Motorhome antenna

Hello. A little advice please....
Taken my magmount from car and put it on van roof. Van roof is aluminium, so I have screwed a steel square plate to roof an attached magmount. Swr on car was great through all channels 27ukfm and great on 555 usb also.
On van the best I can get is swr of 2.5 on ch 1 & 40 and 1.5 on ch20 on 27ukfm. Not tried the multi band radio as too scared. 
Contact between aluminium and steel is good. 
Tried different antennas and just as bad if not worse.
This is a trial to see if it works well before making a hole in roof.
Any help appreciated.
Thanks
Gary

It could be that your magmount on the car wasn't electrically connected to the roof, but was achieving a low swr by means of capacitance.

Deb

Thanks for reply. The antenna on the car works a treat. Thinking outside the box, would the fact it's going from steel to aluminium make any difference? Thanks

Speidy wrote:

Thanks for reply. The antenna on the car works a treat.  Thinking outside the box, would the fact it's going from steel to aluminium make any difference? Thanks

Probably not, but other reasons for a different match might be that the vehicle is the ground plane, the van roof is probably bigger than the car. If the antenna isn't electrically connected to the roof, then the way the coax is routed will also have an effect. To be honest, I'd forget about how the antenna worked on the car and adjust for lowest swr on the van. If you can't get a low swr on the van try grounding the rig to the van body. If all else fails, you could use an antenna analyser to see where the antenna is resonant. Also, check the coax, if it's been jammed in a door, it might be damaged.

Deb

Many thanks for advice. Will try what you suggested. Thanks again.