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NC-173 Transmit / Receive Antenna Proximity...

Discussion in '"Boat Anchor" & Classic Equipment' started by W6ELH, Aug 11, 2018.

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  1. W6ELH

    W6ELH Ham Member QRZ Page

    Hello gang-

    Sorry, but I have another ignorant question to ask: Lately, I've been listening to a local 75-meter AM group, and of the several receivers at my disposal, my old NC-173 just sounds the best. It's connected to a 100' length of #26 wire for an aerial.

    I'm also just about finished putting a Johnson Ranger back into shape, and I'd like to pair it with the National. My concern is this - my transmit antenna lies in very close proximity to the #26 receive only antenna - it runs parallel to and only a couple of feet apart. If I simply throw the "send / receive" switch on the National, does this offer sufficient protection? If I intended for one antenna to share transmit and receive duty, I would rig up a DowKey or other relay, but I'm hoping I can get by without doing so in this case.

    Here's the appropriate section of the manual:

    Screen Shot 2018-08-11 at 10.48.22 AM.png

    Thanks in advance for your guidance! Jim... W6ELH
     
  2. KB4QAA

    KB4QAA XML Subscriber QRZ Page

    Well..... Who knows? With the antennas only a few feet apart, there is a pretty good chance of inducing enough voltage into the 173 antenna to cause arcing and sparking somewhere. I think you would be better off to use a single antenna with a Tx/Rx relay.
     
  3. W6ELH

    W6ELH Ham Member QRZ Page

    Thanks William-

    I was thinking that killing the B+ should mean no amplification and thus no issue, but...

    What's really silly is that I may have already done this in the past. The National is on much of the time that I'm in the radio room, and it's not impossible that I have thrown it into "send" and transmitted via that adjacent antenna, although I do not recall having done so. I don't run over 100 watts, and often much less, so that might help. Hmm...
     
  4. SM0AOM

    SM0AOM Ham Member QRZ Page

    A very simple way to avoid damage to the input circuit of the receiver is to simply
    wire a switch so it short-circuits the receiver input when the transmitter is activated.

    You will not hear anything on the same band where you are transmitting anyway.

    73/
    Karl-Arne
    SM0AOM
     
  5. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page

    Tube type receivers are MUCH less likely to have problems with r.f. from nearby transmitters than do solid-state types.

    You probably will not have any problems. However, installing a relay to ground the antenna connections when transmitting is a good idea. An even better idea is to install a relay, at least a manually operated switch, and use the same antenna for both transmitting and receiving.

    Glen, K9STH
     
  6. W6ELH

    W6ELH Ham Member QRZ Page

    Well, looks like it's a consensus. Thank you very much, fellows.

    This Ranger has a PTT mod which incorporates a 12-volt "ice cube" relay. When I get it powered up, I'll see if there is a usable voltage on one on the contacts which I can snake out to an external relay box.

    Again, thanks! Jim... W6ELH
     
  7. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page

    By the way, the BC-348 series receivers, that have a very similar front end configuration to the NC-173, have a 470,000-ohm resistor to ground from the antenna terminal as a "protective" device.

    Glen, K9STH
     
  8. W6ELH

    W6ELH Ham Member QRZ Page

    Hmm, that's interesting! One reason I wanted to continue to use the receive antenna is that it seems to pick out the 3.870 AM signal in a far superior manner to the end-fed compromise antenna I transmit on. I have a patch panel, so I have been swapping antenna feeds, and it is indeed better. But I think I may have found a way to cheat and get a far longer version of the end fed extended out over the canyon where it will also be much higher due to steep sloping ground. I'm going to have to be sneaky and hope I don't get caught. If I get away with it for a while, perhaps I can string a proper dipole up in the same space.

    Hmm... I think I see some night time "ninja operations" in my future! :cool:

    Jim... W6ELH
     

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