[TowerTalk] Ground strap

w8ji.tom w8ji.tom@MCIONE.com
Tue, 24 Nov 1998 08:50:41 -0500

Hi Jim,

>    I hope this isn't one of the topics that gets repeated so often that
> people are bored with it.  I was talking to a friend about grounding my
> mobile antenna mount to my vehicle and my thought was to get a wide
> braided strap to go from the mount to the truck.  I assumed that I would


> is no bunching due to an odd shape.  So my question is:  what is the
> optimum shape for the conductor making the "ground" connection between
> the mobile antenna mount and the vehicle?

This did go on for a while before. The problem is complex. While inductance
is lower, resistance is higher from non-uniform current distribution (as in
a flat conductor) for a given surface area. Current indeed does bunch up at
the edges of the flat conductor.

Woven conductors, depending on the weave and contact resistance between the
weaves, has much higher loss resistance than a smooth surface. When braid
becomes dirty or oxidized, various effects force the RF current to flow on
the outside. That means the current path is often through hundreds of poor
connections on the irregular surface.

In this case resistance is more important than inductance, but both are
probably very small compared to the rest of the system with a short strap.

 >Does it really matter much
> since mobile antennas are such low-efficiency devices that I wouldn't
> be able to tell the difference anyway? 

Probably so. My 160 meter mobile, with a F-250 HD Supercab long bed truck
(a big truck) has a ground loss ESR of about 20 ohms. Coil loss ESR is
nearly ten ohms (Q=~350). Another ohm or so doesn't mean much one way or
the other. 

The real question is probably how long the different straps or materials
will survive exposed to movement and vibration, salt (maybe), dirt, and

73 Tom

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