DIY Comm antenna?

[gmcjetpilot]

Originally Posted by BobTurner

‘Bandwidth’, as used wrt antenna performance, means how well it performs at 118 and 135 MHz, when cut to spec for 126 MHz. Up to a point, using wide conductors, as opposed to thin wire, can improve this somewhat. ‘Bandwidth’, as used wrt the audio frequencies passed by our com receivers, is a function of the filters used within the radio. With aviation coms the bandwidth is deliberately limited to improve signal to noise ratios.

Thanks Bob, Yes frequency bandwidth not audio bandwidth. That is what I meant Ha ha.

Antenna is tuned optimally for 126.5 Mhz. Wide element will maintain more gain across the band, I recall now. True, but as you say improve somewhat. I recall seeing some numbers, using a spectrum analyzer. What is critical, a good antenna installed properly is +90% of the performance of your COM.

Aviation's slice of the spectrum 118 to 137 MHz is a fairly small band any way. This is why as demand goes up in the same space, they keep splitting the band up. FCC went from 50-kHz to 25-kHz per "channel" step long ago. Now it is 8.33-kHz steps (mostly above FL195 and ICAO and EUR airspace). The 8.33-khz change will not make 25-Khz step radios obsolete fortunately. Remember the KX-170's. Something satisfying about clunking your way through the frequencies. I'd like a KX-170 to put on the book shelf as a book stop.

This is from Jim Weir:

BANDWIDTH
I am frequently asked, “Why can’t I just take a hairfine wire and glue it to the canopy. Won’t that work for an antenna?” The answer is yes, it WILL
work as an antenna. But only at one very precise frequency. When we need to cover a whole BAND of frequencies, then the concept of antenna
“fatness” comes into play. Without going into a long song and dance about standing waves and reflection coefficients, suffice it to say that the
FATTER the antenna elements, the BROADER the bandwidth. Fortunately for we airplane drivers, an antenna doesn’t have to be fat in all three
dimensions. As a matter of fact, thin metal tape will work almost exactly as well as a solid rod of the same diameter.

There is a measurement called “aspect ratio” which is calculated by taking the length of one of the antenna “ears” and dividing it by the antenna
width. For example, a VHF navigation band dipole antenna (yes, we are getting a little ahead of ourselves here) comprised of two 22½” long ears
made out of half-inch wide copper tape would have an aspect ratio of (22.5/0.5) around 45. The rule of thumb is that the CENTER FREQUENCY
for which the antenna is designed multiplied by SIX and then divided by the ASPECT RATIO will be the approximate bandwidth of the antenna. So,
for our VHF nav band antenna centered at 113 MHz. with an aspect ratio of 45, the bandwidth should be somewhere around (113 * 6)/45, or around
15 MHz. This means that the antenna will be “good” (a purely subjective word) from around 105 MHz. to around 120 MHz.. Do you want more
bandwidth? Just use wider tape.

Without getting into the fine details, it is also true that making an antenna FATTER will also reduce its length a small amount. An antenna made up
of #40 wire (approximately the diameter of the hair of the small yak, not to be confused with the large yak) might be 24 inches long to resonate at a
particular frequency. An antenna made of ½” copper tape might be 22½” long to resonate at the same frequency. Unfortunately, there is no rule of
thumb I can give other than a quarter-wave antenna is at LEAST 5% shorter than a calculated quarter wave, and the rest is measurement and
experience.

[FinnFlyer]

Originally Posted by Jpm757

Seriously, here's one from Spruce for $85 bucks! If that's not in your budget then maybe owning an RV is over your head.
.jpg](https://postimg.cc/Cn5MdJdM)

You mean "owning multiple RVs"?

So 5 out of 9 replies so far has been exactly what I specifically did NOT ask for.

One would think that antennas were made of gold and sprinkled with holy water the way they are revered. Simply not so. Sure there is a lot to learn and most of you would rather pay someone than do that. But what part of "experimental" don't you understand?

For me, building the RV-4 is not just about getting an airplane in the air, but an opportunity to explore and learn more in many of the different areas involved. I think I read something about "recreation and education" somewhere ...

I guess home built avionics and antenna theory is off-limits here.

Finn

[N999BT]

Originally Posted by FinnFlyer

You mean "owning multiple RVs"?

So 5 out of 9 replies so far has been exactly what I specifically did NOT ask for.

One would think that antennas were made of gold and sprinkled with holy water the way they are revered. Simply not so. Sure there is a lot to learn and most of you would rather pay someone than do that. But what part of "experimental" don't you understand?

