Here at the KDX Control Desk located in the Vacuum Room of the Internet Building we have a single computer workstation that generates all the audio programming which is distributed by various means to a planned total of 5 amplitude modulated transmitters located at different locations around the building.
We are building a Central Remote Control Point in its own cabinet next to the computer for controlling carrier ON/OFF of each transmitter, and foresee building in as many other remote control features as possible.
In particular, by having our B&K Oscilloscope alongside the Remote Panel we would like to wire a selector switch to enable viewing the modulation of each of the (5) transmitters.
We are asking the part 15 community to suggest methods of tapping the RF antenna feed point of each transmitter and sending it an average of 25-feet by wire to the oscilloscope, without affecting the integrity of the tuned antenna.
Almost anything you do near an antenna will cause some detuning but you can compensate by retuning. If you have to retune very much then your coupling is too "tight".*
For the AM-5000 the best way to tap the output signal is to wind a couple of turns of wire through the antenna loading toroid and connect one end to the center of a coax and the other to the shield. The scope connection is center to input, shield to ground. This method samples a low impedance monitoring point which has little effect on the tuning and also isolates the tx. ground from the measurment system ground..
The other method involves sampling the voltage present at or near the antenna. Voltage measurements near the high impedance radiator of the antenna will affect the tuning but this can be minimized by keeping the coupling "loose". One method is to wrap a single turn of the end of the lead of a 10Megohm resistor around the antenna lead near the transmitter which forms a couple of picofarads capacitor connection. The other resistor lead goes to the coax center and coax ground goes to transmitter ground. Scope connection is as described above.
A third method is to connect a choke (1 mH or so) from the coax center to shield and attach a short wire to the center conductor to form an antenna and place this near the radiating element. This minimizes the disturbance to the antenna system but it will pick up other RF signals as well.
*By way of definitions, tight means a lot of the signal is transferred to the monitor pickup and loose means very little signal is transferred.
Neil
There are several high quality coaxial switches on the market that handle more than 1 feed: https://www.gigaparts.com/mfj-1701.html?gclid=CKybnYOu_NICFQwZgQodvoMGYw
This website gets a gold sticker for efficiency of response!
In only two responses it looks like I now have all the technical instruction needed to build the AM modulation measurement station that I described at the opening.
Neil Radio8Z and John Druid Hills Radio are institutions in the low power field.
While I go forward to build this space module I can't help but wonder...
What do other stations do to check their modulation level?
My AM audio chain is metered with both an analog VU meter and an LED VU meter on the compressor/limiter. Normally, this suffices to set the modulation level but to make sure that these are accurately calibrated and to make sure no negative clipping is happening I connect the scope ground to the transmitter ground and clip the 10X probe tip to the insulated antenna lead wire, set the vertical sensitivity to give a good display, set the horizontal sweep to 2 ms/div and watch for problems. Once the calibration of the meters is verified the scope is disconnected and the meters are used thereafter.
I have a primary transmitter and two backup transmitters, each of which can be connected to the audio chain so each transmitter is initially checked this way and the audio gain of each is appropriately adjusted so they all modulate the same as indicated with the meters..
Neil
This month's QST (April 2017) from the ARRL has an article that might interest you. On page 34 there is a construction article for a RF current sensor that uses a DMM for display. No reason you could not instead use the scope.