Excellent post Ray.
I thought I would relate my experience with older,
copper-braid coaxial cable.
Once upon a time while developing a set of Hi-Q notch
filter elements for use as a duplexer on Six meters I
was in the midst of a "desense test" (you know, inject
a weak, on-channel signal then actuate system
transmitter while observing the decrease in receiver
sensitivity as well as noting the type of 'noise' that
resulted when the transmitter came up).
I noticed that I could significantly 'raise' the noise in the
receiver by moving the 'old' RG-8U braided coaxial cable
when the transmitter was transmitting - waving a goodly
sized piece (8 to 10 foot length of that cable) and I could
make 'noise' that followed the 'wave' action (movement) I
imparted on that cable; the sound was a broadband (white
noise) 'shish' type of sound, as if a thousand (or more)
'connections' were made and broken (as the braid made
ohmic contact with itself during cable movement). Power
level was about 60 watts, freq spacing R to T = 500 KHz.
I never tried performing any 'intermodulation' tests to measure
the 'dynamic range' of that old coax; your post brought this
little observation back to mind and just what problems like
ohmic, rusted, oxidized copper conductors can create.
Jim WB5WPA
----- Original Message -----
From: <Dxhogg@aol.com>
To: <rfi@contesting.com>
Sent: Sunday, December 26, 2004 8:57 AM
Subject: [RFI] COMM0N MODE NOISE OBSERVATIONS AND REPAIRS
----- Original Message -----
From: <Dxhogg@aol.com>
To: <rfi@contesting.com>
Sent: Sunday, December 26, 2004 8:57 AM
Subject: [RFI] COMM0N MODE NOISE OBSERVATIONS AND REPAIRS
>
> I had posted about a week ago about common mode
> noise and what I had thought was a really awesome
> "repair". Well, the repair really works but along the
> road of determining what was really at play creating
> all the different noises I have discovered all of the
> problems in the station was of poor infrastructure.
>
> I had originally wired the operating position all
> station grounds and interstation cabling about 18 years
> ago. Well to put it in simple terms it was all junk.
>
> Wall warts, braided cable for grounds, and poor quality
> coax were the real culprits.
>
> Pay extra attention to ground loops -
>
> - Wall warts are a prime cause of ground loops - cut all
> the cables off the warts and use only the cable
> and use a good quality 10 amp power supply for all the
> ancillary station equipment.
>
> - Multiple antennas and feed lines should all be kept
> galvanically separated from one another till the cables
> are connected at your main station bulkhead. What I mean
> by 'separated' is don't use the ground for a 160 meter
> "inverted L" as a tower ground etc.
>
> Look at your station layout in a ground loop aspect you will
> be amazed [th]at isolating these problems clean up the noise
> floor. I had a terrible noise on 160m that seemed to follow the
> resonant FREQ of the transmit antenna when I would disconnect
> the open wire feed from the tuner the noise on the receive
> antennas would disappear..... Well it turns out that in 1945
> when Technical Materials Corp built this unit using braid as
> interconnecting leads was cool - not so my tuner was a
> rebroadcaster....all the junk braid replaced with 1/4 inch
> tubing and quiet bands again ... this [should be] a wake up
> call look at your station your neighbors WiFI stuff my be
> heard because of a rectifying [junction] in your station...
> simple things like coax connectors with the shield not
> soldered correctly will cause a hell of a lot of problems.
>
> dx on Ray NR1R
>
> _______________________________________________
> RFI mailing list
> RFI@contesting.com
> http://lists.contesting.com/mailman/listinfo/rfi
>
_______________________________________________
RFI mailing list
RFI@contesting.com
http://lists.contesting.com/mailman/listinfo/rfi
|