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Author Topic: MFJ-1700B usage question - multiple transmitters/receivers  (Read 1672 times)
KA4LFP
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Posts: 61




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« on: March 04, 2012, 07:04:10 PM »

Hi all -

I have a potentially stupid question.....
.. but operating under the premise that there are no stupid questions, except for the ones that don't get asked...

I have an MFJ-1700B antenna/transmitter switch -

For those not familiar, here's the manual -
http://www.thiecom.de/pdf/MFJ-1700B.pdf

While the manual does say that you can connect multiple transceivers to it at one time,
it does not make clear as to whether you can or should have those other transcievers
turned ON while you're operating one of the others....

I have several tube HF rigs, including the famed Swan-ThreeDrifty, and most all of these rigs really should be on for a while before being used.
As you Elmers know, the world wasn't always "flip the power switch and go"....


So -- Given the design of that switch, where all unused (non-selected) connections are grounded...
And given the fact that it's a royal pain to get at the coax cables for my rigs
I'd kind of prefer:

- to have the various tube rigs on and stabilized
= not to have to climb behind rigs and unscrew connections
- not have a set of disconnections that must be made up every time I want to change rigs --
.. that DOES seem to be the point of this antenna switch...

So -- the $64.000 question ---

Is it safe to transmit using a rig on switch position "A" using antenna "1" or "2"
while another rig is ON and connected to switch position "D"
I guess it's obvious that what I'm worried about is whacking the receiver front end on the rig that's not currently transmitting...or any other not-so-desirable results for other equipment...

Note -

Rigs are all at or below 200 watts-- I'm not pushing full legal power through this thing,
even though it's rated for that ...


My usual layout:
A - PFT-101EE
B- Swan 350
C - Ten-Tec Omni

1- Hy-Gain AV-18
2- Hustler 5BTV
3- Various wire antennas, including an N4PC loop fed with ladderline..
 



Also -- Given the layout above and the details in the manual (which I don't quite understand, for whatever reason) , do I have to have a PL-259 coax cable run
from Transmitter COM to Antenna COM, if I don't have an SWR bridge in line there?


It almost would appear that there's not actually any connection between transmitter-side
and antenna-side, unless you bridge COM and COM using either equipment or a straight cable.

Thoughts?

73, and thanks...
« Last Edit: March 04, 2012, 07:07:44 PM by KA4LFP » Logged
N4CR
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Posts: 1649




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« Reply #1 on: March 04, 2012, 09:35:58 PM »

Per the first paragraph in the manual... All unused inputs are shorted to ground.

That means that all the rx/tx units that are not selected are shorted to ground.

The worst thing I can think of would be to accidentally transmit on one that isn't selected which would have you transmit into a short circuit. If the unit is designed as described in the first paragraph in the manual you will have no problem with the other receivers being damaged from RF.
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73 de N4CR, Phil

We are Coulomb of Borg. Resistance is futile. Voltage, on the other hand, has potential.
W7SMJ
Member

Posts: 120




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« Reply #2 on: March 04, 2012, 11:02:07 PM »

So -- the $64.000 question ---

Is it safe to transmit using a rig on switch position "A" using antenna "1" or "2"
while another rig is ON and connected to switch position "D"
I guess it's obvious that what I'm worried about is whacking the receiver front end on the rig that's not currently transmitting...or any other not-so-desirable results for other equipment...

Thoughts?

The short answer is maybe.  I have an MFJ-1700C which is similar to your 1700B.  Last night I just noticed that my switch was only providing about 43dB of isolation which is a bit less than what I was expecting.  For some reason I though 60dB was advertised, but I can't find that anywhere in the specs so I'm not sure why I thought that.  I'm only running 100W and 43dB of isolation is "safe" for my other rigs.

I would suggest you try to determine how much isolation your switch is providing by tuning your "grounded" transceiver to the same frequency that your selected transmitter is tuned to.  First verify that the rig you have not selected on the switch is indeed grounded.   Then transmit a signal from the "selected" rig at a low level like 10W - 20W and observe the S-Meter on the grounded rig.  Hopefully you won't have any defelection on the S-Meter and only hear a faint signal.  You can then slowly increase power and watch your S-Meter.  You can then decide if the level of isolation meets your needs and it can be considered "safe".  Note that S-Meters are notoriously inaccurate, so you may not be able to compute what level of isolation the switch is providing, but as long as you stay below S9 or S9 +20, etc. you should be OK.

73,
Scott
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AD5X
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Posts: 1415




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« Reply #3 on: March 05, 2012, 05:11:55 AM »

... I would suggest you try to determine how much isolation your switch is providing by tuning your "grounded" transceiver to the same frequency that your selected transmitter is tuned to.  First verify that the rig you have not selected on the switch is indeed grounded.   Then transmit a signal from the "selected" rig at a low level like 10W - 20W and observe the S-Meter on the grounded rig.  Hopefully you won't have any defelection on the S-Meter and only hear a faint signal.  You can then slowly increase power and watch your S-Meter.  You can then decide if the level of isolation meets your needs and it can be considered "safe".  Note that S-Meters are notoriously inaccurate, so you may not be able to compute what level of isolation the switch is providing, but as long as you stay below S9 or S9 +20, etc. you should be OK.

