HF aerial mounting close to UHF aerial

Without being too perdandic - try to position them about 600mm (2 feet) apart..

If you check out these website links below I'm sure you will find the answers

T iStore

Mobile One Antenna Info

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johnjdh
Reading the Mobile one recomendations of Do's and Don'ts I see they crontradict themselves, at the top it reads

If you put any antenna on a bullbar you should consider a spring to absorb the vibration.

Do's and Don'ts I see
DON'T Use a spring even if mounted on a bullbar - It may damage your radio
The spring is important to absorb the vibration from the engine but will make the antenna out of phase greatly reducing the performance

I guess they don't know what Corrugation is.

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Your other issue will be the performance of the UHF. It is line of sight, and an aerial mounted on the spare tyre will have trouble transmitting forward through the mass of the vehicle. Similar issue to bullbar mounts, but worse with rear mount. Roof is always best, and I noticed the good radio shops have some excellent brackets that mount to the rail on vehicles like the 120 and 100series. I personally use a stubbie aerial on the roof - works well in all directions, and is indestructable.

HF doesn't have this issue, because the signal bounces off the ionosphere.

Hi John. As others have said, put some distance between the antennas - as much as possible.
The UHF will have little effect on the HF but the HF antenna will appear as a big obstruction to the UHF.
As far as mounting location & performance is concerned, you may find my results interesting but not surprising.
In my Landcruiser I have 2 Electrophone TX4000 UHF radios. One is connected to a ground independent 1/4 wave on the bull bar (also have a 1/2 wave). The other is connected via a diplexer to a home brew 1.5 metre helical - 5/8th UHF whip. (Also run a TX835 - the diplexer allows two different frequency radios to use a single dual band antenna.)
Performance is chalk & cheese with the bull bar whip significantly better by many "S" points than the 5/8th whip that is hidden between the spare wheel, high lift jack & the back of the car. There is some loss (about 0.4db) in the diplexer.
Both radios provide similar performance when connected into the same antenna. The dual band antenna works much better on UHF when located on the bullbar.
As the rear mount antenna / radio is only a back up & used for scanning for nearby people, the reduced performance is not a problem. I was looking for maximum separation between the 2 UHF antennas. There is a mob in N.S.W that sells a UHF antenna that sticks on to glass & is very low in visibility - this may be a better option.
Cheers, Tony.

I have the base mounted on the roof-line (gutter) but use a 1/4 wave stubby antenna which is better for areas where obstacles such as buildings and hilly terrain are present.

Then out in flatter countryside I change over to a stainless steel antenna of 6db gain and about 800mm long, which is more suited to longer distances where fewer obstacles are present.

This aerial mounted on the roof performs as good, or better, than a 2.1m AE4706 "white curtain rod" mounted on the bullbar.

This leaves me with a practical spot to mount the sand flag when required. (on the bullbar)