David,
You will still need some type of inductor (or maybe a reistor) to blead off any
statics on the antenna (Guess how I know).
Hans - N2JFS
-----Original Message-----
From: David Robbins <k1ttt@arrl.net>
To: 'Hans Hammarquist' <hanslg@aol.com>
Sent: Tue, Jun 21, 2016 1:58 pm
Subject: RE: [TowerTalk] To get a truly balanced antenna feed
How about a toroidal transformer at the feedpoint? NOT bifilar wound, just
two windings acting as an isolation transformer. I say not bifilar because
the added capacitance between the windings would reduce the impedance from
the feedline to the feedpoint allowing more current to flow... but if it is
isolated by a transformer it should help reduce the imbalanced currents.
That being said... any wire or other metal near a transmitting antenna will
have some current induced in it. even in the case above if the feedline
length is 1/2 wavelength at the frequency you are using it will have a fair
amount of induced current, which can then act as a source of interference
and also get conducted into the shack.
David Robbins K1TTT
e-mail: mailto:k1ttt@arrl.net
web: http://wiki.k1ttt.net
AR-Cluster node: 145.69MHz or telnet://k1ttt.net:7373
-----Original Message-----
From: TowerTalk [mailto:towertalk-bounces@contesting.com] On Behalf Of Hans
Hammarquist via TowerTalk
Sent: Tuesday, June 21, 2016 17:39
To: towertalk@contesting.com
Subject: [TowerTalk] To get a truly balanced antenna feed
I changed the topic as I think this is what we reall y at discussin.
-----Original Message-----
From: Jim Brown <jim@audiosystemsgroup.com>
To: towertalk <towertalk@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
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