On 10/9/2021 12:14 PM, CAROL RICHARDS wrote:
I built several 1:1 Current Baluns from the April 2021 issue of QST and they
work fine.
Hi Carol,
This "balun" is really a common mode choke, and while it adds inductance
to the common mode circuit, that circuit includes the feedline, and in
the common mode circuit, it becomes part of the antenna. By virtue of
its electrical length, an antenna (the feedline) can be inductive,
capacitive, or resistive (at resonance). At frequencies where the
feedline is capacitive, it cancels the some or all of the inductive
reactance of choke (balun) you've added, reducing or defeating its
effectiveness.
The most important function of a common mode choke at the feedpoint is
to prevent noise picked up on the feedline from coupling to the antenna.
For this to be effective, a very large resistive impedance is required.
And because they're resistive, they dissipate power; if the resistance
is not large enough, and if the choke is not exposed to air, it can fry.
Bottom line -- while that design may not fail due to overheating on most
bands (it can on higher bands where the choke resonates but the
resistance at resonance is too low), it's also unlikely to be
universally effective at reducing noise on receive.
I've been studying all of this since 2003, and published what I believe
are far superior designs. http://k9yc.com/2018Cookbook.pdf
This is a tutorial on how chokes work, and a lot more.
k9yc.com/RFI-ham.pdf
73, Jim K9YC
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|