[AMPS] Safety RF choke

[John Fielding]

Phil - your best solution is to connect a shorted 1/4 wave coaxial stub across the output
connector.  For 50 MHz the length will be approx. 995mm of RG58/U cable. 

 If you are feeling adventurous you can connect the cable as an open circuit stub across
your RX input and tune in a strong signal.  Take a pair of side cutters and gradually
reduce the length of the cable until the signal drops into the noise.  Oh I forgot to
mention, start with the cable a bit longer, say 1100mm.  When you have the correct length
strip back about 1/4 inch of the braid and centre and solder together.

You now have a shorted 1/4 wave stub which you solder across the RF output connector.  It
is a DC short circuit but an open circuit to 50 MHz RF.  It is also a good harmonic trap,
at least for 2nd and 4th harmonics, no good for 3rd.  100 MHz energy sees it as 1/2 wave
shorted stub which is converted to a short across the connector.  I hope this makes
sense, look up in the ARRL Handbook if you don't follow the maths of stubs.

You can leave the coax straight or coil it up and tie it with cord or insulating tape, it
makes no difference to the performance.  But avoid using a ferrite rod and anything such
like as they will most probably upset the resonant frequency.

Anyway I use one on my 144 MHz amplifier and it protects from shorted DC blockers,
static, mosqitoes and ring worm!

73	John	ZS5JF

----------
> From: Phil Levin <drpal@waterw.com>
> To: amps@contesting.com
> Subject: [AMPS] Safety RF choke
> Date: 12 August 1999 02:37
> 
> 
> While all you experts are discussing the finer points regarding this
> component I have a question....(SB-200 on 50 mhz)
> 
> I would like to make a new RF safety choke.  Does abt 3 feet (roughly
> 1/4 wave on six meters) wound tightly single layer (solenoid style) over
> abt a pencil thick form using approx. 22 gauge wire sound like it would
> work?  Do I check it's resonant freq. with a GDO in free space when it's
> done?  Do I want this safety choke to resonate on 50 mhz???  Do I have
> any idea in the world what I am doing??  For the ongoing saga...stay
> tuned. Hi
> 
> Tu in advance..I really do appreciate this reflector and the
> help/support I've gotten.
> 
> Phil W2GE
> 
> 
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