Automatic RF sensing antenna switch / relay for 2mtr and 70cms?

Discussion in 'General Technical Questions and Answers' started by G7IVJ, Oct 24, 2009.

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  1. G7IVJ

    G7IVJ Ham Member QRZ Page

    Ladies and Gentlemen,

    I have a dual band 2mtr&70cms high gain vertcial mounted quite high off the ground and this is used on my Yaesu FT-897D normally monitoring the 2m FM calling channel which here in the UK is 145.500 called S20

    Anyway, I recently have purchased a low powered handheld and my idea is still to use the vertical antenna connected to the 897D monitoring the FM calling channel all the time, but at the same time use the handheld scanning around the 2m band on the same single antenna.

    I could simply do this by installing a T piece connector off the coax and connecting to the two radios - however, if I transmit on one of the radios I know it will damage the other.

    So what I would like to buy is an RF sensing antenna switch so I can receive off both radios at the same time on the one antenna, and when I transmit on either of the radios it disables the other port and therefore wont damage the other radio.

    Anyone point me in the direction of where I can buy such devices for 144mhz and 430mhz and also what is the actual name of the product ?

    Many thanks for your help... Simon
  2. KB3X

    KB3X XML Subscriber QRZ Page

    Last edited: Oct 24, 2009
  3. G7IVJ

    G7IVJ Ham Member QRZ Page

    Thanks for the reply.

    I know what a duplexer is, a duplexer is not what I am looking for.

    I am wanting to 'share' on RX one antenna on a single band on two or more radios, ie

    1 x 2m vertical antenna

    1 x 2mtr multimode
    1x 2mtr handheld

    To be able to receive at the same time on both radios - but when I TX on one of the radios it does not damage the other radio.

    I guess it is an RF sensing antenna switch set up in reverse, 2 radios off one antenna (same band)
  4. M3HKU

    M3HKU Ham Member QRZ Page


    Dont duplexer / diplexers have to operate on two different band / freqs for the tx side of things. For example one side passes 2m and the other 70cms ?

    I may be wrong but if you wanted to be able to TX on either radio this would not function as they are both 2m sets.

  5. K2WH

    K2WH XML Subscriber Volunteer Moderator QRZ Page


    Since both radios will be used on the same band, you cannot use the Diplexer as another posted to you. They are for radios on different bands. An RF switch is fairly easy to build, and involves only a few parts. However, you will need a coax relay that is activated by the RF sensing unit. The RF switch or sensor can be as simple as a single transistor that is biased on via rectified RF from your transmitter which in turn turns on a small DIP relay or a larger relay which will "Key" or turn on the coax relay to route proper rig to the antenna and disconnects the other from the antenna. I would suggest a Google search for a schematic for one. I'm sure there are many different and simple designs.

    In other words, RF is sensed, rectified to DC since RF is an AC signal, then turns on a single 2n2222 transistor which in turn allows a simple multi-pole relay DIP or plug in relay to activate which in turn keys the coax relay. However, timing is important and it is important to disconnect the receiving rig before the transmitting rig puts RF into the line. This could damage the other rigs receiver. So it is important the coax relay switches first.

    Or a simpler method might be to use 2 different antennas.

    Last edited: Oct 24, 2009
  6. G7IVJ

    G7IVJ Ham Member QRZ Page

    Thanks Bill - that is exactly correct and what I am trying to do is buy a device ready made that does this... Any ideas?
  7. M3HKU

    M3HKU Ham Member QRZ Page

  8. AI3V

    AI3V Ham Member QRZ Page


    You are playing with fire!:D

    By the time the RF is sensed, and the relay switched, you will probably damage either the rx of the big rig, or, the tx of the handheld, or both.

    Seperate rx and tx setups use what is called a sequencer, to delay the transmission of rf untill the relay is safely switched.

    Unfortunatly, there is no way to interface a sequencer to your setup.

    Best to stick with 2 antennas.



    On 70cm SSB I use a hf rig driving a Microwave modules Transverter, a mast mounted preamp, and a solid state linear.

    When I transmit, the sequence is as follows:

    1-Press PTT
    2-Preamp turns off
    3-Changeover relay switches
    4-amplifier keys
    5-hf rig keys, and rf is generated

    The steps are seperated by about 50ms

    When I let go the PTT, the reverse happens

    1-hf rig un-keys, and rf stops
    2-amplifier drops out
    3-changeover relay switches back to rcv
    4-preamp turns on
    Last edited: Oct 26, 2009
  9. ZL1UZM

    ZL1UZM Ham Member QRZ Page

    Your RF sensing switch would have to switch faster than the speed of light.
    You need a switch on top of your PTT that activates before the PTT does.
  10. WA7KKP

    WA7KKP Ham Member QRZ Page

    What you need is an electronic T-R switch

    These used to be quite common on HF, in spite of the fact that under certain circumstances they could cause TVI . . .

    I'duse a receive-only 'scanner" instead of the HT for scanning the band, through some sort of electronic preamp/isolator. I don't know if they're easy to build at 2 metres or not -- probably tricky at best.

    Diplexers and duplexers won't work -- duplexers are only good for a pair of frequencies (repeaters) on the same band, and diplexers are for different bands.

    Gary WA7KKP
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