[BC] AM transmitter lightning sensitivity issue

[Cowboy]

On Wednesday 21 May 2008 01:41 pm, Gregory Muir wrote:
>  Within my station group I have a Harris Gates Five AM transmitter which 
>  exhibits an unusual sensitivity to local lightning storm events.  Upon the 
>  appearance of a storm, the transmitter will begin to automatically lower 
>  it's output power through the power adjustment range with each lightning 
>  strike until it finally goes off the air after reaching the lowest power 
>  setting.  It is obvious that the unit sees VSWR events which causes it to 
>  react.  But what is interesting is that it will react to the same events 
>  even when the storms are 5-10 miles distant from the site with similar 
>  results.        

 My initial thought is that your particular set of parts happens to be arranged
 in a way that does a step-up on any residual lightning transient that happens
 to get past the ball gaps.
 ( we rarely check response that far from resonance )

 First thing I'd do, is close those gaps as much as possible.
 Second, would be to check the output gap in the transmitter, and
 make sure it is also as close as possible without arcing on mod peaks.
 This one can be considerably closer than tower balls, since it isn't subject
 to the dirt and humidity that the tower bases are.

 Next, I'd be wanting to scope the TX output during a storm, and confirm
 that it really is high level spikes on the RF output lead.
 If so, I'd be installing additional cheap gaps ( like Autolite 665 plugs ) at
 various places in the system to try and tame it down that way.

 Lastly, talk to your design consultant, explain the problem, and see if they
 can't add some series inductance at the true common point, or whatever
 other point they feel might be optimum.

 I'd also be looking at the incoming AC power lines for possible transient
 problems there, and add some protection at that point.  

 Random thoughts from the peanut gallery.......

-- 
Cowboy