I have installed a 135 foot dipole which I tune for all bands but 6
and 20 (I have dipoles cut for those bands.)

Well, one of the locals and I were discussing a loop antenna. He
insists that the loop is better fed with 65 feet of 75 ohm coax and
then 50 ohm coax to the tuner for the radio. I was saying there would
be less loss by using a 300 ohm twin lead since it was being tuned for
all the HF bands and 6 meters, with open wire or ladder line being
better options. He insists that if the loop is properly made, it is
better to feed it with the 75/50 combo which properly matches the
antenna.

any thoughts?


--
73 for now
Buck
N4PGW

Buck wrote:

I have installed a 135 foot dipole which I tune for all bands but 6
and 20 (I have dipoles cut for those bands.)

Well, one of the locals and I were discussing a loop antenna. He
insists that the loop is better fed with 65 feet of 75 ohm coax and
then 50 ohm coax to the tuner for the radio. I was saying there would
be less loss by using a 300 ohm twin lead since it was being tuned for
all the HF bands and 6 meters, with open wire or ladder line being
better options. He insists that if the loop is properly made, it is
better to feed it with the 75/50 combo which properly matches the
antenna. any thoughts?

The 75 ohm matching section is for a single frequency (narrow-banded).
On that single frequency, it is probably just as good as the twin lead
model. However, on other bands than the one for which it is designed,
it will probably not function as well as the twin lead model. Your
"local" just proves that a little knowledge is a dangerous thing.
Do you want a multi-band antenna or a narrow-banded single-band antenna?
--
73, Cecil http://www.qsl.net/w5dxp


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In article ,
Buck wrote:

I have installed a 135 foot dipole which I tune for all bands but 6
and 20 (I have dipoles cut for those bands.)

Well, one of the locals and I were discussing a loop antenna. He
insists that the loop is better fed with 65 feet of 75 ohm coax and
then 50 ohm coax to the tuner for the radio. I was saying there would
be less loss by using a 300 ohm twin lead since it was being tuned for
all the HF bands and 6 meters, with open wire or ladder line being
better options. He insists that if the loop is properly made, it is
better to feed it with the 75/50 combo which properly matches the
antenna.

any thoughts?

I think that's going to depend very much indeed on the size of the
loop and on the band(s) on which you're trying to use it.

If I recall properly, a 1-wavelength resonant loop will have a
feedpoint Z of around 100 ohms. A quarter-wavelength section of
75-ohm coax will transform this down to something close to 50 ohms,
and you'll have a good match.

This specific case doesn't generalize, though. If you use the loop on
the second-harmonic band, its feedpoint impedance will be a higher (I
think), the "quarter-wavelength" 75-ohm section will now be about a
half-wavelength long and will mirror the feedpoint impedance to the
50/75-ohm junction. You'll see a higher SWR (maybe 2:1 or worse?) at
the junction and thus at the transmitter. Same problem, even moreso,
on higher-numbered harmonic bands. It'll likely be usable, but I'd
guess that losses may become significant depending on the coax type
and length.

On non-harmonically-related bands, the situation could be even worse
than that, with a very high or very reactive impedance at the
loop/75-ohm-junction point, and a high SWR and significant losses all
the way to your tuner.

65 feet of electrical length is roughly a quarter-wavelength at 80
meters, but that neglects the velocity factor of the cable. A setup
using this sort of matching arrangement with 65 physical feet of
75-ohm coax would behave somewhat differently depending on whether the
75-ohm coax was solid-dielectric or foam/air dielectric, due to the
difference in the velocity factors.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

He's confusing the operation of a single band loop, to
your planned all band operation. If single band, I would
prefer his method. If all band, I would prefer yours. The only
downside
to the 300 ohm line is it gets pretty lossy when wet. But 450 ladder
line with
"windows" would cure most of that problem. On the upper freq's
with a high SWR, the 300 ohm line would have less loss than coax.
On VHF/UHF, the difference is drastic. If you feed a TV antenna with
rg-6
coax and balun, you will have a good bit more loss than using 300 ohm
line.
On UHF the difference will be fairly huge. But expect a blackout when
it
rains hard. The local ATV machine would turn to snow. Once the rain
quit, it would gradually come back. :/ If you are working all bands,
and
using a tuner, there is no reason whatsoever to tune or match the
antenna to
the line. Waste of time...
MK

On 7 Jul 2005 23:34:43 -0700, wrote:

The only
downside
to the 300 ohm line is it gets pretty lossy when wet. But 450 ladder
line with
"windows" would cure most of that problem.

450-ohm ladder line losses can also be quite substantial when wet.
Check out Wes' study on this subject at:

http://users.triconet.org/wesandlinda/ladder.htm

73,

Danny, K6MHE

email: k6mheatarrldotnet
http://users.adelphia.net/~k6mhe/

On Thu, 07 Jul 2005 22:18:12 -0400, Buck wrote:

I have installed a 135 foot dipole which I tune for all bands but 6
and 20 (I have dipoles cut for those bands.)

Well, one of the locals and I were discussing a loop antenna. He
insists that the loop is better fed with 65 feet of 75 ohm coax and
then 50 ohm coax to the tuner for the radio. I was saying there would
be less loss by using a 300 ohm twin lead since it was being tuned for
all the HF bands and 6 meters, with open wire or ladder line being
better options. He insists that if the loop is properly made, it is
better to feed it with the 75/50 combo which properly matches the
antenna.

any thoughts?

PS. The loop we were discussing was cut for 80 meters and was to be
used with a tuner to tune all HF bands.


--
73 for now
Buck
N4PGW

"Buck" wrote in message
...
On Thu, 07 Jul 2005 22:18:12 -0400, Buck wrote:

I have installed a 135 foot dipole which I tune for all bands but 6
and 20 (I have dipoles cut for those bands.)

Well, one of the locals and I were discussing a loop antenna. He
insists that the loop is better fed with 65 feet of 75 ohm coax and
then 50 ohm coax to the tuner for the radio. I was saying there would
be less loss by using a 300 ohm twin lead since it was being tuned for
all the HF bands and 6 meters, with open wire or ladder line being
better options. He insists that if the loop is properly made, it is
better to feed it with the 75/50 combo which properly matches the
antenna.

any thoughts?

PS. The loop we were discussing was cut for 80 meters and was to be
used with a tuner to tune all HF bands.


--
73 for now
Buck
N4PGW

I have no problem loading up (Linear, with Pi network) my 75 m loop on 18
MHz, but performance wise it is a dog. Much inferior to the 18 MHz dipole,
which is not as high as the top of the loop. You might want to run EZNEC on
this. Received signal strtength on other bands is WAY down.. Works fine on
75DX, not so hot for locals.
Tam/WB2TT