Relay Selection Help Please

Discussion in 'General Technical Questions and Answers' started by N4MU, Mar 1, 2020.

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  1. N4MU

    N4MU Ham Member QRZ Page

    Could someone suggest a small, encapsulated relay that would be available from Newark, Mouser, DigiKey et al that meets the following criteria.
    I plan on building a custonized, homebrew antenna/transceiver switch. Power is up to 600 watts PEP and coil should be 12VDC. SWR at the antenna end is also a factor. Latching/non-latching TBD. Multiple pairs of contacts probably beneficial (i.e. DPDT, etc.)
    I just have trouble searching these sites and trying to narrow down from a gazillion items.
    Thanks for suggestions/info.
  2. K7TRF

    K7TRF Premium Subscriber QRZ Page

    I use these for remote antenna switching in nominally 50 ohm systems:

    These work fine for antenna switching applications.

    For rig switching, port to port isolation becomes important so you might consider configuring the relays in a Contactor, aka X configuration which provides more isolation than single contact switching. Board layout also becomes important if you want as much port to port isolation as possible. It's also possible to run multiple relays on each port (one set up as an X configuration contactor to select the output port and one on the output port to further isolate and ground the unused port). Port to port isolation is important when switching 100 watt or higher power rigs so that switch leakage from the transmitting rig doesn't damage the non-selected receivers.

    For instance if you have 50 dB of isolation between the active switch and the other ports the receivers on those other ports and you transmit with a 100 watt rig (+50dBm) the other receivers will be seeing a 0dBm signal on their antenna connectors. That's roughly 73dB over S9 which is a lot of signal and though that should still be below damage limits for most rigs it doesn't leave a lot of margin. If you're specifically designing a rig selector switch I'd target 80dB to 100dB of port to port isolation and that takes a bit more effort than simple antenna switching applications.
  3. WA7PRC

    WA7PRC Ham Member QRZ Page

    When I modified my 80m cage inverted vee (fed w/ about 85' of coax) to function as a vertical w/ top hat on 160m, I used a SPDT vacuum relay to do the switching. Since it needed to be outdoors, I fitted it into a weatherproof PVC outlet box:
    The relay is a Jennings RJ1A w/ 12VDC coil, powered via a "bias tee" in the shack. While mine is SPDT momentary, there are LOTS of other vacuum relays. Max Gain and RF Parts (and others) offer vacuum relays.

    An alternative is one of the many multiple relay boxes from Ameritron, DX Engineering, et al. They tend to use relays w/ contacts that are open to atmosphere. Instead of "Form C" contacts, they use "Form X" contacts to minimize reactance (thru HF).
  4. WA4SIX

    WA4SIX Ham Member QRZ Page

  5. WA7PRC

    WA7PRC Ham Member QRZ Page

    A nice thing about vacuum relays is, even small ones withstand yuuge voltage that the OP might see w/ high VSWR + high power. And, since the contacts are in a vacuum, they'll NEVER oxidize. :)
  6. KH6AQ

    KH6AQ Ham Member QRZ Page

    The relay used by AD5X in his ALS-600 QSK mod should work. It's an Omron G6A-274P-ST40-US-DC12 rated for 1000 volts and 3 amps carry current. The current rating is sufficient for 600 watts CW/SSB and 450 watts RTTY. The price is $5.08.

    Mouser link

    Relay datasheet

    AD5X article
    Last edited: Mar 1, 2020
  7. N4MU

    N4MU Ham Member QRZ Page

    Thanks to all above!!! All good ideas and facts. The Ameritron/MFJ would be the easy way out of course. I have had them in the past...even had the Heath equivalent at one time. I want to steer clear of the current ones because last time I had one I could still hear moderate signals on my receiver from the non-selected ports. I was quite surprised...and disappointed. Also, I would rather not use a bias tee. I will use a CAT cable for control/status to the remote device. Keep 'em coming and thanks!
  8. WA7PRC

    WA7PRC Ham Member QRZ Page

    That's a small signal relay w/ contacts rated 250VAC/220VDC (see page 3). It's similar to the relays suggested by others mentioned in this thread. With high VSWR mentioned as an operating condition I envision lotsa arc'in & spark'in. A small vacuum relay is rated to withstand several KILOVOLTS and is the only type of relay I would use to switch HV RF. :)

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