Dan wrote:
> Jim, obviously you are quite knowledgeable about antennas and have
> mentioned most of the factors to be considered. You did not mention the
> importance of the available ground system. If you have a lossless tuner
> and your ground system is sea water I suspect that a 160m antenna made
> of very conductive material could be reasonably efficient but would have
> a bandwidth that was extremely narrow as well as very high voltages and
> currents in it.
For the purposes of discussion, assume some non-perfect ground (and, in
any case, the "salt water marsh" for great vertical performance is more
in the far field for ground reflection, than in the "close to the bottom
of the radiator" counterpoise..
> Any reasonable ground system would seriously compromise the efficiency
> due to very low radiation resistance compared to ground resistance.
But, I don't know that this effect is hugely different as a function of
radiator height at least for comparing, say, 20 ft against 40ft. Sure..
a 3 ft radiator compared to a 100 ft radiator.
> On the high frequency end, even 43 feet is so long that much of its
> radiation is at too high an angle to be optimum. It seems to me that
> there is no best answer to your question unless you place some relative
> value on what is acceptable to you for each band.
That's exactly the sort of discussion I'm looking for. Sure, a long
radiator is terrible for high bands, but my (limited) practical
experience on, say, 10-12-15 m has been that either there's propagation,
in which case the proverbial wet noodle and a tuner works, or there's
not, in which case nothing will help. (Curse the lack of sunspots)
So, I suppose the question is more on lower bands..
> I would prefer to think of a set of 1/4 wave verticals paralleled at
> their base, fed with a single coax, and suitable line chokes to reduce
> shield radiation.
That is an alternate strategy, and moderately reasonable. I was looking
for a "single radiator" though: mostly for conceptual simplicity, but
also, I suppose driven by a desire for visual unobtrusiveness.
The non resonant verticals would have a high enough
> end impedance that the SWR would be acceptable on the desired band.The
> Hytower is a reasonable commercial version of this idea. On the lower
> bands it is an effective antenna but it is designed to use 3/4
> wavelength verticals on the higher bands which results in high angle
> radiation there. Just my opinion.
And trapped verticals like the xBTV are another approach to shortening
the radiator on higher bands.. The SteppIR vertical is another approach..
But.. returning to the original query.. single radiator, arbitrary
adjustable tuning network at the base.. what's the "best" single length
(where "best" is defined as you wish...)
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