[AMPS] PIN diodes

[km1h@juno.com]

On Tue, 12 Jan 1999 19:01:11 +0200 "John Fielding"
<johnf@futurenet.co.za> writes:
>
>Pin diodes are great for rf switching at the kW level.  However, they
>have a few shortcomings due to the circuitry you need around them to
>make them work.


I owned a pair of QSK-1500's many years ago. They could not even handle a
2:1 VSWR on 80M at 1200W. Several contesters also bought that model and
blew them up repeatedly at under 1500W. I believe the designer is now
selling antennas that fall down in NE winters.


>
>The biggest problem is the rf chokes which are needed to feed in the 
>dc
>and provide isolation from the rf.  Sod's law predicts that no matter
>which choke you use, somewhere there will be a spurious resonance - a
>bit like the plate choke problem with shunt supplied tubes!!


Since we are talking DC bias would a RF lossy ferrite toroid be an
option? Another thought is that since the Ameritron RCS-4 remote coax
switch works well at the 2KW level with a Pi-wound 1mH choke why not a
PIN switch?  High DC currents?  

There are certainly ample published articles that stress that high power
PIN's are not designed for low frequencies period. Low means under about
10MHz. It took me quite awhile to have a PIN developed that would work
well at RX levels down to 455Khz as a means to dramatically improve
ricebox 2nd and 3rd IMD along with white noise leakage in common TX/RX
paths; however the end results were worth the efffort.

73  Carl  KM1H



>However, with a bit of playing around with the circuit values and 
>types
>of chokes can get you around this problem - with some pretty
>spectacular "fire-works" whilst you experiment!!
>
>The other parameters you mentioned can probably be worked out with 
>help
>from the various handbooks.
>
>Good luck with your design.
>
>John	ZS5JF

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