Top mounted comm antenna, slider
 

I am working on my RV7 aft top skins and would like to make preparations for any possible antennas. I see most people are mounting there comm antennas on the bottom but I don't mind seeing mine on the top, as long as I don't run out of room since it will be a slider and I am thinking a gps puck will also be back there. I am wondering how far the slider comes back, I found the question in another post but no answer. I might just add a doubler for now and decide later, you are welcome to convince me otherwise.

Why on top??

Not much comm traffic from above you, airport control towers are on the ground. Ditto most everything you might want to talk to, except ship to ship.

I think a belly mount has several advantages, drag, aesthetics and reception. Also should you decide on a Super Tracks installation (which really improves the slider) there is very little room for a top mount antenna.

Antenna on top
 

If you do put in a doubler I would suggest you but put it as far back as possible. Additionally I highly recommend you purchase and install the Supertracks slider mod which extends the travel of the slider back another 9" or so. Its one of the most useful changes I have made to my airplane.

The super tracks modification sounds very useful indeed. My slider was completed and an antenna installed before that was an option. One of my Comm antennas is mounted below (bent whip) and another mounted above. The top antenna is a combination Comm/GPS antenna.
Absent the Super Track, I was able to modify the slider so that the rear block could be released, allowing the canopy to tip forward. This allows full access to the baggage compartment.

The super tracks mod has crossed my mind, You guys are starting to convince me, its hard to argue with the majority.

In case you need more convincing, do the Super Track mod. You won't regret it. Also consider the Almost -14 mod for more seat adjustment options.

My COM antenna is bottom mounted under the co-pilot seat and it works well. ADS-B in (left side) and transponder (right side) antennas are just fwd of the spar in my -7A. Some also mount them in the vent line holes at the fwd fuse, which are not used if you go with the Rocket style vent in the wing root.

I agree with Mike, most communications from your plane is with people on the ground "UNDER" the plane. :D

My COMM antenna was mounted on top when I bought the plane but recently put a Rami AV-17 "bent whip" belly mount antenna on and I swear I can hear more distant communications now. Could be the placebo effect also. :D

Shawn

I have one on top and one on bottom, the top definately makes ground communication easier. In the air who knows, unless you had identical radios with exactly the same sensiivity (not likely) there really is no way to tell, but the top antenna seems to work equally well from what I can tell.

The issue may be speaking to a Tower Ground while taxing or on the runway.

A bent whip bottom mounted is poor when close that large expanse of "ground".

Note the comment on the bottom of the FAA TUS airport diagram.

http://155.178.201.160/d-tpp/1806/00430AD.PDF

I wonder if it's related to how much rebar is in the concrete?

There might actually be a reason behind the usual certified aircraft combination of one upper and one lower comm antenna. :)