[TowerTalk] Fwd: Re: Antennas for 80m

[Jim Brown]

See responses interspersed.

 >Do any of your tech notes reflect on the problems of grounding a 
roof-mounted
 >vertical?

>The problem, of course, is that the ground lead becomes part of the
>radiator and detunes the antenna and, unlike the feedline,
>decoupling the ground lead can be an issue.

Your question reflects some serious misconceptions with respect to
how the EARTH interacts with a vertical antenna. The earth is very
lossy. If we allow the EARTH to be part of the antenna, it burns
transmitter power, making transmitted signal weaker. We use radials
to shield the fields produced by a a vertical antenna from the earth,
and also provide a return path for antenna current so that this does
not happen. It IS important to connect the coax shield at the base
of the antenna to the earth, but this is for LIGHTNING PROTECTION,
not to make the antenna work. There should NEVER be a choke in that
lightning protection line.

Radials also serve to decouple the coax from the antenna, but only
partially. This is also the function of adding a coax choke at the
feedpoint.

>Are the issues the same whether it's a radial-free antenna (R5,
>R8000 etc.) vs. a radial-dependent (HF2V, 5BTV, etc.)?

What I've described mostly applies to quarter-wave antennas. A few
antennas are designed to function as vertical dipoles, so they don't
need radials to carry return current, or as a return for the fields.
BUT - in my experience, vertical dipoles don't work very well on
the HF bands.

Bottom line -- verticals for 80 and 160 want to be on the ground and
have radial systems or serious counterpoises or both. And, as others
have noted, verticals much shorter than a quarter wave need loading,
preferably top loading.

There are good discussions of these issues in the ON4UN book, and in
the ARRL Handbook and Antenna Book.

73, Jim K9YC