Hello Gary:

Seems I've simply lost the ability to communicate any more. My point was
that seeking the most efficient antenna (as defined by maximum power
transfer into space) ought not to be the guiding principle. We want
maximum power delivered to the other station. All other things the same,
a higher radiation resistance would mean lower ohmic losses. But all
other things are not the same when antenna length is increased.

Yes, radiation patterns on real boats will differ from radiation
patterns in space. But even on a real boat, a high percentage of energy
is radiated at quite high angles when the antenna is a half-wavelength.
Yes, the half-wave will be more "efficient" in getting energy off the
boat because the radiation resistance is higher than with a shorter
antenna. But if it doesn't get the signal to the other station because
the radiation angle is too high, then it's not really optimal. Look at
some numbers on vertical radiation patterns. You can easily lose 6 db at
useful, low radiation angles by going from a quarter-wave to a
hslf-wave. There is no way you'll ever recover that through the
half-wave's higher radiation resistance, although that high-angle stuff
could actually be a good thing if you're not in the middle of the
Pacific trying to work Europe. And we have not even addressed the
consequences of a sloping antenna on both horizontal and vertical patterns.

No quarrel, of course, with your observation that as the length of an
antenna falls below a quarter-wave, the radiation resistance (and thus
radiated efficiency) falls. Those losses are one of the parameters that
one needs to weigh against other considerations. At the same time, a
reduction of length to say, .2 wavelengths would probably not even be
detectable (i.e., 1 dB). Also, even as the length decreases, the
radiation pattern remains basically that of a quarter-wave antenna.

I probably need to repeat that I have not advocated "shorter" antennas,
"longer" antennas, quarter-wave antennas, half-wave antennas, vertical
antennas, horizontal antennas or much of anthing other than an analysis
of the desired signal paths and the basing of an antenna design and
frequency combination on that analysis. Well, I have also cautioned
against blindly increasing antenna length. Sort of struck me as a
motherhood kind of thing.

Onward . . .

Chuck



Gary Schafer wrote:
As Bruce says, "tuners get very lossy with short antennas". But that
is not the only problem with short antennas. The antenna and ground
system become very lossy with short antennas. Below 1/4 wavelength the
radiation resistance of the antenna drops drastically. It can be less
than an ohm. That equates to very high losses. The antenna system in
those cases may be only a few percent efficient.

It is far better to have a longer antenna that gives a much higher
radiation resistance even if it may not be the optimum length as far
as radiation pattern is concerned. If you can't get the power to the
antenna the radiation pattern doesn't much matter. You still won't get
out very well.

On a typical boat the radiation pattern is going to be far from ideal
with whatever length antenna you have due to all the surrounding
objects on the boat.

The difference in radiation patterns between a 1/2 wavelength and 5/8
wavelength antennas are minimal. About the only real difference is the
feed point impedance they present.

As far as antennas greater in length than a quarter wavelength, they
start to produce multiple lobes in the pattern. Which on a boat may
not be a bad thing. As you mention, sometimes higher angles are
desired depending on the distance trying to be covered.

A longer antenna on a typical boat is most always going to be more
efficient than a short antenna even if the longer antenna produces
multiple pattern lobes.

Regards
Gary



On Tue, 09 Nov 2004 00:29:53 GMT, Chuck wrote:


Antennas are really a lot like boats: No
boat will do everything well and no
antenna will either. Boats and antennas
that try to do everything usually fail
across the board.

FWIW, SGC-237, -230, and -231 tuners
need 23 feet only to tune from 1.6 MHz
to 3.3 MHz. Above 3.3 MHz, these SGC
tuners require only eight (8) feet.

The Icom AH-4, for example, needs 23
feet only to tune down to 3.5 MHz, but
will tune from 7 MHz up with Icom's
AH-2b whip (8.2 feet long).

But it doesn't matter what lengths the
tuners require if there is no desire to
operate in that frequency range, and
chances are excellent that recreational
boaters will not be found at the very
low frequencies.

As has been pointed out, some antenna
lengths will be more taxing for an
autotuner than other lengths. Your
objective is not to make life easier for
your tuner, especially when doing so may
move you farther from your real needs.
You may not even need a tuner! Your
objective is to achieve your
communication goals.

You might give some thought to posting
on one of the cruising newsgroups to ask
experienced cruisers for their thoughts
on things like "if you had only one
frequency to operate on, what would it
be? Among other things, that might be
the basis for an antenna you can stow
for emergencies. But tell them where and
how you'll be cruising and what you want
the ssb for (email, emergencies,
boat-to-boat communication, etc.) Then
return to the antenna design questions.

Keep to it!

Chuck