1
\$\begingroup\$

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.

\$\endgroup\$
9
  • \$\begingroup\$ @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). \$\endgroup\$
    – Connor Wolf
    Commented Aug 12, 2013 at 20:01
  • \$\begingroup\$ A frequency range would help \$\endgroup\$
    – JIm Dearden
    Commented Aug 12, 2013 at 20:19
  • \$\begingroup\$ @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 electronics.stackexchange.com/questions/78530/… \$\endgroup\$
    – JYelton
    Commented Aug 12, 2013 at 20:34
  • \$\begingroup\$ Do you need to verify the power output of a transmitter and antenna, or just a transmitter? \$\endgroup\$
    – Phil Frost
    Commented Aug 12, 2013 at 20:47
  • \$\begingroup\$ And, how accurately do you need to measure this power? Keep in mind, cheap and accurate are mutually exclusive. \$\endgroup\$
    – Phil Frost
    Commented Aug 12, 2013 at 20:49

2 Answers 2

Reset to default
2
\$\begingroup\$

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

\$\endgroup\$
1
\$\begingroup\$

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.

\$\endgroup\$
2
  • \$\begingroup\$ Sure, but how will you calibrate it? If the system is known to work now, maybe baseline measurements can be taken? \$\endgroup\$
    – Chris Stratton
    Commented Aug 13, 2013 at 2:52
  • \$\begingroup\$ @ChrisStratton that's the way I would do it - the OP's demands are quite lenient. \$\endgroup\$
    – Andy aka
    Commented Aug 13, 2013 at 7:07

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.