Comm antenna and location for metal plane

Discussion in 'Instruments / Avionics / Electrical System' started by 12notes, Aug 15, 2018.

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1. Aug 15, 2018

12notes

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My Hummelbird build has been moving along (I'll update the build log at the end of the month, and I've been looking at antennas. I'm looking for a good, cheap antenna. I swear I found one for under $50, but now can't find the site it was on. Aircraft Spruce has the Ravi RV-534 for$108 (with the install kit), but I'm wondering if there is a good, cheaper antenna people are using that I'm missing. It's an all metal airplane, so the RST antennas are out. I'm not opposed to making one, but I'd rather not put the effort in if it can be purchased cheaply.

I've got a couple of options in mounting it, I can go on top of the vertical stabilizer or on the turtledeck right behind the canopy. I'm leaning towards the turtledeck. Any advantages/disadvantages to either? I'm planning on using a Becker AR4201 or Flightline FL-760 type radio (Europe is changing to the 8.33kHz channel spacing, the used units have been dropping in price on eBay.)

2. Aug 15, 2018

Aerowerx

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3. Aug 15, 2018

12notes

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That may have been it. I had been looking under the COM/VHF category, where it doesn't show up for some reason. Does anyone have any experience with this one? I'm not a huge fan of the connection method, but if it works well, I can get over it for that price.

4. Aug 15, 2018

BoKu

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You can make a 1/4-wave whip like the ones that ACS used to carry for about $4 in materials. I'll send you a kit for$25.

5. Aug 15, 2018

Pops

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You can still make the RST type antenna for a metal aircraft. Use the copper tape to the length in RST information, sandwich between 2 pieces of wood, (maybe 1/8"x 3/4") epoxy with the copper foil in the middle and sand to streamline shape and fiberglass with light weight cloth. Mount to a female connector, bulkhead fitting, ( as close as possible) that mates the connector on your coax cable. If limber, just add more wood on the sandwich until you like the stiffness. Monut antenna in a whole in the metal skin. Make an aluminum doubler for the inside of the skin.

Last edited: Aug 16, 2018
6. Aug 15, 2018

pictsidhe

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I'd make a slot antenna
cut a suitable slot, connect cable, glass it. Dimensions may need tweaking when glassed.

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7. Aug 15, 2018

BoKu

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Sounds like a great thing to try as an experiment. But I don't think I'd have a lot of confidence it would work first-time-every-time.

8. Aug 16, 2018

pictsidhe

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Build a ground test one first, then. If you can find a local ham to collaborate with, you'll likely get something that works great.

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9. Aug 16, 2018

Daleandee

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I had one like that and, like you mentioned, I wasn't thrilled with the connection. I recently upgraded by making my own from a 102" stainless steel CB antenna whip and a bulkhead connector. I can't find a connector exactly like the one I have on my plane as it has a BNC connector on it but here's one from Amazon that is the same except for the SO-239 connector. If your coax cable has that type connector this one would work:

https://www.amazon.com/dp/B01HHAGM7...&pd_rd_r=15ed4545-a0eb-11e8-b894-65785e22d3f3

I obviously had to shorten the 102" whip quite a bit (1/4 wave frequency length is ~23" depending) but I already had it on hand. A shorter whip antenna could be used as long as it has the 3/8-24 thread and long enough to be cut for a good SWR reading at aviation frequencies. This one could be shortened and is said to fit the connector:

https://www.ebay.com/itm/PROCOMM-31...:2hwAAOSwwWJavsBj:sc:USPSPriority!29461!US!-1

FWIW ... I bent mine and mounted it on the belly of my all metal airplane just ahead of the spar tunnel. I'm using a Flightline 760 radio with a Clarity Aloft headset. Works well and gets good reports.

Dale
N319WF

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10. Aug 16, 2018

Aerowerx

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Or just use a length of wire.

11. Aug 16, 2018

Aerowerx

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Well, maybe. A lot of the younger hams nowadays are "appliance operators". They didn't learn the theory as was required when I started in it.

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12. Aug 16, 2018

Pops

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If you have the antenna tuned for the middle of the frequencies using a small dia wire will make more lost on the higher or lower part of the band. Higher SWR on each end of the band than if using a wider driven element. 3/8"/ 1/2" wide copper foil works better. Also tune length of your coax cable for the lowest SWR. Don't just cut what ever length you need to go from the radio to the antenna. Needs to be as close to the fundamental length of one wavelengh for the middle of the band. 1/2 wavelength, 1/4 wavelength, 1/8 wavelength, etc. for the lowest SWR for the length needed. Short version.

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13. Aug 16, 2018

Dan Thomas

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Call an aircraft wrecking yard. They have lots of comm antennae. Lots and lots. And then some.

Don't put it atop the fin. Very poor ground plane. On the turtledeck is better.

14. Aug 16, 2018

Derswede

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As the bird is all metal, you already have your ground plane ( pardon the pun). Mount it on the turtleneck, ground the coax braid to the metal using a star washer/connector underneath, and solder all connections, close up with heat shrink. You can buy a generic whip for a few bucks from most any two-way radio mobile shop. Cut to resonate frequency. Make sure it is a stainless steel whip. I would suggest RG/58 at a minimum, prefer "mini RG-8". Make sure all connections are tight, strain relieve the coax near bends and the antenna mount. Doing so will keep losses and noise to a minimum and keep it reliable as well.

Derswede

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15. Aug 16, 2018

Pops

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Correct. If mounted top or bottom of cabin or at root of wings you will have a slightly more range off the wing tips and towards the rear and less towards the front. (slightly more range in the direction of the largest area of ground plane).
Its not as good mounting the antenna on the bottom of the fuselage pointing down. Dan's turtledeck location is better for all around best performance.

16. Aug 17, 2018

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