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Author Topic: coax switch  (Read 2881 times)
KJ4NBM
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Posts: 4




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« on: December 08, 2009, 07:09:17 PM »

My station setup right now is pretty modest with just one HF transceiver. I'm soon going to add a second rig. I want to run both transceivers to my MFJ versa tuner which only has the one transmitter input. Will it be fine to use a 2 position coax switch to switch between the two transceivers?
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K9KJM
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« Reply #1 on: December 09, 2009, 12:26:41 AM »

Sure. That will work.

If I were doing it, I would shop around for a three position coax switch, With the center position that puts the antenna to ground, To add a level of safety from lightning. Put the switch near where the coax enters the building, And ground the switch to a good outdoor ground system.

I have found that most any coax switch will usually work out just fine at HF.
But don't try to use the "cheapo" coax switches at VHF or UHF!  Only the stripline types should be used for the higher frequencies.

Do be careful when using two radios that you shut the one OFF that you are not using!
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K9WI
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« Reply #2 on: December 10, 2009, 10:06:10 AM »

Yes it will work, but you could accidently transmit the non-selected rig into an open circuit.
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KH6AQ
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« Reply #3 on: December 10, 2009, 02:24:00 PM »

It will work. MFJ has a suitable 2 position switch.
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KJ4NBM
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« Reply #4 on: December 11, 2009, 12:47:25 PM »

"Yes it will work, but you could accidently transmit the non-selected rig into an open circuit."

Yeah, I thought about that. I'm always careful to make sure where the switches are set on the mfj tuner and start with low output for tuning.

My other concern is RF bleeding across the switch from the transmitting rig into the front-end of the other transceiver and doing damage. Some people say to make sure the rig you are not using is turned off.
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W8JI
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« Reply #5 on: December 14, 2009, 10:26:42 AM »

"Yes it will work, but you could accidently transmit the non-selected rig into an open circuit."

Yeah, I thought about that. I'm always careful to make sure where the switches are set on the MFJ tuner and start with low output for tuning.

My other concern is RF bleeding across the switch from the transmitting rig into the front-end of the other transceiver and doing damage. Some people say to make sure the rig you are not using is turned off.

I don't know why turning off the other radio would make any difference. Of course I don't know why it would be a problem if it was on either. I think the idea you have to turn off the other radio is a myth.

Let's consider a 100 watt radio with a really poor switch. The worse cases I've ever tested at HF were -25 dB between ports and I've tested many dozens of types. Most are -35 dB to -50 dB, and some are in the -80dB range and higher.  The worse case isolation is always on 30 MHz, and they get better on lower bands. That's only 1/4 watt worse case worse band, or about +25 dBm.    Most signal generators will put out +25 dBm and I've never seen that level blow up a radio, ever.   

I'd be very confident in telling you any switch you would buy, unless it was seriously defective, would have more than enough isolation. Also I'm very confident if there is enough level to cause damage, which is usually well over one watt, it won't make a difference if the radio is off or on. The problematic semiconductors and resistors are almost always connected no matter if power is on or off.

By the way, the worse switches I've tested ground the unused ports! They have a problem because of ground loops inside the switch.

Tom
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KJ4NBM
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« Reply #6 on: December 14, 2009, 01:48:29 PM »

Thanks Tom, you seem to have a good perspective on the issue from actual testing experience. It sounds like most switches will give adequate isolation between the ports. I'll probably go with an AlphaDelta or Daiwa switch.
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K1XV
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Posts: 69




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« Reply #7 on: December 16, 2009, 07:05:43 PM »

The absolute best would be a Bird 74 Coaxwitch, sometimes available at hamfests and ePay.
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