RF earth and operating from the car.

Yesterday, whilst down the beach with the mobile whip, I noticed that the matching of the antenna was not always reliable.  Rather oddly, I found that attaching a short wire to the base of the antenna brought an improvement to about 1.2:1 SWR from an occasional high of over 1.7:1.

I thought this additional wire might adversely affect performance, but I continued to get good signal reports on FT8 from Argentina - even though that direction had no seawater or beach to help me.  But it's probably not the best solution, and may give rise to some signal loss.

I came home to read Les Moxon's ever-useful 'HF Antennas for All Locations', which has a good section on losses in mobile antenna systems.

The situation with mobile whips is in no way a simple affair to understand, even though ham outlets tend to suggest it's just a case of sticking a magnetic mount and whip on top of your car and away you go!  The difference in capacitance between antenna and car body, and then the capacitance to ground, not to mention earth losses, are complex matters that lend themselves more to empirical, rather than theoretical understanding.

Luckily, I am not actually 'mobile mobile', but static mobile.  So this gives a little room for improvements that a moving car couldn't accommodate.

I decided to make a good earth connection between the HF rig and the car's metalwork.  I drilled a small hole in the car's boot metalwork, grinding the paint away with a rotary tool.  I used a self-tapping screw to give a good physical connection via some copper braid.

Then I made an earth connection for the coax braid side of the antenna system that can be made and unmade using DC crimp connectors used for vehicle applications.  I made the connection to the magnetic mount by drilling a hole for a stainless steel bolt in the aluminium bar that connects each magnet.

Testing down at the beach again showed that these two improvements to earth were worth the effort.  Now the matching is reliable, and doesn't show strange variations that appeared without them.  The SWR is now a consistent 1.2:1, which is very good for a mobile whip.

Of course, if I move the car to another type of ground, which, inevitably in comparison to a beach, will be much poorer, the system will change.  In general, the antenna will tend to resonate too high in the band.  But because I only really operate at the beach, I don't need to worry about all that.

First call with the new earthing instantly yielded a JA giving me a -11dB report from 15W FT8.  Another good result!

Another idea from Les Moxon's imagination is to add short radials above the car roof, to shorten the return current path and so reduce earth resistance losses.  I will try that one of these days.