Vor V-Shaped Tail Antenna Physics

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Vor V-Shaped Tail Antenna Physics

Postby Jim Collins » Wed Jul 04, 2018 7:16 pm

Hello All,
I have a question that I need the exact answer to. In measuring my VOR antenna at the end of the coax inside the instrument panel, I find it is not working too well at all. Which accounts for the VOR's very short range. I am an electrical engineer and I have the equipment to do these measurements. Anyway, I'd like to fix it but I can find nothing on the internet as to how the coax couples to the antennas. I assume it is some sort of gamma matching system but I don't know. If anybody knows the exact answer or has really good pictures of the antenna before its mounted in the rudder, that would be most helpful. Also any ideas on how to remove the antenna and mount a new one would be helpful. Thank you
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Re: Vor V-Shaped Tail Antenna Physics

Postby DaveF » Wed Jul 04, 2018 10:36 pm

There's a 1:1 balun on the coax at the antenna. Its purpose is to match the unbalanced coax to the balanced antenna. The antenna is a balanced dipole, slightly folded to create a somewhat omnidirectional receive pattern. Gamma match is for impedance matching unbalanced antenna to unbalanced line, isn't it?

The traditional aircraft VHF antenna balun is constructed from a quarter-wave length of coax, and is called a Roberts Balun. https://www.hamradio.me/antennas/airplane-navigation-receiver-antenna-balun.html

The coax balun is fairly narrowband and doesn't have great VSWR because the coax is 50 ohm and the dipole is 70, but it's adequate and doesn't wear out. If you're having poor reception the problem is likely from decayed coax or corroded connections.
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Re: Vor V-Shaped Tail Antenna Physics

Postby Jim Collins » Wed Jul 04, 2018 11:00 pm

Dave,
Thank you for the reply. Unfortunately I am still confused. At DC there is a short between shield and center conductor and the 1/4 wave stub idea would still be open at DC. So I still think it is some kind of gamma match (which can go from unbalanced antenna to balanced line which is done all the time in ham radio). I met a guy at the local avionics shop who tried to describe it but did not really know how it worked and stated that it is supposed to shorted at DC and it sounded like a 1/2 wave gamma match which would be a short at DC. But after all of that, it looks like the skins have to come off of the rudder to fix it and I am not ready for that.
Thanks again Dave
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Re: Vor V-Shaped Tail Antenna Physics

Postby G280driver » Wed Jul 04, 2018 11:06 pm

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Re: Vor V-Shaped Tail Antenna Physics

Postby Jim Collins » Wed Jul 04, 2018 11:53 pm

Wow,
This is turning into a lively conversation. Thanks for the information on Baluns. When you go through the books on the subject (which I did), you find that there are many different types of Baluns and matching networks. Some baluns introduce a DC short and some don't. Also there are some Gamma and Delta matching networks that can also result in a DC short or not. So I know I have a DC short but because I don't know the exact system used, I don't know if the short is supposed to be there or not. I suspect Dave is right in that my problem is most likely due to corrosion or a bad section of coax. It just hard to get a starting point without knowing how the antenna was designed to work in the first place and everything is under the rudder skin.
Thanks again - its helping me think through the problem.
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Re: Vor V-Shaped Tail Antenna Physics

Postby Bruce Fenstermacher » Thu Jul 05, 2018 10:46 am

Jim, I'm not an electrical engineer. However my best flying buddy with nearly 50 years experience working antenna design for the military and other major industry, is. He insists the balun construction depicted or described in 43.13 and other places such as those mentioned already here, won't work. 8O

Stupid me just built the balun as depicted and guess what. Works pretty well.

Apparently there is more than one way to skin a cat or build a balun good enough for our application but not bounce signals of the moon and hit a dime back on earth. It's a black art to me, my eyes gloss over when he starts telling me how he worked a guy (he's a ham) in Australia with his modified '70s CB radio and a coat hanger antenna.

I doubt all this is behind rudder skins you can't get to. I imagine by your description you antenna is mounted at the top of your vertical stabilizer, under the cap. The cap is riveted on from the factory and more often than not these rivets are replaced with screws when an antenna is installed. Your also likely going to have to remove the rudder to remove the stabilizer cap.

