PSN-L Email List Message

Subject: Re: Ham radio interference
From: "James L. Gundersen" jgundie@.......
Date: Sat, 20 Feb 2010 14:15:15 -0800

On 2/20/2010 9:00 AM, Thomas Dick wrote:
> Anyone got some suggestions; experiencing interference while 
> transmitting ---
>                               my ham radio setup; FT101, FT757 and 
> Icon 207
>                               low band filtering, torroids on in power 
> and grounds -- ground uses 6 ft copper coated rods 
> ...                            only about 20 ft from a lake
> antennas; vertical for 2 meters at 60 feet, ground plane for ten 
> meters at 80 feet, 160 meter, 80-40 inverted V
> my seismic epuipment; two 20 sec Lehmans, AS1 with magnetic induction, 
> Larry Cochrane Amp, AD converter(in metal cases and grounded outside 
> again with a separate 6 rod), 4.5 HZ geophones .. with isolation and 
> battery backup.
> When I transmit on any band. drifting of the base line display occurs 
> and spikes appear. I used to run 400 watts on 29.6 mHz (a repeater) 
> but abandoned that after 25 yrs when I couldn't cure this problem. I 
> recently put up the 80-40 meter antenna and have a 1:1.2 SWR.... still 
> the interference is there! Any suggestions?
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My experience has been interference is usually caused by the interfering 
source getting into the most sensitive part of electronics, typically 
the input of input amplifier.  A voltage difference of ~10 mVrms will 
"overload" the transistors (bipolar technology; MOS tech much less 
susceptible but they have diode protection in the amplifier's front 
ends) and literally be rectified (detected) creating a dc current at the 
RF modulation frequency.  (With AM its an audio frequency inteference 
signal and FM probably a DC when transmitted).  The trick in concept is 
to reduce the interference by adding inductance in series combined with 
capacitance to "ground" to eliminate the interference.

The source impedance of the interference is usually around 50 to 200 
ohms at the susceptible circuit level.  A 1nf will give a shunting 
impedance of 1 ohm at 2M but only 44 ohms at 80M hence you'd like to use 
10nf or more if the circuit's normal performance won't be affected.  
These caps can be differentially across the input or to a "ground" or 
better yet both.  .  I should also note that the "good" ground that is 
often referred to may elusive in some cases.  The ferrite beads in 
series with the source of the interferenc (if you actually know what the 
source path is; usually the input wiring) like a low impedance to ground 
to work into.  Most ferrite beads have too little impedance at 80M so 
you may need more of a ferrite core.  I should also note that one inch 
of wire is about 0.5 ohms at 80M but 22 ohms at 2M in case you don't 
have experience with this problem.

Conceptually you would like to put a metal enclosure around the 
electronics for the seismograph and have "good" filters on all the 
inputs and outputs including power at the walls of the unit.  This will 
"guarantee" no EMI problems.  With this approach you don't need a 
"ground" because the walls become the common reference point and in fact 
should be a good ground.  I mention this because it might not be that 
hard for you to put the circuits inside an enclosure (metal box) and get 
the results you need.  To get a little different perspective of the 
problem your dealing with and it may not be a "single" problem, your 
400W transmitter has about 150 Vrms signal to the antenna and your 
seismograph is sensitive to 10 mV of interference.  The translates into 
needing over a 100db of isolation.  And since you are transmitting it 
means even a small loop of wiring (etch or etc) in the seismograph 
circuitry in the will be coupled into by the transmitted signal.  If you 
have a scope, you can make a loop of wire (or a dipole too) and see what 
magnitude of signal your making in the air.

Interference problems can be very difficult to solve sometimes even for 
"experts".  Your problem can be solved depending on you options.  If you 
continue to have problems we can go into the details of your problem more

Good luck.



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