A "pulse forming network" is a simple LC lumped approximation to a short
circuited transmission line. A voltage step is applied to it, and after
the round trip time of the "transmission line", the input drops back to
zero. The resulting pulse is applied as the supply voltage to a radar
transmitting tube such as a klystron or magnetron, which produces an RF
output pulse of that width.
Because a PFN limits you to a fixed pulse width, I doubt they're used
much any more -- there are lots of advantages in being able to vary the
pulse width.
I worked on heavy ground radar on the DEW line in the mid '60s, where
this technology was used. On the radar I worked on the most(*), a pulse
of several kV was applied to a PFN by connecting it to a power supply
with a thyratron. The thyratron was extinguished when the trailing edge
of the pulse occurred. The pulse width was a few microseconds. This was
applied to the primary of an oil-filled transformer which the magnatron
plugged directly into. The transformer stepped the pulse voltage up by
ten times and applied it to the cathode of the magnatron, which then
oscillated for the duration. The applied power during the time of the
pulse was over 5 MW, giving 5 MW output power. The pulse repitition rate
was between 2 and 3 ms, so the average power was in the several kW
range, and the range of the radar was several hundred miles. The insides
of the PFN would physically constrict when forming a pulse, so it
produced a fairly loud audible buzz of 400 Hz or so when the radar was
operating.
(*) See
http://www.radarpages.co.uk/mob/rotor/fps6.htm or
http://www.radomes.org/museum/equip/fps-6.html. I don't think the ebay
PFN came from this exact same model of radar, but it's from one of
comparable size and vintage.
Roy Lewallen, W7EL
Paul Burridge wrote:
Hi, fellow radio enthusiasts,
Take a look at this:
http://cgi.ebay.co.uk/ws/eBayISAPI.d...306285 9&rd=1
Stare adoringly at it, then tell me what is meant by a "Pulse Forming
Network" - I'd be interested to know what this design feature is.
Thanks,
p.