Icom IC-751A external RX antenna overload

Discussion in 'General Technical Questions and Answers' started by W0BTU, Oct 4, 2010.

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  1. W0BTU

    W0BTU Subscriber QRZ Page

    Somewhere, I read about a problem occurring when the Icom IC-751A external antenna input is overloaded with too large a signal, thereby damaging the radio. The scenario is when there is a separate receive antenna that is too close to the transmit antenna, especially with an external linear amp feeding the tx antenna.

    I've searched eHam, QRZ, Google, and Bing until I'm sick if it, and I just cannot find it. :mad:

    I know I've read a post somewhere, which included comments by W8JI that back-to-back diodes across the RX input were insufficient to prevent damage to the radio (a relay opening the RX input was required), but I just can't find this anywhere.

    Can you please help me find that thread?

    Also, I have a chance to buy an IC-765 and am wondering if that radio has the same issues.
     
  2. W9PSK

    W9PSK Ham Member QRZ Page

    Doesn't the radio automatically mute the receiver when the PTT switch is pressed?
     
  3. W0BTU

    W0BTU Subscriber QRZ Page

    Yes.

    Since that post, I did find this bit of wisdom:
    http://www.eham.net/ehamforum/smf/index.php/topic,43503.msg298683.html#msg298683

    "I used my IC-751A with the "receiver patch cable" removed from the rear panel, so I could use a separate RX antenna (very common for 80m/160m ops), and connecting a Beverage antenna to the RX jack while transmitting with a kilowatt to a vertical about 150 feet away blew up several parts of the rig. That was fun. ;-)" - WB2WIK​

    I'm building a 1500 watt amplifier, and I have Beverages, and I want to know how not to fry my 751A.
     
    Last edited: Oct 4, 2010
  4. N2VWW

    N2VWW Ham Member QRZ Page

    Put a relay inline with the receive antenna so when the transmitter is keyed the receive antenna is shorted to ground.
     
  5. W8JI

    W8JI Ham Member QRZ Page

    Muting almost always has nothing at all to do with preventing damage. That aside...


    I have some 751A's.

    The first problem you will have is spurious signals. The 751A, like most transceivers, shares stages in the radio with receiving and transmitting. When the signal exceeds the hold-off threshold on switching diodes, the diodes leak transmitter RF picked up by the receiving antenna into the transmitter's early stages. This results in RF feedback causing the radio to oscillate on spurious frequencies.

    A considerable margin beyond where the RF causes a ratty signal, you can damage front end parts. The most likely damage is to the attenuator pad and early filter stages including diodes. It takes about 1/2 watt or more to do this, so you need at least 1500/.5 = 3000 = 34 dB of attenuation or loss in the feedback path.

    I would use both a solid state limiter circuit and a fast disconnect relay, not just one of the two.

    73 Tom
     
  6. AF6LJ

    AF6LJ Premium Subscriber QRZ Page

    Tom as usual hit the nail on the head...
     
  7. K4KWH

    K4KWH Guest

    ANY receiver can be damaged by RF overload/receiver paralysis, not just the 751A. It's just something you must allow for in planning your radio and antenna installation. While it was not as critical in the tube era, receivers could be damaged or paralyzed in this manner. We used various relays and change-overs for our separate xmitters and receivers in those days. Today's transistor rigs are more sensitive, so one must be even more mindful not to overload components. Good luck.


    J
     
  8. K7ZRZ

    K7ZRZ Ham Member QRZ Page

    This discussion has a familiar note with me, as I have recently acquired a Yaesu FT-1000d. With a second antenna connected to the BPF-1 sub-receiver coax connector, and operating with my amplifier of 1000 watts output, I can hear a distinct buzz (sounds mechanical) inside the radio when I talk. It took me a while to diagnose that buzz, but I have now decided that it isn't very healthy to continue to operate this way.... and have disconnected that coax.

    Does any of the above information apply in any way to my situation. And if what you (Tom) are recommending, can you please explain a bit about the "solid state limiter circuit and a fast disconnect relay"

    Moving about so often, as I do, I'm not so much in need of a second antenna connected. But I have noticed that the sub-receiver sounds pretty bad in comparison to the main receiver (in which I have replaced two filters with 2.8 k from Inrad), and I'm wondering if I might have already damaged its (sub-receiver) front end doing what I did.

    Thanks,
     
  9. W8JI

    W8JI Ham Member QRZ Page

    To make a system like this reasonably failsafe for everyone is no easy task. It is easier if it is custom for one particular case, because every system is different. In some cases just slapping two diodes across the feedline works. In other cases it takes a relay, and it other cases the relay won't work! In some cases grounding the receiver input decreases isolation, in other cases it improves it.

    If the relay is not sequenced correctly, there will be a brief period where transmitter RF is able to get into the receive port during switching. This can cause a strong spurious signal for an instant, or can damage components. There also is risk the system can come unplugged from the relay line, and then there will be no disconnect at all!

    People also tend to screw things up and out of habit use a clamp diode across the relay coil. The only safe relay would one that is default disconnected, so if power fails the receive antenna is automatically off. If the coil has a back-pulse canceling diode it slows the relay release, and that means it leaves the antenna on the receiver until the flux in the relay decays.

    Also grounding the receiver input only works where there are no ground loops. We always want to OPEN the receiver line, and as a general rule leave the antenna floating. It often helps isolation to short the receiver input, but only if the layout doesn't have ground loops. There are cases where grounding the antenna or receiver decreases isolation, so it isn't always an advantage unless the layout is good.

    Because of all these problems, and because it is cheaper to repair an external diode system than things in the radio, a failsafe using a lamp fuse and a diode limiter in conjunction with the relay is a good idea.

    The problem with a diode clamp is at about .5 volts peak or less, which is only 0.35 volts RMS or less, most diodes conduct. This voltage would not always be signal voltage, but rather a vector sum of all the voltages present in the antenna system. It is really easy to get .35 volts. To work around that the diodes have to be biased into cutoff, yet clamp very hard when a signal is present. In my installation my diodes clamp at 3 volts peak.

    So I used this with close in antennas:

    RX ant==fast relay NO===fuse lamp==diode 3v limiter===radio

    I lost one IC751A to RF on the receiver antenna. After spending several hours working on the filter board (hard to get at) I decided to be safe. :)

    73 Tom
     
  10. W0BTU

    W0BTU Subscriber QRZ Page

    Thanks, Tom !
     
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