making a common mode filter
 

Ed G wrote:
I am having a heck of a time locating a source of good information on
how to make an effective common mode filter using the snap-on type ferrite
beads such as available at RAdio Shack.

Someone needs to characterize those beads.
What is the one-turn choking impedance at
10 MHz?
--
73, Cecil http://www.w5dxp.com

making a common mode filter
 

I am having a heck of a time locating a source of good information
on
how to make an effective common mode filter using the snap-on type
ferrite beads such as available at RAdio Shack.

Someone needs to characterize those beads.
What is the one-turn choking impedance at
10 MHz?


That is a good point. I'm not even sure of the vendor of the numerous
split beads I have on hand. The ferrite is about 1/2" thick, 1" in
diameter, and a 5/8" hole diameter. Don't know the material. The
plastic housing is labeled " F / R " with the slant bar being a
lightning bolt, and the part number is: 80640* . (that's an asterisk
after the zero.)

Ed

making a common mode filter
 

That is a good point. I'm not even sure of the vendor of the numerous


After some additional Google research, I found them. Made by Fair-
Rite Products Corp. PN is probably: 2643806402. Product description
matches unit on hand, and lists it as type 43 material with a Z around
where I want to operate of about 20 ohms.

Ed

making a common mode filter
 

On 2 Sep 2007 17:53:37 GMT, Ed G
wrote:



That is a good point. I'm not even sure of the vendor of the numerous


After some additional Google research, I found them. Made by Fair-
Rite Products Corp. PN is probably: 2643806402. Product description
matches unit on hand, and lists it as type 43 material with a Z around
where I want to operate of about 20 ohms.


Hi Ed,

Read the fine print. This Z probably relates to some "standard" size
item, such as a small bead and not your clip-on. The boon here is
your clip-on will probably present more resistance to common modes.

73's
Richard Clark, KB7QHC

making a common mode filter
 

Ed G wrote in
.95:



That is a good point. I'm not even sure of the vendor of the
numerous


After some additional Google research, I found them. Made by
Fair-
Rite Products Corp. PN is probably: 2643806402. Product description
matches unit on hand, and lists it as type 43 material with a Z around
where I want to operate of about 20 ohms.

My guess is that you got the 20 ohms (for an unstated intended frequency
of use) by extrapolation of the published data from 10 to 250MHz. You
probably also read that the core material is recommended for 25 to
300MHz. Both of these would suggest that the material is not the best for
your intended frequency, whatever that is.

So, for an effective choke, you may need 25 to 50 of these things. They
are only a half inch long each, so that is just 13 to 25 inches in length
in total, not unusual for that type of choke.

Owen


Ed

making a common mode filter
 

After some additional Google research, I found them. Made by
Fair-
Rite Products Corp. PN is probably: 2643806402. Product
description matches unit on hand, and lists it as type 43 material
with a Z around where I want to operate of about 20 ohms.

My guess is that you got the 20 ohms (for an unstated intended
frequency of use) by extrapolation of the published data from 10 to
250MHz. You probably also read that the core material is recommended
for 25 to 300MHz. Both of these would suggest that the material is not
the best for your intended frequency, whatever that is.

So, for an effective choke, you may need 25 to 50 of these things.
They are only a half inch long each, so that is just 13 to 25 inches
in length in total, not unusual for that type of choke.



Actually, since I probably will be using RG8X I can most likely
loop a couple turns through each bead... which should preclude the need
for quite as many.


Ed

making a common mode filter
 

Ed G wrote in
. 192.196:



After some additional Google research, I found them. Made by
Fair-
Rite Products Corp. PN is probably: 2643806402. Product
description matches unit on hand, and lists it as type 43 material
with a Z around where I want to operate of about 20 ohms.

My guess is that you got the 20 ohms (for an unstated intended
frequency of use) by extrapolation of the published data from 10 to
250MHz. You probably also read that the core material is recommended
for 25 to 300MHz. Both of these would suggest that the material is not
the best for your intended frequency, whatever that is.

So, for an effective choke, you may need 25 to 50 of these things.
They are only a half inch long each, so that is just 13 to 25 inches
in length in total, not unusual for that type of choke.



