[ARC5] Making Stable VFO for 2 MHz

[Mike Feher]

I agree with Ken 100%. Those coils were never intended to be used as
frequency determining inductors, especially near 2 MHz. What is the self
resonant frequency? I bet it also varies from unit to unit. This reminds me
of a young engineer with an MSEE who worked for me about 30 years ago. He
needed a BPF in his design project at work so I gave him Saal's book on
normalized filter element values. He managed to get all of the math right,
but, the filter did not work. When he showed it to me I could not believe
it. His air wound inductors were wound as multi layered solenoids with a
very low self resonant frequency due to all of the distributed capacitance.
Once I pointed that out, and he re wound his inductors, everything was fine.
Regards - Mike

Mike B. Feher, N4FS
89 Arnold Blvd.
Howell, NJ, 07731
732-886-5960


-----Original Message-----
From: arc5-bounces at mailman.qth.net [mailto:arc5-bounces at mailman.qth.net] On
Behalf Of Kenneth G. Gordon
Sent: Wednesday, June 29, 2011 7:40 PM
To: arc5 at mailman.qth.net
Subject: Re: [ARC5] Making Stable VFO for 2 MHz

On 29 Jun 2011 at 15:21, J. Forster wrote:

> A Vishay RF choke from Mouser, about the size of a 1 Watt resistor.
> I'm working on the keying at the moment.

I know those things quite well. In my experience, they are VERY 
unstable inductors. They are horribly temperature sensitive and were 
never designed to be part of a frequency-determining network. They 
work OK as an RF choke in certain circuits, but I would never attempt 
to use one in a VFO.

What is the value of the inductance which you need?

Ken
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