At 02:18 PM 11/2/1999 -0600, you wrote:
>BUT,
>
>Couldn't (wouldn't) maximum output likely correspond to a rather low-Q
>setting of L1, rather than the usual 10 or 12? With a high L to C ratio
>in the pi-net, efficiency is improved at the expense of harmonic
>attenuation (low, loaded tank Q).
>
>So, it seems to me, just tuning all three components for maximum, full
>bore output, could end a person up with an undesireable loaded Q tank
>situation.
>
>Other comments please. (My danged ISP dumped all my e-mail, so I had to
>retrieve the responses from the archives after I realized what had
>happened--and the ISP admitted to it--sorry for the delay).
>
>73, Roy K6XK Iowa
>
>
Roy, certainly everything you mentioned above is absolutely possible and
true. But the point of using a variable inductor is to allow maintaining
the Q of the network as you tune around a band. With a fixed (ie. tapped
inductor), the design Q only occurs at or near the frequency you designed
the inductor tap with. As you move away from that design frequency, you
can adjust the C1 & C2 caps but not the inductance in a conventional tapped
inductor. This causes the Q to move around. You should first consider
whether this is even worth worrying about.
The plan for a roller inductor would be this : you would either find the
preliminary settings as pointed out by the other Phil, or with more
conventional homebrew methods such as noise bridges, calculations,
inductance meters etc. This is what you would do anyway if you were using
a tapped inductor. Then, in actual operation, you would 'touch-up' the
inductance from that point as you move around the band. By preventing the Q
from going too high, you maintain efficiency(ie. you avoid excessive tank
circuit losses associated with higher Q's). By preventing it from going
too low, you ensure maintaining a certain minimum of harmonic supression.
The other advantage of a variable inductance is that it is possible to
obtain a match into a wider range of load impedances than you otherwise
could with a tapped inductor. Note however that there were several posts
on the reflector early this year about the woes of roller inductors in this
application. Several examples of burnt roller contacts were cited.
Apparently, antenna tuner caliber roller coils don't cut it this
application over the long haul. The best defence for this is an expensive,
high quality roller coil such as the ones currently being sold at Surplus
Sales of Nebraska. If the price seems high, consider the cost of a truly
capable band switch - its not as much as the coil, but its a good part of
the way there.
Phil T.
>
>
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>
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