Comm Antenna Location and Performance

[Bill.Peyton]

I am at the point of closing the baggage floor. Prior to closing it up I need to select the locations for the comm antenna. So I am looking for recommendations and real world experience with both bent whips on bottom verses one on bottom and one on top. Antenna theory says that you should have a quarter wave ground plane to work effectively, which would be 24.5 inches radius, so centering the antenna as closely as possible could achieve this. In addition, it would seem that a bottom location would not perform well at all on the ground. Any takers on experience with either ?

Bill

 

[AltonD]

I have two located on the bottom, right below the seat pans. No issues, including talking to ground at least a half mile away. Don't forget, at some point in the plane's life, you will need to get to the BNC connector to unplug the cable and possibly replace the antenna.

 

[w1curtis]

These are my locations. I like the idea of having one comm antenna on the top side. Not as pretty, but I'm more comfortable with that. Even Cirrus with an all composite airframe chooses to put one comm, GPS and other antenna on top.

 

[dmaib]

#1 comm under the pilots seat and #2 comm under the baggage compartment right side. 300+ hours with no problems. In and out of lots of controlled airports.

 

[Bavafa]

Both comm antenna are under the seat (left and right) and have had zero issues with Radio for the 300+ hours on the plane. I can even pick up my clearance on the ground from ATC by pulling to the end of the taxi way which sits some what on top of a hill.

 

[David Shelton]

I originally had two COM antennas on the belly, beneath the passenger foot wells. Ideally, they shouldn't have been so close together, but they worked just fine. Eventually, I had to relocate them to make room for my belly pod. I lmoved them to the bottom of the wings and they worked great there too.

 

[Ron B.]

Nothing to do with radios but I'm in the process of installing baggage tie downs. Sure wished I had delt with them before the floor was riveted.

 

[Wayne Gillispie]

Com Antenna Location

I am installing Com 2 on top above passenger seats(same as Cirrus). I am installing Com 1 on the bottom in the tunnel either just in front of F-1034A or just in front of main spar. I have ran RG-400 for either location just in case I change my mind or have magnetometer issues with top antenna. Magnetometer shelf is high in tailcone just behind F-1007 bulkhead. This allows access in case of bonding issues down the road and keeps them as far away from gear legs/steps as possible. I will install 24" ground plane using copper foil tape or aluminum microgrid(also used for lightning strike protection by Boeing and Embraer).

 

[Bill.Peyton]

Wayne,
I had planned on locating my upper antenna 18" behind bulkhead 1006, which is right behind where the cabin top attaches. If you install the copper foil, where you planning on glueing it on the top of the surface, or inside? I would be slightly concerned with the ground plane being located the thickness of the cabin structure below the base of the antenna. I considered using aluminum grid on the topside just under the base of the antenna, but thought it would be too much work.
Bill

 

[Wayne Gillispie]

Top com antenna

I am going to install ground plane on the inside. RF energy passes right through the fiberglass cabin top. I will dig out the core 1/2" around the four screws and fill with flox otherwise the core could be damaged upon tightening hardware. Ensure csk screws are bonded to antenna with star lockwashers, then install aluminum doubler on bottom side of microgrid with lockwashers under nuts. Fabricate a fiberglass cover for RG-400 and hardware.

I may also just use a 24" x 24" piece of .016" 2024-T3 for the ground plane and doubler all in one. I have a backup plan with the two locations on the bottom. Fiberglass and avionics is all new to me. Looking forward to the cabin top in another month or so. Maybe one of the guys with com antennas on the cabin cover can provide some assistance for us too.

 

[Kellym]

I am going to install ground plane on the inside. RF energy passes right through the fiberglass cabin top. I will dig out the core 1/2" around the four screws and fill with flox otherwise the core could be damaged upon tightening hardware. Ensure csk screws are bonded to antenna with star lockwashers, then install aluminum doubler on bottom side of microgrid with lockwashers under nuts. Fabricate a fiberglass cover for RG-400 and hardware.

I may also just use a 24" x 24" piece of .016" 2024-T3 for the ground plane and doubler all in one. I have a backup plan with the two locations on the bottom. Fiberglass and avionics is all new to me. Looking forward to the cabin top in another month or so. Maybe one of the guys with com antennas on the cabin cover can provide some assistance for us too.

A ground plane needs to be physically close to the base of the antenna. Having 3/4" of fiberglass separating the two is not good. You could bond some aluminum tape radials on top of the cover for a ground plane and put a thin layer of resin over it, except where the actual electrical connection is made.
You would also be better off using aluminum tube spacers through the cabin top to protect the core than carving it out and replacing with solid. Same as you did for the windshield center brace. Probably don't need a doubler if you use washers on the under side of the cabin cover.
Keep in mind that locations near the top of the cabin peak are high drag as that is where the air is moving fastest. Further back on the fuselage will be less drag for same antenna. Doesn't matter on a c172, but is more of an issue when you cruise at 170kts. Given that you only need an ELT antenna and the com antenna on top, there should be plenty of room on the tailcone or your -10 without having to penetrate the fiberglass cabin cover.
Kelly