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Author Topic: Adding a bypass switch to a tuner  (Read 3337 times)
KF7ATL
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Posts: 51




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« on: January 12, 2012, 09:08:23 AM »

I would like to add a bypass switch to my MFJ 901B tuner. Is that possible, and has anyone done it?

Garth, KF7ATL
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W5DQ
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Posts: 1209


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« Reply #1 on: January 12, 2012, 09:38:35 AM »

A simple solution would be to use the MFJ-1705 'RF BYPASS SWITCH, DC-60 MHZ, 300W'. Although I personally don't like the switch used (it's a slide switch), the design should do exactly what you want to do. There is also a high power model with a -H suffix which only $10 more and appears to have a better looking switching element (as far as I can see on the MFJ website). I have not used either of these so I can't say good or bad about the performance.

Gene W5DQ

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Gene W5DQ
Ridgecrest, CA - DM15dp
www.radioroom.org
K0IZ
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Posts: 737




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« Reply #2 on: January 12, 2012, 06:11:27 PM »

Although it would cost a little money, you could add two coaxial relays to input and output to allow transfer.  No changes to tuner, and very low loss.
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WB6BYU
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Posts: 13026




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« Reply #3 on: January 12, 2012, 08:52:12 PM »

One of my home-brew tuners uses a big DPDT toggle switch for the bypass function.
You do have to worry about the voltage and current ratings of the switch, though, b
because half of it will be in the output circuit where voltages and currents can be high.
A ceramic wafer switch would be better at higher power levels.

If you don't want to drill a hole in the front panel of your tuner, just mount the switch
in a separate box with 4 connectors so the signals go directly from input to output in
one position, while those two ports connect to the two ports that go to the tuner in
the other position.  In fact there are some commercial switches built with this type
of function.

For example, with 4 ports (IN, OUT, A and B), you'd wire it to connect IN - OUT and
A - B in one position, and IN - A and OUT - B in the other.  A would go to the input
of the tuner and B to the output.  Then you can also use the same switch to connect
two antennas to two rigs in either combination.
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AB4ZT
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Posts: 172




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« Reply #4 on: January 14, 2012, 06:50:05 AM »

I have an original MFJ-901 that I added a bypass switch to a few months ago.  Since the tuner case provides the shield side of the connection, you can use a SPDT switch.  I used a decently sized radio shack paddle switch mounted on the left side towards the rear - there is room enough back there.  On the switch, the incoming center conductor goes to common center pin on the switch, one pin then to the input of the tuning circuit, and one pin to the outgoing center conductor.  It has worked fine for me.

73,

Richard
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ND6P
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Posts: 69




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« Reply #5 on: January 22, 2012, 08:47:24 PM »

A pair of two-position coax switches will work also.
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KM3F
Member

Posts: 496




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« Reply #6 on: February 01, 2012, 01:43:58 PM »

Working with RF power where matching and power requirements are  important, I would not consider a commom toggle switch.
Use a single coax transfer type to switch the coax from the radio to another application.
Mount it on the rear.
Takes a power source to operate the relay winding  but handles all the parameters without upsetting the transmission line impedance from the radio to the next application.
Hook up using short Coax jumpers.
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K8AG
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Posts: 349




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« Reply #7 on: February 04, 2012, 08:05:58 AM »

You might look for a dpdt knife switch.  A ceramic one would be best.  They are impressive in the shack.  But of course then you have exposed RF contacts.

73

JP
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