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Author Topic: Common Mode Choke advice for a homebrew OCF (Carolina Windom)?  (Read 7216 times)

Posts: 114


« on: January 11, 2012, 06:13:05 AM »

I am currently building an OCF wire antenna to the measurements of the commercial "Carolina Windom 80 Special" feed with coax (RG-58 as I have a roll here to hand and I cant get RG8X locally at present) to fit in my restricted space, and just as an experiment.

At 66 feet over all it will be feed at a point 37.8% for one end giving me a 25 foot short leg and a 41 foot long leg. I have made a homebrew 4:1 "Ruthroff" (voltage) balun to place at the feed point, I selected this design in order to let the "magic" 10 foot vertical coax radiator radiate.  This design is also supposed to work on 80m with an atu but I wouldn't expect too much from it.

Just need some advise on the second "top secret black box" on the bottom of this 10 foot section.

I was going to wind an air core 1:1 "Ugly Balun" on the feed line here which was from 21 feet of RG 58 close wound on a 1 5/8 inch (40mm) PVC former, the coil is about 10 inches in length with about 45 turns, but on further research I think I will opt for a choke made with the RG58 feed line close wound on 3 X approx 3/8 inch (10mm) ferrite rods taped together I have seen some built on internet sites using 10 turns on a single rod,  I can easily fit 15 turns on the rods available so my question is.

Is it better to place as many turns as possible on the ferrite rod choke to stop as much CMC as possible travelling back into the shack on the coax braid?  The shack is located on the second floor.

The finished choke will be housed in a PVC housing on the feed line and weather proofed.

I am just running barefoot around 100 watts so high power handling is not a real issue.

Also I have read that with the commercial version CW it is recommended to cut the feed line to a length of 86 feet from the choke to the shack which I can do, any thoughts on this ?



Posts: 2438

« Reply #1 on: January 11, 2012, 09:37:33 AM »

I wouldn't recommend an ugly balun for that antenna.  An ugly balun is primarily an inductor.  I tried a bunch of various values of inductance in a simulation and all of them resulted in high common mode currents on the feedline.  Some of the values completely destroyed the SWR curve on all bands while other values had minimum impact even though the feedline current was high.  You need a very good ferrite core choke for that antenna that will give very high impedance an all bands that you plan to use it.

I have also seen a bunch of different recommended dimensions for this antenna, and I'm not sure what it is really supposed to be.  Using the dimensions you quoted my simulation shows the 20 and 15 meter resonant points to be too low.  However, the amount of feedline common mode current does effect those resonant points, and that is probably why I see various dimensions recommended for this antenna.   It's difficult to remove all the common mode currents from the feedline with this antenna, and because of that, the feedline length effects the resonant points as well.  The 86 ft number was probably recommended by someone that had large common mode currents on the feedline.  My simulation shows insignificant effects of feedline length when a good current mode choke is used.

Jerry, K4SAV

Posts: 593

« Reply #2 on: January 11, 2012, 11:15:07 AM »


A Carolina Windom of the "radioWorks" design will not give the lowest SWR on all bands.  It doesn't matter because you have to use a tuner with this antenna and an SWR above 1:1 at the balun is what causes the 22ft. feed line section to radiate which
provides the enhancement to low angle radiation.  Basically what you need for a choke at the bottom of the 22ft. section is a line isolator that chokes all rf on he frequencies of operation (in your case 40M, 20M, 15M, 10M).  You should be able to find design data for such a ferrite choke thru Google.

Have fun,


Posts: 54

« Reply #3 on: January 11, 2012, 01:19:33 PM »

Try this link:

You are building what I am just finishing up.  I made a choke similar to the one Len Carlson, K4IWL, suggested in his article with ferrite tubes over coax and put in a PVC tube.  I made mine a little different (poetic or "engineering" license  Smiley ) mainly for construction and mounting purposes.  It isn't supposed to need a tuner on 40, 20, 15 and 10, but mine does.  It is also supposed to be able to work on 80, 30, and 17 with a tuner.  I need a tuner on all bands; probably something I have done wrong; but I'm still learning.

Gerry, KK4GER 

Posts: 17476

« Reply #4 on: January 11, 2012, 06:36:56 PM »

Quote from: KG6YV

... an SWR above 1:1 at the balun is what causes the 22ft. feed line section to radiate which
provides the enhancement to low angle radiation. 

This statement indicates a misunderstanding of how transmission lines work.

The radiation from the vertical section has nothing to do with the SWR at that point.
It is possible to have a perfect SWR and still have the line radiating, or have a high
SWR with no radiation. 

