Message-ID: <19980915224019.21138.qmail@www07.netaddress.usa.net>
Date: Tue, 15 Sep 1998 22:40:19
From: Dan Schaefer <dfschaefer@usa.net>
To: lancair.list@olsusa.com
Subject: coax leakage, grape juice, noise filters for MT prop
Mime-Version: 1.0

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I had several further thoughts to add to yesterday's note but ran out
of time - so , as I was saying......

To John Cooper re: the noise filter inductor for the MT prop control
unit. Caution is advised if, as you surmised, the box does have a 
switching converter power supply of some sort. Certain types of con-
verter topographies will come unhinged with just a little too much 
inductance in series with the power input. Remember, the guy that 
designed this thing was figuring that the input (power) was going to
be connected directly to the battery fed (low impedance) bus. I've 
made a living in the past designing these kind of circuits and all I
can say is: Be careful. If a problem should develop, that large 
electrolytic capacitor you tried by itself would probably help as 
long as it is placed on the LOAD side of the choke. Also, by the way,
the ferrite is probably the best choice over the tape core as the DC
in the windings (depending on the no. of ampere-turns) would likely
saturate the core and render the whole thing to little more than an
air core inductor. The ferrite has an inherent air gap so it won't
saturate.

John, you mention the leakage of radio frequency (RF) energy from the
coax cables and I got the impression that it was somehow being related
to the quality of the bulk cable. This may be where the RF is leaking
into the other circuits but it's highly unlikely that the cable is the
root cause. If the cable and connectors are all fairly good, i.e., not
kinked, squashed or broken, etc., and the antenna is reasonably well
designed, not much RF will get away. However, the biggest culprit in
this area is almost always the antenna. If your antenna is sorta 
typical, it won't match the cable characteristics perfectly, which
will result in a percentage of the energy your transmitter squirts at
it when you hit the "Transmit" button reflecting right back up cable.
(If interested, check out the definition and meaning of VSWR). This
causes the largest part of the RF "leaks" off the cable. Use antennas
with measured VSWR of 2:1 or better (a good avionics shop should be 
able to do the measurement) and gauges shouldn't hear a thing.

To Scott Dahlgren. The next time I take this mini-bladder flying, I'm
going to try your grape juice idea. Be nice if it works. I go trout
fishing every year in eastern Idaho from here in SoCal and I can tell
you that there's not many places to land for a pee break over central
Nevada. Even with carefully limiting my coffee intake in the morning
to one cup (safety dictates at least one), after a little over three 
hours in the saddle, I NEED to stop! And I can tell you, the owner of
the FBO up there at Elko doesn't appreciate Lancairs taxiing to the 
building where the lavatories are at 100 MPH! Hope the grape juice is
the answer. And, oh yes - I do have a fishing pole (fly rod) carrying
tube built into the tail cone of N235SP.

Dan Schaefer


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