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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 Premium Subscriber 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
  11. AI3V

    AI3V Ham Member QRZ Page


    A duplexer would work, IF the OP wanted a SINGLE BAND soloution, Using only ONE transmitter, and ONE receiver, some 1/4 wave lines, and PIN diodes will work fine, and is easy to build.

    It's done every day in millions of RADAR sets around the world.

    His problem is multiple transmitters/receivers operating on multiple frequency bands.

  12. ZL1UZM

    ZL1UZM Ham Member QRZ Page

    Can you please tell us where we can find a switch like that, that can switch faster than 3.3x10^-9 seconds (assuming a distance between transceivers of 1m)?
  13. K2WH

    K2WH Premium Subscriber QRZ Page

    Yes it would provided he never ever changes frequency and maintains the 600khz seperation. Beyond that, nothing would work.

  14. K2WH

    K2WH Premium Subscriber QRZ Page

  15. AI3V

    AI3V Ham Member QRZ Page

    Nope, don't need any frequency separation at all, The RADAR set uses a common antenna to send and receive, all on the same (or nearly the same for Doppler sets) frequency.

    The basic idea is that a 1/4 wave section of transmission line, will reflect the opposite impedance from end to end.

    Also remember that a 1/2 wave section will reflect the same impedance end to end.

    The RF causes the diodes to switch from a insulator with no rf- to a short with rf, by arranging the receiver,transmitter,line sections, and diodes correctly you can switch automatically.

    Here is a better diagram:

    The Radioactive TR and ATR tubes are only needed with high power like in a pulsed RADAR, PIN diodes are OK for low powers.


    P.S. note that in this example, they use Tritium as the radioactive source, clever from a profit standpoint, as the short half life of tritium will limit the tube life, even if sitting on a shelf in the supply building!!!
  16. VK6ZGO

    VK6ZGO Ham Member QRZ Page

    Or a box in line with the PTT which extends a command to the changeover to operate first,then a delay before the Tx operates.--Easy enough for the 897but a tad difficult for the handheld rig.
    It would also have to have a short delay at the other end to make sure the Tx was off before it switched back to the other Rx.

    73 VK6ZGO
  17. K0RGR

    K0RGR Premium Subscriber QRZ Page

    I agree with the post that this is not workable without serious modification to the radios involved. Indeed, the switchover must happen BEFORE either transmitter starts to put RF on the line! This is, indeed, done in weak signal work to switch remote preamps in and out of the line using a sequencer.

    I suppose you could create some sort of monstrosity that would block the RF from getting on the line to the antenna until after the switchover was completed. If you used a solid state switch, you should be able to achieve the switchover fast enough to avoid damaging the transmitter. Please observe the weasel word "should" in that statement. You might be able to use phase delay lines to avoid that problem.

    The microwave component discussed here is called a circulator or isolator. There are 3 port circulators and 4 port circulator. A circulator redirects the RF energy applied to one port to the next adjacent port. So, the transmitter could be attached to port 1, and the antenna on port 2. Any reflected energy from the antenna would be redirected to port 3 instead of port 1. You could have a receiver on port 3, or a load to absorb any reflected power from the antenna.

    If you could construct a practical isolator for 2 meters, you could use two of them - one on each radio. Microwave isolators use ferrite materials, and would be huge at 2 meters, not to mention the magnets required to make it work. There is a way to make isolators using waveguides and I've seen designs for isolators made from coax. NOTE: several manufacturers offer something that they call an 'isolator'. This is basically a big RF choke for the outer shield of the coax and it is NOT what I'm talking about. The 'isolator' I'm talking about is actually what's known as a circulator. I found one commercial unit from Wenteq that would probably work, but I can't find a price on it. If you have to ask, you probably can't afford it.
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