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"Doc" == w5lz writes:
Doc Jack, As stated, the only answer anyone can make to your
Doc questions is, [2] I don't know. SWR meters and antenna analyzers
Doc are nice, and handy, things to have around, but they require a
Doc little thought in their use. They aren't the "be all, end all"
Doc of antenna instruments by themselves. Probably the most 'handy'
Doc measuring device for dealing with antennas is the yardstick (or
Doc meter stick for those that would rather). Antenna analyzers are
Doc probably the most 'handy' gadget for finding out band/frequency
Doc an antenna is made for that I've seen in years. But, and there
Doc are several 'buts' that have to be taken into consideration when
Doc using one.
I knew it wouldn't be as easy as "plug it in and turn it on", but the
details are a little hard to find and a little harder for me to
understand. Thanks for explaining.
Doc If you connect an analyzer to the feed line of your antenna you
Doc are measuring where the whole 'system' (feed line and antenna)
Doc are 'resonant', not just the antenna. The feed line 'modifies'
Doc what the analyzer 'sees' of the antenna, changing the answer to,
Doc "What is the resonant frequency, and the input impedance?".
I knew this much, which is why I mentioned "antenna/feedline" in my
original post.
Doc To make the antenna 'right', you have to get rid of the feed
Doc line. OR, make it 'disappear' electrically. You can do that by
Doc making the feed line an electrical 1/2 wave length at whatever
Doc frequency your antenna is supposed to work at (or multiples of an
Doc electrical 1/2 wave length if one isn't long enough to reach from
Doc the antenna to where you are doing the measuring). The analyzer
Doc readings then are for the antenna only since an electrical 1/2
Doc wave length of feed line is 'invisible' to the analyzer. (Takes
Doc a different feed line length for each band.)
This isn't as bad as I thought on first read. The bands I want to
reach with this antenna are 80, 40, 20, 15, and 10. Four of those
five bands collapse into a single case, and the fifth one will
collapse as well due to the odd harmonic thing with 40 and 15, right?
This means a single feedline of 40m should work for all five bands.
My station isn't 40m from my antenna feedpoint, though, so I'll have
to make coils of feedline -- some at the feedpoint, and some at the
station -- will that cause problems?
[... Doc's thoughts on efficiency ...]
Yes, I've got to work with what (little) I've got, true enough.
Doc Use that yardstick to measure the length of your multiband
Doc antenna's elements. That will give you a rough idea where they
Doc 'should' be resonant (barring any loading coils, that makes it a
Doc little more difficult). Plugging those lengths into the 'magic'
Doc formula, F = 234 / length(feet), will give you a 'ball park' idea
Doc of frequency for 1/4 wave lengths (one half of each antenna).
Doc Then it's just a matter of 'tweaking' the lengths for each band.
Doc That doesn't do anything about input impedance, just resonance.
Doc To match the input impedance is a separate thing, and there are
Doc several methods of doing that. When both length and impedance
Doc matching are done, you will have the most 'efficiency' for the
Doc antenna in ~that~ particular configuration. It may not be
Doc exactly what you want, but that's more a result of how/where the
Doc antenna is mounted. Anything and everything can change the
Doc usefulness of an antenna, which is due to the radiation pattern,
Doc which is/can be affected by how/where/when the antenna is put up
Doc (at night, in a snow storm, at the North Pole is the best
Doc 'when'). Having used 10 words where 2 may have been more
Doc 'efficient', I'll shut up... 'Doc
I can see a long weekend in my future. Plug in a noise bridge, check
the resonance, lower the antenna, change its length, raise the
antenna, repeat. Since it's a multiband fan dipole, I'll have to tune
each leg for its own band, right?
Doc PS - A mobile antenna is usually only around 3 - 20% efficient
Doc compared to a 'properly' set up fixed antenna. They still work
Doc okay.
Doc, some days I want to take down all the copper in the yard, buy
myself a mobile antenna, and stick it on a big piece of sheet steel.
Jack.
- --
Jack Twilley
jmt at twilley dot org
http colon slash slash www dot twilley dot org slash tilde jmt slash
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