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
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