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   Home   Help Search  
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Author Topic: RF triggered coax switch  (Read 2868 times)
W4ZDI
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Posts: 37




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« on: July 08, 2009, 05:31:09 PM »

I would like to use the SDR-14 receiver for generating a panoramic frequency display while using my Transceiver.  I realize there is no tracking available with this sort of setup, but considering the prices for RF Analyzers  on the market, this would be a bargin.

Their link is : http://www.rfspace.com/SDR-14.html

My problem is that they do not offer a T/R coax switch to eliminate the transmitter RF from burning up the front end.

Can anyone recommend a reliable, RF triggered switch?

Thanks,

Pierce W4ZDI
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KB9CRY
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Posts: 4283


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« Reply #1 on: July 08, 2009, 06:47:21 PM »

Home Brew
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W4ZDI
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Posts: 37




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« Reply #2 on: July 09, 2009, 04:18:14 AM »

Sorry, the correct link is:

http://www.rfspace.com/SDR-IQ.html

About $500 less.
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WB2WIK
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Posts: 21837




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« Reply #3 on: July 09, 2009, 09:43:04 AM »

If it's RF triggered, the first pulse of RF to trigger the switch would instantaneously hit the front end of the spectrum analyzer.  Not a good thing.

Better is to use no switch and no moving parts, just a 20 dB directional coupler you transmit through (no loss there), and maybe a 20 dB pad on the -20 dB port to knock the 100W down to 10mW (+10 dBm), and just leave that connected to the analyzer all the time.  If the analyzer sensitivity is -120 dBm, this will reduce it to -80 dBm but you'd still see all the strong signals on the band.

The RF-activated "switch" will always be risky if you're dealing with a sensitive and unprotected front end.

No, a "hard-wired" switch (not RF actuated) that switches first before the TX can produce any output power would work fine.  Most rigs have such a provision, in the form of their amplifier key line output which keys first, before the TX does, when you provide a transmit enable signal from the mike or key.

WB2WIK/6
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DIPOLE
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Posts: 176




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« Reply #4 on: July 09, 2009, 01:44:57 PM »

Look at the MFJ-1708 RF switch.
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K0BT
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Posts: 27




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« Reply #5 on: July 09, 2009, 03:41:12 PM »

I agree with WB2WIK.  By the time the RF switch can react to the RF, the RF has already overloaded the front end of the receiver.  Even using fast PIN diodes, you are basically gambling on the receiver being able to tolerate abuse for a longer time than it takes for the RF switch to trigger.  

On a TS-2000 (and most other modern transceivers) the amp control line will deliver +12v about 10mS before the transmitter delivers RF in full break-in mode, with about 25mS delay in semi break-in.  If you use that to drive an electronic switch, you should be OK.  You could drive either an electronic switch or a fast reed relay, but a heavier relay is too slow for this application.  

The coupler method is safer.  If the switch fails, you'll be replacing parts in your receiver.  

Bob
K0BT
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W4ZDI
Member

Posts: 37




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« Reply #6 on: July 09, 2009, 06:55:00 PM »

I wasn't aware that provisions for a delay in switching is available on these transceivers.  I'm about 50 years behind times - the last transmitter I used was a 450W BC-610 in '57 that belonged to the Navy club station KH6AHQ.  Been busy working for a living since then.

As you say, for safety it would be best to divide the signal down to a harmless level.  I may use that in conjunction with a delayed switch that would cut out all of the attenuation.

thanks for the help,

Pierce W4ZDI
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