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Author Topic: Need a simple RF sensing relay circuit  (Read 10834 times)
HS0ZIB
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Posts: 436




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« on: July 09, 2011, 09:40:40 PM »

I have a mobile SSB rig, 15 meter band, 2/10 watts output into my antenna,

I have purchased a 'homebrew' linear amp off Ebay which boosts the output power to about 30 watts, (from 2 watts input power - 10 watts is rather high for the input).

This amp does not include any switching circuit to allow the received signal to be routed directly back to the rig receiver when I am not transmitting.  The rig does not have easy access to a 12 volt 'switching' line that is activated when I am transmitting.

So it sounds like the easiest solution is to provide an antenna switch/relay that is activated when RF is sensed during transmit.

Can anyone point me towards a suitable circuit for this?  The amp is speced at 1.6 - 30MHz, so a circuit that will operate with an input frequency over this range is appreciated.

Thanks

Simon
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KA4POL
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Posts: 2128




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« Reply #1 on: July 09, 2011, 11:27:23 PM »

Just one of the many circuits out there: http://www.kolumbus.fi/oh5iy/HW/Homebrew.html
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AD4U
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Posts: 2186




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« Reply #2 on: July 10, 2011, 05:14:41 AM »

In SSB the RF activated relay will not key until you speak and produce some RF, not when you key the mic.  To keep the relay from chattering in time with your SSB modulation, you will need some type of delay circuit to keey the relay keyed between voice peaks.

Another way to key the amp is to build a simple circuit that keys the amp from the mic contacts when you key your mic.

Not knowing anything about your rig, the amp, of the lay-out of your mobile installation makes it hard to make suggestions.

Dick  AD4U
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KE3WD
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Posts: 5689




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« Reply #3 on: July 10, 2011, 07:19:41 AM »

The whole thing sounds like an illegal CB situation.   

For realworld use on the 15 meter Amateur band, a relay, keyed by the PTT, buffered by at least one transistor and proper snubbing at the coil would be the right thing to do.


73
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AD6KA
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Posts: 2238




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« Reply #4 on: July 10, 2011, 08:34:26 AM »

For a simple, low power setup like you describe, this should work well. 
Ramsey RFS1 - RF Sensed T-R Relay Kit
http://www.ramseyelectronics.com/cgi-bin/commerce.exe?preadd=action&key=RFS1


Too bad you can't just get the schematic from the free kit manual downloads,
but they remove the schematic from the download.
GL ES 73, Ken  AD6KA

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W8JI
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« Reply #5 on: July 10, 2011, 02:20:59 PM »

The problem with an RF keyed relay is the relay does not move until AFTER RF appears.

This means the amplifier relay, nearly 100% of the time when it turns on, switches while hot with RF.

A PTT connection is much better.

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HS0ZIB
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Posts: 436




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« Reply #6 on: July 11, 2011, 02:00:27 AM »

Yes, since it is very difficult to purchase even common components, (resistors, capacitors, diodes etc) in this part of the world, it might be easier to use the microphone 12 volt switch line to activate the relay.

Quote
The whole thing sounds like an illegal CB situation

Not sure what this comment means.  You are welcome to come and live out here, where the nearest Tandy/Radioshack is 12,000km away and see how you cannot assume even the simplest of components, tools and test equipment will be available. Why, just this morning I had to fashion a power shower out of 20 banana leaves, a tin can and 14 savages with coconuts full of cold water

Simon (in deepest jungle...)
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KE3WD
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Posts: 5689




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« Reply #7 on: July 11, 2011, 08:00:56 AM »

Simon, what I meant by that comment is that the COR Carrier Operated Relay in an amp for SSB is usually seen in those cheap little CB amps.  That's all. 

Go with Push To Talk.


73
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XV4TUJ
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Posts: 20




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« Reply #8 on: July 12, 2011, 10:11:24 PM »

Hi Simon,

I agree with other than the cheapest, most reliable and easiest solution is to make to wire going out from the mic PTT and use them to switch a transistor that will drive a DPDT relay.
Specially if you don't have access to RF parts, a RF VOX could be hard to implement and will never function great in SSB.

BTW, if you need part you can order from futurlec.com . They have their warehouse in Thailand and shipping is good to me in VN. Price are great and shipping costs fair. Their customer service is responsive. They don't have any RF parts, though.

73,
Yan - XV4Y.
---
http://capheda.wordpress.com/
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KE3WD
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Posts: 5689




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« Reply #9 on: July 13, 2011, 05:10:46 PM »

To implement a simple NPN transistor PTT circuit, you could cannablize just about any old transistor AM. FM radio.  Look for a 2SC828 or C945, something along that line will be plenty stout.  The needed resistors and perhaps a small decoupling cap should be in there also. 


73
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WB6BYU
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Posts: 13580




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« Reply #10 on: July 13, 2011, 05:53:34 PM »

I used the Ten-Tec 405 amp with my 505 Argonaut, and I always wished I had direct keying of the amp
on SSB rather than relying on the RF-sensed relay.  By the time I got the delay set long enough so it
didn't chatter, it stayed on too long after I was finished talking.

Then, when the CBer 2 apartments down got on the air, there was enough received signal to trip the
relay (at least while the amp was in receive mode) so the amp would start clicking on and off by itself.

I'd certainly recommend looking for some way to turn on the amp with the mic button instead of
RF sensing if you can manage.
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KE3WD
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Posts: 5689




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« Reply #11 on: July 14, 2011, 07:47:21 AM »

It isn't rocket science to go inside an amp like that TenTec, locate the RF sensing stuff, deactivate and bring a PTT line out.  All the parts are probably already there for that, although it might be wise to add an additional transistor buffer to the relay circuit, the schematic should be examined first to see what is going on, of course. 


73
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HS0ZIB
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Posts: 436




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« Reply #12 on: July 17, 2011, 12:41:45 AM »

I agree that using the 12 volt switching line is the best solution, especially for SSB.  I've managed to find a small relay, so I'll test this all out, (I have been busy today optimising/repairing my homebrew 15 meter mobile antenna after the whip section disappeared during a late night journey a few days ago!)

Simon
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AA4HA
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Posts: 1640




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« Reply #13 on: July 20, 2011, 09:47:27 AM »

You could try a small winding on the antenna lead, feed it into an OP-Amp (in differential mode). Use back to Zenier diodes to limit the voltage and use the op-amp output to drive a transistor. The feedback resistor on the op amp can be used to set the sensitivity and you can change the number of windings on the antenna lead (sort of line a transformer with the antenna lead making a single pass through the center of the coil, (look up "current transformer").

The impedance bump would be very small, you do not want to get crazy with a bunch of windings as this is going to act like a transformer and you do not want to put a few hundred volts into the op-amp (a load resistor across the op-amp input may also be a good idea).
« Last Edit: July 20, 2011, 09:49:26 AM by AA4HA » Logged

Ms. Tisha Hayes, AA4HA
Lookout Mountain, Alabama
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