Ian White, GM3SEK wrote:
> Roger D Johnson wrote:
>> Tony King - W4ZT wrote:
>>> I don't know if any of you have looked into this particular product but
>>> at first glance it appears that this is one solution for the tuned input
>>> problem many of us face and at the right price too:
>>>
>>> <http://www.ldgelectronics.com/manuals/AT-100AMP%20Manual.pdf>
>>>
>>> 73, Tony W4ZT
>> The purpose of the tuned input is to provide a "flywheel" effect to
>> smooth out the variations of amplifier input impedance over the
>> operating cycle. The Q of the circuit determines how much "flywheel"
>> effect we have. The lowest recommended value I've seen is 2. Solid
>> state transceivers want even more...perhaps 3 to 5.
>>
>> The LDG tuner is an L network in which the Q is dependent on the ratio
>> of impedances to be matched. For normal input impedances, the resulting
>> Q will be very low. It's hard to write the formula in text format but
>> in words it's as follows: divide the higher impedance by the lower,
>> subtract 1 and then take the square root. For example to match an
>> impedance of 100 ohms to 50 ohms, the resulting Q is only 1!
>>
>> With a 3 terminal network you can choose the Q independently of the
>> matching and would seem to be the way to go.
>>
>
> The recommendation for an input tuned circuit Q of 2-4 came from a 1961
> article by Eimac authors in QST. That recommendation was specifically
> based on a pi tank, and it was also pointed out that the output
> capacitor provides a direct shunt path from cathode to ground for
> circulating harmonic currents. But when an L-network is being used in
> impedance step-down mode, it doesn't even have an output C.
>
> Now it may be that the designers know all about these things, have
> checked their effects on amplifier IMD performance, and have found some
> reason to ignore the Eimac recommendations. If that is the case, it
> would be good to hear why.
>
>
>
> 73 from
> Ian GM3SEK
These are very good points that raise questions that must be answered
before I would consider using such a tuner as the input circuit. I'm
with you Ian, I'd like to see the answers to the questions.
It's been said many times that the best input circuit is a pi-network
which will provide the fly-wheel effect to help smooth the dynamic
changes in input impedance. As Skipp025 said, on another list, "The
automated antenna tuners might tend to hunt around looking for the best
match." If that happens, we've just moved the problem from the
transceiver to the amp.
Bill, WA4LAV, has an interesting suggestion about shunting the cathode
with a capacitor and I wonder what everyone thinks of that?
73, Tony W4ZT
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