In article ,
Reg Edwards g4fgq,regp@ZZZbtinternet,com wrote:

The whole purpose of the choke balun is to allow the ordinary tuner to
be safely grounded.

The tuner is grounded automatically by virtue of its short connection
to the unbalanced transmitter.

Or have I misunderstood you?

It depends. There are two arrangements using choke baluns that I'm
aware of - balun at the input, and balun at the output.

It's fairly common to put a choke balun at the output of an unbalanced
tuner, to convert the unbalanced output to a balanced form. This
seems to work pretty well when the tuner is looking into loads that
aren't too high, low, or reactive in nature. For high-R or
highly-reactive loads, it may be difficult to create a choke balun
with a high enough choking reactivity to force adequate balance on the
line. In this approach, the tuner can be connected to the transceiver
via a direct cable, and its chassis can be connected to the station RF
ground.

The other approach is to put a choke balun at the input of the tuner,
"float" the tuner chassis clear of ground, and connect the balanced
line directly to the "unbalanced" output of the tuner. This
pseudo-balanced arrangment can provide good balance once the tuner is
adjusted, because the balun "sees" 50 ohms on both sides... but it
adds the complication of having to float and insulate the tuner. I've
heard of some people taking a commercial tuner, and putting it on
insulators inside a secondary outer chassis. The outer chassis can be
connected to the station ground, and or to the transceiver coaxial
feedline braid _before_ going through the choke balun.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

Dave Platt wrote:
In article ,
Reg Edwards g4fgq,regp@ZZZbtinternet,com wrote:


The whole purpose of the choke balun is to allow the ordinary tuner to
be safely grounded.

The tuner is grounded automatically by virtue of its short connection
to the unbalanced transmitter.

Or have I misunderstood you?


It depends. There are two arrangements using choke baluns that I'm
aware of - balun at the input, and balun at the output.

It's fairly common to put a choke balun at the output of an unbalanced
tuner, to convert the unbalanced output to a balanced form. This
seems to work pretty well when the tuner is looking into loads that
aren't too high, low, or reactive in nature. For high-R or
highly-reactive loads, it may be difficult to create a choke balun
with a high enough choking reactivity to force adequate balance on the
line. In this approach, the tuner can be connected to the transceiver
via a direct cable, and its chassis can be connected to the station RF
ground.

The other approach is to put a choke balun at the input of the tuner,
"float" the tuner chassis clear of ground, and connect the balanced
line directly to the "unbalanced" output of the tuner. This
pseudo-balanced arrangment can provide good balance once the tuner is
adjusted, because the balun "sees" 50 ohms on both sides... but it
adds the complication of having to float and insulate the tuner. I've
heard of some people taking a commercial tuner, and putting it on
insulators inside a secondary outer chassis. The outer chassis can be
connected to the station ground, and or to the transceiver coaxial
feedline braid _before_ going through the choke balun.

Hi Dave and all,

For Reg. I've used the method that Dave speaks of above over the years
with very good Sucess but as he says the tuner must be isolated from
ground in order to produce the needed balance at the out put. The
advatage as stated is that the Balun (once the turner is tuned to the
frequency being used will see pretty much 50 ohms on both sides. It has
work very well here .

73 Dave Kc1di

Dave Platt wrote:
The other approach is to put a choke balun at the input of the tuner,
"float" the tuner chassis clear of ground, and connect the balanced
line directly to the "unbalanced" output of the tuner. This
pseudo-balanced arrangment can provide good balance once the tuner is
adjusted, because the balun "sees" 50 ohms on both sides... but it
adds the complication of having to float and insulate the tuner.

There's considerable debate on that configuration. If we assume
the common-mode current doesn't change when we move the choke
from output to input, i.e. it is a systematic problem, then the
choke is exposed to exactly the same common-mode current on output
and input. Common mode current travels on the tuner chassis and the
coax shield input and coax shield output may be one inch from each
other with a dead short between them.

And here's something that no one, to the best of my knowledge,
has mentioned. If one succeeds in balancing the currents at
the input of an unbalanced antenna tuner, the currents at the
output will automatically be unbalanced because the current on
the coax center wire will suffer a greater phase delay than
the current on the coax inner braid thus forcing some current
to the outside of the coax. This effect should be easy to measure.

Link coupling solves the common-mode problem. Unfortunately, the
MFJ balanced tuners do not relieve the stress on the choke. Take
a look at the schematic to see why.
--
73, Cecil http://www.qsl.net/w5dxp

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