Hello:

Will be putting up a receive-only antenna (30 MHz) in my backyard.

The antenna will terminate in a Balun, and then a lightning arrestor,
immediately before the coax (8X probably)
run back to the house some 30 feet away.

Have read in one or two places a brief comment that it is a good idea to
make a few turns of the coax
right before the start of the antenna. Termed a "Choke".

If so, what is the purpose ?
Needed even if there will also be an arrestor ?

If a good idea, how many coax turns, of what diameter ?

Also a good idea at the other end, by the receiver ?

Thanks,
Bob

I don't think a coax choke is suitable at HF frequencies. I've seen
them used as low as FM broadcast pirate stations. The length of coax is
proportional to the frequency in use.


Robert11 wrote:
Hello:

Will be putting up a receive-only antenna (30 MHz) in my backyard.

The antenna will terminate in a Balun, and then a lightning arrestor,
immediately before the coax (8X probably)
run back to the house some 30 feet away.

Have read in one or two places a brief comment that it is a good idea to
make a few turns of the coax
right before the start of the antenna. Termed a "Choke".

If so, what is the purpose ?
Needed even if there will also be an arrestor ?

If a good idea, how many coax turns, of what diameter ?

Also a good idea at the other end, by the receiver ?

Thanks,
Bob

wrote:

I don't think a coax choke is suitable at HF frequencies. I've seen
them used as low as FM broadcast pirate stations. The length of coax is
proportional to the frequency in use.

Coax chokes are quite common in HF antenna designs.

dxAce
Michigan
USA

On Tue, 31 Jan 2006 15:22:33 -0500, "Robert11"
wrote:

Hello:

Will be putting up a receive-only antenna (30 MHz) in my backyard.

The antenna will terminate in a Balun, and then a lightning arrestor,
immediately before the coax (8X probably)
run back to the house some 30 feet away.

Have read in one or two places a brief comment that it is a good idea to
make a few turns of the coax
right before the start of the antenna. Termed a "Choke".

If so, what is the purpose ?
Needed even if there will also be an arrestor ?

If a good idea, how many coax turns, of what diameter ?

Also a good idea at the other end, by the receiver ?

Thanks,
Bob


A choke passes DC unimpeded and blocks higher frequecy AC. Unless
your SWL antenna is mounted on an FM transmitter tower I don't see any
advantage. Lightning wouldn't notice it.

Most Baluns are actually autotransformers and all parts are at DC
ground (provided your coax outer conductor is grounded and the
termination is properly affixed). This is the best solution to
protecting you from anything other than a direct hit. A lightning
arrestor will keep your house from burning down but won't save your
radio's front-end.

On Tue, 31 Jan 2006 15:22:33 -0500, "Robert11"
wrote:

Hello:

Will be putting up a receive-only antenna (30 MHz) in my backyard.

The antenna will terminate in a Balun, and then a lightning arrestor,
immediately before the coax (8X probably)
run back to the house some 30 feet away.

Have read in one or two places a brief comment that it is a good idea to
make a few turns of the coax
right before the start of the antenna. Termed a "Choke".

If so, what is the purpose ?
Needed even if there will also be an arrestor ?

If a good idea, how many coax turns, of what diameter ?

Also a good idea at the other end, by the receiver ?

Thanks,
Bob


If transmitting, a choke can keep stray RF off the outside of the coax
shield. There's not much point to one on a receiving antenna.

bob
k5qwg

In article ,
"Robert11" wrote:

Hello:

Will be putting up a receive-only antenna (30 MHz) in my backyard.

The antenna will terminate in a Balun, and then a lightning arrestor,
immediately before the coax (8X probably)
run back to the house some 30 feet away.

Have read in one or two places a brief comment that it is a good idea
to make a few turns of the coax right before the start of the
antenna. Termed a "Choke". If so, what is the purpose ?

You are looking at the Ham stuff, which is geared for transmitting. The
transmitting situation needs to take a few more things in account that
are not as important in receiving. The main idea here is to prevent
common mode RF current on the outer shield coupling to/from the antenna
as you may end up with a hot radio and shack. If you are burrowing the
coax on the way to the BALUN there is no need for this and the BALUN
already performs this function.

Needed even if there will also be an arrestor ?

Not needed in your situation.

