Hi David,
Thanks for the reply and the great information. I can put together some
specifics and more pictuers of the inside of the deck, and using a copper foil
wouldbe an easy way to test it. The 3cx800 tube height is taller than the
3cx400 so this might play a role. Perhaps the stripline size difference in the
k1fo's amp is due to the cabinet size, he does use a larger rf chamber than
what I have.
Just a quick question, what is a "GDO" you mention near the end of your
message?
Get some rest, 73,
Paul
----- Original Message -----
From: David Kirkby <david.kirkby@onetel.net>
To: Paul Decker <kg7hf@comcast.net>
Cc: amps@contesting.com
Sent: Thu, 05 Jan 2012 21:30:30 -0000 (UTC)
Subject: Re: [Amps] Stripline design
Sorry, I did not finish this before accidently sending.
I'll revisit this later for you, but it would help if you post the
relevant designs. One parameter you have not given is the height above
the chassis for the lines. That's critical, as it sets the impedance.
You can change the line impedance to compensate for the change in
length. A wider line will have capacitance per metre, and a lower
impedance value Zo.
The other issue is that if the line is open-circuited for RF at the
far end, then the formula is slightly different
XL=Zo cot(2 Pi l /lambda)
Using ATLC, you can compute the impedance of the line accurately
http://atlc.sourceforge.net/
you will need to enter the width, height and locations of the enclosure.
My guess you need to sort out if the lines are a half-wave or
whatever, show if they are grounded or open as far as RF is concened,
then set about chaging the impedance of the line to compensate for the
change of length. Lowering the line towards the chassis, or making it
wider will both lower the impedance. Raising the line, or making it
thiner will both increase the impedance of the line. But there's no
simple formula, as the presence of the chassis around this tends to
have a large effect. Hence numerical modelling is one way to do it.
It's impossible to say whether you need to make the line wider or
narrower withoug know if this is operating as a half-wave or
three-quarter wave line. Nine inches sounds too long to be a quarter
wave.
If you can provide some extra information, we can work out what way
you need to change the line.
It might be wisest to make a temporary line out of some self-adhesive
copper tape on carboard, and see if making it wider brings it closer
to resonance or further away using a GDO.
Hopefully that lot is not too garbled,
I must get some sleep here.
Dave, G8WRB
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