On Dec 27, 4:43*pm, Tuno wrote:
Absent any better ideas, I took the whip antenna I once used on my
Honda Pilot and mounted it on a 2-by-2 foot piece of sheet metal from
Ace Hardware. I attached the latter to the RV top using industrial 3M
dual lock tape at the corners, and strips of 3/16" weather seal
underneath to prevent rubbing.

I have no clue how big the ground plane is supposed to be, or if it's
needed for transmit as well as receive, but I could hear pattern
traffic at an airport 30 miles away.

2NO

Saying it is' a "whip" does not tell us much. For 1/4 wave (radiator
about 2 feet long) and 5/8 wave designs (radiator about 5 feet long)
start with a ground plane at least the length of the radiating
element. A 1/2 wave (radiator about 4 feet long) antenna could still
be a whip but does not need a ground plane. If the old antenna had a
coil loaded base etc. you must use that on the new installation, e.g.
you can't just mount the whip unloaded on a threaded bolt.

An antenna transmit and receive properties are identical. A ground
plane if required works the same in both directions.

A lot depends on how much work you want to do, how large an antenna
you are willing to put up with, how you want to trade off beam shapes
(how much is the antenna beam oriented towards the horizon vs.
vertical), etc. etc.

Personally if mounting on the roof I'd use a 5/8 wave with a metal
sheet ground plane or I'd hoist a 1/2 wave antenna up on a PVC conduit
or fiberglass pole. Ham radio stores (like Ham Radio Outlet) often
have staff that can help.

Darryl