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2 close antenna's, what Tx protection do I need?

Discussion in 'General Technical Questions and Answers' started by W1EZE, Jun 11, 2021.

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  1. W1EZE

    W1EZE XML Subscriber QRZ Page

    I have antennas that are in close proximity, what sort of protection do I need for each transceiver?
    Rigs are Yaesu 991a, Icom 735 and a Xiegu G90.
    Thank you & sorry for the noob question.
  2. K8DO

    K8DO Ham Member QRZ Page

    During my contesting years with 5 radio transmitting at time I use the ICE model 419B. Having given up contesting 15 years ago I do not know what is currently available.
    W1EZE likes this.
  3. K8XG

    K8XG XML Subscriber QRZ Page

    First how close is close? On the same pole? 20 feet apart 40 feet apart? How much power you running? assume no more than 100 watts?

    I disconnect any other radio that is on the same pole as another when I use it. My g90 is on a 80 dipole that is at the top of my 2- meter /440 mast. I do not disconnect it or the 2/440 rigs to use either one.

    If I TX on an antenna 43 feet away 40meters from that dipole the g90 is connected to and and on 80m from my 991a/7300/fr-2000 it will chop up the receive on a different band. I will not do that on the same band. If I fire up my amp, I even have disconnected the 2/440 rig upstairs, had the firmware messed up on it once from that even with the antenna 40 feet away. all other radio disconnected when the 1Kw amp is on even at 500 watts
  4. KB0MNM

    KB0MNM Ham Member QRZ Page

    The technical name for what you may need is called a blanker. This is something that you might construct yourself, if you are extremely handy with electronics. Any strong signal transmitted on an antenna in close proximity to another antenna tends to 'desense' the receivers of other rigs. That is to say, it may produce such a strong signal that the other receivers can have either a temporary lack of sensitivity to signals or even permanent damage. If you want the 'other two' rigs to be 'listening' while you transmit, filters may be needed with thoughts about both fundamental and harmonic overload. If not, then each transmit PTT ( abbreviation for push-to-talk ) should intentionally disconnect ( A.K.A. *blank* ) the two other rigs. In a better system, the blanking is done and there is a very slight delay added to the transmit to ensure that the other receivers are indeed disconnected. The tough part is paying for appropriate 'antenna relays' which are generally specialized relays or contactors or triacs which can handle about 3 times more wattage than you might expect. The reason for this is simple: Standing Wave Ratio- also known as reflected power or 'inductive kick' in switching. To start with, you will need the microphone pin-outs for the PTT circuits, and an understanding of how to add relay or sense circuits to them. You will also need a small power supply for the P.T.T. sense/relays and receiver blanking switches ( usually coaxial ). Indicator lamps are a nice touch.
  5. K8XG

    K8XG XML Subscriber QRZ Page

    Depending on how much power you run, you may cause front end damage on the radios still left on but just switched off the antenna switch. Had a few of those. i wont leave them on anymore or connected to the switch that has a 60db rating... Have had many friends with 2/440 radios need front end diodes replaced.

    I have had the full set of array systems RF filters for field day [80/40/20/10] you still needed distance to make them work well to not get receiver ripping/trash into the other radios but they knocked it down about 2-3 times over not having them.
    KB0MNM likes this.
  6. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    If you're working HF (the 991A can work VHF also, but the Icom and Xiegu cannot), why would you have an antenna connected to each one?

    Better to use a coax switch so you can select one rig at a time and isolate the others. If you have multiple antennas, then two coax switches: One to select the antenna you want to use, the other one to select the rig you want to use.

    Doing this, it doesn't matter how close the antennas are.
    K0UO likes this.
  7. K8XG

    K8XG XML Subscriber QRZ Page

    Perhaps Radios located in different places in the house running at the same time off different antennas like in my case? Yes I have 3 RF rigs in the shack but I have others in the house located downstairs and on the patio....
  8. W6MK

    W6MK Ham Member QRZ Page

    K1LKP and K0UO like this.
  9. W1EZE

    W1EZE XML Subscriber QRZ Page

    These are receive only, which would work with the 735 but nothing else.
  10. W6MK

    W6MK Ham Member QRZ Page

    Of course. The receive circuit is the vulnerable one. A power-limiting circuit
    would make no sense in a transmit feed line.

    Such a device (available from several ham sources and easily constructed from designs available on line) provides protection for receive-only antenna feeds and works with any rig that has a separate
    receive antenna input.

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