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  • #31
    Years ago, our flying club had a C-172 that had dual Nav/Com radios. Com1 used a belly-mounted antenna, while Com2 used an antenna mounted above the cabin (between the wings, basically). While the airplane was on the ground, Com1 was almost useless talking to Ground or Tower, but it was amazingly good in the air. Com2, on the other hand, was fine on the ground, but would occasionally receive very garbled and weak transmissions, even in the pattern. I played around in the air with it once, and found that if I changed heading by 30º, Com2 would clear up and work fine. I concluded that the issues with Com2 probably occurred when the wing was directly aligned between the antenna and the ground station, and that Com1's antenna was likewise being obstructed by the airframe while on the ground. But given my vast knowledge and experience in this area (i.e. - none at all) my logic could be complete malarky...

    Antenna placement is an interesting discussion. I have concerns about stuff hanging down too far at the bottom of the fuselage, having lost a dipole transponder antenna landing in some moderately tall grass (NOT in a Bearhawk - this was a Grumman Traveler with far less ground clearance). I keep thinking that the ideal solution would be dual antennas (top and bottom), but I suspect that there are reasons it isn't generally done that way... Like maybe the two sources would interfere with each other? I should probably ask that question over on the AeroElectric forum...
    Jim Parker
    Farmersville, TX (NE of Dallas)
    RANS S-6ES – E-LSA powered by 100 HP Rotax 912ULS

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    • #32
      Its not uncommon to see on Piper products. I took these at OSH last year. Its a green Tri-Pacer and a red Super Cruiser.
      IMG_5649.jpgIMG_5673.jpg
      Brooks Cone
      Southeast Michigan
      Patrol #303, Kit build

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      • #33
        While in CAP, we learned to use wing-shading to track ELTs when the COM antennae were mounted on the wing roots. Do a 360 circle at about 30deg bank and listen for the drop in signal. There is some directionality and signal shading for that antenna location, but it provides a good ground plain and a shorter cable run (ie cheaper & easier). The top of the empennage provides better all-around coverage, but you have to add ground plains, mounting brackets, and longer cables. Do one of each and let us know which works best.

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        • #34
          Another one, anyone who has installed nav antennas not in the wingtips, is everyone just mounting them to the tail like you see in about everything?

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          • #35
            Originally posted by zkelley2 View Post
            Another one, anyone who has installed nav antennas not in the wingtips, is everyone just mounting them to the tail like you see in about everything?
            Looks to me like you can fit a VOR antenna between the belly skin and fuse. I'm going to roll my own out of some stripped coax. Here's a discussion I found helpful:

            http://forums.matronics.com/viewtopi...82a848f6e1fb16
            Dave B.

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            • Archer39J
              Archer39J commented
              Editing a comment
              Perhaps, regardless I think a terrestrial backup is a prudent move, and GPS outages aren't exactly uncommon. Also RNAV approaches aren't considered "precision" if I recall correctly and I don't see ILS disappearing anytime soon.

            • JimParker256
              JimParker256 commented
              Editing a comment
              Dave, my post wasn't intended to criticize your decision to include the capability. I agree that it's a darn good idea if you're going to be flying IFR, and an easily defended decision even if you're VFR-only. When I purchased my current airplane ('65 Champion 7ECA), it had one ancient KX-170B NAV-COM radio and a KI-201C VOR/LOC indicator. The NAV portion was not operational when I bought the plane, and I felt it was important enough that I paid the local avionics shop to swap it for a used, but working KX-170B they had on their shelf.

              I was more lamenting the FAA's plan (or lack of plan) to do away with supporting VORs in general. Around here, when they break down, they are no longer being repaired. A couple of the VORs that I used to use for backup navigation all the time have been out of service now for over 2 years, with no apparent plan to fix them.

              And, yes, every time I read about yet another GPS Interference Test that is planned, I wonder what the heck the FAA NEXTGEN plan calls for them to do when someone SERIOUSLY hacks the GPS system, and it's not just a test that they can discontinue once they have a couple of mid-airs... I predict it will happen first in either Denver or El Paso, since they are both pretty close to the origin of those tests...

            • Bcone1381
              Bcone1381 commented
              Editing a comment
              Thanks for the reference, Dave!

          • #36
            Resurrect this from the dead. I bought one of the wisker type VOR/LOC antennas and tried to mount it somewhere in the belly between the fabric and the fuselage, but there is nowhere to do that it doesn't touch the metal fuselage. So I mounted it back toward the tail on top of the airplane.
            It works great, so long as I'm pointed 90 degrees from a VOR station. It shadow's really bad pointing at the station, not as bad, but still not great tracking away.

            Has anyone else put in an NAV antenna and had success (60miles) of range tracking to/from and arcing(perpendicular to)? What antenna and where did you put it?

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            • #37
              Hi Zkelley2,

              Thanks for resurrecting the thread - I'm asking some of the same questions you are. I'm at the stage where I need to weld tabs, and I'm wondering how many antennas I can mount without welding any tabs on to the fuselage.

              Did you get any further with the idea of putting a COM antenna onto the wing root fairing?
              I was also thinking about just mounting it on to the top of the wing root trailing edge box, but the root fairing sounds even better. Can anyone send me a link that will allow me to calculate how big the ground plane needs to be? Does it need to be distributed "evenly" around the footprint of the antenna, or can a long thin bit of metal do? What about the fuselage tubing right near the wing attachment lugs?

              Obviously, by these questions, people can guess I have only the vaguest idea of how antennae work, any guidance (as usual!) greatly appreciated.

              Thanks guys,

              James
              The Bob Barrows Bearhawk: "It's big like a Boeing... but better built."

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              • #38
                James, with the comm antenna, I mounted it on top of the wing root fairing, which is essentially a long kind of skinny strip of metal. It's a straight DPA red tail antenna. Works great there. Have talked to another airplane in the air at about 80 miles. No complaints from any ground stations.

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                • #39
                  You're an absolute champion, ZK - nothing beats advice based on real-world experience!

                  You've sold me on both the antenna and the location, especially as it saves me from welding tabs, and worrying about the comms until later down the track.

                  Back to mounting the wings...

                  James


                  The Bob Barrows Bearhawk: "It's big like a Boeing... but better built."

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