Hi Frank,
I just don´t see a reason to make chokes self resonant.
If one really needs that a parallel resonant LC circuit can be formed with a
normal choke and a capacitor.
On my 160m dipole the choke at the feed point is wound with RG 141 on a
stack of two large ferrite toroids which gives me a Z of about 10k.
In addition the outer conductor is connected to the tower after a quarter
wave length and the coax run inside the tower and grounded to one leg.
The same tower is direct fed as an elevated GP at the 25m level against two
sloping radials with the outer connector bonded to the tower.
I never saw any common mode current when the common mode chokes XL was at
least 100 times the coax Z.
73
Peter
-----Original Message-----
From: donovanf@starpower.net [mailto:donovanf@starpower.net]
Sent: Freitag, 30. März 2018 19:30
To: topband@contesting.com
Subject: Re: Topband: 160M Balun
Hi Peter,
What you need in a common mode choke is adequate resistive
component to absorb common mode signals as heat. The choke
doesn't need to be at resonance to achieve adequate choking
resistance, but it doesn't hurt either. It takes some thought
and technical understanding to determine how much common
suppression is enough. And common mode chokes aren't the
only way to adequately suppress common mode signals.
A low impedance radial system provides good common mode
suppression. Grounding the coax feed line to a center-fed dipole
1/4 wavelength or 3/4 wavelengths from the dipole feed point also
provides good common mode suppression, certainly more than
you ever need for a dipole. This assumes that the feed line runs
at approximately 90 degrees from the dipole. Pulling the feed line
significantly away from 90 degrees significantly increases common
mode. An off-center-fed dipole is much worse because its very
difficult to build a choke with adequate power handling capability
and good choking performance.
You can't understand your needed common mode signal suppression
without understanding your signal environment and the directive
performance you want to achieve from your antenna.
For example:
A multi-transmitter or SO2R station needs much more attention to
common mode suppression than a single operator low power station
A highly directive multi-element Yagi needs much more common
mode suppression than a dipole or a vertical.
Coax feed lines properly installed and bonded to a tower has less
common mode signals than a coax feed line suspended well above
the ground.
Perhaps the worst case need for common mode suppression is
directive receiving antennas with very low signal levels such a small
terminated loop (K9AY, Flag, etc) or a small "magnetic" loop. Both
require extreme attention to common mode signal suppression,
including a strong preference for buried feed lines, control lines
and power cables.
73
Frank
W3LPL
________________________________
From: "Peter Voelpel" <dj7ww@t-online.de>
To: "Jim Thomson" <jim.thom@telus.net>, topband@contesting.com
Sent: Friday, March 30, 2018 4:47:44 PM
Subject: Re: Topband: 160M Balun
Why do you want it resonant?
73
Peter
-----Original Message-----
From: Topband [mailto:topband-bounces@contesting.com] On Behalf Of Jim
Thomson
The 160m CM choke needs to resonate lower in freq. It requires more cores
to shift the res freq down.
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