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Author Topic: using two radios so close to one another ??  (Read 2670 times)
GM1ZVJ
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Posts: 152




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« on: June 17, 2002, 06:19:25 PM »

HI,
    Having now set up my ft920 and all my audio gear I would like to use my other radio an ft707 to monitor the ft920`s outgoing audio.is it safe to use them side by side at very low power output ?? or not in the same room ??
                              regards,
                                      john.....
                                 " gm1zvj / mm3zvj "
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WB2WIK
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« Reply #1 on: June 17, 2002, 06:30:00 PM »

Of course it's safe.  Provided the rigs are not directly connected in any way, and you're just receiving the FT920 via a wireless path, you cannot possibly hurt the FT707 this way.  However, you may still overload the FT707, not in a damaging way, but in a way that might create received signal distortion.  It would undoubtedly be best to "receive" the FT920's transmitted signal when no antenna at all is connected to the FT707, so the received signal is greatly attenuated by the rig's shielding.

WB2WIK/6
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W8MW
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Posts: 330




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« Reply #2 on: June 18, 2002, 09:02:54 AM »

The popular point of view when it comes to qualitative evaluation of your transmitted signal is ... get a friend across town to listen.  This is an option.  But it seems to suggest that an amateur sitting inside his own shack cannot judge his own signal. Quite to the contrary, all it takes is second receiver in the shack and some time spent listening to the characteristics of one's own signal. Years back when we had separate transmitters and receivers, listening to our own signal was standard practice.  Transceivers robbed us of that ability.  

That setup you described will work just fine.  I do the exact thing with different equipment.  My monitor transceiver does require a short length of wire attached to the antenna connector, approximately 12 inches.  Receiver overload has never been a problem.  

This is the ideal way to monitor your own signal and know exactly what you are putting on the air.  It is  superior to those built-in monitors which don't detect the final RF to the antenna.  You can take this approach to the next step by using an audio mixer to monitor received signals from the main rig and your own signal from the monitor rig.  I use a small Mackie mixer to do this, driving a good quality set of headphones.

73, Mike W8MW
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K3GC
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Posts: 119




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« Reply #3 on: April 02, 2012, 11:24:17 AM »

Old topic, new question.
My station transceiver is a Yaesu DX5000.  I recently acquired a Kenwood TS-520 (now that takes me back a ways.)
My intention is to use the TS-520 to monitor quality of output.  The problem is that even with no antenna attached and the DX5K at 10 watts the TS-520 front end is seriously overloaded.
How might I go about attenuating the TS-520 receiver so that it is no longer overloaded?
Thanks,
Gene KW4GC
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KA5IPF
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« Reply #4 on: April 02, 2012, 12:35:04 PM »

Put a dummyload on it. One of the small ones that screws directly on the back of the radio.
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KG6AF
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« Reply #5 on: April 02, 2012, 12:47:35 PM »

QST had an interesting discussion of this topic in their January, 2011 issue.  Rig-to-rig isolation came up in a review of an MFJ Transmitter Antenna Crossover switch.  Rigs tend not to be spec'ed with a maximum allowable input voltage on the antenna input, so the numbers aren't terribly precise.  Two data points:

 - The writer surveyed some hams (don't know how many) and was told that most rigs should be fine with an input of 20dBm (1/10W, or about 2.2V across 50 ohms).

 - The ARRL lab protocol says not to exceed a level of 10dBm (10mW, or about 0.7V across 50 ohms).  According to the article, this level has never been observed to damage a piece of receiving equipment.

 - Some air-coupled signals were measured at W1AW.  The highest such signal observed was at 20dBm, between a Yagi hooked to a 1.2kW transmitter and a receiver hooked to a 160-meter dipole.

In short, it seems pretty unlikely that you're going to damage your monitor rig's front end.

