kl7aj
05-04-2007, 05:17 PM
Another DA disaster from the case files.
Leon Rodale, chief engineer of WWKK, thought he had seen it all. #He belonged to that exclusive handful of engineers who had "Chiefed" a full gallon (50KW) Directional AM station.
This one stumped Leon AND a few of his colleagues...for a while.
WWKK has two identical approximately 3/8 wave #towers, spaced 1/4 wave, a simple cardioid pattern. #The monitoring and feed system is purely conventional...a current/phase sample loop 1/4 wave down from the top of each tower, a T-type ATU at each tower base, 3" heliax from the power splitter/phasor to each ATU.
Normal operation parameters call for an 81 degree phase angle and a 95 percent current ratio. #(It's a fairly deep cardioid, but not a perfect null.) #The nominal base current readings are 15.7 amps and 9.9 amps.
One day, Leon notices the phase and loop current ratios being a bit out of tolerance...the ratio is down to 89 % on tower two. #There was a bit of rain the night before, but nothing significant...he chalks the change to normal seasonal ground conductivity. #He readjusts the phasor/splitter to bring the currents into tolerance.
Alas, when he does so, the base currents are now out of tolerance! #He scratches his head for a couple of days on this. #No amount of adjustment seems to bring the loop currents AND the base currents into FCC tolerance at the same time. #Station management is breathing down his back, because they are currently running an illegal station. #
Only one thing, in Leon's estimation, can cause the discrepancy between the base currents and the loop currents....the impedance of one or both of the towers has changed. #
That night, Leon lugs out his trusty General Radio impedance bridge and associated signal generators and fittings to the tower field. #He carefully measures each tower, and compares the readings to the original antenna proof. #Everything seems to be perfectly in line.
He suspects that, perhaps there is an intermittent or heat related problem.
He reassembles the ATU's and fires up the transmitter on ten KW...the minumum power the beast will put out without serious modification. #The loop and base current ratios remain unchanged....wrong but unchanged.
Leon takes one final desperate measure. #He replaces the base current thermocouple ammeters AND the common point meter (the latter, just for good measure).
The base currents BARELY fall within limits...if he allows the Loop currents to linger at the OPPOSITE limits. #He is now legal.....barely. He HATES to skirt the edges of legality like that....for decades the antenna has operated right on the money.
The array operates in this legal but lame mode for FIVE WHOLE YEARS. #
Finally, an aha moment comes. #Leon solves the problem, and the array is as good as new.
What POSSIBLY could have been the problem?
eric
Leon Rodale, chief engineer of WWKK, thought he had seen it all. #He belonged to that exclusive handful of engineers who had "Chiefed" a full gallon (50KW) Directional AM station.
This one stumped Leon AND a few of his colleagues...for a while.
WWKK has two identical approximately 3/8 wave #towers, spaced 1/4 wave, a simple cardioid pattern. #The monitoring and feed system is purely conventional...a current/phase sample loop 1/4 wave down from the top of each tower, a T-type ATU at each tower base, 3" heliax from the power splitter/phasor to each ATU.
Normal operation parameters call for an 81 degree phase angle and a 95 percent current ratio. #(It's a fairly deep cardioid, but not a perfect null.) #The nominal base current readings are 15.7 amps and 9.9 amps.
One day, Leon notices the phase and loop current ratios being a bit out of tolerance...the ratio is down to 89 % on tower two. #There was a bit of rain the night before, but nothing significant...he chalks the change to normal seasonal ground conductivity. #He readjusts the phasor/splitter to bring the currents into tolerance.
Alas, when he does so, the base currents are now out of tolerance! #He scratches his head for a couple of days on this. #No amount of adjustment seems to bring the loop currents AND the base currents into FCC tolerance at the same time. #Station management is breathing down his back, because they are currently running an illegal station. #
Only one thing, in Leon's estimation, can cause the discrepancy between the base currents and the loop currents....the impedance of one or both of the towers has changed. #
That night, Leon lugs out his trusty General Radio impedance bridge and associated signal generators and fittings to the tower field. #He carefully measures each tower, and compares the readings to the original antenna proof. #Everything seems to be perfectly in line.
He suspects that, perhaps there is an intermittent or heat related problem.
He reassembles the ATU's and fires up the transmitter on ten KW...the minumum power the beast will put out without serious modification. #The loop and base current ratios remain unchanged....wrong but unchanged.
Leon takes one final desperate measure. #He replaces the base current thermocouple ammeters AND the common point meter (the latter, just for good measure).
The base currents BARELY fall within limits...if he allows the Loop currents to linger at the OPPOSITE limits. #He is now legal.....barely. He HATES to skirt the edges of legality like that....for decades the antenna has operated right on the money.
The array operates in this legal but lame mode for FIVE WHOLE YEARS. #
Finally, an aha moment comes. #Leon solves the problem, and the array is as good as new.
What POSSIBLY could have been the problem?
eric