[TowerTalk] re: Best Coax

[Eric Gustafson]

Hi Dave,

I think that the most important word in your response is
"almost".

I fear that this is the same trap that Collins fell into when
they designed the receiver in the KWM380.  They sacrificed noise
figure to get high IP3 on the grounds that the external noise in
the HF region was the limiting factor and cited the canonical
curve of such in one of the standard reference books as
justification.  On this basis, They figured that a 12 dB noise
figure was just fine for HF.  I looked up the reference and the
caption under the curve said "AVERAGE" noise level near
"populated regions".  The average is the average because
sometimes the noise is higher than that and sometimes it is
lower.  Sometimes a _lot_ lower.

I then understood why I could go over to my buddy's house (W7FPX)
and listen to a completely dead 20 meter band on his KWM380 while
easily copying many QSOs on the same band at the same time on the
same antenna through his old trusty R4C.  It turns out that the
noise level is frequently far enough below the published expected
values in the HF region to make the system noise floor
established by the sum of the receiver thermal noise level and
any losses ahead of the receiver be the limiting factor.

While this may not concern the casual operator much, it can be of
critical importance for a rabid gonzo obsessive DXer or your
average contester. ;-)  Actually, I'm neither of those and I still
don't want the feedline limiting my receive sensitivity any more
than absolutely necessary.  I confess that I do live out in the
boonies in a fairly quiet location.

73, Eric  N7CL

--
------------------------------------------------------
Eric Gustafson  N7CL      | The mountains are high and
6730 S. Old Spanish Trail | the Emperor is far away.
Tucson, AZ 85747-9498     |
                          | You can't work 'em
   n7cl@mmsi.com          | if you can't hear 'em.
------------------------------------------------------


>From: k6ll@juno.com
>Date: Tue, 11 Apr 2000 10:14:25 -0600
>
>A while ago I wrote:
>
>
>>Remember that these dB's are only for Transmitting, not
>>Receiving.
>
>Here's what I meant by that. Receiving at HF, signal to noise
>ratio is almost always established by atmospheric background
>noise, and nothing you do in the cable can change it. A 2dB
>cable loss is like switching in a 2 dB attenuator on your
>transceiver. The s-meter goes down a little, but the snr, and
>your ability to copy a weak signal, stay the same.
>
>Dave Hachadorian, K6LL
>Yuma, AZ
>K6LL@juno.com

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