PSN-L Email List Message

Subject: Re: update on radio interference
From: ChrisAtUpw@.......
Date: Wed, 9 Aug 2000 15:27:43 EDT

John.Tacinelli@........ writes:

> A little more progress has been made on my radio problem. I have discovered 
> that the radio tower is less than 2 miles from here.  

    OK. We know that 'good ole timers' KOLM on 1.52 MHz is supposed to 
radiate 10 KW. If the maximum field in V / m is E = (7.02 x sqrt(Power))/ d, 
where d is the distance in metres, assuming that the radiation is isotropic. 
1 mile ~ 1.6 Km, so lets estimate d at 3000 m. This gives E = 0.23 V / m. If 
the aerials are designed to keep the radiation low, E could be twice this, 
but it still = high field problem = trouble. 

    1 Arrange the apparatus to be reasonably compact and reduce the cable 
lengths as convenient. 
    2 Connect the various 'boxes' and the frame with heavy 3/8" wide braided 
copper 'strapping', particularly to the input connection of the preamp.
    It is unlikely that either will remove the problems completely, but they 
should help and you may not be able to solve the problem without doing this.

> Also I hooked up a 
> small speaker to the amplifier and I can hear the radio station on the 
> seismograph.  I tried unplugging the coil and I could still hear it 
> it was much reduced.  When the pre-amp was unplugged I couldn't hear it at 
> all.  So it looks like the coil is acting as an antenna for the pre-amp.  
> cable from pre-amp to amp may also be doing so but to a lesser degree (it 
> shielded).  

    Great! The next thing to try is to disconnect the coil and it's cable and 
short out the input where it comes into the preamp and see what effect this 
has. Check that the 'low' side of the input connector is earthed at the 
input. If this doesn't effectively silence the output, you may have to put RF 
chokes on the power supply lines inside the preamp box. You may need to 
solder a thick copper wire between the input connector and the output earth 
return - even wide copper track on a PCB may not have a sufficiently low 
resistance. This is a common 'fix' in audio amplifiers. Also, connect up the 
input cable on it's own with the end shorted and move the end around in a 
circle. There may be a direction which gives ~zero pickup.
    If you get ~zero interference with the input shorted, an input filter 
will solve the problem. A word of warning about decoupling capacitors. Every 
capacitor has a tiny bit of inductance and it can resonate. You need to keep 
well away from these frequencies. A 1nF disc ceramic with 1/2" leads 
resonates at about 40 MHz. A large 10 nF disc resonates at about 14 MHz. I 
don't have figures for the multi layer ceramic caps, but they are available. 
Radio engineers may use several small capacitors in parallel to get a low 
impedance at a high frequency.

>  I called the electronics company and they are working on it. They seemed 
> doubt that shielding it would work since AM is long wavelength. 

    I don't see why not.
>  While I was listening to it, I made some observations.  When I touched the 
> intrument the magnet would move and the radio signal would go out.  I guess 
> the instrument signal overpowers the radio signal when it is moving.  

    Which shows that the radio signal is coming through with the seis 

> Also, 
> my touching the coil or preamp seemed to affect it too. With the coil 
> unplugged, I could hear better if I touched the pre-amp. However with the 
> coil plugged in touching it would often stop the signal. It seemed very 
> touchy.  

    This suggests that you have an input / earthing problem. Have you been 
able to contact any helpful Ham radio operators? If there is one locally, he 
will probably have had problems with KOLM as well....


    Chris Chapman

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Larry Cochrane <cochrane@..............>