PA5Y,
In my opinion, either material would work fine for your amplifier.
Keeping interturn capacitance low isn't a big issue with so few turns
for a 50 Mhz PA. With the copper tubing, you can always flatten the end
and then torch solder a flat bar at the end to hold parallel capacitors.
The flat material might take more space to wind a coil edge to edge. In
the 1980s, I used to work with a device called a Tennsmith metal roller,
which even put a crimp in flat stock to make a more rigid coil of metal.
For RF choke, you should measure its impedance if you have a device to
do that, and try to design so that it has a parallel self-resonance near
and above your operating frequency. You will work it on the low side of
its resonance, which is the safest. As it gets mounted in the circuit,
this may change, so measure it in the box, but with the hot end
disconnected from the tubes. You can also use a grid dip meter and the
first sharp resonance will be the one representing parallel resonance.
73
John
K5PRO
Message: 1
Date: Thu, 31 Aug 2017 22:48:37 +0200
From: Conrad PA5Y <g0ruz@g0ruz.com>
To: amps@contesting.com
Hello again, I have gathered almost everything that I need for my
3CX800A7 conversion from 2m to 6m.
I have choice of materials for L1.
I have some 6mm soft copper pipe, some 4mm soft copper pipe and some 3mm
silver plated copper wire. All the tubing is C102 copper and forms very
nicely. VE7RF suggested that I use some flat strap and make L1 a flat
wound coil to reduce capacitance between turns, it is also very easy to
drill for blocking caps.? This makes sense to me.
The strap that I have is 17mm wide and 1.5mm thick but I have some
concerns. It was used as bus bar material and is much harder than the
C102 copper tube that I have. It is also considerably darker in colour.
It forms into coils quite easily and is not particularly springy so I am
fairly sure that it is not Beryllium copper. I suspect that it has been
annealed. I am worried that the resistivity will be high but surely as
it was used for busbars it won't be lossy, will it?
Soon I will have a DER EE DE-5000 LCR meter which I am told are very
good value for money and I can measure component Q so I should see the
difference between the flat bar and the copper tube, if there is a
problem. I know for sure that the copper tube is C102 soft copper. I
would be interested in hearing from anyone who has this instrument and
what they think of it.
Any advice? In the past I have made a number of QRO VHF amps for 6m and
without exception the efficiency has been good. This is the first one
that I have actually designed myself and I expect the loaded Q to be
10-12, previous amplifiers had big tubes, lots of stray C and had a
higher Q and were mainly copies or adaptations of published designs.
This time I want to design and truly understand what I am doing.
I am still somewhat mystified by Anode RFCs for VHF amplifiers, I can
plug numbers into the GM3SEK spread sheet but I do not know exactly how
to design a good choke. Most of the stuff that I found on google was
concerned with multi-band HF amplifiers and the perils of series
resonance. What are the rules of thumb. I can do the calculations but
what are my goals?
Hopefully this will give the reflector a kick and start some interesting
discussion.
Regards
Conrad PA5Y
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