As there are as many ways to mount an antenna as there apparently are to build a balun, pictures what you have will help us help you.
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Re: Vor V-Shaped Tail Antenna Physics

Postby lowNslow » Thu Jul 05, 2018 1:49 pm

I'm with Bruce, it is a black art. I do notice however that most of Comant antennas have a "integral ferrite balun", so no black magic required.
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Re: Vor V-Shaped Tail Antenna Physics

Postby c170b53 » Thu Jul 05, 2018 2:11 pm

Bruce, you probably meant mounted in the vertical fin. Moisture in the connectors or in the coax likely the culprit.
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Re: Vor V-Shaped Tail Antenna Physics

Postby Jim Collins » Thu Jul 05, 2018 2:32 pm

Dave,
Good idea about the pictures. Silly me assumed all the 170's from the factory had the same VOR antenna installed. However, I tried to attach a photo but I could not get it to work. I know it can be done, I just need to know how.
Thanks
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Re: Vor V-Shaped Tail Antenna Physics

Postby Bruce Fenstermacher » Thu Jul 05, 2018 3:15 pm

Jim Collins wrote:Dave,
Good idea about the pictures. Silly me assumed all the 170's from the factory had the same VOR antenna installed. However, I tried to attach a photo but I could not get it to work. I know it can be done, I just need to know how.
Thanks
Jim


Jim, no antenna was standard from Cessna. They were all options. Cessna did have a few "standard" optional installations they did. Notably the Narco V VOR antenna mounted on the ceililng of the cabin.

As for posting photos. See several of the instructions posted here: viewforum.php?f=12

If your taking pictures with your phone, they will need to be reduced in size prior to attaching them.
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Re: Vor V-Shaped Tail Antenna Physics

Postby Jim Collins » Thu Jul 05, 2018 4:30 pm

Dave,
Thanks again, I seem to be bothering you a lot. I hope the attached file works
Jim
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Re: Vor V-Shaped Tail Antenna Physics

Postby Jim Collins » Thu Jul 05, 2018 5:35 pm

Sorry Bruce, Somehow I got you and Dave mixed up

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Re: Vor V-Shaped Tail Antenna Physics

Postby DaveF » Thu Jul 05, 2018 5:50 pm

Looks like you have a Cessna 180 antenna mounting block. And it looks like you'll have to drill out the rivets and remove the fin cap to get at the antenna and coax attachment.

As Bruce said, after doing the antenna repair you can reattach the cap with screws or pulled rivets.

The attachment ends of the antenna elements are threaded. They pass through the skin and into the outer holes of the block, and are secured to the block with nuts, which live in the cut-out region of the block. The balun has ring lugs crimped onto its leads that fit onto the threaded ends of the antenna elements.
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Re: Vor V-Shaped Tail Antenna Physics

Postby Jim Collins » Thu Jul 12, 2018 12:53 am

Thanks to all,
I got the VOR antenna fixed. I had to take off the rudder and then take off the Vertical stabilizer cap. I found the Balun's weight was hanging off of the antenna connection and after all the years it fatigued and broke off. So nothing was connected to the antennas. I made a new Balun with fresh coax cable and reconnected everything and now (according to my antenna meter) it works just like new. The Balun that was in the plane was a little different from the others shown in this message stream but it works just the same and is a little easier to make. The loop section is 1/4 wave long which equals about 50cm. Photo below
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Re: Vor V-Shaped Tail Antenna Physics

Postby n2582d » Tue Jul 17, 2018 5:20 am

Jim Collins wrote:... When you go through the books on the subject (which I did), you find that there are many different types of Baluns and matching networks. Some baluns introduce a DC short and some don't. Also there are some Gamma and Delta matching networks that can also result in a DC short or not. So I know I have a DC short but because I don't know the exact system used, I don't know if the short is supposed to be there or not. ... It just hard to get a starting point without knowing how the antenna was designed to work in the first place ... .

“... Gamma and Delta matching networks ...”. It’s ALL Greek to me. Fascinating stuff! Looking at the various balun illustrations it seems to me that this black art isn’t even completely understood by avionics guys. The first and second illustration are the same but how does one explain the various differences found in those and the other two illustrations below?
507CCE25-F5F4-4CD5-8420-D7549586311B.jpeg
AC43.13-2B Fig. 3-6
A199540D-DED5-42E7-BB5C-4858DAC91CFB.jpeg
https://www.hamradio.me/antennas/airplane-navigation-receiver-antenna-balun.html
A34F6C2F-474C-48EC-9618-D83288165850.jpeg
Aero Electric Connection example
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Jim Collins’ example
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