Roy,
There often arises confusion about Q. In this case, we need to be concerned
about Q of the final circuit and Q of the inductor itself. We usually want
Q of our individual reactive components to be as high as possible
(infinite). When we calculate values for the pi or pi-L network (where the
desired Q is no longer infinite, but more like 10 - 12), the derived
component values assume zero-loss capacitors and inductors. Practically,
inductors with a Q value of 100 to 300 are easily realized. Several
references have indicated that "square" inductors maximize the Q. That is,
length equals diameter. That is why in some amplifier designed with
switched inductors, you'll notice that the turns that are in use on 10
meters are wound with a smaller diameter.
But what if our roller inductor is set for a non-optimum Q position? We may
have a Q of, say 40 or 50 instead of hundreds. This will have a minimal
(although measurable) effect on final Q of the pi or pi-L network. A
network designed for a Q of 12 assuming perfect inductors may end up
measuring "only" 11.8. The effect of this in terms of bandwidth is probably
not noticeable. For practical purposes, the main reason to be concerned
with Q of the actual tank inductor is heating. If the inductor is low Q,
then it therefore will be losing some power, and getting warm as a result.
In a 1.5kW amp, it's easy to exceed solder melting temperatures. And has
been mentioned previously, even if you do see a performance degradation
caused by the Q being too low, you'll tend automatically to retune for a
better setting.
For the majority of hams, as long as the inductors are staying cool enough
to not melt solder (and you should really never depend on solder as the only
mechanical fixing of tank components anyway), and cool enough to not cause
frequency drift, then you're OK on Q.
FF
>From: "Roy Koeppe" <royanjoy@ncn.net>
>To: "Amps Reflector" <amps@contesting.com>
>Subject: [AMPS] Re: Elementary question...(Shame)
To: <amps@contesting.com>
>Date: Tue, 2 Nov 1999 14:18:42 -0600
>
>
>Phil, thanks for the following:
>
>"A roller coil gives the advantage of being able to tune the tank
>for maximum efficiency at any power setting and frees one from many
>hours of tapping a fixed coil in place in a new amp.
>
>It is very simple; you tune the tank circuit for maximum power output.
>With a roller coil, you just have one more item to tune.
>
>Antenna tuners with roller coils work the same way. A tapped coil
>may not be close enough to give a perfect match in every case."
>
>(((73)))
>Phil, K5PC
>
>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
>
>
>
>
>--
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>
>
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