Unun & common mode choke in series?

[Mikejo]

Ii’m making an EFHW antenna.

I want to make a toroid wound unun, and also make a toroid wound common mode choke.

Is there any good reason why I cannot place the common mode choke directly after and connected directly to the unun in series fashion, and then connect my coax to the common mode choke?

 

[Mikejo]

Correction I’m making…

 

[prcguy]

It sounds like your asking if the unun and choke can be right next to each other with very little coax if any. I did this once with the transformer and common mode choke inside the same box and it didn't seem to match up very well. I removed the choke from the box and the antenna tuned as normal and also with an external choke a few feet down the coax. The internal choke was an FT-240-31 core with about 9 turns of RG-316 coax.

The EFHW using a 64:1 or 49:1 transformer is said to need a little counterpoise to work against and that would normally be the coax exiting the transformer. If you place a choke balun right at the transformer it may eliminate the counterpoise effect and change things. On the other hand I've placed chokes in the coax just a foot or two from the transformer with no affect on the tuning or SWR.

 

[Mikejo]

Interesting, but on the unun, there will be a terminal for ground for a short counterpoise, do you think that if I attached a short wire there it would mitigate the effects similar to what you were experiencing?

 

[prcguy]

I think it should but since I've not specifically tested that it will be your experiment and we will all be waiting anxiously for the results.

Here is some info on the subject, although I don't completely agree with it there is some actual testing beyond theoretical. In my brief read it seems a counterpoise off the ground end of the transformer secondary of about .05 wavelengths at the lowest frequency could be useful. That would be about 6.5ft long for 40m.

AA5TB - The End Fed Half Wave Antenna

The End Fed Half Wave Antenna explained by AA5TB.

www.aa5tb.com

Interesting, but on the unun, there will be a terminal for ground for a short counterpoise, do you think that if I attached a short wire there it would mitigate the effects similar to what you were experiencing?

 

[Mikejo]

I think it should but since I've not specifically tested that it will be your experiment and we will all be waiting anxiously for the results.

Here is some info on the subject, although I don't completely agree with it there is some actual testing beyond theoretical. In my brief read it seems a counterpoise off the ground end of the transformer secondary of about .05 wavelengths at the lowest frequency could be useful. That would be about 6.5ft long for 40m.

AA5TB - The End Fed Half Wave Antenna

The End Fed Half Wave Antenna explained by AA5TB.

www.aa5tb.com

Well OK, sounds to me like it may just work. However, it may be a while before I can give it a good try; right now it’s just in the theoretical stage. Thanks for your help, 73

 

[prcguy]

It only takes about 10 min to wind a transformer or a choke. I figure you can have them in a box and in the air before dark today? Maybe with a few EU contacts to boot!

Well OK, sounds to me like it may just work. However, it may be a while before I can give it a good try; right now it’s just in the theoretical stage. Thanks for your help, 73