[PCR-1000] ZAPTRAP

[pcr-1000@mailman.qth.net]

Hi all, me again,

> "Tim Tapio" wrote:-
> 
> How do you ground such an antenna?

First, let's be clear about one thing, the engineering required the protect a 
radio installation, from a direct strike to the tower, needs to be thorough 
and rugged  (read expensive!). But a badly installed station can suffer damage 
from a strike as much as a mile away while a careful, sensible installation 
can survive a strike 100 yards (metres) away.

The best ground for the signal line of an HF antenna is a fairly heavy duty 
parallel tuned circuit from line to ground or, for a VHF antenna, a quarter 
wave shorted stub, in each case, of course, tuned to the band in use. Think of 
the voltage spike as a broad-band signal (literally from DC to light!) and the 
tuned circuit as limitting this to a narrow band with corresponding reduction 
in energy. ATU's with a DC path from antenna line to ground are therefor much 
to be prefered.

That leaves, for consideration, the outer(s) of coaxial cabl(s). The RF 
ground, in the case of an MF antenna, is likely to be as good as we can aspire to 
for lightning protection..

The mast/tower should be thoroughly grounded at it's base, if it's a wooden 
(or plastic) pole, it's a good idea to run a heay conductor down the pole and 
put in a long spike at the base - have a look at your local, wooden, 
electricity or telephone poles.

It's best if all the metal on the tower is electrically part of the tower.

The outer of coaxial feeders should be bonded to the tower earth as they 
leave the tower; otherwise some sort of grounded entry plate should be provided at 
the shack. If the shack is more than about 10 metres from the tower, do both. 
A convenient way of making that ground , for VHF feeders, is to use two type 
"N" plugs and a flanged, chassis feedthrough, with the ground connection taken 
from the flange. At HF, the corresponding UHF fittings will serve.

When there is a strike near a vertical metal structure, an EMF is induced in 
it and this depends on the inductance (not much the resistance) of the 
structure - pole or tower in our case. That voltage is then piped into the shack on 
the OUTER of coaxial cable(s). Think of the tower as an antenna, grounded at 
its base, with a top feed connection to the shack! To prevent this the outer of 
the feeder(s) should be connected to the base of the tower and a little time, 
and money, spent on that ground.

Also read section 36.6 of the ARRL Handbook (best) or section 12.100 of the 
RSGB Handbook.

73, Heath.

Note to moderator - how long do you want this to go on for ?


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