[CQ-Contest] Bandpass filters

Ward Silver hwardsil at WOLFENET.com
Fri Dec 3 08:17:35 EST 1999

> > Anybody have any experience with this same problem? I have notice
> > the LEDs flashing before to the cadence of the RF, so I thought perhaps
> > they were acting like noise generators. 

It's quite possible that an unbypassed LED can generate wideband spurious
emissions in the presence of strong RF.

I had a similar experience with Tailtwister rotor control boxes - the
rectifiers in the control box are unbypassed.  The strong local AM BC
signals (there are 6 within five miles) came down the control lines, were
cheerfully mixed together and evenly distributed all over 40-meters, in my
case, and probably elsewhere.  The solution was to bypass all of the
control lines at the terminal strip to the control box with 1000 pF
ceramic caps. Problem solved.  This could cause serious spurious problems
at multi-radio setups wherever similar rotor boxes are in use.

In this case, it sounds like RF is leaking in via the control port.
Bypass all the lines to ground - outside the case if possible - with 1000
pF ceramic caps.  Make sure the case is solidly grounded, as well.  If the
bypassing can't be done outside the case, do it at the back of the control
port connector and put a big ferrite core on the control cable, as well.
If the LEDs are still flashing after this treatment, I'd go after the
power supply next.

73, Ward N0AX

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