For me, building the RV-4 is not just about getting an airplane in the air, but an opportunity to explore and learn more in many of the different areas involved. I think I read something about "recreation and education" somewhere ...

I guess home built avionics and antenna theory is off-limits here.

Finn

Hear hear! Its not just about throwing money at a problem. I do know of one racer that made a com antenna that doubles as a pitot tube! No extra drag there! What I don't understand is how to change the impedance of an antenna.

[AX-O]

Originally Posted by FinnFlyer

I was planning on making my own comm antenna to be mounted on the turtle deck of my RV-4 below the canopy and following the canopy forward.

I thought a thin wire would do it, but after reading some Kitplane articles it looks like the thinner the antenna, the narrower the bandwidth. So 1/2" or wider copper tape seems indicated, but can't really attach the tape to the canopy and still have the turtle deck as a ground plane?

Mounting it on the side of the canopy, the rollover bar will be close to the antenna tip.

Wing tip mounting does not lend itself to a vertical antenna either (and a longer cable run to boot).

Any ideas (other than spending $120 or more on a Delta Pop antenna hanging out in the breeze)?

Finn

Finn,
this is easy to do. I did it on my -4 when I was racing at reno. All you have to do is grab a standard RG-XX coaxial cable and strip the shielding and grounding wire off. Leaving the center wire (and shield). I can't remember the length that gets stripped off top of my head but I think it is around 24.5 inches for the correct freq tuning.

for the purpose of racing, I installed it in the wing tip (horizontally). I tested the COMMs to be clear over 5 nm. Way more than what i needed for racing.

You would need to figure out a grounding plane and a way to hold the wire. Maybe a plastic rod or fiberglass. Have fun with it. That is one of the reasons of the EXPERIMENTAL category.

don't worry about the folks saying that if you can't afford an antenna, you should not be in the hobby. We all have our requirements, needs, desires and goals. Just because they don't find value in it, it is not an excuse to nay say.

If I stopped every time some one asked me why am I making something i could just buy, I would not have the knowledge nor experience I do today. Post some pics, do some testing.

[rvmills]

Finn,

Like Axel, my motivation was racing Reno, plus fast X-C speeds, and the fun of reducing drag wherever possible...that experimenting thang! Early on, I started with two bent whips (comm 1 and 2) and two stick and ball antennas (xpndr and APRS)...all on the belly. Bob Axsom and Larry Vetterman shook their heads and fingers at me at AirVenture Cup (in a friendly way) and started me down the antenna-hiding journey. Pete Howell and I tried several (some kinda nutty) ideas for radios and APRS, and after a while I ended up with:

- One bent whip on the belly for comm 1, which comes off for races
- One bent whip in the left wingtip, which becomes comm 1 for races
- One bent whip in the right wingtip, which is always comm 2
- A small Delta Pop blade for the xpndr on the belly (its a clean antenna)
- While I still had APRS, I used a Ryan Howell dipole slid down the right gear leg

All of them work pretty dern well.

Comparing ATIS reception ranges, the belly antenna has a 40+ nm range, and the wingtip antennas have a 25ish nm range. The tip antennas can be a little more directional, but overall they're pretty good.

The wingtip bent whips are from the local avionics shop "junk drawer", cost was nada. You can find them cheap or free (used or discarded), and looks don't matter in the tips. A couple of pics below. To make them fit, I needed to remove the base plate, then solder the coax center lead to the end of the antenna. Then I connected the antenna case and the coax shield to the ground plane, which is about a 10"x10" aluminum sheet bonded inside the wingtip. I did the right side first, and put the ground plane on the bottom inside surface. The left side was done a while later, and I tried putting the ground plane on the upper inside surface of the tip, just for the experiment. Performance is about the same. Very workable for racing, and sufficient for normal ops, with a bit less range. A larger ground plane might help that.

Once Steve Smith and I finish the tapered composite wings, I need to figure out how to build an antenna for the smaller tips that will work. Jason Rovey's pitot antenna is another possibility, and a canopy antenna is yet another...more for me to learn on those options. Hey Pete, you ready to spool up the antenna skunk works again?

Have fun Finn...lots of ways to skin this cat. The belly whip is the best performer, but there is some fun to be had in this experiment too!

Cheers,
Bob

[BillL]

Originally Posted by FinnFlyer

. . . . May even build my own SWR meter

Finn

When I fooled around with antenna decades ago the SWR was very helpful, like a torque wrench building an engine. May do without one, but not ideal. It will at least keep you from burning up a radio, which was my objective. Sometimes a 1/4" length brought it from really bad to quite good.

