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Re: Source for F to N/SO239 Adapters, et. seq.



"'Mike Murphree'" <n4cnw@knology.net> wrote:

> As a test this morning, I stretched out 150 feet of 9913 and 150 feet
> of R6G I had surplus, and then respectively hooked a dummy load on
> one, then the other. Each was hooked to the 736 when the dummy load
> was attached. The RG6 was WAY noiser than the 9913.
>
> Maybe what my experience is showing relates to a noisy urban
> environment and noise rejection as much as anything else.

If I understand your test setup, the receiver is connected to 150 feet
of coax and there is a dummy load on the end of the coax run.  Then you
listened for noise on the receiver.  So your dummy load (a 50 ohm non-
inductive resistor, no doubt) is functioning as the "antenna".  Total
noise heard with RG6 was less than total noise heard with 9913.

I'd like to think, as you imply, that the RG6 is noiser because the
shield is poorer and more noise gets in that way.  I'm not quite sure
how to demonstrate this, other than perhaps taking a signal source and
bringing it up to the coax and seeing if it can be heard through the
shield (far away from the dummy load, close to the receiver).

An alternative interpretation is that both cables have equivalent
shielding, but the 9913 attenuates the "signal" from the dummy load
(pseudo-antenna) much more, so less noise reaches the receiver.  You
could put a power meter at the far end of the coax, just before the
dummy load, and see how much power reaches the dummy load to see if one
cable is a lot lossier than the other.  (If there is more or less of
the power transmitted from the radio reaching the meter, there would be
the same magnitude of difference in loss from "antenna" to receiver.
So the objective test is "backwards" but valid because of the
symmetry.)

Yet another possiblity is that the 9913 was accidentally shorted at
one end or the other (or both, but that would be presumably even
less likely), effectively preventing the "signal" from the dummy
load from reaching the receiver.  This could be checked with an
ohmmeter at each end of the cable (one end at a time).  Not casting
any stones at you from inside my glass house, I just know that I
have personally made up a few coax cables in my ham career that had
this problem!

If it really does turn out to be poor shielding in the RG6, it would
be interesting to see what improvement there would be using RG6-QS,
a quadruple-shielded version of RG6.  I used this stuff when I brought
the cable TV wiring into my shack (for the cable modem, NOT to watch
TV!), since it was going to run close to my antenna cables (mostly
LMR-400).  I see no evidence of "crosstalk" at this point, though I
could easily attribute that to the very high quality of the LMR-400
shielding rather than any superior shielding of the RG6-QS.

BTW, back to the ORIGINAL subject, I had been considering building a
little watertight box with an N connector on one end, an F connector
on the other end, with the DC power injection circuit inside, and
use a Male-F to Male-F adapter to hang it right off the IF side of
the 2880, thereby accomplishing both the power insertion and the
connector adaptation together.  Now I'm not so sure, I may just go
with the F to N adapter instead and drill the hole in the 2880 for
DC power.  Why N instead of SO239 if a few tenths of a db of loss at
the connector is not an issue?  An N connector has intrinsically a
somewhat better chance of being (and staying) watertight, even though
either one can be assembled well enough to keep moisture out pretty
well if you are extremely careful.

John Toscano (KB0ZEV)
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