Ariel mounted on roof rack?

Hi all.

I use an Albrecht CB with IIRC a Sirio 27 bolted through the front of a full length roof rack on my off road vehicle.  The coax from the ariel base is connect to the vehicle roof using an adaptor to suit a PL259.  Doing this enables me to remove the rack and then move the ariel to the connector at the rear of the vehicle, I use a switchable splitter within the car to change to the other coax, this keeps the SWR around 1.6 on the car and around 1.8 when on the rack, I say 'around' because it rarely is the same when I swap from one position to the other, it could simply be the environment I am in when swapping.  Despite these numbers range seems to be limited to around 3 miles in open country when the ariel is on the car perhaps even less when on the rack although it is difficult to measure accurately.

Given the ariel/rack is being earthed via the coax to the vehicle body, any thoughts on extending the range?  I don't see any chance of increasing the range by drilling into the roof and fitting an independent rack to car body earth, but someone may have an idea on this?  I have tried a fibreglass style ariel but this did not seem to help so went with the Sirio.

I will throw this in as well, I am not a lover of the ariels with the large spring at the bottom or centre loaded, this is a mental 'thing'.........yeh I know, don't go there.

Many thanks.

Regards

Dave