RadioBanter - View Single Post - Quic and short thoughts on local noise issues.

June 10th 05, 01:16 PM

wrote:

Visiting our friend who has just received her Astronomy PHD was
educational.
Here thesis was on HF radio astronomy. She used a large dairy farm she
inheirited and sold off most of the pysical asssets to pay for school.
She ended up making most her money by selling he sod.

She did her work on the lower HF (13.36 - 13.41 MHz) allocation.
RA is a study in weak signals. She had a RC test transmiter on the
ISM 13MH that she used to generate "known" far field reference
signals. The two frequencies are so close that her RA antenna
received teh ISM perfectly. She was unaware of HiFer operations
and was releived because she had received several "odd" signals
over the last 4 years and couldn't determinie what they where.

Back the the noise part. While "Ground loops" may be an issue with RF
devices they are a killer issue with weak signal audio issues. Ms. C
found that she had to use isoation transformers on ALL of the audio
lines.

This means the AF from the reciver to the USB Ardvark "sound card".
From the audio out of the PC to a HP analyser etc. She found that she
even had to use a 1:1 RF transformer and NOT carry the braid from the
antenna to the R390. By using jumpers she showed me the jump in the
the auido noise floor.

Whne we got home, the first thing Idid was to install a bunch of
"ground isolation" transformers in all of my audio circuits. A PITA.
I simplified by only hooking up one R2000 and one audio chain. No
DSP no filters but even so it took 4 stereo or 8 transformers
to isoalte all of my audio "grounds". The only loops where now
casued by the power/safety grounds and the coax braid. I added
a MC 1:1 .1~400MHz transformer leaving only the power and safety
ground. I then made a special power cable that only ran the +12.

Whne I started jumpering to restor the wiring to what I had, the
noise in my PC audio FFT water fall went up with EVERY ground loop
I created. The "ground isolation" units I used wehere crude automotive
sound system transformers. Ms C used Jensen 10K:10K with internal
interwindind shield. I have a bunch of Byer transformers that are
similar.
I started added the Byers and as I grounded the interwinding shields,
the noise dropped again. I also found out that as Ms C said, "HiFi
cables
are junk", they most often use spiral wrapped cooper wire and are not
braided.
After much experimentation Ms C found that RG174 made very good audio
cables.
The 75 ohm version of mini coax workd just as well.

Now for most SWL activities this extreme level of isolation isn't
likely to
be productive, or at least not worth the money. But if, like me, you
are
interested in weak signal "oddities" it might be worth the effort.
Only
time will tell.

In the near future I will share the results of some experimnets made
on triax and twin ax feedlines to eliminate the vertical noise "picker
upper" of the Doty antenna. In my location the 20' vertical run picks
up over half of my QRM.

Interesting. What did she hear radio astronomy wise?

Here I have my matching transformers mounted at the 9' level where the antenna
is and have never used the 'Doty' method with the vertical component.

dxAce
Michigan
USA

Terry