The radiation is because the shield of the coax cable is connected to one side of
the antenna - either directly or through a voltage balun that doesn't do a good job
of isolating it.  Because of that connection the outside of the coax IS a part of the
antenna.  How much current flows on it will depend on what else is connected to
the feedpoint, and what impedance the short wire presents compared to the others.

All that, of course, affects the load impedance, and thus can affect the SWR.
But you'd expect that - there are, in effect, 3 wires connected to the feedpoint
and you are changing one of them, so of course the SWR may change.

Whether the radiation from that section of the feedline really improves the low
angle radiation is a different matter:  it's probably best to treat that as an
invention of the advertising department.

Posts: 114


« Reply #5 on: January 12, 2012, 02:57:43 AM »

Thank you for all your responses thus far guys!!

I have like probably all of us on here a junk box full of salvaged unknown ferrite material cores of many shapes and sizes. Unlike in the USA ferrite material is not easy to obtain over here so I chose to bundle three rods together as one to wind the choke on,

What do you guys think is it better to place as many close wound turns of the coax feed line as physically possible on the ferrite rod choke to stop as much CMC as possible travelling back into the shack on the coax braid??

I have seen some designs on the internet using 10 turns on this particular antenna design but I can easily fit 15 turns on theses rods???

There are some interesting (for want of a better word ;-) ) claims made about the commercial version of this antenna which maybe are best treated as just advertising hype.

It's just a simple 40m OCF as far as I can see.. I doubt there is anything magical involved in the 10 foot vertical coax section. I know it's a 22 foot section on the 80m version but I have seen figures of 18 foot and of 40 foot sections recommended from various sources also just to add to the "magic"... lol

Posts: 2438

« Reply #6 on: January 12, 2012, 09:31:33 AM »

Whether the radiation from that section of the feedline really improves the low
angle radiation is a different matter:  it's probably best to treat that as an
invention of the advertising department.

The advertising department has been know to expand on the original idea.   

Here is some info on Carolina Windoms.

The effect of the vertical section on the 40 meter version of the Carolina Windom is interesting.  On 15 meters that vertical section steals current from the other two wires and changes the phase of the currents in all the wires.  That changes the radiated pattern from a 6 lobe pattern (in the case of an  OCF) to a 4 lobe pattern in the Carolina Windom.  The radiation from the vertical section is insignificant.  It doesn't lower the radiation angle and it doesn't add any significant vertical polarization to the pattern.  Its only function is to alter all the antenna currents.  It's difficult to look at those two patterns and say which is best.  Either antenna can beat the other by up to15 dB depending on where the station is located.  I would probably take the Carolina Windom because I don't like antennas with lots of lobes and nulls and the CW has less.  On the other hand, I would take the OCF because the resonant points hit the bands better and choking common mode currents is easier, or maybe I should say less difficult.

The effect is not the same on 10 meters.  Both the OCF and the Carolina Windom have the same number of lobes (eight) and nulls (eight).  They aren't in the same directions, but overall the performance between the two is a wash in my opinion.

On 20 meters the patterns are basically the same except for a small skew (4 lobe patterns).  The Carolina Windom does limit two of the nulls to 9.3 dB (below the lobes) where they are deeper on the OCF.

The data above was for a Carolina Windom of 41/25 ft with a vertical section of 10 ft.  The OCF was 41/25 ft.  Height 45 ft.

The ARRL Antenna Book mislead everyone with their plot of the 80 meter version of this antenna.  It shows the antenna to be omni-directional within about 6 dB, which of course leads to the idea of the vertical section filling in the nulls.  That bugged me for a long time because I could never get that with my models – until I happened to notice this plot was for 5 degrees elevation over salt water.  With those conditions I could then duplicate the plot.  However not many people erect this antenna over salt water.  A plot for 20 meters over average ground at 50 ft height at 20 degrees elevation shows the nulls to be up to 20 dB below the lobes (which is about the same for an OCF of the same size).  The antenna is not anywhere near omni-directional.

Jerry, K4SAV

Posts: 2100

« Reply #7 on: January 12, 2012, 11:05:12 AM »

     James,along with (Gerry-KK4GER) I also built (Len Carlson's-K4IWL) 66 ft.version exactly to his internet design.Although  I cheated and bought the MFJ 1:1 line isolator for the bottom of the 10 ft.Q section,however I have used several different lengths from 20 to 100 ft. for the feed line in the air,on the ground and every where in between it has made no difference for me.This is my favorite antenna as qrp/dx is my niche.Mine is 25 ft. high and as I only work 40/20/15 meters I don't have to mess with a tuner.This antenna has survived 3 Maine winters with no problems at all.When you decide on the proper magic black box for the bottom of the Q section I'm sure you will be happy this antenna.    73 Jim
« Last Edit: January 12, 2012, 11:10:30 AM by W1JKA » Logged
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