Roger,
Both you and Rick understand things as I do. Rick is simply speaking
figuratively, using exactly the same words I have. That is, an effective
choke nearly eliminates the common mode connection between the feedline
and the antenna. But no choke is perfect, so there will be some small
current if there is imbalance in the system.
73, Jim K9YC
On Mon,7/4/2016 11:40 PM, Roger (K8RI) on TT wrote:
It's my understanding that a choke of (theoretical) infinite Z does
not disconnect the antenna. It would reduce the current flowing on the
outside of the coax (common mode) to zero at that point. It has no
effect on the interior currents
The choke does nothing to the balance of the antenna. It just stops
(more correctly reduces) the CM current caused by a reasonable
imbalance such as a sloping dipole, or that caused by nearby objects.
The greater the imbalance the greater the power dissipated in the core
material. The difference between sloping, center fed dipoles on 80 Vs
40 is very pronounced
Different core materials will likely have different dissipation values.
OTOH it does not prevent that same exterior shield from picking up
signals if long enough, or of the right (or wrong) length. Then a
second and possibly third choke of smaller values might be advisable
making the shield appear as if it were broken up with insulators.
Unless referring to a specific antenna, I'd not expect Z to be
relatively important. For a specific type of antenna Z & R vary with
height, particularly on 160 and 75. Sometimes by large amounts.
73
Roger (K8RI)
On 7/4/2016 Monday 2:08 AM, Jim Thomson wrote:
Date: Sun, 3 Jul 2016 11:23:10 -0700
From: "Richard (Rick) Karlquist" <richard@karlquist.com>
To: Steve Hunt <steve@karinya.net>, Reflector -tower
<towertalk@contesting.com>
Subject: Re: [TowerTalk] Fair rite materials for choke baluns
On 7/3/2016 10:05 AM, Steve Hunt wrote:
Rudy,
http://www.karinya.net/g3txq/temp/52_mix.png
It achieves Rs>5000 Ohms from 10Mhz thru 28MHz, and Rs>8000 Ohms from
12MHz thru 21MHz.
Steve G3TXQ
It is more useful to think about Rp than Rs. Rp determines the
power dissipation for a given voltage. Although Z is a figure
of merit for how well the antenna works, Rp is a figure of merit
for how much power can be handled. On the G3TXQ curves, note
that where Z=Rs, it is also true that Z=Rp. Away from that
frequency, the general trend is that Rp monotonically increases
with frequency, at least up to resonance.
It's nice to brag about extremely high Z values, but even if
Z=infinity, all you have accomplished is that you have
disconnected the feedline from the antenna. It doesn't
guarantee there will be no feedline currents. Also, it merely
PERMITS the antenna to be balanced, if there is no external
conductor otherwise perturbing it. It does not guarantee
balance.
Rick N6RK
## Ok, Im lost. I thought we wanted Z > RS > XS .
Put another way, RS ideally should be the same as Z... or as close as
you can get to Z. And RS should definitely be > than XS.
What is RP ??
Jim VE7RF
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