[TowerTalk] OCF dipole

[jimjarvis@comcast.net]

Yup...complete agreement, Tom.

The line isolators will give you a couple hundred ohms, at best.
My solution...and it's a seriously compromised one...is to use 
clip-ons which are 2x the amount of what's in the RW line isolator,
at 4 places in the feed system.   That is...either side of each of two
ground rods...one where the feeder comes to ground, and the other
at the house entrance, about 100' away.

Now...that said...this CAN'T be an efficient system.   I'm warming the earthworms with a serious amount of power.   What it is, is relatively simple and easy to install, and a compromise which worked reasonably well on multiple bands.  

Actually, looking at the topside 'unun' or 'balun'...whatever it is....it can't be limiting feedline current much, if at all.   If it DID, the antenna wouldn't work on higher frequencies worth a damn.   The feedline is part of the system.  

So, while I'm singing the praises of the Carolina Windom, in reality, it is certainly less efficient than any centerfed halfwave up and in the clear.   But it is also much more useful on harmonically related bands than a center fed dipole would be.  

Reading Belrose's QST articles will provide useful design, with data to back it up.  
I highly recommend them.

n2ea



-------------- Original message -------------- 

> > In reality, what is needed is an "un-un", and Sevick's 
> work may prove helpful, as well. 
> 
> We better be careful here. 
> 
> A current balun is always useful as an "un-un", but an 
> "un-un" is not always a useful current balun. Some un-uns 
> have no common mode isolation, and would be totally useless. 
> 
> In the OCF application the antenna requires a decoupling 
> device that provides a *very high* common mode impedance. 
> The only device guaranteed to do that is an exceptionally 
> good current balun. Certainly a string of beads would never 
> be adequate. The more we move the feedpoint towards an end, 
> the more complex or critical decoupling becomes. 
> 
> > As both K4SAV and W8JI observed, an unbalanced antenna 
> will tend to have current on the outside of the feedline. 
> The CW is no exception. My solution is use of ferrite 
> chokes in the feed system, on a ground point directly below 
> the antenna. The coax shield is grounded through a 
> bulkhead connector fastened to the ground rod. Clip on 
> ferrites both upstream and downstream of this point tamed 
> the common mode problem. 
> 
> If that is true, the Carolina Windom lacks adequate feedline 
> decoupling. That isn't any sudden surprise. Measuring a RW 
> line isolator, you'll see it doesn't even have 10% of the 
> claimed impedance of 50,000 ohms over most of HF. It takes a 
> lot more than a string of beads with a few hundred ohms 
> common mode impedance to decouple an OCF antenna. 
> 
> A voltage balun or any other single core non-unity ratio 
> balun won't work either. It has to be a current balun or 
> common mode choke, and a good one. 
> 
> 73 Tom 
>