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Author Topic: MFJ-259B connection  (Read 880 times)
N2CJ
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« on: March 16, 2007, 09:56:06 PM »

I recently purchased an MFJ-259B SWR Analyzer. I don't know how I ever lived without this thing. It is great for adjusting all kinds of antennas. I don't even know how to use most of the features of this unit.
I want to be able to connect it in the shack to adjust a manual antenna transmatch without having to transmit. I would like to use a coax switch to switch between the rig and the tuner. The manual says an RF switch may be used for this purpose, provided that the switch has "better than 50 dB port isolation".
The manual has many warnings about not applying RF or any external voltages to the analyzer, because it uses "zero bias detector diodes which are easily damaged by external voltages over a few volts." I am assuming that a quality switch from a reputable manufacturer provides the required isolation, but I wonder if there isn't some circuit which could be installed between the switch and the analyzer which would protect the analyzer from errant volatages, rf, transients, lightning, etc.

Any ideas?
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NI0C
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« Reply #1 on: March 18, 2007, 06:38:51 AM »

I've had my MFJ-259B Analyzer hooked in on the "radio" side of an MFJ-1700C antenna switch for over a year, with no problems.  In fact the analyzer and two transceivers get routed through my amplifier input using the switch.  

This arrangement makes it very convenient to check antennas for resonance point changes, damage, etc.

I think MFJ has since replaced the 1700C with an equivalent model with coax connections on the back of the unit.  The sitch is capable of selecting one of six radios and six antennas.

73,
Chuck  NI0C
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N3OX
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« Reply #2 on: March 22, 2007, 06:48:35 AM »

"I wonder if there isn't some circuit which could be installed between the switch and the analyzer which would protect the analyzer from errant volatages, rf, transients, lightning, etc."

You'd have to worry about it affecting your measurements, especially if you built something faster or that triggered at a lower voltage than the diodes themselves ;-)

I agree with Chuck... just do it.  Don't buy the cheapest switch and you'll be absolutely fine.

Besides, they may be "zero bias detector diodes" which sound a little exotic but they're only $0.34 apiece in small quantities from Arrow Electronics, and not impossible to replace if you have some tweezers, a good soldering iron, and a steady hand.

On a related note, I would suggest not shuffling along the carpet in the winter and then touching a wire that's connected to the antenna input of the MFJ-259B !!

Dan
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
K5FH
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« Reply #3 on: March 23, 2007, 04:10:44 PM »

I also use my MFJ-259 for transmatch adjustment.  What I do is use SO-239-to-BNC adapters on the 259 and on the transmatch and unplug (via a BNC adapter) the xmtr input on the transmatch and plug the 259 into the transmatch.  After the adjustments are made to the transmatch it takes a couple more seconds to unplug and reconnect for on-air.  Not quite as quick or easy as using a coax switch but I never have to worry about forgetting to throw the coax switch and sending 100W into the 259.
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