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TX/RX swith for one radio, two antennas

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by W4EAE, Jun 17, 2019.

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  1. W4EAE

    W4EAE Subscriber QRZ Page

    I live on a city lot with a pretty good amount of RF noise. I am talking about 'static' that from time to time will blanket 2MHz-15MHz. I have no problem getting my signal out with 100w, but sometimes only the strongest stations are readable to me. This is particularly a problem in that I am a net controller for an 80m SSB traffic net that gets 50+ check-ins daily.

    After investigating and experimenting with various possible solutions (different antennas, different installations, different locations on my 1/2 acre property [narrow and deep]), I have found that 100ft wire arranged in a loop on the ground gets rid of the noise and allows me to copy everyone--with lower signal level, but GREATLY improved signal to noise ratio.

    What I am now after is pretty accurately described in the thread title. My radio has only one antenna connection for HF, and I am not about to spend $2K on a new radio that I do not need just to get a second antenna connector. I was somewhat surprised to find that such switches seem to be available for SDRs, but none are speced for usage the other way round.

    Any ideas (about a switch)?
     
  2. WA8UEG

    WA8UEG XML Subscriber QRZ Page

    You could just use a simple 2 position antenna switch, you will have to manually switch it between tx & rx. You could also use a dow key type relay that you switch between TX & RX with a conveniently located toggle switch. Even if your rig had 2 antenna ports you would still need to switch between port 1 and 2.
     
  3. W4EAE

    W4EAE Subscriber QRZ Page

    I was really hoping to find something triggered by PTT. This may be the route I have to take though.

    Reading some manuals from Yaesu and Kenwood, there are several rigs which allow you to set TX and RX to different SO239 ports on the radio.
     
  4. W1VT

    W1VT Ham Member QRZ Page

    The MFJ 1025/1026 can be used as a PTT controlled switch for a receive antenna. It can also phase the receive antenna with the transmit antenna.
     
  5. WA7ARK

    WA7ARK Ham Member QRZ Page

    What is your radio?

    I built an external, PTT-controlled, antenna relay for my ICom 7300 for about $3 cost.

    Basically, I use an automotive sugar-cube Form C relay , a snubber-diode wired across the relay coil, a couple of S0-239s and a small Bud Box.

    My Icom radio has an RCA phono plug Keying output that can drive the relay directly.
     
    WB5YUZ likes this.
  6. W4EAE

    W4EAE Subscriber QRZ Page

    The radio is a Yaesu FT-991A. Keying output is on a 9-pin DIN, and would be easy to get to phono.
     
  7. WA7ARK

    WA7ARK Ham Member QRZ Page

    Look at your owner's manual for the detailed specs on what the keying circuit inside the FT-991A can drive. The keying circuit in my Icom can sink 0.25A when keyed, and stand off 35Vdc when in receive mode, so it can drive the relay coil directly without any additional circuitry.

    We need to determine if your Yaesu can drive the relay coil direcly, or if it needs some help? The relay is 12Vdc, and requires about 135mA to pull it in.

    About your noise problem on receive? Describe your transmitting antenna in detail, including any grounding, feedline lengths, how it enters the house, tuners, Common-mode current chokes, baluns, etc.
     
    Last edited: Jun 17, 2019
    WB5YUZ likes this.
  8. W4EAE

    W4EAE Subscriber QRZ Page

    Transmitting antennas are two:

    80m OCF coax-fed (4:1) dipole at 70ft, with the short leg sloping to 18ft.

    Coax 120ft RG-58. (I have tried 80ft runs as well, with the coax slopping toward the house.

    Coax enters the house through a window-fitted panel. Panel (and therefore coax shield) is grounded. 1 1/2ft RG-58 pigtails on the inside for the radio. Radio case is grounded.

    The noise is not constant, but intermittent (mostly for hours at a time, not minutes). The noise is even, not pulsing or variable. On SSB & AM is sounds like static. On the waterfall, it looks like static. It is also incredibly wide-banded, 80m-20m. When it is present, it is overwhelming. When it is not present, I have beautiful audio and a dark-blue waterfall.

    In regards to common-mode current, chokes make no difference to the noise. I have made coax chokes of varying turns, and employed a My Antennas common mode choke that has worked quite well in portable situations to no avail here.

    The only tuner I ever use is the one built-in to the radio. It is off more often than on.

    I have killed the mains and run off of a battery, and it makes no difference. I even convinced my neighbor to let me kill her mains as well, and it made no difference.

    I have satisfied myself (and everyone else who I have consulted with locally) that the noise is being received by the antenna itself, not being picked up by the coax. I have lowered the antenna in increments, but the noise does not disappear until it is nearly on the ground (and not very useful for transmitting). This is what spurred me to try the long wire on the ground. I did a web search for an 'antenna on the ground' and found lots of others employing this type of antenna, but in a loop. When I finally set it all up, eureka!

    The last 80m net I ran, I used a second transceiver connected to the loop on the ground for the QRP check-ins and was able to copy every single one of them. None were readable on the transmit antenna. To make this work, I actually called for QRP check-ins, then quickly connected the coax to the receiver, then disconnected it again before transmitting. I did not use it for the rest of the net because it was so cumbersome, and had to take 9 check-ins via relay. I am confident that those relays would not have been necessary if I could have continued to receive on the loop.
     
  9. WA7ARK

    WA7ARK Ham Member QRZ Page

    The OCF antenna almost certainly uses a voltage balun (poor at blocking common-mode current on the coax) at its feedpoint. The reality is that if there is common-mode current on the coax during transmit, it means that the coax can also pick up RFI down near the house end, conduct it back up the coax toward the antenna, where the noise excites the antenna wires, and thereby couples into the inside of the coax to contaminate the real signals you are trying to receive.

    The "grounding" provided by the window panel and its ground rod will help by shunting some of the common-mode currents (transmit or receive) to earth. However, it may help to add a high-quality coaxial common-mode choke CMC (aka 1:1 current-mode unun) onto the coax a few feet along the coax from the house toward the antenna. I am surprised that if you added the CMC in this part of the coax, it didn't help.

    Do not put the CMC right below the OCF transformer, unless the OCF maker specifically says that this is ok. I tried this with a poorly-designed OCF antenna (Buckmaster), and it stopped working.

    Well, it is simple to build the automatic T/R relay as described above. Have you found the keying specs on the Yaesu?
     
  10. W5LZ

    W5LZ Ham Member QRZ Page

    The simple answer is a two position antenna switch. Wanna make it 'automatic'? Build some delay into it or it'll be chattering a whole lot.
     

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