Comm Antennal Location

[LetMeFly]

I'm installing the Advanced Aircraft Electronics VHF-5T comm antenna in my vertical stab, like I've seen some others do on the forum. The pictures I've seen has the antenna high in the VS so the cable connection will be sealed inside the tail. Unless they have installed an access panel, they wont be able to change the cable if there is a problem in the future. I was thinking of installing mine lower so the cable connection is in the area of the inspection panels at the vert/horz tail intersection. This does allow the lower part of the antenna to be near the elevator push/pull tube, the trim motor, and the rudder cables. If I secure this so it's clear of all that, does anyone see a problem with the location?

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[FoxbuilderWill]

I'm installing the Advanced Aircraft Electronics VHF-5T comm antenna in my vertical stab, like I've seen some others do on the forum. The pictures I've seen has the antenna high in the VS so the cable connection will be sealed inside the tail. Unless they have installed an access panel, they wont be able to change the cable if there is a problem in the future. I was thinking of installing mine lower so the cable connection is in the area of the inspection panels at the vert/horz tail intersection. This does allow the lower part of the antenna to be near the elevator push/pull tube, the trim motor, and the rudder cables. If I secure this so it's clear of all that, does anyone see a problem with the location?

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I'm going to be building a homebrew antenna that I will mount internally. My dad who is a ham radio guy tells me there are ways to tune the antenna to compensate for nearby metal that causes interference. So if you know a ham radio guy he might be helpful.

If it were my plane I'd wire up the radio move the fuselage outside and have a friend with a radio go a certain distance and test it to see how it worked. Rotate the plane to see if the signal was stronger in different directions. Probably need to have the wings and tail on it to make it an accurate test.

[Eric Page]

I'm one of the ones who installed that antenna in my vertical stab. Mine is attached to the top-most rib. As you pointed out, that puts the feed point in a location that will require an inspection hole to access. With the antenna in that location, testing* with a spectrum analyzer shows a VSWR of about 2:1. That's a bit worse than I had hoped, but not bad enough that I'm worried about its performance (someone else on this forum reported good range from the same antenna in that location). Anyway, in a plane that tops out at 100 mph you really don't need to be able to hear or talk to the back side of the moon.

That said, installing it lower in the tail will surround the antenna with a lot more steel tubing. I don't know what effect that would have on performance but it definitely won't help. Keep in mind that no matter whether you mount the antenna high or low, and even with easy access to the feed point, you'll still be stymied by the lacing tape or zip ties along the coax run that you won't be able to reach. If something goes wrong, getting a bad antenna or coax out won't really be an option unless you're prepared to partially re-cover the plane. I'm planning to install an inspection ring near the antenna feed point during covering but not cut out the fabric unless it becomes necessary to reach it in the future. I'll paint an extra cover plate along with the plane and store it with my other spares.

WRT testing the antenna by connecting the radio to it and rolling the plane outside, the person giving you radio checks will have to be at a distance near the far limit of reception to give you any meaningful information, and at ground level you won't know whether low performance is due to antenna mounting location or obstacles like trees and buildings. 10-12 watts is a lot of power, so anyone nearby with a handheld radio will hear you 5-by-5 even if they're in one of the low points of the antenna's radiation pattern.


* I tested my antenna through an 18" length of coax (which put my body near the antenna), with the fuselage inside my garage and without the horizontal stabilizer installed. So, take my 2:1 VSWR result with a grain of salt; it's probably not quite​ that bad and it will change a bit once the horizontal stab is installed and it's tested outside through the final coax installation.

[Geek]

I'm one who also put the antenna higher up in the vertical stab. There have been a couple people who have done this and reported good performance and let's face it - a dipole antenna is a good tried and true antenna with excellent performance. My first question would be how are you going to secure the lower pole of your antenna that is down with the trim motor and rudder cables? I would be leery of having things around where my control interfaces and parts that move are located. John McBean saw my install and asked me about what my plan was to access the antenna if I ran into issues later. My answer was simple. I'm leaving the 'normal' position on the aircraft open for an external antenna open so that if the antenna I'm using has issues, I'll cut the cable off at the top of the tail access and re-route it to the 'new' antenna location. I also agree with Eric's comments on testing methods/results and such. (I measured my VSWR with a network analyzer Eric and it came in at about 1.7:1. and think you are correct that the mismatch will get better with the longer cable. 2:1 is about 10% reflected power and that longer cable will help)

Gary

[LetMeFly]

...My first question would be how are you going to secure the lower pole of your antenna that is down with the trim motor and rudder cables?...

Yes, I'm still working that out, I have some ideas. If I decide to continue down this route, I'll post that for discussion too.

Now I'm off on the road (literally) to Oshkosh on the 20th!

[Delta Whisky]

I too put mine as high as possible and get really good results with aviation's lowest cost radio. I tie off the lower end to a block of wood (hysoled in place) that will be untied so the antenna can be moved out of the way should the HT or elevator have to be removed.

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[LetMeFly]

Thanks for all the replies. I've decided to mount the antenna up higher in the tail, like Delta Whisky shows. How did you guys route the wire from the trim sensor to avoid the antenna? Since the wires for the sensor use shielded cable, do I need to worry about that?

Thanks. :)

[Eric Page]

Don't route it right up against the antenna, and cross it at right angles; otherwise I wouldn't give it a second thought. It carries virtually no current and the voltage on the wires changes slowly and only when the stab trim is moving. It won't bother an antenna.