[Amps] Line Isolators for RF feedback

[Harold B. Mandel]

Dear Peter,


Some fourteen years ago, Rich Measures wrote an article that appeared in
QST magazine called A BALANCED BALANCED ANTENNA TUNER.

This design firstly moves the balun to the input of the impedance
transformer
network and then incorporates two separate but symmetrical variable
inductors
to operate on both halves of the open-wire feed line arrangement.

In applying Mr. Measures' design to my shack the first problem
encountered
was the QRO-O and the fact that mostly RTTY is favored (with attendant
long-winded periods of transmission).  The balun coil to provide the
balanced 50-ohm feed to the impedance transformer needed to be made of
10AWG silver-teflon, closely wound on a PVC core.

Secondly, the original antenna design was an Off-Center-Feed 188 meter
dipole with the feedpoint at the 62 percent point, which provided 184
feet
and 414 feet (roughly). This arrangement worked, albeit inefficiently,
with
a NYE VIKING MB-V-A that uses a double-core output balun to provide
a semblance of a balanced output, but the power running through it
quickly
saturated the cores, heated things up and made for squirrely tuning,
especially
in the 80-meter RTTY band.

The solution was to re-cut the dipole in order to make both halves
symmetrical
so that the dual-inductor ATU would feed equal currents through the
network.

The 56uH roller inductors with the 1,000pF vacuum variable shunting the
balanced
outputs then provided a sure means to finding the sweet spot for a dipole
presenting
a high impedance to the output of the tuner.

Respectfully,

Hal Mandel
KA1XO  
On Tue, 10 Aug 2004 09:23:21 +0200 peter.chadwick at Zarlink.Com writes:
> I found that the measured common mode impedance ona supposedly 
> identical 
> arrangement to the W2AU balun gave rather less common mode 
> suppression. 
> However, the losses about 5%, or about 0.22dB. That still represents 
> 50 
> watts at 1kW however, and the beads at the balanced end did get 
> warmer 
> than the ones at the unbalanced end. Overall, the temperature rise 
> wasn't 
> that excessive - after 5 minutes, you could still comfortably hold 
> the 
> beads. Of course with 75 of them, that's less than a watt each on 
> average.
> 
> Now that is in a matched system. In an arrangement with a high SWR, 
> you 
> can get a very large 'common mode' voltage across the balun, with 
> resulting currents to be suppressed that can cause problems. For 
> this 
> reasons, I'm very leery about baluns with high SWR. Far better to 
> have a 
> proper balanced tuner.
> 
> The balanced L network that Rich favours can, for some antennas, 
> give 
> wider bandwidth without retuning than the classic parallel or series 
> tuned 
> circuit. Feeding my 80m dipole on 40 (the feeder is 64 feet of open 
> wire 
> line), the balun feeding a balanced L network has lower working Q 
> than a 
> parallel tuned circuit, while on 80, the tuned circuit is better.
> 
> Not,Rich, that you'd approve - the balun feeding the L network is a 
> 
> ferrite bead balun, and the L network inductors are wound on 
> powdered iron 
> toroids  3 inch o.d., 2 inch i.d. and 1 inch thick!  I did do the 
> sums 
> though to check that the flux density would be low enough to avoid 
> problems.
> 
> 73
> 
> Peter  temporarily SM/G3RZP
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