Jack,
As stated, the only answer anyone can make to your
questions is, [2] I don't know.
SWR meters and antenna analyzers are nice, and handy,
things to have around, but they require a little thought
in their use. They aren't the "be all, end all" of
antenna instruments by themselves. Probably the most
'handy' measuring device for dealing with antennas is the
yardstick (or meter stick for those that would rather).
Antenna analyzers are probably the most 'handy' gadget
for finding out band/frequency an antenna is made for
that I've seen in years. But, and there are several 'buts'
that have to be taken into consideration when using one.
If you connect an analyzer to the feed line of your antenna
you are measuring where the whole 'system' (feed line and
antenna) are 'resonant', not just the antenna. The feed line
'modifies' what the analyzer 'sees' of the antenna, changing
the answer to, "What is the resonant frequency, and the input
impedance?". To make the antenna 'right', you have to get
rid of the feed line. OR, make it 'disappear' electrically.
You can do that by making the feed line an electrical 1/2
wave length at whatever frequency your antenna is supposed
to work at (or multiples of an electrical 1/2 wave length
if one isn't long enough to reach from the antenna to where
you are doing the measuring). The analyzer readings then
are for the antenna only since an electrical 1/2 wave length
of feed line is 'invisible' to the analyzer. (Takes a different
feed line length for each band.)
**[A thought about 'efficiency' here. Don't worry too much
about
efficiency, it isn't that important really. Of course you want
the most efficient antenna you can have, but that can change
with
any number of things, even with the exact same antenna. (Watch
the
fur being rubbed in the wrong direction with that statement!
LOL)
I'm talking about efficient 'results', not the characteristic
efficiency of a particular antenna. An antenna should be
mounted as
high as possible, away from anything around it. But, you can
only
put one in the space you have available, not always what would
be
the 'best' height/clearance, (right?), so make the best of what
you have and live with it.]**
Use that yardstick to measure the length of your multiband
antenna's elements. That will give you a rough idea where they
'should' be resonant (barring any loading coils, that makes
it a little more difficult). Plugging those lengths into the
'magic' formula, F = 234 / length(feet), will give you a 'ball
park'
idea of frequency for 1/4 wave lengths (one half of each
antenna).
Then it's just a matter of 'tweaking' the lengths for each band.
That doesn't do anything about input impedance, just resonance.
To
match the input impedance is a separate thing, and there are
several
methods of doing that. When both length and impedance matching
are
done, you will have the most 'efficiency' for the antenna in
~that~
particular configuration. It may not be exactly what you want,
but
that's more a result of how/where the antenna is mounted.
Anything and everything can change the usefulness of an
antenna,
which is due to the radiation pattern, which is/can be affected
by
how/where/when the antenna is put up (at night, in a snow storm,
at
the North Pole is the best 'when').
Having used 10 words where 2 may have been more 'efficient',
I'll
shut up...
'Doc

PS - A mobile antenna is usually only around 3 - 20% efficient
compared
to a 'properly' set up fixed antenna. They still work okay.

'Doc wrote:
PS - A mobile antenna is usually only around 3 - 20% efficient
compared to a 'properly' set up fixed antenna.

Dang Doc, a mobile 104" whip is more efficient than that
on the CB band. :-)
--
73, Cecil, W5DXP

Cecil,
Only if it's got one of them 'Wizz-Band, super-dupper,
all weather, triple-by-pass, monster coils' in it! Ain't
that right?
But then again, any 'full sized' 1/4 wave antenna is
more efficient than the 'usual' loaded mobile antenna for
bands lower than about 15 meters. That's also 'right', is
it not?
'Doc

'Doc wrote:

Hope you didn't miss my smiley face. I just couldn't resist
pointing out that a RS CB whip is more efficient than the
numbers you quoted. :-)

Only if it's got one of them 'Wizz-Band, super-dupper,
all weather, triple-by-pass, monster coils' in it! Ain't
that right?

you said:
A mobile antenna is usually only around 3 - 20% efficient

I don't see any mention of a coil. :-)

But then again, any 'full sized' 1/4 wave antenna is
more efficient than the 'usual' loaded mobile antenna for
bands lower than about 15 meters. That's also 'right', is
it not?

you said:
A mobile antenna is usually only around 3 - 20% efficient

I don't see any mention of size or frequency. :-)
--
73, Cecil http://www.qsl.net/w5dxp



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Cecil,
But did you see my 'tongue in cheek'? Looked
sort of like a ball player three months behind
with his 'Red Man'...
'Doc

'Doc wrote:
But did you see my 'tongue in cheek'?

Nope, I didn't, Doc. "Seeing" is not one of my strong
points lately given cataracts and macular degen.
--
73, Cecil http://www.qsl.net/w5dxp



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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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Jack,
Yep, lots of fun with the up/down/up/down thing, but that's
just the normal part of tuning almost any antenna I can think
of, off hand. For a multiband antenna, multiply all that up
and down stuff for each band (probably). And since each 'part'
of the antenna will affect the other 'parts', repeating the
whole mess is something to count on till all of them are
'right'.
One way of changing the input impedance of a dipole is to
change the 'angle of the dangle' of each 'element'. Making the
angle between the legs of a dipole smaller reduces the input
impedance. So playing with the 'dangle angle' of each part of
the multiband antenna can be one of the simpler ways of doing
the impedance matching. Something to remember is that the
input impedance for all bands will probably never be 'perfect'.
Settling for the 'best' you can get is probably what the
majority
of people do, and just don't worry about it too much. While
looking for the 'best' you can get is the idea, working for
'perfection' is usually a wasted effort.
The thing about using an electrical 1/2 wave feed line is
mostly
for tuning purposes. Once the antenna is tuned correctly the
length
of feed line (coax type) isn't very important, since it isn't
being
used to do any of the impedance matching (right?).
I don't remember what else you mentioned in your post. It's
late,
I just got off work...
'Doc

'Doc wrote:
Yep, lots of fun with the up/down/up/down thing, but that's
just the normal part of tuning almost any antenna I can think
of, off hand. For a multiband antenna, multiply all that up
and down stuff for each band (probably). And since each 'part'
of the antenna will affect the other 'parts', repeating the
whole mess is something to count on till all of them are
'right'.

OTOH, I raised my all-HF-band dipole only once and achieved
a decent match on all HF bands without an antenna tuner.
--
73, Cecil http://www.qsl.net/w5dxp



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Cecil,
Miracles do happen!
'Doc