UHF antennas

Johnf.
The myth about antenna's for hilly and flat ground must have been started by a antenna salesman, the polar patern of the 4db and the 6db are both suitable for eather terian. The most important thing about mounting your antenna is to get it above the vehicle because the signal is blocked by any metal object more than 1/10th of a wavelenth wide. Look at the police and ambulance antennas, always short and above the roof line and away from the interference generated by the motor. When lives depend on radio many hours of testing is done and the result is there for all to copy.
Eric.

I don't know why everybody has large aerials on there vehicles, I have a 13cm uhf 1/4 wave aerial on a magnetic mount thats sits on the rear shelf INSIDE my car and I TX & RX repeaters some 60km away also simplex around 10kms.

Back to the original question.
No you won't be able to get an effective splitter. Signal can be split between 2 antennas however this requires an engineering solution for each particular setup. Reason being that the signal has to have the correct phase relationship at each antenna in order that you don't get cancelation of signal. Dificult to explain but basically where mulitiple antenna are used together they have a splitter and phasing setup in order that they work correctly. eg Have a look at phased arrays for TV. (The setups with multiple antenas one under the other).
You could use a coaxial switch ( has to be a coaxial switch) to selct one antenna or the other (make sure you are not transmitting when you switch, the smoke might come out),

Cheers
Ken

I just carry it in the car and swap ariels when I cant hear anymore.....

Takes a whole 10 seconds

John, have you had a look at GME website, their 4700 series aerials would suit your requirements. Five whips, from about 400mm up to 2 M, with gains up to 6 dB, all on one spring base. Screw on & off in few seconds.

It's easy to get sidetracked from original query, when others make statements, that might suit them, but wouldn't apply in extreme conditions. Catch you later...

A splitter would not help you as even if it was engineered to suit your two different antennas, you would only be getting approx half the signal into each antenna, thereby reducing your useful transmitting range.

Co-phasing is simpler and uses two identical antennas to increase the signal strength in a particular direction (at the expense of other directions) but that's not what you are trying to acheive.

Well, I've welded a tag onto the front of the bull bar on my FJ60 which carries both 2dB and 4,5dB colinear, groundplane-independent antennas. Contrary to what some may think, the radiation patterns *are* different. I use the 2dB for hilly areas and the 4,5dB when things are flat. Definitely makes a difference. I just use a switch mounted under the dashboard where the glovebox is. 1 second job to change over.