In article .com,
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/in...tId=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

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.

Terry

Terry

On 22 Dec 2005 07:11:38 -0800, wrote:


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.

Terry

Terry

If I started digging a 12 feet deep trench for the shortwave radio
wire they'd have me committed.

David wrote:
On 22 Dec 2005 07:11:38 -0800, wrote:


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.

Terry
If I started digging a 12 feet deep trench for the shortwave radio
wire they'd have me committed.
----------------
Yea, 12 feet would be interesting.

As most readers with half a clue might have suspected I meant ~12
inches.

Terry

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

Telamon wrote:

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
---------------------------
The MW engress was noted in experiments in 3 diverse locations with
several different
receivers ranging from a R390, AOR7030+, R2000, and, a R5000. In one
location I used
also used several ham (Kenwood) general coverage transceivers. And yes
the MW
signals, with the exception of my home QTH, were only present with the
coax
connected to the receiver. And even at home the signal level jumped
enough to
clearly show the engress was fromthe coax. Interestingly a borrowed
~300' spool
of TV triax was reeled out and with only the outer shield groudned at
both ends,
with the inner shield grounded at the receiver end, with the far end
groudned or
ungrounded, had no engress.

The WWV dipole experiment was ~20 years ago and I used a borrowed
R5000,
a R392 and my trusty R2000.

At my home QTH I do get one MW station that is about 3 miles away and I
swear
they must throw a lobe, they are ND daytime only, right across me. The
AOR7030+
would receive this MW station on external 12V without an antenna. Only
on internal
power, wiht the antenna input terminated, did the AOR reject the
bugger.

I built a rat wire (1/4" square openings) faraday cage and can get the
local MW on
my R2000s, my R390 and my DX398. Faraday cages do not offer 100%
isolation.
It is weak but there. This is with all receivers powered from gell
cells and the antenna
terminated in a 50 ohm non inductive load. The signal was week, but
still there, just
barely, but there none the less.

I am working on trade to get a ~15 year old HP spectrum analyser that
will allow
me to measure the real signal strength of various events. I am also
working on
another trade for tuneable RF voltmeter that measures from 0.1uV and
from 10KHZ
through ~15MHZ. Not perfect, but at least I should be able to get
repeatable numbers.

Like I said this whole excercise has made me very interested in
"transfer impedence".
Science is not magic, but sometimes the rules are arcane enough to look
like magic
until you get enough info to start making sense.

For most applicaton minor coax imperfections shouldn't mater. Unless
the local noise
floor near the coax is on the order of volts per meter any leakage will
be, I think,
pretty minor. But when trying to reject strong local noise emitters
like PCs VCRs and other QRM any leakage can be problematic. There are
days I swear I am going to
move to the woods and live in a mud hut to et away from all the EMI
crap.

Terry