I can't imagine that you wouldn't see a large S-meter deflection at 10-watts.  10 watts is +40dBm.  If you have 60dB isolation, the receive signal would be -20dBm.  Since S9 is -73dBm, you would be seeing S9+53dB if you have an accurate S-meter capable of reading that level.  In any case, the signal leakage through even a high isolation switch will be LOUD.  But I think most receivers can handle up to +10dBm or more without damage.

Phil - AD5X
« Last Edit: March 05, 2012, 08:17:28 AM by AD5X » Logged
W7SMJ
Member

Posts: 120




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« Reply #4 on: March 05, 2012, 11:32:24 AM »

I can't imagine that you wouldn't see a large S-meter deflection at 10-watts.  10 watts is +40dBm.  If you have 60dB isolation, the receive signal would be -20dBm.  Since S9 is -73dBm, you would be seeing S9+53dB if you have an accurate S-meter capable of reading that level.  In any case, the signal leakage through even a high isolation switch will be LOUD.  But I think most receivers can handle up to +10dBm or more without damage.

Phil - AD5X

Yes, you are absolutely correct, not sure what I was thinking! Huh

KA4LFP - Unless you have some attenuators at your disposal, you're unlikely going to find any meaningful data by following my suggestion above.  In my paticular case +9dBm is the max acceptable level for me and the 43dB of isolation I'm seeing brings me close but within that range at 100W.  If I was running 200W like your situation I would exceed my acceptable level and be right at the +10dBm Phil mentioned above.

So unfortunately we're back to the "maybe" answer since we really don't know the level of isolation your switch provides. 

73,
Scott
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KA4LFP
Member

Posts: 61




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« Reply #5 on: March 05, 2012, 06:37:30 PM »

Thanks guys!

You pretty much answered (well, did answer) my first question, which was "is it safe"

Seems like you all think it is, albeit with some caveats about how loud the receiver will hear the transmitting rig--
which evidently is certainly not enough to do harm.. or let out any magic smoke, hihi ...

And yes - transmitting when the particular rig  isn't selected would be bad -- it sure would be a transmit straight into a dead short.. so will avoid that..  probably by labelling not only the front of the rig with a tag indicating what switch position to have, but also labelling the switch itself....


Carrying on --- do I assume that with multiple rigs and multiple antennas
that I do need either an SWR bridge/wattmeter or a straight thru cable from COM on the transceiver side to COM on the antenna side?

Looks to me that if I had one rig, I'd hook that to Transmitter COM, and switch antennas on the other side ---
but because I have multiples on each side, I have to have a cable from COM to COM.


After that patch cable or SWR/watt meter was  in place between the two COM SO-239s, I could then have a transmitter set on position A and an antenna on position 1 and they'd connect.

Correct??
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W7SMJ
Member

Posts: 120




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« Reply #6 on: March 05, 2012, 10:43:53 PM »

Carrying on --- do I assume that with multiple rigs and multiple antennas
that I do need either an SWR bridge/wattmeter or a straight thru cable from COM on the transceiver side to COM on the antenna side?

Looks to me that if I had one rig, I'd hook that to Transmitter COM, and switch antennas on the other side ---
but because I have multiples on each side, I have to have a cable from COM to COM.


After that patch cable or SWR/watt meter was  in place between the two COM SO-239s, I could then have a transmitter set on position A and an antenna on position 1 and they'd connect.

Correct??

Yes, you are correct!

73,
Scott
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WN2C
Member

Posts: 422




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« Reply #7 on: March 06, 2012, 12:08:28 AM »

This question was asked in a recent QST (Dr.is in in column I think) but the one thing that was not mentioned is turning on the attenuator on the non connected radios to protect the front end further.  I ask... won't it help?  Or am I way off base?

Inquiring minds want to know...
73 de Rick wn2c
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N4CR
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Posts: 1649




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« Reply #8 on: March 06, 2012, 04:21:34 PM »

This question was asked in a recent QST (Dr.is in in column I think) but the one thing that was not mentioned is turning on the attenuator on the non connected radios to protect the front end further.  I ask... won't it help?  Or am I way off base?

Inquiring minds want to know...
73 de Rick wn2c

It depends on how the attenuator is incorporated into the circuit. If it's before the first RF amp it would help. And IMO, that's the right place for an attenuator but there's no guarantee that's how your radio is designed.
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73 de N4CR, Phil

We are Coulomb of Borg. Resistance is futile. Voltage, on the other hand, has potential.
KA4LFP
Member

Posts: 61




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« Reply #9 on: March 06, 2012, 07:05:08 PM »

Thanks for all the help, guys!

I'll post pics here sometime when I get the station fully configured, and you can see how your advice panned out!

73, de KA4LFP
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