[TowerTalk] Fwd: Fwd: OCF Dipole Questions

[Hans Hammarquist]

 I wouldn't think about a transformer at the feed-point. The transformer would be some type of monster with a high impedance AND a high current capacity which spells "heavy". As the feed-point frequently is as a high voltage point there will be common mode current on the feed line. It is "just" a matter of accepting it and, maybe place the floating tuner and the choke close to the antenna. 

I can see to put an outdoor tuner on top of the tower and let the OCF dipole start from there. The, now remote controlled" tuner will need some RF isolated control lines as well. I think I will let the radiating feed line be included in the picture.

Hans N2JFS

 

 

-----Original Message-----
From: Jim Brown <jim at audiosystemsgroup.com>
To: towertalk <towertalk at contesting.com>
Sent: Mon, Dec 12, 2016 7:50 am
Subject: Re: [TowerTalk] Fwd: OCF Dipole Questions

Only to the extent that less RF current flows into equipment with Pin 
One Problems. But that feedline still radiates -- the only thing that 
changes is the tuner chassis is now hot with RF, and the tuner modifies 
the current distribution on the line depending on how it's tuned. And 
none of this changes the fact that off-center-fed antennas are 
inherently noisy BECAUSE they are imbalanced, nor does it change the 
fact that it is simply not practical to choke open wire line at the 
feedpoint.

What COULD block feedline current is a real transformer on a low loss 
ferrite core, IF the transformer had very low capacitance between 
windings (because common mode current would couple through interwinding 
capacitance). This means that windings could NOT lay next to each other 
as is often done (bifilar, but connected as a transformer), but would 
have to be on opposite sides of the core. And, of course, to handle 
power, a beefy, low loss core would be needed. Something like Fair-Rite 
#67, which N6RK has used for high power transformers.

73, Jim K9YC

On Fri,12/9/2016 3:46 PM, Hans Hammarquist via TowerTalk wrote:
> One way to get control over the common mode current is to place a good common mode choke between the tuner and the transceiver and let the tuner "float" electrically. Don't ground the tuner and leave it far from any grounded equipment. You don't stop the contribution from the feeder to the radiated feild but you reduce the RF in the shack generated by the common mode current.


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