Currently I have a coax fed trapped 75/40M dipole up between two
trees. The spacing of the trees (a bit over 110 feet) allows this
antenna with no extra length allowed, except for minimal support line and
insulator length.

I plan on replacing this dipole with 450 ohm ladder line feed doublet
so I can operate on more than just 75/40M, however a full 1/4 wave length
for 75 would be too long to fit between these trees.


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 K7AAT

A G5RV would fit and meet your needs for all band operation with a tuner.

"Ed" wrote in message
. 93.175...

Currently I have a coax fed trapped 75/40M dipole up between two
trees. The spacing of the trees (a bit over 110 feet) allows this
antenna with no extra length allowed, except for minimal support line and
insulator length.

I plan on replacing this dipole with 450 ohm ladder line feed doublet
so I can operate on more than just 75/40M, however a full 1/4 wave length
for 75 would be too long to fit between these trees.


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 K7AAT

Fred W4JLE wrote:
A G5RV would fit and meet your needs for all band operation with a tuner.

Hi Fred,
The standard G5RV is a pretty good antenna for 80m, 40m, 20m, & 12m.
Not bad on 15m, but the SWR on the coax for 30m, 17m, & 10m is
greater than 40:1 according to EZNEC.

I have optimized my G5RV for 40m, 17m, & 10m operation by making the
series balanced section 36 feet of 450 ohm ladder-line.
--
73, Cecil http://www.qsl.net/w5dxp

I am running a 100 ft bowtie antenna ok with link coupled transmatch, fed
with 450 ohm twin lead and it tunes 75 thru 10 meters
Each side of center feedpoint has a pair of 50 ft wires, that are connected
at the ends with a piece of 6 ft wire.
"Ed" wrote in message
. 93.175...

Currently I have a coax fed trapped 75/40M dipole up between two
trees. The spacing of the trees (a bit over 110 feet) allows this
antenna with no extra length allowed, except for minimal support line and
insulator length.

I plan on replacing this dipole with 450 ohm ladder line feed doublet
so I can operate on more than just 75/40M, however a full 1/4 wave length
for 75 would be too long to fit between these trees.


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 K7AAT

On Sun, 16 Oct 2005 18:28:36 GMT, Ed
wrote:
I plan on replacing this dipole with 450 ohm ladder line feed doublet

Hi Ed,

The answers to your questions are found in the quote above.

If those bands on the old antenna were already resonant, and they
matched, then you didn't need 450 Ohm ladder line.

If you are going to use 450 Ohm ladder line on a generic doublet, it
stands to reason you won't be resonant much anywhere (or you don't
count on it) and you anticipate tuning and use this line for low loss.

The only thing left to be said is about the lobe structure at any
particular frequency. Long, long antennas at high frequency develop
sharp lobes and nulls. That being said, it is more a matter of where
you are pointed and where you want to be heard. The solution to that
is to use a fan tail design for the higher bands' sake.

Using a doublet, ladder line, and a tuner is classic - it works fine.

73's
Richard Clark, KB7QHC

If those bands on the old antenna were already resonant, and they
matched, then you didn't need 450 Ohm ladder line.

If you are going to use 450 Ohm ladder line on a generic doublet, it
stands to reason you won't be resonant much anywhere (or you don't
count on it) and you anticipate tuning and use this line for low loss.

Thanks, Richard.

My main concern was that trying to tune a 100 foot dipole on 75M
might cause problems due to the dipole being shorter than halfwave on
75M. From what I am now concluding from your comments, and others',
this probably won't be an issue?


Ed

On Mon, 17 Oct 2005 02:48:13 GMT, Ed
wrote:

My main concern was that trying to tune a 100 foot dipole on 75M
might cause problems due to the dipole being shorter than halfwave on
75M. From what I am now concluding from your comments, and others',
this probably won't be an issue?

Hi Ed,

An antenna can be too long, an antenna can be too short. Somewhere
over the span of all the HF bands, one antenna qualifies for one of
those two conditions. The issue is can you cope?

A tuner can usually resolve the problem of match, but it cannot do
anything about line loss for certain situations. Using the ladder
line answers that.

Finally, unless you demand the point shaving of eking out every tenth
dB for contesting, the combination of a 100 foot doublet, a tuner and
ladder line will give you just as good service as a tuned dipole.

73's
Richard Clark, KB7QHC

Ed wrote:
My main concern was that trying to tune a 100 foot dipole on 75M
might cause problems due to the dipole being shorter than halfwave on
75M. From what I am now concluding from your comments, and others',
this probably won't be an issue?

Walter Maxwell of "Reflections" fame recommends a minimum length
for a dipole of 3/8 wavelength. 102 ft is 3/8 wavelength on
about 3.6 MHz so it should and does work well. That's the length
of my dipole.
--
73, Cecil http://www.qsl.net/w5dxp

"Ed" wrote in message
. 93.175...

Currently I have a coax fed trapped 75/40M dipole up between two
trees. The spacing of the trees (a bit over 110 feet) allows this
antenna with no extra length allowed, except for minimal support line and
insulator length.

I plan on replacing this dipole with 450 ohm ladder line feed doublet
so I can operate on more than just 75/40M, however a full 1/4 wave length
for 75 would be too long to fit between these trees.


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 K7AAT

If you are not satisified with the antenna you have then I would put up
about 130 feet of wire and feed it in the middle with the balanced feed. If
you can not get up the full 130 feet or so of wire, put it all up and let
the ends hang down. That is the support ropes could be attached 10 to 15
feet from each end and then you let whatever is left over hang down. Just
don't let the ends reach close enough to the ground where someone could
reach them. Shock and RF burns.

If you go to the 100 feet and feed it with open wire , you are trying to
make the antenna into a g5rv if you use whatever length of feedline the g5rv
calls for. Not really that good of an antenna for working all bands.

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
--