Hi Nick

Generally speaking a 1/2 wave on 70cm will have 1/9 odd the capture area
on 2m and thus less range for the same RX input levels. This is of
course for an LOS situation. I'll admit I have never compared 2m/70cm in
a mountainous vehicle to vehicle situation. I expect reflection would be
the primary mode. How well it works then depends on the size of the
reflectors! You have probably heard that some pine tree needles are the
exact length to attenuate 70cm signals!

I did some experiments years ago comparing horizontal polarization
mobile to base on 70cm. Quite an interesting outcome. The general figure
of merit was that horiz was better than vert by about 9-12dB once you
went past LOS. The signal also didn't chop around as much ("Picket
fencing") eg a vertical signal may swing 30-40dB but the horizontal only
5-10dB. Horiz antennas are "interesting" on 2/70 though. Mine was a
three leaf clover design about 400mm dia on 70cm.

Most important is height, Get the device out of clutter. I think if I
was doing it I'd get one of those long fiberglass whips and put my
halfwave at the top. Probably go for a simple slim-jim or jpole design.
Dual band designs will of course be a compromise so it may be smart to
just go and buy something. A vertical one and a half wave vertical
radiates really badly exactly where you don't want it! (ie the 2m
antenna on 70)

Also consider using SSB rather than FM. Don't know the figure of merit
there but 12-15dB sounds about right!

Apologies for not being specific to your request.

Cheers BOb VK2YQA

Nick wrote:
I am wanting to build 1/2 wave end fed ground plane independent aerials for
2M and 70cms for use on
touring motorcycles for bike to bike comms.