Modulation Monitor Pickup

[w5dud]

Anyone have a good design for the pickup when using different power level transmitters,have one I built but when going down in power not enough rf to drive the monitor, Thanks Dudley/W5DUD

[KF1Z]

For what monitor, and for what power level?

[w5dud]

From 1Kw down to 200 watts, Have several belar, gates and a general radio 1931 , The belar and gates take the most drive,

[KF1Z]

Oh, ok, well, I'm not at all familiar with any of those...

But I wonder how much voltage do any of them take, for a good reading?

[N2DTS]

For my mod monitor takeoff, I use a wide spaced variable cap, about 50PF?
It drives the rca (bellair?) mod monitor fine from 50 watts up to 700 watts carrier.
Its built into the station control, and hooks directly up to the TX line.
Voltage rating of the cap is about 6000 volts.
Its worked fine for 30 years...

Brett

[w5dud]

Thanks Bret I will build one tonite, have in the past always wound coils for the pick up voltage ,Thanks Dudley

[N2DTS]

I should say, i only operate AM on 80 and 40 meters, but have tried 160.
As you go up in frequency, the cap passes the rf better, so it might not work real well above 40 meters at high powers.

I used to have an old General radio mod monitor that had coils you had to switch for bands, which is a no go for me....

Brett

[k7yoo]

The cap hooked to a coax Tee works FB. You can even make a switch box with different caps for different power levels. For the past 20 years I have been using a gimmick cap made out of RG 59,58,or 8X coax. Hook a 2' piece of coax to the tee using a PL 259. Do not solder the braid to the fitting but pull it off the cable until only about 12" is inside the jacket/braid. Solder the braid to the center pin of the other PL 259 that feeds the monitor. The easiest way is to solder a piece of solid wire to the center pin and solder the braid to that. This forms a cap equal to the pf per foot characteristic of the cable. If you are a little hot you can pull the center conductor out to reduce the coupling. When you get it all tweaked shrink tube or tape the unit up.
For scope use I use cheap Tech school castoffs and hang a short antenna on the bnc input. A 1' piece of stiff wire is all the sniff you need for our normal QRP power levels. Adjust the scope for a pleasing trace and modulate on.
Skip

[Jim, W5JO]

Who is it, Gates that uses a roller inductor after the tank circuit tapped to the output RF.  The other end of the roller is not tied to anything and output is taken from the roller.   Plug the monitor into the connector from the roller and turn everything on after putting the roller at the far end of rotation from the input.  Rotate the roller until you have the meter set at the red line.

[W7TFO]

I have lots of parts from old AM broadcast rigs, and as originally designed they have a ubiquitous high-uH choke coil tied directly across the output line and ground with a variable tap going out to the freq and mod monitor units. 

Some vintage monitors had to have some 20 Volts @ 50 Ohms to get going as not all of them had tuned front ends.

The reference to the one that had one end going nowhere was probably in reality grounded by a metal spacer rather than a porcelain insulator as used on the 'hot' end. 

They also provided a safety factor in the event a cap shorted between B+ and the output ckt.