Re: [Amps] Line Isolators for RF feedback

[R.Measures <r@somis.org>]

On Aug 10, 2004, at 12:23 AM, peter.chadwick@Zarlink.Com wrote:

> 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,

Peter - -  I favour using whatever is simplist that does the job.   I 
use ferrites, but I know their limitations and ability to generate 
harmonics when they are saturating.   There will be a ferrite core in 
the input circuitry of the current amplifier that I am presently 
constructing, however, the manufacturer rates the core material at 
50MHz max and I am using it at 21MHz, max.  However, the balanced-L 
antenna tuner that I am constructing to use with this amplifier will 
use an air-core RG-213/u coax-choke ugly-balun wound on plastic pipe.

> 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.

-  Powdered-iron does not have the saturation problem that ferrite has. 
 The trade-off is that iron has a lower Mu than ferrite.
cheerz
> 73
>
> Peter ?temporarily SM/G3RZP

Richard L. Measures, AG6K, 805.386.3734.  www.somis.org

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