Topband: IMD Problems with Alpha 87A

[Terry Hahn]

I recently installed an Alpha 87A at my station and have run into a serious IMD problem coming from a broadcast station located approximately 3 miles LOS from my QTH. The broadcast station is running daytime power of 50 KW and night time power of 1 KW on an assigned frequency of 1100 KHz.  

The problem only occurs when the station is running their daytime power level (50 KW).   On my end the IMD is detectable only on 80 and 160 meters and pops up as soon as I switch the Alpha from standby to the operate mode.  In the Operate mode the IMD is running nearly +20 on the S-meter making it almost impossible to operate.  

 We sent an email to Alpha explaining the problem and they in turn explained the mechanics of just how signals are  passed through the Alpha and what they thought our problem was.  

Alpha said that signal path changes depending on the position of the Standby/Operate switch.  In the Standby mode the connection is via an electromechanical relay contacts directly to the exciter and in the Operate mode the amplifier uses pin diodes for T/R switching.

The problem is only present in the Operate mode when the pin diode T/R switching is used. In order to isolate the problem Alpha suggested several things. First, they recommended that we check to see if there was any RF traveling in the coax shield.  They recommended we put a common mode filter in the line, which we did without success. They also recommended putting a highpass filter in the line to see if that would help.  BINGO - That fixed the problem.  I happened to have an ICE 402X highpass filter laying around and that did the job. 

Alpha's recommendation to fix the problem is to install a notch or highpass filter in the antenna input line to attenuate the amount of RF the amplifier is seeing.   I have looked around and the only highpass filter of this type I could find was the ICE 402X and it's only rated to handle  around 300 watts.  

Has anyone experienced this problem before, and if so, please let me know how you resolved it or any other recommendations would be greatly appreciated..  

73 de Terry W7KW