[AMRadio] Home-brewing construction considerations

John Coleman jc at pctechref.com
Thu Jan 17 16:06:27 EST 2008

Larry you are correct about the theory and if two capacitors are used they
must be balanced also.  I have generally found that balance is not always
what it seems and if secondary single or double fixed capacitor can be
avoided then that is best but it sometimes is not avoidable. And stray
capacitance from a bad layout can really through you off.  It is just one of
those things that has to be dealt with on a individual basis.

John, WA5BXO  

-----Original Message-----
From: amradio-bounces at mailman.qth.net
[mailto:amradio-bounces at mailman.qth.net] On Behalf Of ne1s at neandertech.com
Sent: Thursday, January 17, 2008 1:41 PM
To: Discussion of AM Radio in the Amateur Service
Subject: Re: [AMRadio] Home-brewing construction considerations


1) A single capacitor plate-to-plate wound probably not upset the balance
appreciably - in fact if the cap, and connections to it, were totally
symmetric it wouldn't at all. The fixed vacuum caps I've seen have an
outside and an inside cylinder, so stray capacitance to the surroundings
from the outside cylinder would probably be larger than the strays to the
inside cylinder. However, as long as the cap was reasonably spaced from
surrounding stuff, I don't think it would be of any practical consequence
on the lower bands where you'd be using the auxiliary fixed cap anyway.

2)If you were to use two caps, with a common connection tied to the rotor
of the butterfly cap (you said stator - did you mean rotor?), their values
would need to be double (not half) the value of a single cap, because they
are effectively in series.

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