If a good idea, how many coax turns, of what diameter ?

As an example if you were to do this for a dipole up a mast 3 to 10
turns about a foot diameter depending on the frequency.

Also a good idea at the other end, by the receiver ?

If the coax has a run through the walls of your place and 30 feet
through the air instead of 30 feet through the ground then yes both ends
and also spaced at intervals along the run.

This is easier done with a ferrite clamp on choke as opposed to coiling
the coax.

--
Telamon
Ventura, California

On 31 Jan 2006 15:12:40 -0800, "bpnjensen"
wrote:

A choke passes DC unimpeded and blocks higher frequecy AC. Unless
your SWL antenna is mounted on an FM transmitter tower I don't see any
advantage.

I will disagree here. Although a single coax choke of specific design
won't solve all problems across all bands, a choke of the right size
will give you an obvious reduction in common mode noise at certain
minmum requencies and upward.

I installed one on my own antenna at home. I was hearing common mode
noise across all bands from zero to 30 mHz. i fashioned a coil choke
about 7" diameter on a piece of ABS pipe, wrapped neatly enough times
to get about 30 feet of the coax onto the coil, and mounted it just
below the antenna. This has made a noticeable difference for all bands
from 11 mHz and up...the background is considerably quieter than
before. A note here - this thing can get pretty hefty, fast, if you
use substantial coax of the 3/8' variety or larger. Messing with this
thing as an integral part of a relatively stiff run of coax can cause
minor cussing and swearing if you do not have some assistance (and
maybe even if you do ;-)

For lower frequencies, a larger coil able to hold a greater length of
coax would have been necessary; I didn't feel that the difficulty in
fashioning one of these and trying to get it up on top of the mount I
have would have been worth the trouble. I have since reconsidered this
decision many times and one of these days the neighbors just might see
big ugly coils of coax up under my two antennas. Whether my wife goes
along with this, I can't tell you.

Bruce Jensen

Wouldn't a ferrite work better?

If transmitting, a choke can keep stray RF off the outside of the coax
shield. There's not much point to one on a receiving antenna.

bob
k5qwg

How about for keeping common mode noise from the house reaching the antenna?
Of course this is better accomplished by a 1:1 isolation transformer nearer
the RX.

Dale W4OP

You more or less made my point, i..e. these coax chokes are kind of
unwieldy in the HF range. However, if it works for you, you can't
argue with success.


bpnjensen wrote:
A choke passes DC unimpeded and blocks higher frequecy AC. Unless
your SWL antenna is mounted on an FM transmitter tower I don't see any
advantage.

I will disagree here. Although a single coax choke of specific design
won't solve all problems across all bands, a choke of the right size
will give you an obvious reduction in common mode noise at certain
minmum requencies and upward.

I installed one on my own antenna at home. I was hearing common mode
noise across all bands from zero to 30 mHz. i fashioned a coil choke
about 7" diameter on a piece of ABS pipe, wrapped neatly enough times
to get about 30 feet of the coax onto the coil, and mounted it just
below the antenna. This has made a noticeable difference for all bands
from 11 mHz and up...the background is considerably quieter than
before. A note here - this thing can get pretty hefty, fast, if you
use substantial coax of the 3/8' variety or larger. Messing with this
thing as an integral part of a relatively stiff run of coax can cause
minor cussing and swearing if you do not have some assistance (and
maybe even if you do ;-)

For lower frequencies, a larger coil able to hold a greater length of
coax would have been necessary; I didn't feel that the difficulty in
fashioning one of these and trying to get it up on top of the mount I
have would have been worth the trouble. I have since reconsidered this
decision many times and one of these days the neighbors just might see
big ugly coils of coax up under my two antennas. Whether my wife goes
along with this, I can't tell you.

Bruce Jensen

On Wed, 01 Feb 2006 03:01:58 GMT, "Dale Parfitt"
wrote:



If transmitting, a choke can keep stray RF off the outside of the coax
shield. There's not much point to one on a receiving antenna.

bob
k5qwg

How about for keeping common mode noise from the house reaching the antenna?
Of course this is better accomplished by a 1:1 isolation transformer nearer
the RX.

Dale W4OP


You're absolutely right. Somethimes my fingers get ahead of my
brains...

bob
k5qwg