[HomeBrew] Grounding Question

[James Kearman]

Conductors function differently at rf than at dc. For dc (or 
line-voltage ac) we are mostly concerned with measurable dc resistance.

The magnetic field generated around a conductor pushes rf currents 
toward the surface. The higher the frequency the greater the "skin 
effect." So the rf resistance of a conductor may be much higher than the 
dc resistance. Braided conductors consist of many very thin wires, each 
of which is subject to skin effect. The net rf resistance can be very 
high. We use braided outer conductors on common coaxial cables for 
flexibility.

Now, if you are using a mobile antenna in fixed service, the situation 
is quite different than when the antenna is mounted to a vehicle. In 
mobile service the vehicle acts like one plate of a capacitor, the other 
plate being the ground you're driving across. You need a very good rf 
connection to the vehicle.

At home, you want a good dc connection to ground for safety purposes, 
but you can achieve good rf coupling to the ground by using as many 
radials as possible. If you're using fewer than about 64 radials they 
can be 0.2 wavelength long, or even shorter, without noticeable 
degradation. A bunch of short radials is still better than a car body 
for hf!

I cannot overstate the need for a good radial system. Rf current flowing 
into your antenna returns via the ground. The greater your ground 
resistance, the more power is dissipated in the ground. That's power 
that never gets radiated. A lousy ground with high resistance can fool 
you. The transmitter sees that high resistance as if it were a dummy 
load. Because the total resistance (antenna radiation resistance + 
ground loss) may be about 50 ohms, you think, Wow, what a great match! 
In truth, most of that resistance is lossy. As it is lossy, you may also 
get wide SWR bandwidth. For short antennas, wide bandwidth is a sure 
sign that something's amiss.

A short antenna should have a low radiation resistance. A 
quarter-wavelength vertical, for example, has a radiation resistance of 
about 34 ohms. Going shorter means more current flows, making it 
necessary to further reduce ground-system losses. Thus the use of 
low-loss copper strap in mobile installations, and mucho radials at home.

Yes, you can make contacts with a crummy ground system. Some so-called 
multiband vertical antennas only radiate 10% of the power delivered to 
them. More hams are radiating QRP power levels than they realize! 
Improving the ground system greatkt improves antenna efficiency. Even at 
today's copper prices, wire is cheaper than an amplifier.

For more information I highly recommend http://www.w8ji.com/

73,

Jim, KR1S
http://kr1s.kearman.com/