On 3/11/2021 5:57 AM, Dave Sublette wrote:
recommendation for good quality 72 ohm coax with foam dielectric for a V/F
of 78% or so. It has to be foam in order that the feedline be long enough
to reach the center of the array where the control box is located. The
connectors on the box are SO-239. The PL259 can be soldered or crimped, I
don't care. But if soldered, shielding must be copper.
So 72 ohms, 78% v/f, good quality. Where to get it?
Dave, K4TO
75 ohms is what is generally available, not 72 ohms.
Everyone likes to talk about insertion loss. This is actually
specified. Some people, like you, like to talk about velocity factor.
That may or may not be specified. What seems to be overlooked
is characteristic impedance, listed as "nominal characteristic
impedance." What out for that word "nominal".
I recently built an antenna project using RG/6 from a big
box store because I was in a hurry. I got strange results
and eventually I tested a 100 ft piece of it with a quality
75 ohm termination on the end. Over frequency, the VSWR
oscillated between 1:1 and 1.3:1. I tested other pieces of
it, and they were all very repeatable, but not 75 ohms.
This coax was proudly marked "3 GHz, for satellite TV".
I then tested some coax that I got VERY cheaply on ebay.
It had less than 1/2 the mismatch. Who knew?
Although the original question was about an 80 meter antenna,
my work involved 160 meters. When the frequency got down to
the 160 meter band, both samples of coax exhibited an almost
doubling of the mismatch, which I suspect is related to the CCS
center conductor. I believe what is going on is that the
Zo is going complex, and exascerbating the problem. Remember
that steel is magnetic!
This problem is still under investigation at this time.
Complex Zo is usually an indication of dispersion. When
making phasing lines, many people including me have been
guilty of measuring the frequency where the coax is a 1/4
wavelength, and then extrapolating that data point to
determine what the delay is at, say 1830. The trouble is
that if the delay line is NOT 90 degrees, and has dispersion,
then you will get the wrong answer. You MUST measure the
delay at the operating frequency to get it correct, when
using dispersive line. Fortunately, with today's VNA's,
this is now trivial.
Especially for 160 meters, it is probably better to get
all copper line like RG-59 if you really want to get good
results.
Has anyone else measured characteristic impedance of RG/6?
73
Rick N6RK
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