Required Antennas?

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Will Aldridge

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I'm trying to decide what antennas to put in my bird. Right now now I'm only planning on a single comm antenna, a gps, whatever xpdr/adsb is necessary (i live under class b) and the antenna for the ELT. im trying to decide if a nav antenna is worth it? I have the RST antenna kit and info and although structurally my bird should be pretty transparent to radio waves, the control system (mostly metal push rods) will interfere with the antennas ( at least from my reading of the RST manual) and my plane is small enough that getting the ends of individual antennas at least 2 feet from other metal bits of significant size is somewhat problematic. So in the interest of simplicity, lightness, and lack of space have i covered my bases? Is a vor/nav antenna something that one day i might really wish i had? I only have about 205 hrs in my logbook so i don't have much experience that way. Other than flight training I've only really used gps for navigation and in 10 years when this plane gets off the ground will they be phasing out vor?

Last question, apparently the RST manual was written before the new ELT and adsb stuff came on the scene, anyone know the geometry for those?
 

Will Aldridge

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20170728_172515.jpg20170728_172143.jpg

Just to help you visualize the space constraints the above screen caps show the major metal bits. 1/2 of the comm antenna is going to be in the vertical stab, the other half in the h-stab. Im worried that the wiring going to the position light in the tip of the rudder might interfere with that.

The GPS antenna Is going in one wingtip and the xpdr in the other. I am a little worried about the wiring for the wingtip lights interfering with those.

So as you can see there's not much real estate left that's technically far enough away from metal stuff, that I'm wondering where the ELT should go and i don't even know if another antenna is needed for the adsb beyond the xpdr one.
 

Toobuilder

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A couple of random thoughts:

Why are you running the Xpdr antenna all the way out to the tip? Thats a lot of coax. Why not on the belly on BL 0?

You do not need a position light on the rudder. A white light on the aft side of the wingtips works too.

I run one of those archer Nav antennas in the wingtip of the Rocket and it works fine despite the high voltage strobe wiring right next to it. If you dont have a NAV radio today, then I wouldnt worry about it. Ive done a practice ILS and some VOR to VOR nav just for fun, but GPS has served me well for practical navigation.
 

Will Aldridge

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A couple of random thoughts:

Why are you running the Xpdr antenna all the way out to the tip? Thats a lot of coax. Why not on the belly on BL 0?

You do not need a position light on the rudder. A white light on the aft side of the wingtips works too.
I've got pushrods running within a few inches of the belly most of the length of the fuselage and i assumed they would interfere with the antenna. Also i didnt think parking it somewhat between the gear legs would be a good idea. If practice has shown that not to be an issue I'll reconsider.

As for the wingtip lights i chose Whelen lights for aesthtic purposes and i didnt think they had the white light in the back. I'll have to go back and look at that more closely.
 

Pops

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You are worrying about nothing. If measuring under lab conditions you might find some very small reduced range in a very small areas but nothing you could ever tell while flying. I have been using the RST antennas since about 1980 and they work as good if not better than any high priced antenna you can buy.
 

Will Aldridge

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Thanks Pops, I know that anybody who produces a product had to do a certain amount of CYA so if someone complains he can say "See it's right here in the manual". Fortunately it sounds like real world is a little more forgiving. I think I'll put the XPDR just under the pilot and offset to the side a little and mount the GPS above and behind the pilot. There's some seat belt and overturn structure not shown but I'm guessing it probably won't matter that much as long as it can see the sky. That just leaves ELT and ADSB.
 

BoKu

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...1/2 of the comm antenna is going to be in the vertical stab, the other half in the h-stab...
Okay, making the dipole turn a right angle is just strange. Com signals are normally vertically polarized, so I would recommend that you make your dipole as straight and vertical as practical, with the ends of the tape ribbons as close together as practical.

For the HP-24 com antenna, we put an RST dipole inside the Kevlar rudder. We started long and cut the ends down while plotting SWR on a graph; the lowest was at 22.8" when centered on 123.4 MHz (half way between typical glider frequencies of 123.3 and 123.5). As far as keeping the antenna away from conductive bits goes, we broke about half the rules in the RST book, and our antenna still works very well. The SWR is around 2:1 (okay but not great), but the effective range is up around 100 miles for air-to-air transmission and reception.

--Bob K.
 

Will Aldridge

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20170728_194820.jpg

Bob, hopefully the salient parts are visible. This shows the leading edge and forward strake removed. The upper diploe of the comm antenna ( green lines) is mounted to the forward spar of the vertical stab, the lower dipole is mounted just below the surface of the hstab. I did it this way to keep the ends of the dipole away from the elevator control tube which you can see. Correct me if I'm wrong but the antenna is in the vertical plane just not the longitudinal vertical plane.

