Re open-wire fed antennas:
If the antenna is well-balanced and fed via a true balanced antenna
tuner (preferably link-coupled) then there shouldn't be a problem with
common mode currents.
Yes, you have to worry about nearby objects unbalancing the antenna,
which is less of a problem with a choked coax-fed dipole. But being able
to use a very simple single antenna from (for example) 7 to 28 mHz. with
relatively good efficiency is advantageous.
My experiments with baluns seem to indicate that just using a current
balun to go from an unbalanced transmitter or tuner to a balanced line
works poorly unless you cancel out the reactance with a balanced network
on the antenna side of the balun. All these tuners with "balanced"
outputs provided by a balun (often a voltage balun) do not work well at all.
My best result with balanced lines has been with the Johnson Matchbox.
It's a shame that there doesn't seem to be an equivalent available today
(and it wouldn't be easy to procure the parts to build one).
73,
Victor, 4X6GP
Rehovot, Israel
Formerly K2VCO
CWops no. 5
http://www.qsl.net/k2vco/
On 16/07/2020 6:26, Jim Brown wrote:
On 7/15/2020 1:36 PM, CUTTER DAVID wrote:
It's all about size. Bigger core helps,
Dave,
No, it is NOT about size. It is about design of the entire antenna
system, including the antenna, the feedline, and other parts needed to
make the SYSTEM work. The principal characteristic of a common mode
choke is the resistive component of its common mode impedance at the
operating frequency(ies) where it will be used. Further, dissipation in
the choke occurs at least as much in the WIRE that is wound around the
core as in the core itself.
There is another fundamental error in many antenna systems that ONLY
looks at matching to the transmitter at the transmitter, ignoring the
match between the antenna and the transmission line, using high
impedance, parallel wire line, and using a random center-fed or
off-center fed horizontal wire on all bands. Yes, the transmitter can be
made to supply power to the feedline, yes, it will get to the antenna,
and yes, it will radiate. But it may not receive all that well due to
common mode current on the line from noise sources in our own homes and
those of our neighbors. THAT is the problem with using a decades-old
design for a world where there was 20 dB less noise than most of us face
today.
so a core that is OK for ssb
and cw might be undersized for AM or some data modes. Just like linear
amplifiers.
So it is NOT the size of the core, it's the design of the antenna
system. HFTA author and retired ARRL Antenna Book and Handbook editor
Dean Straw, N6BV, published an excellent piece in QST 6-8 years ago
called "Don't Blow Up Your Balun," in which he pointed out the
differential mode dissipation in chokes, which can be extremely high if
the choke is at a very high current point in a mismatched line. When he
passed it to me for review, I noted that these losses were in addition
to the common mode dissipation, and he revised the piece to reflect that.
you can't
label something poorly designed because it doesn't pass the BOK test.
BOK?
73, Jim K9YC
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