[NLRS] shack local noise issue - 2M - help?
[email protected]
[email protected]
Mon, 16 Dec 2002 08:18:53 -0600
Bryan, my guess is that this is not a "earth" grounding issue. Two
thoughts come to mind.
The first is related to the questions that have been already asked about
the quality of the jumper and its connectors .... you didnt say what kind
of connectors are in use, but I'll assume PL259s. Make sure that the coax
center and braid are solder to the connectors properly. I have also
experience problems with the center pin on the PL259 floating inside an
SO239 so make sure that the PL259 center pin is clean (free of flux) and
inserts snugly into the SO239 before the PL259 barrel is then screwed down
.... sometimes I take my pliers and actually squish the center pin EVER SO
SLIGHTLY in order to assure a snug fit with the SO239. A floating center
pin on a PL259 inside of a SO239 can cause all sorts of woes.
A second thought is that there may be nothing wrong with the coax but
instead something going on with the frond end of the TR751a. By placing
different lengths of coax ahead of the RF gain stage, you may be throwing
different complex input impedances at it that cause this problem. If you
have a 3 dB pad, try putting that between the coax run and the input
antenna port (right at the rig) and see if it calms it down and acts more
rationally. Perhaps the preamp or first RF gain stage in theTR751a is a
bit unstable and reacts to changes in input impedance. This would also go
a longs ways towards addressing the question as to why you have such a
background noise problem ...... have you tried a different 2m rig .....
perhaps you can borrow one to try out if its the rig or your environment.
73, Jon
W0ZQ
"Bryan LaPlante"
<bryan.laplante@pra To: "Nlrs posting (E-mail)" <[email protected]>
gmatek.com> cc:
Sent by: Subject: [NLRS] shack local noise issue - 2M - help?
nlrs-admin@mailman.
qth.net
12/15/02 08:05 PM
Please respond to
bryan.laplante
Well, tried to work some meteor scatter this morning and finally got fed up
with the high background noise levels I have. So, started to do some
isolation work and found something that has me a bit puzzled.
With an audio power meter, no antenna connected, set audio gain so that I
read 0dbm of noise from the speaker.
Add my first section of coax (RG 8x, commercial jumper, 1 foot long) - no
change in noise (left unterminated)
Add my second section of coax (RG 8x, my connectors, another 1 foot long) +
3 db increase in noise with a commercial shielded 50 ohm termination at the
end. Worse if unterminated. Huh?
What I'm finding is that with the two sections of cable, this 2 foot cable
is acting like an antenna (despite the 50 ohm terminator), and picking up
background noise, birdies, etc. Of course, this is a 'shack' so there are
lots of local sources - PC, power wires, etc. nearby.
I'm astonished! Each connector should be maybe a .1 db degradation, and RG
8x isn't great, but 1 foot of it is nothing. From theory, I should be
seeing maybe 1 db of added noise. I have checked the coax for continuity
and shorts - checks fine.
I'm thinking what is happening is that signals are coupling onto the shield
of the coax, and then somehow coupling into the rig. The rig is a kenwood
TR751a 2M all-mode.
Note - I don't have the rig connected to a station ground. The station is
on the 2nd floor, and from what I've read it isn't really practical to
provide an effective station ground at this frequency over that distance.
Suggestions?
Hardline?
Move the shack to the basement?
Install a station ground and only work 10M and lower?
Stamp collecting?
KB0OBT - Bryan
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