I guess i could run the lower dipole in the top of the strake but then it would be paralleling the control tube. Is one better than the other? Construction wise the latter is easier.
 

Toobuilder

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Looks like you have plenty of height on the vertical stab to get full length on the antenna. Especially if you kink it to follow the dorsal LE.

Im looking at doing a wood vertical and rudder on the Rocket so I can bury 2 com and one ELT antenna, so I can relate to what you're going through.
 

Pops

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Okay, making the dipole turn a right angle is just strange. Com signals are normally vertically polarized, so I would recommend that you make your dipole as straight and vertical as practical, with the ends of the tape ribbons as close together as practical.

For the HP-24 com antenna, we put an RST dipole inside the Kevlar rudder. We started long and cut the ends down while plotting SWR on a graph; the lowest was at 22.8" when centered on 123.4 MHz (half way between typical glider frequencies of 123.3 and 123.5). As far as keeping the antenna away from conductive bits goes, we broke about half the rules in the RST book, and our antenna still works very well. The SWR is around 2:1 (okay but not great), but the effective range is up around 100 miles for air-to-air transmission and reception.

--Bob K.
I also get around 100 miles reception with my RST antenna and a Icom A-6 handheld radio.
 

BoKu

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I'm sure you can make that work, but it looks more complicated than it needs to be. If the rudder is taller than about 46", I'd just put the antenna in the rudder. We gave our dipole a slight V angle so that the junction is near the hinge line, halfway up the rudder, and the ends are at the upper and lower ends of the trailing edge. That kept the moment of inertia of the antenna as low as practical (for mass balancing), but kept the antenna ends as far as practical from the metal rudder drive parts at the bottom and the metal elevator drive parts at the top. Most of the antenna is closer than Weir recommends to conductive bits, but it still work plenty well enough for all practical purposes.

--Bob K.
 

Toobuilder

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I've got pushrods running within a few inches of the belly most of the length of the fuselage and i assumed they would interfere with the antenna. Also i didnt think parking it somewhat between the gear legs would be a good idea. If practice has shown that not to be an issue I'll reconsider.

As for the wingtip lights i chose Whelen lights for aesthtic purposes and i didnt think they had the white light in the back. I'll have to go back and look at that more closely.
Pushrods up in the belly wont hurt... The txpdr antenna needs only to see down. You will likely be laying in a few strips of copper foil tape to create a ground plane anyway. That should hide the pushrods.

Whelen makes several configurations of tip lights. I have both the traditional nav/strobe as well as the nav/strobe/tail assemblies laying around. That said, its pretty easy to add some LED tape lights to the TE of the tip. A trip to the "bling" section of Pep Boys can be enlightening (forgive the pun).
 

Will Aldridge

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I'm sure you can make that work, but it looks more complicated than it needs to be. If the rudder is taller than about 46", I'd just put the antenna in the rudder. We gave our dipole a slight V angle so that the junction is near the hinge line, halfway up the rudder, and the ends are at the upper and lower ends of the trailing edge. That kept the moment of inertia of the antenna as low as practical (for mass balancing), but kept the antenna ends as far as practical from the metal rudder drive parts at the bottom and the metal elevator drive parts at the top. Most of the antenna is closer than Weir recommends to conductive bits, but it still work plenty well enough for all practical purposes.

--Bob K.
I've got a pretty short rudder, only about 41" tall, so that won't work. I've got some metal plates that are holding the tailspring that makes a more vertical mounting tricky but maybe doable.

Btw has Weir ever published a design for a 406 Mhz ELT?
 

Pops

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Jim Weir is the reason I get Kitplanes mag. One sharp cookie. I have built 9 of his audio panels and Marker Beacon Recievers and also built the Marker Beacon Tester many years ago and have used it hundreds of times on MB receiver test.
 

Will Aldridge

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Jim Weir is the reason I get Kitplanes mag. One sharp cookie. I have built 9 of his audio panels and Marker Beacon Recievers and also built the Marker Beacon Tester many years ago and have used it hundreds of times on MB receiver test.
Sorry if i was a little unclear, does Jim Weir specify the length of the dipoles for the 406 mhz ELT antenna? I didn't see that in his manual.

One other question, what if i wrapped the copper tape around a dowel in a helical pattern? I've seen something similar done on my truck's antenna. The foil is still the correct length but can be stuffed in a smaller space.
 

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