Lumpy wrote:
Roy Lewallen wrote:
Something's seriously wrong there. A shorted quarter wavelength of
coax should read very high impedance when the far end is shorted and
very low when the far end is open. The only time it should read 50
ohms with the far end shorted or open is if it has many dB of loss.
And no even half decent coax should have anywhere near that much loss
in a quarter wavelength.

Maybe I'm not testing correctly.
100' length of new coax. Tests ok for no shorts or open.

MFJ set to 2.4MHz gives me a Z (on the advanced
menu) of 50 ohms with distal end of the coax
either open or shorted. Actually shorting the
end with a screwdriver shaft drops the Z from
50 to 49. I assume 1 ohm is within normal limits.

Same settings gives me an R (on the main menu)
of zero ohms either open or shorted.

I get exactly the same results on several
pieces of coax, old/new, RG8 or 58. Resistance
is zero, impedance is 50, shorted or open at
the resonant quarter freq.

Craig 'Lumpy' Lemke

www.n0eq.com

At 2.4 MHz, the line is pretty close to 3/8 wavelength, not 1/4 --
assuming it has solid and not foamed dielectric. At that length with the
other end shorted or open, the impedance would be very nearly 50 ohms,
all reactive, which means the resistance would be zero as you're seeing.
So those measurements are correct. Your meter is showing the correct
values when measuring resistors (purely resistive) and the shorted/open
coax (purely reactive), so there's nothing obviously wrong with the meter.

The remaining possibility for your problem with the antenna is external
RF, as someone else mentioned earlier. I find it nearly impossible to
use my 259B for antenna measurements at my QTH because of strong AM, FM,
and TV signals -- ironically, the "antenna analyzer" is great for a lot
of other chores, but not for actually analyzing antennas. It's pretty
likely that's what your problem is too. When measuring antennas, I have
to use filters which introduce minimal impedance disturbances at the
measurement frequency while providing attenuation of the offending signals.

Roy Lewallen, W7EL