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asw20c 02-12-2020 07:08 PM

Antenna Question
 
I figure there are folks on this forum that understand antenna voodoo and can explain something that makes no sense to me. When I started on my slow-build wings, I knew that I wanted ground-based navaids as a backup for satellite navigation and approaches; i.e I wanted to be able to navigate by VOR and make ILS approaches if I had to. Stein was happy to sell me a Bob Archer VOR antenna that fits in the wingtip (pick one) that they said would do VOR navigation and ILS approaches.
Fast forward and I'm now at the stage where it is time to install the antenna. I did a quick internet search and found that VOR frequencies are 108-118 MHz, and the instructions that came with the antenna say it covers that range and more (I think 108-136MHz) so I know it is good for both VOR navigation as well as localizers. However glide slope frequencies are 329-335 MHz which clearly this antenna was not designed to receive.
I called Stein today to ask them about this and they said that if you have a modern navigator, like a GTN650, that it is able to sort out the localizer and glide slope frequencies from this one antenna and that they said other people have been using these same antennas with no complaints.
Can anyone explain this? Is there a misunderstanding between me and Stein? Can anyone here vouch for the fact that they have one of these antennas and can make an ILS approach? i.e. a PRECISION approach?

Skid 02-12-2020 07:13 PM

Quote:

Originally Posted by asw20c (Post 1407476)
I figure there are folks on this forum that understand antenna voodoo and can explain something that makes no sense to me. When I started on my slow-build wings, I knew that I wanted ground-based navaids as a backup for satellite navigation and approaches; i.e I wanted to be able to navigate by VOR and make ILS approaches if I had to. Stein was happy to sell me a Bob Archer VOR antenna that fits in the wingtip (pick one) that they said would do VOR navigation and ILS approaches.
Fast forward and I'm now at the stage where it is time to install the antenna. I did a quick internet search and found that VOR frequencies are 108-118 MHz, and the instructions that came with the antenna say it covers that range and more (I think 108-136MHz) so I know it is good for both VOR navigation as well as localizers. However glide slope frequencies are 329-335 MHz which clearly this antenna was not designed to receive.
I called Stein today to ask them about this and they said that if you have a modern navigator, like a GTN650, that it is able to sort out the localizer and glide slope frequencies from this one antenna and that they said other people have been using these same antennas with no complaints.
Can anyone explain this? Is there a misunderstanding between me and Stein? Can anyone here vouch for the fact that they have one of these antennas and can make an ILS approach? i.e. a PRECISION approach?

I have made countless approaches in my RV with said antenna in my left wingtip, works great.

dpansier 02-12-2020 07:54 PM

The Glide slope frequency is near the third harmonic of the localizer frequency and antennas perform fairly well on odd harmonics, an antenna designed for 108 to 118 MHz will also be useful on 329.3 to 335.0 MHz.
The ground based Glide Slope antenna beams most of the 40 watt signal toward the aircraft which is only about 5 miles away so it does not take much of an antenna to receive the signal.

BobTurner 02-13-2020 12:28 AM

As Don said, antennas that are resonant at half-wavelength are also resonant at 3/2, 5/2, etc., wavelengths, so they work well at 330 MHz. The 50 ohm matching network may be off a bit, but, as Don said, the GS is relatively strong. There are 1000’s of flying airplanes with dipoles for 110 MHz up on their tails that also use the same antenna for 330 MHz GS. My SL30 doesn’t even have a separate GS antenna input - it expects to use the nav antenna. BTW, I have an Archer in my wingtip, it works fine - vor, loc, and GS.

asw20c 02-13-2020 07:13 AM

I knew someone here could explain it. Thanks guys!

JHartline 02-13-2020 03:01 PM

I’m in the “I can’t explain how it works, but it works” category. I have a GTN650 with the Archer antenna in the right wingtip. I have flown multiple ILS approaches, including one in a pretty good rain shower and it’s rock solid every time. Have not tested the localizer or GS receiver to see how far out I can get the signal but I have always been able to fly any published approach I attempted. I’m very happy (and confident) with this setup.

