On Sun, 16 Oct 2005 18:28:36 GMT, Ed
wrote:



My QUESTION(s): How effective would a balanced fed dipole of about
100 feet length be on 75M ? Should it tune OK with a tuner such as the
new MFJ balanced tuner? Or, if I were to put either the old 40M traps,
or wind coils for the new dipole to shorten 75M resonance, would it then
tune OK on all bands from 75M down with balanced feed?

Ed,

When you analyse the performance of a centre fed dipole antenna
system, you will find that for most practical dipoles, the challenge
is getting most of the power to the feed point, rather than the ohmic
losses in the radiator itself.

By the time the dipole is long enough to obtain acceptable performance
from a practical feed system, the loss in the radiator itself is
typically less than 0.5dB (unless you contrive do something pretty
silly).

It turns out that it becomes quite challenging to implement an
efficient feed system (especially over a wide frequency range) where
the dipole is less than about 35% of a wavelength at the lowest
frequency.

Now, the 100' you nominate is so close to the length of a G5RV that
you could draw some information from analyes of the G5RV. I wrote one
that focusses on the performance of common distortions of G5RV's
design and examines particularly the feed system performance, which I
suggest to you is the most significant influence on antenna system
efficiency in this case. Unlike many other analyses of the G5RV, this
does not just consider ham band performance, it models performance
every 100KHz from 1MHz to 30MHz, so you can see a continuous view of
the behaviour. The article is at http://www.vk1od.net/G5RV/ .

But you weren't asking about a G5RV were you. Look at Figure 10 and
the "Classic tune feeder" topic which is a 30m dipole fed with 23m of
ideal 600 ohm open wire feeder. (Ladder line won't perform quite that
well, especially when wet). That topic is directly relevant to your
proposal.

Nothing in the feed system affects the pattern (in three dimensions),
that is determined by the length of the dipole, and its environment.

Owen
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