RadioBanter - View Single Post - Info - Icom IC-R75 with Kiwa Mods and Antenna Supermarket Eavesdropper SWL Sloper

December 22nd 05, 11:56 PM posted to rec.radio.shortwave

In article . com,
wrote:

Telamon wrote:

There is not much mystery to coax shielding. If putting ferrite on the
coax is making a difference then you may not have a proper termination
on the end. This is fixing a symptom not the problem. Disconnect the
antenna on the far end and using very short leads place a 50 ohm
resistor in place of the antenna across the coax end. Make sure no
conductors are near that resistor like the end of the disconnected
antenna. Tune the receiver through the band and you should hear nothing
other than the radios birdies and internal noise. Any question here
where the noise is coming from disconnect the coax from the radio to see
if it is still there.
snip
--
Telamon
Ventura, California
---------------------------
Several minor points. Very few receivers has a true 50 Ohm input Z.
With exception of active antennas, ver few antennas present a 50 Ohm
Z to a coax.

Real world experience, several years ago I was hired to design and
install a WWV time recovery system. My initial effort used a dipole
cut for 10MHz. I uses a 1:1 balun to match the balanced dipole to the
unbaanced coax. The receiver was coupled to a dedicated PC ISA card
and I had major problems with PC RFI getting back into the antenna.
Experimenting with a portable SW receiver showed the PCs RF was going
up the shield.

With the antenna disconnected, and with or with out a 75 ohm load, I
had no RF from the PC gtting into the receiver. This was in late
1986. I was able to use a feritte rod wound with (maybe) ~20 turns
and a variable capacitor to resonate the LC to 10MHZ. This showed me
that even with a matched antenna, and decent receiver, R2000 (I am
prone to using equipment I am familiar with) RFI flowing up the coax
could be a source of major interference.

I finally went with a homebrew active antenna but I still had to use
the 10MHz trap.

But even with ~100' of every coax I checked, terminated into the
proper Z, with a 1.5:1 matching transformer for the use of 75 ohm
coax to feed the receiver, I had some engress from local (within ~10
miles) MW stations getting into through the shield. This lead to my
research on "transfer impedence". I still don't have any more
"understandable" data on that effect. Lots of math that causes my
ears to bleed.

Based on my experiences I think that more people have self inflicted
RF via their coax then commonly accepted.

As John Doty points out, a very good shield RF supression can be
gained by simply placing the coax under at least ~12' of soil. If the
soil dies out completly, this effectiveness will be degraded.

Practically speaking there is not much you can do about the receiver
input not being 50 ohms but it should be close enough to terminate the
coax to the extent that the the coax would be able to shield properly
unless it is a really bad design.

So with the coax terminated with a 75 ohm resistor (for a 75 ohm system)
instead of the antenna the receiver still picked up a local AM station
AND the station was not there if the coax was disconnected from the
radio input? Both tests need to pass for you to feel confident that this
was a case of the local station making it past the shield. It's possible
that the local stations signal could be on the mains making it way into
the PC radio. If you are correct about this then it would be a pretty
rare situation.

Dry ground does not have mobile ions for electric fields to push around
thereby absorbing the energy.

--
Telamon
Ventura, California