mounting antennas on bullbars

Ideally they should be separated about a wavelength for the UHF antenna (about 65cm).
In practice, it probably won't matter all that much if they're a bit closer, although the high power from your HF radio may cause some issues in the receiver of your UHF radio down the if they are really close.

Hi Coot,
UHF CB is around 470MHz, mobile phones depends on who your provider is, but NextG is 850 MHz, the rest use 900, 1800 and 2100 Mhz (incl Telstra).
So a wavelength at 470Mhz is about 640mm.
As others have claimed, their systems work fine with close-mounted antennas, but in reality their performance will not be optimal.

cheers
Gerry

Would it matter if you are only using one at a time?
Would one pick up the other seeing as their frequencies are so far apart or is it that this does not matter?

ob

Hi ob,
It will still affect the performance, whether the second antenna is transmitting or not. Some of the transmitted signal will be absorbed into the adjoining receiver.

Basically, an antenna design, in part, depends on its surroundings. Add metallic bits close around it that weren't part of the design and they affect the performance, especially if the offending bit is around 1/4 wavelength or multiples of that. But any length, mounted close to the working antenna will affect it. And it will cause some transmitted power to be reflected back into the transmitter (usually with no damage). You may be able to tune out the affected VSWR (reflection), but the radiation pattern will still be affected.
This is used to good effect on yagi antennas (eg, TV antennas) where a slightly larger than 1/2 wavelength rod placed 1/4 wavelength behind the dipole (antenna) causes the signal to be reflected almost completely; ie no signal behind the reflecting rod. Similarly, slightly shorter rods placed in front of the dipole will enhance the signal, but narrow down the beam width and improve the gain. This is all part of careful design, otherwise you don't get the desired effect.
But conducting elements of random length and random spacing placed nearby will cause all sorts of unpredictable radiation patterns.
Hope this helps.
Here endeth the lesson....
Gerry