[TowerTalk] best coax/receiving

[Eric Gustafson]

Hi Dave,

I seem to have sparked a small flame here.  I didn't intend to.
And, I'm not an HF Hardline (except for extremely long runs)
advocate either.  In fact, I don't really disagree with you for
the majority of cases or even for any particular location most of
the time.  But I'm pretty certain that there _can_ be instances
in the HF region where the system NF needs to be far lower than
is generally accepted to be the case based on the background
noise level that is present at most locations or most of the
time.  And in those instances, the loss in the feed system can be
significant at even just a few dB.

I think you know me well enough to know I'm not just trying to be
contrary or sharpshoot you.


>From: k6ll@juno.com
>Date: Wed, 12 Apr 2000 08:29:53 -0600
>
>I conducted a little experiment around midnight last night to
>see just how much background noise I had on 10 meters, just to
>make sure I wasn't missing something through my cheesy RG-213
>coax.

If your RG-213 is cheesy, that is obviously the problem.  You
should probably replace it with some non-cheesy RG-213 ;-).  I
always wondered why the rodents liked to chew on that stuff so
much.  Maybe it isn't that they just like PVC after all...


>
>I listened to the background noise in AM mode. It sounded
>relatively "white."
>
>I rotated the antenna around to where the noise was minimal, but
>it didn't make much difference.
>
>The noise did not illuminate any LED's on the TS-850's bargraph
>s-meter.  The meter is quite generous.

I'm using an 850 these days too.  What happened to your 830S?  My
S-meter is no longer generous though.  I set S-9 to 100 uV.  Then
with the zero reset to idle below S-1 (no signal with the RX
terminated with a load), each S-unit turns out to be close to 6
dB.  I find that really generous S-meters are pretty useless.

>
>I cranked in 6dB of attenuation on the TS-850, and the audible
>noise went way down. I then cranked in 12 dB more attenuation,
>and the noise went down a lot more. Then I removed all the
>attenuators, and of course the noise came way back up.
>
>Then I cranked in 12 dB of attenuation and the noise went way
>down. Then I cranked in an additional 6dB of attenuation, and
>the noise didn't change. I repeated this test using the TS-850's
>AIP function, which is supposedly 10dB of attenuation. The noise
>just barely changed when I cranked in more attenuation.
>
>I believe this is telling me that, with the existing conditions
>in my neighborhood, on a dead band, at midnight, I had about a
>10 dB margin of atmospheric noise before I began to be limited
>by the receiver. I could have been using 300 feet of RG-58 and
>it wouldn't have made any difference in my ability to receive a
>weak signal.

I have told myself the same thing for many years.  But nowadays,
I'm not so sure that just being able to hear the presence or
absence of the antenna and feedline is a good indicator of how
far you are from the internal / external system noise limiting
threshold.

For example, in the KWM380 case I mentioned previously, the
background noise level increased noticeably for _both_ the 380
and the R4C when the antenna system was connected.  But for the
R4, the noise increase brought signals with it.  However, I _do_
believe that if you _don't_ hear a noise level increase when the
antenna is connected, the limit is being established internally
to the system.

All I can say is that I personally observed more than one
instance when the difference between a receiver with a 12 dB NF
and one with a 4 or 5 dB NF (I'm guessing here about the R4's NF)
made the difference between not even detecting that signals were
there (even once we knew they were and where to listen) and being
able to easily copy a SSB voice conversation (no s-meter action
but we could understand every word they said).  This was on a
frequency as low as 14.180 MHz in the early evening during a
sunspot minimum.  The location was on the south central edge of
Tucson (probably not significantly less noisy than your Yuma
off-mesa location) and the antenna was a TH6DX at 45 feet with
about 70 feet of RG-213 feedline.


>
>I really envy you folks who have such a quiet location that you
>have to use hardline for receiving.

So do I.  My location is quiet but not that quiet.  But if, for
example, I had a dedicated receiving antenna that had very low
gain but a good or controllable pattern (say a small loop on 80
or 160 remoted far enough from the transciever to not interact
with the TX antennas), I might be concerned that the feedline
losses could degrade the overall system NF.




>If I had a location like that, I might be able to win a contest
>once in a while.

You seem pretty competitive from your present location.

73, Eric  N7CL

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