[DaleB]

There's nothing at all wrong with building your own antenna, assuming some basic things are true...

You will need to design and build it to withstand the physical loads it will see during flight, landing, etc. Obviously.

As has been discussed before, make sure you have a good ground plane and decent quality, low loss coax. That doesn't mean it has to be gold plated connectors and hardline, but don't use old TV cable either... and if you're mounting it to the skin, just get a good electrical connection.

You'll want to tune it for the center of the COM band. A VHF antenna analyzer is a wonderful tool for this! Many ham operators have them. Get one to help you out, if you're not one yourself. They typically cost more than a decent COM antenna. I do see, however, that the NanoVNA is being sold for a whole lot less. It will tell you more than an SWR meter or antenna analyzer will. I've never used a VNA, and a lot of stuff is way over my head -- but it will tell you where the antenna is resonant.

As for tuning, start a little long and gradually trim to size. Shortening the antenna will raise the resonant frequency, lengthening it is usually inconvenient.

[FinnFlyer]

Originally Posted by AX-O

Finn,
this is easy to do. I did it on my -4 when I was racing at reno. All you have to do is grab a standard RG-XX coaxial cable and strip the shielding and grounding wire off. Leaving the center wire (and shield). I can't remember the length that gets stripped off top of my head but I think it is around 24.5 inches for the correct freq tuning.

Yep, that would work very well for a narrow range of frequencies. In fact you avoid the (small) loss from the BNC connector.

However, for the full 118-137 MHz range, the part from Jim Weir that George quoted in post #11 above comes into play.

Another quote on antenna materials from Jim I find interesting:
"As a matter of fact, copper tape has come to be used almost universally as a plastic plane antenna element. Metal ships, too, use copper tape for antenna elements, but wrap it in a fiberglass rod to keep it from fluttering about in the breeze. Yep, cut one of those pretty white fiberglass ?COM? antennas in half and the odds are that you will find a piece of copper tape curled up inside."

As for wing tip mounting, it seems feasible: use end rib (covering the holes with foil) as ground plane with rod going diagonally up (or down) and back (or forward). Major disadvantage would be the dead spot on opposite side caused by fuselage and wings. Probably mostly an issue on the ground and at low altitudes.

BTW, as covered in another thread, not all external (fiberglass) rod or whip antennas are created equal. Some actually hide circuitry (impedance matching, network?) in the antenna base. Probably considered proprietary, but anyone here in the know are welcome to share

As for drag, I'm still amazed at the stated difference between the simple TED transponder rod and the "shark fin" antenna. The whip antenna is a round rod too, right? So its drag ...

Thanks for the encouraging words. I'll keep you posted.

Finn

[dpansier]

The type of antenna depends on what mission you have for the aircraft, assuming metal aircraft construction.

If your mission is racing or Sunday morning pancake runs, hidden DIY antennas may meet your performance requirements where 5 to 8 miles of reliable range is adequate.

If the mission is interfacing within the ATC system where communication is much more important, a commercial general aviation antenna mounted per the mfg recommendations would be my choice. Antennas such as Comant, Rami, Dorne & Margolin,and Delta Pop are designed to achieve maximum communication reliability.

The difference between a simple rod / strip type and a commercial VHF Com antenna is a matching network contained in the base, the characteristics of matching network and the element length work in conjunction to provide as close as 50 ohm impedance possible to the transmitter across the design frequency. The simple rod / strip type antenna characteristics allow it to be cut or tuned to only one frequency with the performance falling off on the frequencies furthest from the cut frequency.

Low cost, Ham type VSWR meters are available on the internet and are fine to tune simple antennas however I do not recommend cutting the element on a matching network type antenna without access to the network and a vector network analyzer or sweeping type antenna analyzer . Changing the length of the element without changing the network characteristics can cause the VSWR to be out of limits on the design frequency as they both interact.

Many on the forum here have Ham Radio or EE backgrounds and understand the challenges of covering a 19 MHz bandwidth (118 to 137 Mhz) and achieving a reasonable VSWR from end to end. Consider 2 meters where the band is 144 -148 MHz and how difficult it is to get reasonable VSWR across the 4 MHz bandwidth without a matching network.

So I guess it comes down to your expectation of performance and what you are willing to accept.

[GalinHdz]

Originally Posted by dpansier

So I guess it comes down to your expectation of performance and what you are willing to accept.

Hence the saying "You get what you pay for. "