[Boatanchors] A "Funky" Low Pass Filter, an analysis.

[Philip B Atchley]

Hi All,
For a couple years now I've had a low pass filter in the junkbox here
that was given to me to use with any Ham transmitter I decided to put on
the air.  I once connected it up to a transceiver, but it seemed to have
a moderate amount of loss on receive so I removed it from the antenna
line.  Never did transmit through it.  GOOD THING!!

It is an Omega Industries Model 101840.  Today I decided to open it up
and see if it had an open connection or something (remove 6 screws).  It
didn't!  I "did" notice that it's built like the proverbial  brick
outhouse with two doorknob capacitors rated at 15KV and a coil that looks
like it's the spring from a Mac Truck (slight exaggeration ;-).  It IS
extremely heavy duty compared to other LPF filters I've opened.  

So, I decided to model it on the (freeware) AADE filter analysis program
(thanks AADE).  Entering the parameters of the filter was fairly simple. 
It's a pi filter with 200pF 15KV capacitors on input/output with the
"truck spring" in between.  The truck spring computed to be .73uH using
another freeware program designed to calculate coils.

SURPRISE!  At 50 Ohms it appears to have a "knee" of about 15MHz, 3dB
loss at 16 MHz and 6dB loss at 20MHz!  

NO WONDER THE RECEIVER DIDN'T LIKE THIS FILTER, especially on the higher
HF bands!

Anybody have any idea what this filter was designed to be used in?  
Apparently something operating below 15MHz.  It is quite heavy duty and
probably good for beau-coup power.  I've decided to rob it of it's
components and save the neat heavy aluminum box with SO-239 connecters on
either end for a future project.  

73 from the "Beaconeers Lair", 
Phil, KO6BB

DX begins at the noise floor!  Merced, Central California

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