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Industrial Relay's for Remote Antenna Switch

Discussion in 'Homebrew and Kit Projects' started by K8ATG, Aug 13, 2017.

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  1. K8ATG

    K8ATG Ham Member QRZ Page

    Hello All,

    I'm interested in building a remote antenna switch for switching 4-5 antennas to a common coax running from the bottom of my tower to the shack. I have a handful of industrial type relays laying in my junk box that I hope I can use for this.

    My thought was to use a multi rotary controlled selector switch and energize each relay coil that is dedicated to an antenna port. My question is that will the contacts in the relay change the impedance to where I get unsuitable SWR?

    My design would be to have 5 SO-239 chassis mount connectors on a common bus and then the center conductor would have a few inches of solid 12 copper running through the contacts on each relay. Rotor control cable would be used to energize and de-energize the relay coils based on the position of the selector switch.

    Is this more complicated than just what I'm thinking or is my idea worth pursuing?

  2. N3DT

    N3DT Ham Member QRZ Page

    Check this out. I got one and he has a list of Mouser parts to buy, I think I put the whole thing together including the Hammond wx proof box for about $150. It's real nice. I've been pumping my 80B through it and no problems. Just don't run it into high SWR. Not bad on 6M too, perfectly acceptable. There's also a $60 one on ebay from eastern EU that doesn't look too bad, includes all the parts. But the SJ2W one is a fine piece. Mine's been up over a year now. I love how it fits in the Hammond box. Save you a lot of grief. I use a rotary switch with it and telco wire to the relays.

    I've seen the automotive relays used for switches, but I doubt if they have very good SWR on the higher bands. Depends on what you want.
    N2EY and K8ATG like this.
  3. WA7PRC

    WA7PRC Ham Member QRZ Page

    The least added reactance uses relays with "Form X, Y, or Z" contacts, sometimes called "bridging" contacts. Form A, B, or C contacts typically require a wire to make connection from the movable contact to the interface terminal. That adds series reactance that can matter on higher frequencies.

    This shows the various contact forms: (page 5). The Ameritron RCS-8 (datasheet) that came with my used tower uses relays with Form Z contacts. It automagically grounds unused ports.
    N0TZU likes this.
  4. AA7QQ

    AA7QQ Ham Member QRZ Page

    When you say, "Industrial", could you give a part number? I have "Industrial" relays that are so large, that very few would use them above 40 meters.

  5. WA9WVX

    WA9WVX Subscriber QRZ Page

  6. AI3V

    AI3V Ham Member QRZ Page

    Yes you can use "industrial relays" :)

    Yes, they will affect the vswr.

    You didn't mention frequency, or just how big these relays are, but at HF, you should be fine.
    Keep the total length of wire in the switch, including the relay wires, to a fraction of a wavelength , swag 1/50 or less and it will work like a champ.


    A note about relay impedance.

    relays that are not specifically made to have a particular RF impedance in some cases have been found to have a 50ohm z by folks that build RF switches.

    Use the exact relay specified, or be ready to measure and experiment.
  7. AA7QQ

    AA7QQ Ham Member QRZ Page

    Go to w6pql's page & get the relays he uses. They are decent for what you want to do. Otherwise, you may have to figure compensation for lead & arm length.
    An example is that some AmeriMfj amps & others need a capacitor to compensate the lengths for receive, or the SWR will be off with the amp in bypass.

  8. N3DT

    N3DT Ham Member QRZ Page

    The SJ2W relays short the unused antenna to ground too. Only thing that doesn't happen is when all relays are off, the output (single port) is not hooked to anything, it's open and still connected to the feed line.
  9. NM7G

    NM7G Ham Member QRZ Page

    AI3V is right on target.

    Cluster SO239s as close to relay(s) as you can possibly place them, even if that means you need to craft a bracket or two. I've successfully used garden variety industrial relays, both open frame and sealed at 21 MHz without impedance problems, but I took pains to keep wiring very short. I'm talking about 100 watts out. 1500 watts could present a problem if RF current carrying portions of relays are too thin, or the actual contacts are puny.

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