On 1/20/11 6:31 AM, Rik van Riel wrote:
> On 01/20/2011 12:44 AM, W7ZZ wrote:
>> N6RY and I (mostly N6RY) designed a very broadband open sleeve 80 dipole,
>> fed with coax, that works very well. I have remodeled this for 40, 30 and
>> 20 meters. I would be happy to send anyone the EZNEC files for all versions
>> of the antenna upon request, but I'm leaving for vacation, so it would be
>> ten days or so before you'd get a reply from me. So far, only the 80 m
>> antenna has been built and is in operation. the 40, 30 and 20 versions are
>> models only so far.
>
> I wouldn't think 40m is a wide enough band to need such an open
> sleeve dipole.
>
> On the other hand, it should be possible to mount a 30m parasitic
> element inside a 40m folded dipole, and get both bands on the
> same antenna.
>
But here's the question...
could you also put a clever lumped matching network at the feed of that
folded dipole (perhaps making use of the fact that you've got two
potential feedpoints to work with in a folded dipole) that would have
lower loss than the parasitic element.
I've seen some designs for which the whole system essentially has two
resonances (granted, for wireless bands, but the principles are the
same) and used very few components in the matching network (3, as I recall).
After all, from a "radiator" standpoint, there's no particular advantage
for the antenna being close to 1/2 wavelength as opposed to something a
bit different. That is, the pattern of a 17 meter long dipole on 40
meters isn't much different from a 20 meter dipole, nor is it much
different for 30 meters. Sure, if you get to radiators that are 1/10th
of a wavelength or close to a wavelength long, you start to see bigger
pattern changes.
There are some old IEEE papers on optimum matching networks for multiple
frequencies that I should try and dig up. One advantage of the lumped
scheme is that you're not relying on mutual coupling between elements,
which can get tricky and construction tolerance sensitive when you start
to get 3 or more resonances (like adjusting a multi coupled cavity filter)
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