Reduce RFI with a Common Mode Choke

Using my 130 foot End Fed Half Wave for 80 meters (works on all bands) I do not use a counterpoise but rather have an 8 foot ground rod driven into the ground right next to the autotransformer. I do not experience any common mode current interference with this system. When I used to operate with an off center fed dipole (Windom) up at around 90 feet I used to experience RF getting into my audio at times. Winding about seven or eight turns of the RG-58 coax around a large ferrite clam shell solved the problem. I located the ferrite clam shell about 25 or 30 feet below the balun. The clam shell allowed me to add it onto the coax without having to disconnect anything or thread it through a toroid.

 

What is the difference in using coax vs copper wire as shown here

 

Results-wise, not much that I have discerned. Winding the choke using coax cable reduces the number of connections usually -- one connector at the radio, and a connector/connection at the antenna. The choke can be wound onto the existing cable -- no connectors needed. ...otherwise should yield the same results in choking.

One advantage of the wire vs coax is that the coax has a limit on radius bend / diameter and the center conductor can migrate and short out to the shield, especially with foam coax, when wound too tightly. The wire method reduces the risk of a short.

 

None whatsoever.

 

Tom is a very smart and talented ham but I was always a bit dubious about his test so we did one as well using both a VNA and network analyzer for analysis. One was 11 continuous turns on a mix 31 core and the other 10 (5 to a side) turns with the crossover which is counted a one turn for a total of 11. You can see by the scan that there is minimal difference. I believe the loose, close coupled turns used by Tom might have contributed to his results but who knows. The scan is of total choking impedance generated by the windings and the line with the dots is the continuous winding.

Bob KZ5R
Balun Designs

 

VNA stands for Vector Network Analyzer. Buttery butter?

 

We use the dual port VNA to measure insertion loss but I agree one of the ports measures the same as a network analyzer. We used both to double check our measurements.

 

why wouldn't you?

 

Because out in the open air the toroid has an opportunity to run cooler. The more common mode you have the hotter the toroid will get especially if it cannot cool and can get hot enough to initially change its permeability and if it continues it can crack and break. Apparently the toroid's weather just fine out in the open. Also RG400 will handle much more power and the Teflon dielectric will not migrate.

 

That's a great graph Bob, that's the kind of information that we need, real data that was researched by someone who knows what they are doing. The more evidence like this that we have will enable us to make better choices when we are deciding between alternative designs. Thank you for taking the time to do this and making it available for everyone.

73

 

No reason for it. Adds a failure point and extra connector loss. The enclosure traps heat, and heat is the enemy. The added connectors are just failure points and more loss. The torrid and coax are weatherproof. The other issue is using RG58 coax when RG-400 is what is called for.

Everything one needs to know is professionally written and tested in the "Choke Cookbook." Coax chokes are easy and do a fair job. A bifilar wind with PTFE SPC solid wire produces excellent results but is more difficult to wind and attention to detail. Use high-quality PL and SO-259 connectors, Ft240-31, and RG-400 coax, and you can obtain commercial results at a DIY price, don't mess it up with inferior materials and practices.

 

Jerry W2FMI (sk), who didn't just write the book on baluns, he wrote books on baluns, noted that the W1JR "Reisert Crossover" had the main advantage of allowing the input and outputs of a balun wound around a toroidal core to be located opposite each other.
In a word, "convenience."
BTW, I don't think W2FMI counted the "Reisert Crossover" as a turn.

 

Thanks for the direct link to the cookbook. Lots of other good info on Jim's website: http://k9yc.com/publish.htm

 

Interessant et efficace utilisé depuis très longtemps dans les descente d'antenne pour diminuer les retours. Bravo.

 

You are right, so why don't you try it? Fact there is more than one way to skin a cat.