BlackhawkSP 02-14-2020 08:26 AM

Quote:

Originally Posted by asw20c (Post 1407476)
I figure there are folks on this forum that understand antenna voodoo and can explain something that makes no sense to me. When I started on my slow-build wings, I knew that I wanted ground-based navaids as a backup for satellite navigation and approaches; i.e I wanted to be able to navigate by VOR and make ILS approaches if I had to. Stein was happy to sell me a Bob Archer VOR antenna that fits in the wingtip (pick one) that they said would do VOR navigation and ILS approaches.
Fast forward and I'm now at the stage where it is time to install the antenna. I did a quick internet search and found that VOR frequencies are 108-118 MHz, and the instructions that came with the antenna say it covers that range and more (I think 108-136MHz) so I know it is good for both VOR navigation as well as localizers. However glide slope frequencies are 329-335 MHz which clearly this antenna was not designed to receive.
I called Stein today to ask them about this and they said that if you have a modern navigator, like a GTN650, that it is able to sort out the localizer and glide slope frequencies from this one antenna and that they said other people have been using these same antennas with no complaints.
Can anyone explain this? Is there a misunderstanding between me and Stein? Can anyone here vouch for the fact that they have one of these antennas and can make an ILS approach? i.e. a PRECISION approach?

Stein is correct. The Bob Archer hooked to the GTN 650 will sort out the higher frequency from the Archer antenna quite nicely. I have one under the fiberglass tip on my RV-10 and it works perfectly.

On my RV-8, I actually made the GS antenna with a tuned length of stripped coax and ran the GS antenna down the inside of my left gear leg fairing and it works perfectly on the Garmin 430 WAAS. The reason for the doing it that way was the 430 has a second B&C connector for the GS antenna input. Others have used splitters on the one antenna and that works too, but the signal strength may be less.

Pilot135pd 02-14-2020 09:19 AM

Quote:

Originally Posted by BlackhawkSP (Post 1407845)
....On my RV-8, I actually made the GS antenna with a tuned length of stripped coax and ran the GS antenna down the inside of my left gear leg fairing and it works perfectly on the Garmin 430 WAAS....

I've been researching this for a while now and I'm going to do it for my VOR and GS this weekend but my plan was to install it in front of the canopy arc like in this thread

http://www.vansairforce.com/communit...t=40633&page=3

For the last 3 weeks researching this my doubt has been, and I'm still trying to it figure out, why everyone is using 16" because 234/113 Mhz (middle of frequency range) = 2.07 feet not 16" !

What length did you strip on your RV-8 and why?

BlackhawkSP 02-14-2020 10:57 AM

Quote:

Originally Posted by Pilot135pd (Post 1407858)
I've been researching this for a while now and I'm going to do it for my VOR and GS this weekend but my plan was to install it in front of the canopy arc like in this thread

http://www.vansairforce.com/communit...t=40633&page=3

For the last 3 weeks researching this my doubt has been, and I'm still trying to it figure out, why everyone is using 16" because 234/113 Mhz (middle of frequency range) = 2.07 feet not 16" !

What length did you strip on your RV-8 and why?

First of all, the GS frequencies are 329 to 335 MHZ. If you used the listed 234/113, that's why your math is not working out.

Using the formula of WAVELENGTH X FREQUENCY = SPEED OF LIGHT (C) and run the numbers for the middle of the band for GS frequencies, the math works out as 35.55 inches for a full wave antenna. This makes the exact length of the stripped portion of a 1/2 wave antenna at 17.78 inches. The 16 to 17 inches of the stripped center conductor of a piece of coax stuffed down the gear leg of my 8 has worked perfectly for years.


Would do it again in a second:-).

I had to get the calculator out and shake off a few cobwebs in the brain on that one:-)....

BobTurner 02-14-2020 01:21 PM

Quote:

Originally Posted by BlackhawkSP (Post 1407886)
This makes the exact length of the stripped portion of a 1/2 wave antenna at 17.78 inches. The 16 to 17 inches of the stripped center conductor of a piece of coax stuffed down the gear leg of my 8 has worked perfectly for years.
...

This just illustrates what Don said, above: with a 40 watt gs transmitter 5 miles away, almost anything, including a coat hangar, will work. A ‘proper’ half wave dipole for GS will have 9” of insulated center conductor (no shield) oriented left-right, and 9” of shield (no center conductor) running right-left, with the intact coax running away at 90 deg (usually up). 18” of center conductor running down a gear leg is a poor 50 ohm match, limited horizontal polarization, yet often works okay. Modern receivers are incredibly sensitive.


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