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Author Topic: RF proximity  (Read 1197 times)
M5AEO
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Posts: 270




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« on: March 08, 2013, 08:33:11 AM »

Would someone please advise.  I have 2 transceivers, a FT897 and a IC718.  One feeds a dipole, and the other feeds a vertical.  The antennas are only about 10 to 15 feet apart.  My question is: am I damaging one rig while operating the other?  I only run 50 to 100 watts (usually SSB), and the two radios are usually on different bands, but should I take the precaution of unplugging, or at least turning off, the unused rig?

Any advice gratefully received!

Jonathan M5AEO, London.
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N6AJR
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« Reply #1 on: March 08, 2013, 08:42:58 AM »

I have been just that same thing for years.. no problems
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K5LXP
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« Reply #2 on: March 08, 2013, 11:00:08 AM »

It's rather difficult to couple significant power between antennas unless you set out to do so.  The only time I've seen one radio damaged by transmitting energy from another was when they were inadvertently directly connected (patch cables misconnected, or whips touching together in a mobile).  That's not to say you won't get some "undesired operation" while the one is transmitting in the form of overload but that's usually it.  Turning off the receiving radio won't help.  The energy is still coming in through the port and will take out what it's going to take out whether the radio is powered or not.

Mark K5LXP
Albuquerque, NM
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K2DC
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« Reply #3 on: March 08, 2013, 01:16:56 PM »

Jonathan,

   That's difficult to assess without a measurement or rigorous analysis of the coupling between the two antenna systems.  The fact that the two radios are typically on different bands doesn't have much bearing because what's most at risk is the receiver front end, which is susceptible to any very high level HF RF.  However, you have the feedline losses to both antennas, and the fact that the antennas are cross-polarized working in your favor.  I'm not saying it's a completely safe situation, but at 50W or so SSB it doesn't sound very high risk.

73,

Don, K2DC

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WB2WIK
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« Reply #4 on: March 08, 2013, 01:46:10 PM »

I wouldn't worry about it at all.

I've run multiop/multiband contest stations on HF where we ran 1500W output on all bands and antennas were stacked on the same mast on the same tower for different bands.  Using bandpass filters reduced interference, but even without the filters we could still operate that way (with some interference between stations).

Never damaged anything.

Turning the unused rig "off" probably does absolutely nothing to reduce the risk of damage.  Even "off," the receiver is still connected to the antenna.

Older vacuum tube gear was way more robust and could handle a lot of RF voltage, more so than most of the modern SS stuff; however even with the modern SS rigs I haven't been able to damage any doing what you describe.
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G3RZP
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« Reply #5 on: March 08, 2013, 02:46:51 PM »

I had a 2 turn tuned loop with 1.5 metre sides for 160. With 400 watts to a vertical 70 feet away, I measured 80 volts rms on the loop. I used a 6DJ8 push-pull cathode follower because this limited the volts as it overloaded without damage. With a transformer, I would have had  about 4 volts rms into 50 ohms for a Q degradation of 2.

Even with the limiting of the 6DJ8, I still arranged for a relay to short the rx input on transmit.

Measurement, rather than guesswork, is the best.
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K6AER
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« Reply #6 on: March 08, 2013, 03:03:05 PM »

Didn't we just go through this discission last week?
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M5AEO
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« Reply #7 on: March 09, 2013, 01:22:10 AM »

Many thanks for all the advice.  I shall probably unplug the unused rig in future, just to be on the safe side.

M5AEO



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N4CR
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Posts: 1666




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« Reply #8 on: March 09, 2013, 05:33:01 AM »

Many thanks for all the advice.  I shall probably unplug the unused rig in future, just to be on the safe side.

M5AEO

When you say unplug, do you mean the antenna or power?

Does either rig desense when the other one is trasmitting? If not, you have absolutely nothing to concern yourself with.
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73 de N4CR, Phil

We are Coulomb of Borg. Resistance is futile. Voltage, on the other hand, has potential.
WA3SKN
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Posts: 5480




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« Reply #9 on: March 13, 2013, 07:14:55 AM »

Get yourself a couple of antenna switches.  They will ground the unused antennas and eliminate any voltage from getting back into the out radio's input! Don't leave the unused antenna hang open so that static charges can build up.
73s.

-Mike.
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