[TowerTalk] To get a truly balanced antenna feed

[Hans Hammarquist]

I changed the topic as I think this is what we reall y at discussin.



-----Original Message-----
From: Jim Brown <jim at audiosystemsgroup.com>
To: towertalk <towertalk at contesting.com>
Sent: Mon, Jun 20, 2016 11:14 pm
Subject: Re: [TowerTalk] Fwd: Fwd: Fwd: OWAs or Fans?

On Mon,6/20/2016 5:32 PM, Hans Hammarquist via TowerTalk wrote:

>
>> 1) My antenna tuner is isolated from ground. 2) the coax from the tuner to the radio is passing a choke. This render a balanced feed to the antenna feeder. I hope that clears that up a little. The technique was presented at a seminar at Boxboro some years ago.

>Don't believe everything you hear or read. Always question the 
qualifications and education of who's talking/writing.


It makes perfectly sense to me, if the tuner is not connected to ground or sees ground through some high impedance path, the output should act very balanced. Optimal, of cause, is if there is a coil inside the tuner, with very low capacitance to round, picking up the magnetic field from a tuned coil. That would give you a nice balanced output. That would give very little common mode current in the feeder (at the feeding point).


The whole discussion comes down to how good my choking coil is and how well the tuner is located not to have a high capacitive or inductive coupling to ground. The tuner is located on a shelf several inches away from walls floor etc. The choking coil is located just underneath the tuner.


I think I need to describe that coil some. (I will measure the impedance of the coil in a few days. (Am away from QTH just now.) The 50 feet of "RG58" type is wound on a 4" schedule 40 PVC pipe. It is tight wounded in one end with a gradually increase in the pitch towards the other end. The idea is to get resonances with the stray capacitance between the turns at as many frequencies as possible. (If I remember it right) the impedance stayed rather high for frequencies between about 6 and 30 MHz. (I will check and confirm this later.) With high I mean >1000 ohms.


>> 4) The feed line is connected to the ground isolated antenna tuner. The isolation contribute to equal, balanced current in the feeder.

>That's Alice In Wonderland.  Balance is determined by the impedances in 
the circuit -- ALL of the impedances. The antenna is unbalanced, so 
current on the feedline will be unbalanced.


Yes, the antenna is unbalanced but the isolated tuner forces the current to be balanced at the transmitter side. The unbalanced antenna gives a common mode current where the feed line is attached to the antenna and results in a common mode current going towards the transmitter. This current has to have a current minimum at the transmitter, and this is not "Alice in Wonderland".


The question is how to minimize this current by putting something at the feedpoint of the antenna.


>> The tuner is, in reality unbalanced but as it doesn't have a return path to ground the high Z is by all means balanced.

>A coil of coax does NOT provide a high impedance to ground. It is an 
inductor, with stray capacitance.

>> My question and interest is to get a CM choke next to the antenna so I can reduce the radiation from the feeder.

>As I've repeated several times, it is not practical to choke the antenna 
you have described. That's why no one who knows how chokes work has made 
a suggestion.

>73, Jim K9YC


So Jim, what would you suggest "balancing" the antenna system? The idea is to reduce (or eliminating) the common mode current on the open wire feeder. The Windom is by nature unbalanced and anything attached to it will have some current induced regardless how well you "isolate" it. 


The original question was if the was some type of common mode choke to put at the antenna feed point that could do the trick. A tuner that is balanced by providing equal but opposite voltages at the output gives a balanced output voltage but does nothing (or very little) to reduce the CM current.


With best 73 de,


Hans - N2JFS