Further to comments from Rich and Bill, over here you can get 'microbore'
copper tube for central heating - it's abt 5mm diameter. 3/16 or 1/4 inch
copper tube is ideal for tank coils; at 500 watts, you'd probably be ok with
1/8 inch copper tube. You don't need the copper in the middle, and tube is
easier to use. 5/8 th inch diameter tube is good at 30KW. Industrial metal
suppliers often stock suitable small bore tube in coils of 50 or 100 foot.
For taps on the inductor, connections to the capacitors and switches, I'd
suggest using strips of copper flashing or copper tape, about 3/16 or 1/4
inch wide, wrapped round and preferably silver soldered.
Make sure that you use RF rated capacitors for padding the pi-L network if
you want to avoid drift problems.
Although I guess there may be some argument about this, I'd suggest a pi-L
network on the output rather than a straight pi network - the extra harmonic
attenuation is worth having. A pi network with a Q of 10 and a class AB1 PA
add up to being a bit marginal on meeting the FCC harmonic specs. If you use
a transmatch, no problem, but if you feed straight into a multi-band beam,
you may radiate a good signal on 28 when you're on 14 MHz.
However, all of this is likely to be a bit of overkill if you you don't use
CW, RTTY, speech processing or overs exceeding 30 seconds!!! I personally
prefer overkill on a home brew amplifier, though.
73
Peter G3RZP
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