Re: Info - Icom IC-R75 with Kiwa Mods and Antenna Supermarket Eavesdropper SWL Sloper
- From: Telamon <telamon_spamshield@xxxxxxxxxxxxxxxxxxxxxx>
- Date: Thu, 22 Dec 2005 03:50:55 GMT
In article <1135194885.112951.276800@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>,
r2000swler@xxxxxxxxxxx wrote:
> RHF wrote:
> >SNIP
> > Tip - Try placing Two Snap-Together Ferrite Core 'on' the Coax
> > Cable from the Antenna to the Receiver : RadioShack Catalog #
> > 273-105
> > http://www.radioshack.com/product/index.jsp?productId=2103222&cp *
> > One about Three to Six Inches from the PL-259 Plug going into the
> > Radio. * One about Three ot Six Inches from the PL-259 Plug
> > connecting to the Antenna.
> snip ------------------ It will take more then a few ferrite cores to
> block noise on the outer shield on coax. See:
> <http://www3.telus.net/7dxr/ircatech/snake.pdf#search='bryant%20lead%2
> 0in%20an tenna>
>
> I was dismayed until I went to the (considerable) effort to "do the
> job right". The techniques shown by John Bryant have reduced my PC to
> antenna noise to below detection, and knocked an offending close by
> MW outlet to a barely detectable level.
>
> "Below detection" means I can't even see it on a FFT display.
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.
If the coax is in a location where the outer shield can induce a voltage
on the very short resistor lead at the end then you can snap on a
ferrite to see the difference it makes. If you have short leads to the
resistor this should not be necessary. This test should show that the
coax is not picking anything up that you can hear on the radio.
Now you should for sure be able to tell if the coax is the problem,
which if it is in good shape should not be generating anything the
receiver will pick up.
Why does putting the ferrite on the coax ends make a difference when it
is connected to the antenna? Most likely if the coax is above ground the
outer shield will pick up common mode signal energy and transfer it to
the nearby antenna to which it is connected. This is the mode that is
most likely causing you a problem that the ferrite on the outer shield
seems to fix.
Firstly you need to cause the antenna impedance to be close to the coax
impedance with a BALUN or UNUN or some other matching device.
If you still have a problem then you have options depending on the
situation.
You could bury the coax so the earth around it will help dissipate the
common mode signals on the outer braid preventing them from coupling to
the antenna.
If the coax is strung above the ground you could distribute ferrite
cores all along the coax run. You can also coil several turns of the
coax near the antenna end and use one or more clamp on ferrite cores on
the coil.
The sequence is then:
1. Test the coax with a 50 ohm resistor. This will show the coax to be
OK and you become aware if the radio internal noise and birdies if any.
You should not pick up any signals from the coax cable.
If OK then the next step.
2. Test the antenna matching device unless the antenna is itself at the
coax impedance and you are not using one. Use a resistor at the antenna
impedance on the far side of the matching device again keeping the leads
short.
You should not pick up any signals from the matching device and coax
cable with the resistor substituted for the antenna.
If OK then the next step.
3. If common mode signals are coupling from the coax shield to the
antenna then use ferrite cores on several turns of the coax coiled up or
bury the coax. Ferrite on the coax at the radio end should not help.
--
Telamon
Ventura, California
.
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