On 5/1/2012 2:05 PM, Larry Benko wrote:
> Jim,
>
> I have made the exact measurement I mentioned below as an exercise.
> Are you questioning the 2:1 SWR or the fact that a 2"1 SWR is a
> mismatch loss of .55dB?
Both. See the family of curves in the ARRL Handbook for excess
attenuation due to mismatch, which has been in every edition of the
handbook since I've been buying it (the 50s), and which I've confirmed
by cranking the equations. The curve for a 2:1 SWR shows an excess
attenuation of 0.1 dB if the matched loss is 0.4dB, 0.2dB if the matched
loss is 1dB, 0.4dB if the matched loss is 10dB. And remember, the
connector is quite small as a fraction of a wavelengh at HF, which is
what this discussion is about, and not much more in the lower half of
the VHF spectrum. If you've measured more than that, I suspect you are
simply seeing the standing waves at the point of the measurement.
Very small values of attenuation can be VERY difficult to measure.
Several years ago, I tried to measure the attenuation of Commscope 3227
from 1 MHz to 1 GHz Getting good data below a few MHz requires a VERY
long sample -- even with 1,000 ft, there is some mismatch between the 50
ohm resistive terminations of the HP gear and the complex Zo of the
cable at those frequencies, which is NOT 50+j0.
Another point. While we like to ASSUME that input stages have an ideal
resistive impedance that matches the cable we're using, many do not.
Ditto for output stages -- indeed, the Zo of most output stages is
nowhere near an ideal match for the cable.
73, Jim K9YC
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