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I have a need to remotely confirm output power on a radio. However I'm poor and want to do cheaply. All I need is a confirmation that it's pumping out in excess of 75 Watts and then have a contact closure that indicates it worked.

When I search online I usually get an over wroked solution that gives me a display of the power out and of course they cost a small fortune.

What could I make or buy that would just sense that the RF output has gone from nothing to >75 Watts?

One solution I came up with was a current sensor that would trigger when the radio starts pulling huge current but I'd rather trigger on the RF out.

Any and all ideas would be great!

Edit: VHF frequency 152-162MHz

I guess I left stuff out, sorry about that. Yes needs to be left attached. The radio is part of a paging system and I want to know its going off. I can't distrupt the transmission. A Bird wattmeter does some sort of coupling with the coax like the diode detector idea. However the parts for a Bird cost an arm & leg and for that much cash I might as well just pay the full price and have the full meal deal.

Chef Flambe
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  • @JYelton - I don't think it's really a duplicate. The question you link is looking to determine if something is transmitting at all. What this question is trying to do is measure the actual transmitted RF power accurately enough to know if it exceeds a certain amount (in this case, 75W). – Connor Wolf Aug 12 '13 at 20:01
  • A frequency range would help – JIm Dearden Aug 12 '13 at 20:19
  • @Connor Ah you're right. I was working off the title thinking *this sounds familiar.* Sorry. I'll remove that and simply note that this question is *related* to http://electronics.stackexchange.com/questions/78530/how-can-i-tell-if-an-rf-transmitter-is-transmitting – JYelton Aug 12 '13 at 20:34
  • Do you need to verify the power output of a transmitter *and* antenna, or just a transmitter? – Phil Frost Aug 12 '13 at 20:47
  • And, how accurately do you need to measure this power? Keep in mind, *cheap* and *accurate* are mutually exclusive. – Phil Frost Aug 12 '13 at 20:49
  • And, does the measurement need to happen with minimal impact on the transmitted signal? Does it need to be left permanently attached? – Phil Frost Aug 12 '13 at 20:51
  • @JYelton I think Phil Frost's circuit is cool and can easily be made/calibrated to trigger a relay within reasonable limits of the 75W. – Andy aka Aug 12 '13 at 21:21
  • If I have power coming out of the radio I know I have power at the antenna. Already confirmed the VSWR for the system. Accurate is open ended...>75 Watts is all I need to know as it will be 100Watts in reality and I just want to know when it turns on. The 75Watt trigger is so I can have a window that is certain it is on but wide enough so that I don't need expensive gear. – Chef Flambe Aug 12 '13 at 21:25
  • Please try to edit your question to be coherent and ask the right question the first time. Future readers do not need to read "Edit:" "guess I left stuff out, sorry". If they really want to see the edit history, they can clicked the "edited" link at the bottom. – Phil Frost Aug 13 '13 at 12:06

2 Answers2

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One idea might be to look at this site. https://sites.google.com/site/lofturj/ad8307-power-meter

The power meter is based on the AD8307 logarithmic amp and the circuit handles powers up to 500W / 500MHz with the aid of a 40dB power tap.

The rest of the circuit is basically measuring the voltage produced and displaying it but you could just use a digital voltmeter.

enter image description here

JIm Dearden
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Close by the antenna and, mounted in a fixed way relative to the antenna, a single-turn coil and parallel tuning capacitor (resonating) should pick-up more than enough signal to forward bias a half-decent diode and generate a dc voltage that can be used by any old op-amp circuit to trigger a relay or send an on/off voltage down a cable to indicate at least 75W is being pushed into the antenna.

I'd even say that the diode detector (implied in the above paragraph) would produce enough volts to send back down another cable to a box by the transmitter that could measure that the dc level is above a certain value.

Andy aka
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