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1-10 of 11 messages
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RF Detection Circuit
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by K2YO on May 19, 2009
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Does anyone have a simple rf detection circuit? I'm looking for a circuit that will simply detect if a strong rf field is present. The purpose is to trigger a relay when the field is present to disconnect the antenna lead to a receiver that would be in the field who's front end circuit would be over loaded.
Thanks for any suggestions.
Bernie
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RE: RF Detection Circuit
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by W5FYI on May 19, 2009
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You didn't say how high an r.f. field, so let me suggest a simple diode protection circuit. A pair of front-to-back parallel diodes between the antenna terminal and ground will conduct when the voltage across them is greater than their forward voltage, thus shorting the r.f. on the antenna to ground.
A slower way is to use a carrier-operated relay (COR). R.f. into a transistor switch turns it on, which energizes a relay. The relay can either disconnect the antenna, or short it to ground. The relay operation is mechanical, so operation isn't instantaneous.
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RE: RF Detection Circuit
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by K2YO on May 19, 2009
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"You didn't say how high an r.f. field,"
I don't have an exact number right now. I'm going to have to make some measurements to be sure. Two applications come to mine. One is a 5 watt HT at point blank range. The other would be a 50W VHF/UHF from a gain antenna 50 feet away.
My main question is what kind of circuit to detect the rf field. I'm only played around with receiving circuits where you want to actually capture the audio. In this case I just want to know if there is rf present. It would also be nice to make the circuit adjustable for how much rf is needed to trigger the disconnect.
Bernie
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RE: RF Detection Circuit
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by VK2HHS on May 19, 2009
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You may want to look at the RF sensing "transmit switch" as used in the amplifier attached.
It may not be exactly what you want but should offer a few clues!
http://www.geocities.com/indrasep/irf510.pdf
de VK2HHS
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RE: RF Detection Circuit
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by W3LK on May 20, 2009
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What frequency is the receiver in question on? If it's HF, you are solving an essentially non-existent problem.
73,
Lon - W3LK
Naugatuck, Connecticut
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RE: RF Detection Circuit
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by WB2WIK on May 20, 2009
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The problem with this idea is by the time the switch activated, the "protected" receivers will have already been exposed, so what's the point?
Can you make a "switch" faster than the speed of light?
What damages a receiver is transient voltage, not "power," so prolonged exposure is no more damaging than the first instantaneous transient burst which contains no energy (in the first microsecond or two). For example, the standard lightning transient signal used for threshold analysis is 8x20uS, where the signal rises to a peak in 8uS and the burst has a period of 20uS.
That is enough to blow the smithereens out of sensitive equipment when the transient voltage exceeds the safe operating voltage of the devices in question. You can't build a switch that reacts and opens or closes in less than 8uS.
Best deal in this situation is to determine what the safe separation distance is from source antenna to receiver antenna to maintain some margin of safety, and just operate within those constraints.
Receivers are pretty tolerant unless you're doing something very unusual. For example, it's common for someone using a VHF hand held to be standing right next to a car with a mobile whip transmitting a 50-100W output signal on the same band. In such cases, separation from 'whip' to 'whip' might only be a foot or two. That's generally enough; in fact, I can't recall a single case where a receiver was damaged in this situation, and I've operated like that hundreds of times.
WB2WIK/6
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RE: RF Detection Circuit
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by WB6BYU on May 20, 2009
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The simple method is to use some sort of voltage limiting
(such as one or more diodes in series) across the antenna
input. This will conduct and limit the voltage applied
to the receiver when the incoming voltage is high enough.
Many receivers already contain such protection.
Otherwise a simple diode detector probably will do what
you want. You can add an untuned amp on the input and/or
a DC amplifier (perhaps an op amp) on the output if you
need more sensitivity. Basically a crystal set driving
a DC load instead of a set of headphones. But it may not
switch fast enough to protect the front end of a receiver.
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RE: RF Detection Circuit
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by K2YO on May 20, 2009
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Lon,
"If it's HF, you are solving an essentially non-existent problem"
Don't be silly, hahaha. There wouldn't be a problem if it was on HF.
Bernie
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RE: RF Detection Circuit
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by K2YO on May 20, 2009
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"The problem with this idea is by the time the switch activated, the "protected" receivers will have already been exposed, so what's the point? "
Feedback!
If you've ever been in an environment where scanners are used to monitor frequencies. If a user keys up an HT in the room and it's one of the monitored frequencies, you get instant feedback. If the scanner is on a similar frequency or the front end over loads, sometimes the intermod can do the same.
OK, so we've had a number of why not to do this, does anyone have an suggestions on how to do this?
Bernie
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RE: RF Detection Circuit
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by WB2WIK on May 20, 2009
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Well, if feedback is the issue then a few hundred mS delay won't matter much, and you can derive an output for that from any receiver.
To drive a relay from a VHF-UHF receiver or hand held, I generally just tap off the "activity" or "busy" LED signal, buffer it with a Darlington, and close a relay. Then, the relay can do whatever it needs to do, including turning on street lamps.
But that in no way "protects" the receiver. It just uses the signal to trigger something else.
WB2WIK/6
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