As for what to do if you're overloading a rig that has no antenna attached, why not hook that rig up to a dummy load with a coax, and then turn the RF gain way down?  If the rig has a switchable attenuator, use that, too.
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WB4SPT
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« Reply #6 on: April 02, 2012, 02:17:02 PM »

How timely;  I just got my T4X back up on the air, and I wanted to check audio before I made a QSO.  I have a TS50 in the shack as well.  Even with the coax off of the ts50 and the attenuator ON, there was quite significant "noise" on channel with the TS50.  It went away with either the Tx on a dummy load, OR the ts50 on a dummy load.  With even an open SO235, there was 60 db over S9 with some noise products heard. 
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K3GC
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« Reply #7 on: April 03, 2012, 06:52:14 PM »

I tried the dummy load - it did nothing for me.  Another recommendation was to short the antenna connector and that worked a bit.  I still get enough distortion that I really can't judge signal quality.
The TS-520 does not have attenuation - reducing RF gain only reduces noise.
Any other ideas?
Gene KW4GC
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WA8MEA
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« Reply #8 on: April 04, 2012, 05:57:05 AM »

How Timely - The Sequel.

About two weeks ago I was yelling for help in here on how to reset the CPU on a Radio Shack HTX-10 ten meter xcvr.

How did the HTX-10 land up displaying such odd numbers?

RF from a transmitter sitting right next to it!

No, you might not blow out the front end.  HOWEVER....you might have to reset your CPU....and lose all of your memory settings in the process.

73, Bill
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VO1FZ
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Posts: 71




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« Reply #9 on: April 04, 2012, 07:44:58 AM »

I'll be the dissenting voice here.
Be careful.
I blew the front end for 80 meters on my ic-735 by transmitting from my ft-920 on the same frequency.  Trick is - they were both on separate antennas at the time.
I do it all the time with a dummy load connected, but even then it overloads my AGC somewhat.
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K8AXW
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« Reply #10 on: April 04, 2012, 08:36:12 AM »

Quote
Trick is - they were both on separate antennas at the time.
  Not a good thing!

Way back in the day, before transceivers, we used a separate transmitter and receiver with an antenna changeover relay.  It was common practice to monitor ones fist or audio (using headphones to prevent feedback) with the receiver.

The procedure was:  Run the receiver RF gain control back almost to zero. Switch on the transmitter which pulled in the antenna changeover relay.  Transmit and readjust the RF gain control to a comfortable listening level. 

No reason why this couldn't be done using two transceivers, as long as the "listening" transceiver isn't connected to an antenna which could create an excessive RF voltage at receiver front end.
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AE5QB
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« Reply #11 on: April 04, 2012, 09:15:04 AM »

I apologize if this is viewed as hijacking the thread but I think my question will add to this discussion. I recently watched that video called The Last Great Field Day (or something like that) in which a club set up 50 AB577 portable towers and had 50 stations on the air.  I understand that this was comprised of both UHF and VHF stations, but I am not clear how it could be possible to have even 2 or 3 stations on the same band operating at the same time without serious problems.  Can anyone comment on how this was handled? 
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WA8MEA
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« Reply #12 on: April 04, 2012, 10:27:45 AM »

I am glad to see some dissension, as VO1FZ has aptly pointed out.  His post brought to my attention something else that happened to me in the mid-80's.

I recall wandering by my television set one day, while transmitting on my 2 meter HT. ("Olden days", when I used a Winegard and 300 ohm twin-line....)  Suddenly, my TV picture was no more!

I took the TV to the place I purchased it, since they repaired what they sold.  He fixed it in a matter of hours.  I asked him about his thoughts on a little 5 watt 2 meter HT having damaged the set.  He stated, without hesitation, that this was probably the case.  Sure, it was probably a "weak" transistor to begin with. (What he replaced....)  However, he recommended that I transmit in another room next time.

73, Bill
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VO1FZ
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Posts: 71




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« Reply #13 on: April 05, 2012, 05:43:12 AM »

I guess I should add to my previous post, that this was purely an accident.  I've been a ham for 40 years and started with non-matched transmitters and receivers along with dow-key T/R relays, so I was well aware of what could happen.   I would have never consciously done that!

My poor old 735 was turned on with the AF turned down and I just plain forgot about it.

Just another Stupid Human Ham Stunt.
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