I always thought that the 8900 was a true dual band radio. The 7800 is not a true dual bander.
Personally if I was using one mobile I'd split off ten metres and use a decent CB whip because I think, for local operation, ten needs all the help it can get.

I cant give figures (I dont think anyone can) but I will say Clear of the roof because in the case of VHF and (even more so) UHF, its not range but whats in the way that counts.
Also remember coax losses start coming (With a vengance) into play.

Cheers de Tony

One of the lessons to be learned here is SWR does not equal resonance.
To quote G3YXM
"A good big mobile whip on resonance on the LF bands will present an impedance of about 20 Ohms to the rig, it won't like this! Any 160/80/40m whip that gives a perfect match on resonance is LOSSY, don't believe the hype! To feed your 20 Ohm super-whip you can either use a transformer, available from retailers or home made, or use capacitive matching as I do"

20 Ohms (ish) is about 2.5:1 SWR as  is 125 Ohms (Watch the radio "fold back")

For SSB on 2 you need a horizontal aerial and if you are mobile, mobile as opposed to stationary mobile then you need a horizontal omni as in "halo" or if you can manage it, a "Cloverleaf " aka " Big Wheel"
See what this lad did in Oz

http://www.qsl.net/vk3bjm/

Looking at what you have and if I was in that situation. Id go for a 1/4 wave (Thin and very springy) and mounted on top.. Where you have your aerial, the roll bar will detune it badly. VHF rule one, get it right up in the clear.
 
73 de Tony

You could always put up a delta loop with the tower supporting  the top corner and feed it at one of the bottom corners .
Someone I knew had one. He showed me a comparison between the delta and a dipole. As he switched between the 2 some came up while others went down.  A GM in Shetland told him he was the strongest G that he had worked!
With wire there is no reason you cant have both (Cheap enough)

My KG699E (4 metres) has got good audio reports from my XYL and 2 friends (I didnt ask for reports they just came out with them)

My Icom 706 Mk2G isnt deaf but does suffer under strong signals. I put that down to having wide FM receive capability thus having a very wide filter at the 1st IF also living down Brant Road in Lincoln doesnt help.

1000 mile tropo's usually occur over sea paths rather than land (Ive worked from Doncaster (IO93MM) to the Baltic coast of Poland.
One thing that may give a clue is the weather between you and the other station, large high pressure system would suggest tropo, thunder storms may suggest Es. Was there any stations in between? What was 70 cms doing? WB2WIK is right in as much as , on 2, tropo can get as far as Es
Certainly smells like Es with nowt in between.
Craig, if you're capable try switching polarity, if  its tropo the difference will be very marked

W8JX, leaves (particually wet leaves) are bad for UHF and UHF lifts more often than lower frequencies.
G3PHO, in the 1990's, worked from his bedroom in Sheffield to Czechoslovakia (as it was then) on 10 Gigs (Dish pointing through the window)

Cheers de Tony

Are you using the same cable on the 2 aerials? If not, try it . My thoughts is corrosion on the coax making the coax lossy thus giving a false "low SWR " reading.
Try running the coax into a dummy load and (using the same meter) compare readings at either end of the coax.
Hardly any difference says coax OK, big difference says coax knackered

I know you are on the opposite side of the Pond but it wont hurt to give Garex an email. I believe the Mydel whip for my KG 699 (4 metres) might be a rebadged Garex

http://www.garex.co.uk/aerials/flexis.htm

The noise intervals made me think the same as W7WX, Electric Fencer. Any livestock near you?