TopBanders,
Here's another solution for improving the efficiency of short verticals that
doesn't seem to be widely known. Add a second (or even a third) identical
short vertical in very close proximity to the first and feed them all in phase.
The phased system radiates as a single vertical with improved efficiency. I
believe this technique is documented somewhere in ON4UN's book. The mutual
coupling between the close spaced verticals drives the radiation resistance up,
which enhances efficiency.
I've been doing this for years on 160 with three 60 foot verticals (actually my
80 meter vertical system) spaced 35 feet apart. When fed in-phase, the
feedpoint radiation resistance at each vertical is around 18 ohms without a top
hat. A single 60 foot vertical system has a radiation resistance of around one
third of that. When the total system resistive loss (ground loss plus other
component losses) is high (much bigger than 5-6 ohms), the efficiency of the
three vertical system would be improved as much as a factor of three (5 dB)
over a single vertical. For two in-phase verticals, the improvement would be
around 3 dB. When the system loss starts out low and the single vertical
efficiency is pretty good, there is obviously less to be gained, but that's
also true when other loading schemes are used with short verticals.
Without a top hat, the in-phase short verticals do exhibit capacitive reactance
at the feedpoint that must be cancelled with a loading coil. In my case the
coil is around 25 microH.
The spacing is not real critical. I use 35 feet because that's how my existing
80 meter verticals were already configured, but you can probably go much closer
and still get the same benefit. In fact, moving the verticals further apart
would eventually reduce the benefit, so closer is probably better. This can be
modeled easily.
Some of you might be wondering why not phase the short verticals instead for
directionality and hence gain. In practice you will find that with any
realistic losses, it's very hard to realize the theoretical transmit gain with
very short verticals, although you can achieve good (lossy) directionality for
receiving. It's better to work on improving efficiency by reducing the effect
of loss in the first place.
73, John W1FV
|