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Monday 31 January 2022

Fun with fitting roof bar mounts for antennas

I've fitted roof bars to my car and mounted the two antennas to them, the bars are coated in plastic and their mounting hardware is powder coated, however the bolts holding the fitting kit parts to the car showed continuity to the bodywork, the CB antenna at the very least needs bonding, which I have done using a piece of 27A DC rated cable, but this may be posing a problem itself so I will need a braided strap, while DC continuity is all well and good there could be reactance in the copper cable used, so a braided battery strap is needed instead.

This would be run from the bolts on the CB mount. then one of the bolts holding the roofbar mount fixing plate onto the mounting points on the car.

Using the wire, which is stranded, I got two low-SWR points either side of the CB allocations (SWR remains high within the band) once the mount was bonded, with the wire at the full spool length (2-meters or so) the SWR was much lower but it really needs bonding.

The 2/70 colinear antenna is offset from the centre towards the driver's side of the car, early tests on the VNA suggested that on 2-meters this antenna is working fine so I do not know where the antenna is getting its ground from on that band, I've not checked this on 70cm as yet.

As the VNA battery ran out during tests today I tried to power it from the car charger, this created a ground loop because the car electrical system has its negative grounded to the body as well as is the case in many modern vehicles.

Tomorrow I will pick up a braided earth strap of suitable length and try this on the car to bond the CB antenna as I suspect this copper DC cable I've used may not be working as expected.

73 de M0WNU/26CT730

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