Wow. Obviously, it is in the antenna circuit. Could be a lightning arrestor, balun or a Standing Wave Ratio bridge for matching the antenna to the transmitter. Seems a bit small for AM Broadcast use, but maybe it is a low power transmitter?
This is something electropaw should be familiar with.
Well, it's got an RF ammeter, so it's obviously going to tell you something about the forward power of the antenna. I'm not sure what the device in the middle is, although it looks like a vacuum switch. I'm guessing when power is applied to the coil (small left hand terminal strip), that the RF relay switches the input and output around and reads forward and reflected power.
If it's only 8A full scale, that's not a ton of RF power. I^2 X 50= 3 kW (assuming a 50 ohm impedance).
The more I look at it, the more it looks like a DPDT RF relay that switches the meter polarity around.
Well, if they're running 1KW, then having a 3KW full scale seems reasonable. The RF relay/vacuum switch does something. I don't think the relay actually flips the meter polarity, but I think it does flip around some sort of bridge or RF sensing coil so you get forward and reflected power.
Using an RF ammeter is a pretty ancient way of measuring antenna power, although it does give you a very accurate representation of what's "going up the pipe." Our aircraft NDB transmitters still use RF ammeters. They need a constant strength signal regardless of antenna impedance, so the system is tuned to produce a specific antenna current. The actual power out of the transmitter may be varied, but we know the intensity of the radiated signal will remain the same.
I think you've got the answer. That terminal block to the left of the relay is probably what actuates it. What amuses me is the two wires are the innards of coax with the shield removed. The Weston 304 meter is an oldie ^_^
Well, that looks like a Jennings vacuum relay plugged into a socket, with the coil brought out to that terminal strip. Without knowing the configuration of the relay or seeing where the other conductors go, I can only speculate on the exact configuration, but it definitely either switches the meter around or perhaps switches the antenna array from non-directional (daytime) to directional (night).
Swapping the meter polarity wouldn't make any difference because it's AC amperes (RF), although it could switch around a bridge circuit. So it's either that, or it's switching the antenna array around and the operator would manually adjust the drive to maintain a certain antenna current into the array.
Is that relay made of glass? It really looks like its made of a plastic called Victron which National used alot in their radios. Though, I don't think Victron would hold a vacuum :P
I was assuming so, but it's hard to tell from that angle. If it were low power, I suppose it wouldn't need to be a vacuum relay, but it would have to stand up to some high RF voltages.
This is something electropaw should be familiar with.
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If it's only 8A full scale, that's not a ton of RF power. I^2 X 50= 3 kW (assuming a 50 ohm impedance).
The more I look at it, the more it looks like a DPDT RF relay that switches the meter polarity around.
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Using an RF ammeter is a pretty ancient way of measuring antenna power, although it does give you a very accurate representation of what's "going up the pipe." Our aircraft NDB transmitters still use RF ammeters. They need a constant strength signal regardless of antenna impedance, so the system is tuned to produce a specific antenna current. The actual power out of the transmitter may be varied, but we know the intensity of the radiated signal will remain the same.
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Swapping the meter polarity wouldn't make any difference because it's AC amperes (RF), although it could switch around a bridge circuit. So it's either that, or it's switching the antenna array around and the operator would manually adjust the drive to maintain a certain antenna current into the array.
It does look like a real kluge :D
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It's also pretty old, I don't believe the mystery equipment was in use. At least I'd hope not.
Wish I'd taken more pictures, Don't know if I'll be in that site ever again.
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