 5 miles to at least 60. The "squealing" feedback that occurred while transmitting with the rubber ducky antenna and connected to an ANR headset was completely eliminated. I'd have to guess that this radio/antenna combination is at least as good as what I've used for some 1,900 hours in "certified airplanes". Obviously, I'm extremely pleased with the results and wish I had built this antenna from the outset.

Also, I didn't get the antenna mounted vertical as recommended - I didn't have the room available in the tailcone. One rod of the antenna is pitched up maybe 10 degrees from horizontal the other is not quite vertical. This less than optimal installation still provided great results and is concealed entirely within the wooden airframe.

My advice to others... Try it, you will have a great antenna.

EAA: This antenna would make for a great "Hints For Homebuilders" project/video.

Good Luck!

Mike

[spungey]

One more thing ... If you're messing with RF, contact your local Ham club!

Really. Those guys are all about antennas and propagation and keeping the FCC happy. They'll have the tools and expertise to help you install, understand, and verify your entire radio system. It'll look good in your builder's log too that you bothered to get semi-pro help on your field strength, spurious emissions, reflected power and so forth. And you might make a friend.

[C 172 Pilot]

The half wave antenna is ground independant. Which means it wouldn't need a ground plane to work off of. A simple connector antenna stashed inside the tail will work great I'd imangine. (Like the coax stub antenna mentioned eariler)! For help, I can try to offer a picture or two if needed?

KB5UKT@ARRL.NET
C172Pilot Mike
KDTO

One more thing ... If you're messing with RF, contact your local Ham club!

Really. Those guys are all about antennas and propagation and keeping the FCC happy. They'll have the tools and expertise to help you install, understand, and verify your entire radio system. It'll look good in your builder's log too that you bothered to get semi-pro help on your field strength, spurious emissions, reflected power and so forth. And you might make a friend.

[C 172 Pilot]

Information is from web site: http://www.nr6ca.org/formulas.html



Calculating dipole and vertical antenna length

The official formula for a half wave antenna in free space is 492/f where f is in Megahertz (MHz) and the length is in feet. For a practical 1/2 wavelength dipole length is feet use general formula of 468/f where f is in megahertz (MHz). A half wave antenna length (in inches) in free space is calculated by the official 5904/f where f is in megahertz (MHz) and the calculated length is in inches. If capacitive loading, end effects, etc. are taken into consideration this formula would then become 5616/f where f is in Megahertz (MHz). These formulas will work fairly well for a dipole mounted at least 1/4 wavelength above ground. Some tweaking might be in order due to capacitive loading from nearby objects. End effect may also change resonance requiring slight adjustment in length. It is best to cut a dipole a few percent longer then the calculated length then prune it for resonance. Starting longer is a LOT better then starting at the calculated length or shorter! It is easy to trim but a bit harder to stretch. Using the official free space formula is likely to be way to long. Proximity to ground, trees, homes, even the feedline greatly influence resonance so make sure you check it. The nominal feedpoint impedance of a dipole is generally 72 ohms but this can vary widely. The lower to ground it is, the lower the impenitence.

For a 1/4 wave vertical the above numbers would be half. Therefore, the height of a simple 1/4 wave vertical can be calculated by 234/f where f is in Megahertz (MHz). Since ground (including radials) act as the other half of the antenna a vertical is really a half wave antenna with a vertical polarization.

Let me add the following:

For composit or wood construction!

Half wave antennas do not need a grond plane, but they are LONG:

120 MHz which is a good place to start or tune your comm antenna to:

5616/120= 46.8 inches

Might start at the floor inside and work up the leading edge of the vertical stablizer? Could even stick out in the breeze.

C172Pilot
Mike
amature call sign: KB5UKT
KDTO