[TowerTalk] Common Mode Choke

[Kimo Chun]

The recent references to articles by W1HIS and K9YC were timely

indeed. Building and improving a multi-multi contest station is

always a challenge. There is always something to learn (and repair).

 

With the recent demise of another Radio Works T-4 common mode

filter, probably due to extended use on RTTY at or above 900W

we decided to get replacements for all used in the station.

The T-4's did work for us. However, it appears we can do better

plus not burn the devices.

 

Fortunately, Ward Silver, N0AX, mentioned something in an ARRL

Contest Rate Sheet plus the references to Chuck Counselman's,

W1HIS, article here prompted me to put a halt to the purchase

until I could read further.

 

I subsequently ordered 40 suppression cores of the Fair-Rite

31 mix, part number 2631803802. Though they are of the 2.4 inch

"toroid" variety they are categorized as "emi suppression cores",

unlike the 43 mix which is used and listed in both categories.

 

I built four filters following W1HIS design using 41" of RG-213

but with 10 cores on each. Measuring them with an MFJ-269 I took

the following readings:

 

28.5 MHZ    Z = 220     X = 83 degrees

21.2 MHZ    Z = 312     X = 83 degrees

14.150 MHZ  Z = 498     X = 82 degrees

7.100 MHZ   Z = 1233    X = 74 degrees

3.65 MHZ    Z = >1500   n/a, beyond range of meter on both.

1.83 MHZ    Z = >1500   n/a, beyond range of meter on both.

 

As specified this mix works well at lower frequencies. I am

not sure about the stray capacitance that comes to play in

the design plus measurement technique. I am also not sure

about the losses for this versus using 43. Still learning.

 

Our antennas are very close together on 80 and 160. Actually,

the 160 sloping vertical and sloper are right under the new

2 el. 80 yagi we used for the first time in CQWWSSB. We do

already have a string of bead cores at the feed point of the

160 vertical. 

 

Interaction between the antennas (at least interference-wise)

was minimal in our full-power tests, today. With the addition

of this common-mode choke in each line it was reduced to an

almost unnoticeable level. We shall see what happens in the

upcoming CQWWCW Contest. I have two more filters I can place

in line should it be required.

 

I will likely proceed with building many more but with the

43 mix for the higher bands. I will still try the 31 mix

choke as well to compare the two. Thank you for the education

and incremental improvement in our station.

 

In the last two weeks we also successfully (we hope) took out

mild interference to two neighbors computer speakers and boom-box

using 43 mix "snap-it" cores on AC cords and speaker lines.

We shall see....

 

I admit this is more up the RFI lists alley but I presume it is

also of interest to those on this list as well.

 

CU in the contests. It will be interesting to see if it helps

with local noise as well.

 

Kimo Chun, KH7U

At the KH6YY contest station (aka, KH7X, KH7Q, etc., formerly KH7R).

 

 

Message: 4

Date: Sun, 12 Nov 2006 09:27:33 -0800

From: Jim Lux <jimlux at earthlink.net>

Subject: Re: [TowerTalk] decoupling feedline from a dipole

To: "Jim Miller WB5OXQ" <wb5oxq at grandecom.net>,

      <TowerTalk at contesting.com>

Message-ID: <6.2.3.4.2.20061112091520.0215bbd0 at mail.earthlink.net>

Content-Type: text/plain; charset="us-ascii"; format=flowed

 

At 09:07 AM 11/12/2006, Jim Miller WB5OXQ wrote:

>I hear that decoupling the feedline can reduce noise so what is the 

>best way to do to.  I have tried commercially made baluns in the 

>past only to have them burn out at legal limit power levels.  Is a 

>choke wound from the feedline like I made for my beam going to do 

>the job or something else.  I am plagued by 10 to 30 db over s9 

>noise on 75 especially using an inverted v at 55'.  Noise seems 

>worse on AM when no carrier is present than on ssb also.

 

 

The W1HIS paper on decoupling ia a good start

http://www.yccc.org/Articles/W1HIS/CommonModeChokesW1HIS2006Apr06.pdf

 

Part of the challenge is that a "single" choke may not do the 

job.  You might need several, at different places along the 

feedline.  The chokes/decoupling act in the same way as breaking up a 

long guy wire with insulators. Except that the choke isn't an 

insulator, but a combination of inductance and resistance.

 

So, heating would be because the impedance is too low (if it were 

"really, really" high, then not much current would flow) and the 

current that flows through the "choke" causes heating in the R component.

 

The usual guideline is that the choke impedance should be >10x the 

feedline impedance, but that's not necessarily an appropriate number 

(i.e. it's really the impedance of the "outside" of the feedline 

that's relevant). But even if you took 50 ohms as the nominal 

impedance.. and had a 500 ohm choke impedance, 1/100th of the power 

is going to flow through the choke, and with legal limit, that's 15 

W.  You're not going to successfully dissipate 15W in a little clamp 

on ferrite (at least not for very long).