Actually, since I probably will be using RG8X I can most likely
loop a couple turns through each bead... which should preclude the need
for quite as many.

Am I right that this is a very short length of coax in a vehicle, and the
VSWR will be low due to the ATU at the feedpoint, and that the ATU ground
/ coax braid will be connected to the vehicle metal framework? So why use
thick coax? Why the choke?

If you need a choke, you could get some cores that are a neat fit over
RG58, at 10MHz appropriate type 31 cores has a Z of 115/inch. That
doesn't address the issue of whether the power / control connection needs
protection.

Actually, I am not sure why you are doing this anyway, is it just in case
or is there a demonstrated need?

Owen

making a common mode filter
 

Am I right that this is a very short length of coax in a vehicle, and
the VSWR will be low due to the ATU at the feedpoint, and that the ATU
ground / coax braid will be connected to the vehicle metal framework?
So why use thick coax? Why the choke?

If you need a choke, you could get some cores that are a neat fit over
RG58, at 10MHz appropriate type 31 cores has a Z of 115/inch. That
doesn't address the issue of whether the power / control connection
needs protection.

Actually, I am not sure why you are doing this anyway, is it just in
case or is there a demonstrated need?

Owen


Well, to be honest, I was primarily planning on using the choke for a
base station application.... will have some sort of balanced dipole or
loop fed by an SG-237 tuner. I wanted the choke for the coax feeding the
tuner.

On the RV, I have a 23' whip on the back (ladder) which I had planned
on feeding with another SG-237. ( Currently it is fed with a home-brew
base loading coil assy, on which I must manually change frequency taps.)
In either case, I thought it might be beneficial to put a common mode
choke on the RG8X feeding that antenna, too, since I do have RF causing
various issues inside the coach while I am transmitting.

On your RV grounding question, yes, the antenna feedpoint and all
related ground areas are very well bonded to the main vehicle chassis
with rather large tinned braid, (2" wide).

Ed

making a common mode filter
 

On 03 Sep 2007 03:26:50 GMT, Ed G
wrote:

loop fed by an SG-237 tuner. I wanted the choke for the coax feeding the
tuner.

Hi Ed,

You will have to choke the control lines just as much.

I do have RF causing
various issues inside the coach while I am transmitting.

That could be as easily through the air, or "ground currents" in the
chassis. All points of the chassis, doors, hood, etc. are not usually
bonded very well. Even the exhaust system can be "floating" to
provide a path from ground, right up into the engine compartment.

On your RV grounding question, yes, the antenna feedpoint and all
related ground areas are very well bonded to the main vehicle chassis
with rather large tinned braid, (2" wide).

Re my paragraph above, does this include doors? A lot may be taken
for granted and hinged points are sometimes insulated, allowing large
panels (capacitive plates) to couple RF inside what is considered to
be an otherwise tight box.

73's
Richard Clark, KB7QHC

making a common mode filter
 

Richard Clark wrote in
:

On 03 Sep 2007 03:26:50 GMT, Ed G
wrote:

loop fed by an SG-237 tuner. I wanted the choke for the coax feeding
the tuner.

Hi Ed,

You will have to choke the control lines just as much.

I do have RF causing
various issues inside the coach while I am transmitting.

That could be as easily through the air, or "ground currents" in the
chassis. All points of the chassis, doors, hood, etc. are not usually
bonded very well. Even the exhaust system can be "floating" to
provide a path from ground, right up into the engine compartment.

On your RV grounding question, yes, the antenna feedpoint and
all
related ground areas are very well bonded to the main vehicle chassis
with rather large tinned braid, (2" wide).

Re my paragraph above, does this include doors? A lot may be taken
for granted and hinged points are sometimes insulated, allowing large
panels (capacitive plates) to couple RF inside what is considered to
be an otherwise tight box.

Richard, getting clear details is like drawing teeth. I suspect the RV
might be fibreglass, and so about as RF transparent as you might get.
Further, something hints to me that the grounded ATU is on top of the
ladder, so it isn't really grounded.

But in the absense of clear details of a single scenario, I can't help.

Owen