Hi Bob,
> First off, you need to take into consideration the "uncertainties"
> of the measurement device.
The AEA unit, like the MFJ unit, measures SWR to determine loss. Return
loss is easily determined by any instrument that measures SWR, and
transmission loss is 1/2 of return loss when the far-end termination is a
short or open.
The main problem with this method of determining loss is impedance
sensitivity. Accuracy of the measurement device is directly controlled by
cable impedance. That error is generally greater than instrument error, at
least it is in a properly calibrated 259B.
Since the lines being measured were loaded with female connectors (which
are about 30-40 ohms) impedance bumps were also present.
If these bumps are by chance arranged at certain periodic intervals (odd
1/4 wl intervals would be the worse case possible) the effect of the bumps
increases at a much greater rate than if they were all located in one area
of the cable. Measurement error is also maximized if the connectors (bumps)
are placed at odd 1/4 waves away from the open end.
I don't want to start another conjugate match thread, but it is important
to point out that even your 75 ohm (and my multi-impedance) multi-thousand
dollar Network analyzer will read loss incorrectly when there are bumps in
the line. To correct your analyzer, you'd have to subtract out attenuation
caused by the mismatch to the source (unless the analyzer does that for
you, and most don't) and consider the mismatch loss as a separate parameter
with an effect that varies with application
This disparity occurs because measurement devices generally provide
near-perfect 50 ohm (or 75 ohm) dissipative source resistances, and most
antennas and rigs do not. A large portion of the source impedance of our
less-than class A rigs can be a non-dissipative. They also might not be
exactly 50 ohms. Because of that, a mismatch might do anything from hurt to
increase system efficiency!
> You may want to contact PL259 manufacturers for loss specifications.
The area of the PL-259 that contributes loss is mainly the area that passes
through the insulation that supports the center pin. The electrical length
of a typical 259 is less than 0.1 inch, since the cable itself is inserted
in the 259 up to the start of the center pin. The 259 is not the problem.
It is the dielectric and large flared outer conductor in the mating part of
the female that causes the mismatch "problem". That area can be as much as
1/2 inch long, and is responsible for transmission of power. Since the rule
of thumb is less than one degree electrical length of accumulated mismatch
area is a non-issue (except for critical measurements), a single SO-239
does not even begin to be a mismatch problem until about 50 MHz.
The "problem" is almost exclusively the impedance bump caused by the 239 Zo
being less than 50 ohms, which will generally show up as line attenuation
on many or most measurement devices even though it does NOT generally
increase attenuation in our applications.
73 Tom
--
FAQ on WWW: http://www.contesting.com/towertalkfaq.html
Submissions: towertalk@contesting.com
Administrative requests: towertalk-REQUEST@contesting.com
Problems: owner-towertalk@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|