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.
> Any reasonable ground system would seriously compromise the efficiency
> due to very low radiation resistance compared to ground resistance.
> 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.
>
If you are willing to settle for the top end being 20 meters then 43' is
just a little long as a 5/8ths wave. I can't model it, but I'd guess
there are probably some lobes that would work on 15 and 10 although not
the best with the majority of the radiation at too high an angle to be
useful. That doesn't mean the antenna wouldn't work, just not the best.
As Dan said, it'd be pretty short on 160, but could still work. Again
work and good are not quite the same thing. OTOH there are many who
swear by trap verticals which are notoriously poor, but if conditions
are right...
73
Roger (K8RI)
> 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. 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.
>
> 73, Dan, N5AR
>
> jimlux wrote:
>
>> Or length, really...
>>
>> Say you have a vertical that can be any arbitrary length, and you have a
>> (presumed perfect lossless) tuner at the base of the vertical. What
>> length should one choose, if you want to operate all HF ham bands?
>>
>> Obviously, there's lots of folks selling 43 foot verticals these days,
>> but I suspect that's a length chosen to make sure the impedance isn't
>> "bad" in the ham bands, when fed with a simple matching network (4:1
>> transfomer, often with some leakage C) and coax to a tuner in the shack.
>>
>> Is there a point of diminishing returns in height (e.g. the difference
>> in performance on, say, 160m, is going to be tiny between 20 ft and
>> 25ft.. it's a "very short" radiator on both).
>>
>> What about higher band performance (a very long radiator is going to be
>> much longer than a 1/4 wavelength, so the pattern will start to have
>> many vertical lobes, which may or may not have a big practical effect).
>>
>> (The assumption of lossless tuner isn't all that unreasonable, at least
>> for comparing the radiation performance..)
>>
>> Jim
>>
>>
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