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Date: Wed, 19 Dec 2012 07:53:43 +1100
Message-ID: <CAKvcm2dAQt2B+43B_7JbYx2bt9SKWm8vZxDf37_3CHDd0o0p0w@mail.gmail.com>
From: Dimitrios Tsifakis <dtsifakis@gmail.com>
To: rsgb_lf_group@blacksheep.org
Subject: Re: LF: Better picture of the 807 based 500kHz TX
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Hi Paul-Henrik,

thanks for the direct link. I am curious to see the pi-tank design.
Once I have decided the power output (will be based on the  HV
transformer I decide to use), I will calculate the matching pi-tank
and then try to make the two capacitances using fixed capacitors.
This, I hope, will result in a fixed, 50 ohm, output. I usually tune
the antenna first using an antenna analyser and only when it is very
close to 50 ohm resistive I connect it to my transmitter, so not
having a plate and load tuning cap will not be a huge disadvantage. I
can always attach a variable cap in parallel to the fixed capacitors
in order to have a bit of adjustment available, but I have a feeling
it won't make a huge difference.

By the way, Owen VK1OD has produced a couple of very handy online
calculators. The first is to calculate properly that initial loadline
- no more guess work! It allows you to select the class of operation
(A, B or C) and will calculate plate current, plate dissipation and
output impedance for a given plate voltage and desired power output:

http://vk1od.net/calc/vlle.htm

Once the output impedance is known, this calculator can be used to
calculate the pi-tank components:

http://vk1od.net/calc/pi.htm

I think they are quite handy as one can experiment with various
configuration scenarios in order to come up with a design that matches
the components available.


By the way, I like the suggestions of Pete M0FMT. Mixing the output of
two xtals in order to get the wanted frequency is an excellent way to
get around the lack of 472 kc xtal problem. Also, grinding a lower
frequency xtal sounds like an interesting solution too! I opened up
one of the FT-241 xtals that was a dud and I looked at the actual
xtal. I think it will be easy to remove the rock and attempt to grind
it. At a prince of US$2.29, one can afford to buy half a dozen and
experiment. After all, it's all about the fun of making a valve
transmitter from scratch - literally!

73, Dimitris VK1SV

2012/12/19  <phl@netikka.fi>:
> LF
>
> The webpage was a bit tricky, here is a direct link to the full size picture
> of the TX:
>
> http://www.cchidgey.pwp.blueyonder.co.uk/images/500KHz%20Transmitter_01.jpg
>
>
> BR
>
> Paul-Henrik, OH1LSQ
>