[AMPS] Roller Inductors vs. Bandswitches ... which is best?

[Peter Chadwick]

Theo says;

>I have the coil, but will need to locate a switch.

Fair Radio Sales (http://www.fairradio.com/index.html) have ex BC375 switches at
$3.95 or 10 for $32. They are on their 'special offer' page. These are big 1p
6way 30 degree indexing types, and they'll handle lots of power, even though the
Signal Corps only used them in a 100 watt transmitter! I've used them up at 3kV,
and the contacts are good for about 15 amps. There was an article in CQ some 10
years ago about ganging them, but for a Pi-L, it might be easier to use a right
angle drive so the coils can be at right angles. 

If you go for a roller, you'll really need two of them to form a Pi-L to meet
the harmonic requirements - although unless Riley's men inspect you with a
spectrum analyser, no-one on the air will probably ever notice if your harmonics
are 30dB down instead of 40. If you don't use multiband antennas without a
tuner, a pi network is probably OK in reality.

The most satisfactory high power variable L I've come across is the technique
where you wind a silver plated phosphor bronze strip from an insulated former to
a metal drum. It can have the problem of being a bit high in stray capacity at
the top end of the range, though.

My biggest thing against suitable size roller coasters is availability of
suitable mechanical drives for them.

A 6 way switch is OK for 9 bands. If you calculate the Q variation resulting
from using one value of L for each band of 28/24 MHz, 21/18MHz, 14/10MHz, it
comes out  as a 'not to worry' size number. That leaves 3 positions for 7, 3.5
and 1.8 MHz.

With the price of those BC375 switches, you could have 4 of them - one for the
input tuning cap padders for 160 and 80, one for the taps on the pi coil, one
for the padders across the loading cap on 160 and 80, and one for the L network
coil. Under those circumstance, it would probably be worthwhile unhooking the
springs on some of the detent mechanisms to get the torque requirements down. 

Hope this helps.

73

Peter G3RZP


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