On Sat, 17 Apr 2004 19:16:52 GMT, zeno wrote:
Am I to understand that a promising antenna (you call a broadband
vertical) can be made from a single 50 foot telescoping tv metal mast,
the base of which is at ground level, utililizing the guy wires as part
of the antenna system, with a system of radials in the ground underneath?
How would I best feed this antenna, and would it be worthwhile with just
this single mast, and what advantages would such an antenna have over
others that I might construct?

Hi Bill,

We will do this by stages.

We are going to maximize what you already have: mast material.

Grounded or ungrounded are immaterial - there are ways of getting RF
into either. If grounded, this may require a drop wire described
below (I was a little too enthusiastic about possibilities in the
first post).

The masts will be guyed. The trick here is to think in advance to be
able to support ANY mission, vertical OR horizontal antennas. BOTH
may be available at the same time.

The usual vertical of 50 feet may resonate at several places in HF,
but almost certainly not in the Ham bands - so we must expect some
form of matching along the way. For ungrounded masts this can be done
within a meter of ground so there is no going aloft to perform any
jobs. For grounded masts you may wish to have a conductive yard arm,
perhaps more accurately described not as a yard but as 18 inches,
dropping a wire to ground for a 160M or 80M Gamma feed, or as a folded
monopole.

Note, I suggest 160M or 80M operation for an obviously too short
vertical. This is where the guy wires come in handy. As described
previously, think of them as the skeleton of an umbrella (no fabric,
but ribs expanded out). With the use of insulators, this can be
achieved so that you can construct what is called a "capacitive top
hat." Such additions to verticals lower their resonance (but this
does not imply it makes them 50Ohms - that is a separate issue). You
want them to be resonant (so they will absorb and radiate power), but
you will need to match them further (no different that any wire
antenna for that matter). For 80M, this antenna will probably offer
20 to 30 Ohms (not bad, but not 50). For 160M, this antenna could
offer 5 to 20 Ohms (becoming difficult, but not undo-able), but in all
likelihood, it will also have reactance (the top hat will probably not
be big enough).

Now, why would you want another 160M antenna? The bad news is that
horizontals in this band are abysmal performers even if you could get
one up the minimum quarter wave (a long way from only 50 feet up).
Verticals on the other hand use earth far more effectively for
communication in this band (how many AM stations do you know of with
horizontal antennas?). 80M sits on the borderline of earth
characteristics where horizontals may do better, but this doesn't mean
that verticals do poorer. Higher bands see earth differently, and
verticals need height (lifted into the air) to compete; or they need
to reside near a lake or the sea.

However, most of this discussion gets very far afield. The point is
to plan ahead, construct top hats (they will mix easily with your loop
project) and think about the gamma drop wires if the masts are
grounded. This is very little investment and you have four
opportunities (this then brings experimentation with phased, vertical
arrays - each mast driven separately through delay lines).

73's
Richard Clark, KB7QHC