On 11/14/2010 12:15 PM, Don Jones wrote:
> I think a "very fine hair" is being split here. When this "hair" is measured
> in dB units instead of linear units the loses become insignificant. While
> yes the old balanced match boxes are technically superior to the
> differential mode match boxes common to the ham radio market today. I always
> weigh discussions like this in terms of linear units vs. dB units.
>
> In linear units the losses always appear larger than what they are in dB
> units. As a EMI engineer by profession I do not worry as much about the
> linear unit based losses until they become significant losses when measured
> in dB units.
But some tuners have shown less than 10% efficiency at the tuning
extremes, usually doing a large impedance transformation at 160 meters.
That's 10 dB loss, and even at 100 watts it leads to smoked tuners.
>
> If you are talking about a 1/2 MW VOA transmitter - then yes you have to
> address those loses, if you are talking about a ham running 900W to a zepp
> antenna the losses are not as significant.
>
> FWIW because of those "loses" the 4:1 balun I run is made by DX engineering,
> it is rated for 160 - 10 meters @ 10kW. I chose this balun in order to avoid
> coil saturation. The balun was spendy, but then in dB dollars I am ahead of
> the game when compared to the purchase price of a kW rated balanced Johnson
> matchbox which I think stops at 80 meters. Not sure if those old tuners are
> designed for 160.
The differential capacitor variable impedance tap used in the Match Box
forces a large tuning C for 10m and low for 80 or 160. That means large
circulating currents which increase resistive losses in the components
on 10m and a reduces matching range on the lowest band.
The problems with a balun on the antenna side of a tuner is that its not
always seeing a 200 ohm load, sometimes its over 1000 ohms, sometimes
its under 5 ohms and those often run the balun beyond its high power
rating from high voltage with the high impedance load that increases
core losses or high current with the low impedance load that increases
conductor losses to the smoke point. And at the frequency range limits,
the balun contributes considerable reactance in parallel with the
transmission line.
The Johnson Match Box was advertised to cover 3.5 to 30 MHz.
>
> Just my two bits. Peace,
>
> Don Jones KO7i
> Arlington, WA
>
>
73, Jerry, K0CQ
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