24 March 2015

Front-end saver for LB antennas


If You are using a separate receive antennas then you need a good protection circuit to eliminate the possibility of blowing out Your receiver's front-end while using these auxiliary receive antennas specialy on low bands.

The protective circuit should be designed to disconnect the receive antenna during transmit, ground the receiver input when transmitting thus keeping RF out of your radio and eliminating the chance of blowing your radio's front end.

The perfect solution designed years ago Gary, KD9SV and I am using this genuine design in the alu-box for years without any problems.
Now I need more of these circuits because of more radios is planned in the ham-shack (SO2R etc.) and also the contest team OK5T need to improve the low bands setup...

Thanks to Martin's (OK1RR) work I was able to use his slightly redesigned Front-end Saver based on KD9SV design using his PCB layout.

Very well designed PCB with one-side bus wiring is easy to assemble and it will work for the first shot if you will not make the mistake...

The suggested PRMA 1A12 relays > molded rugged construction with an internal protection diodes are used.

Steel painted box from local electronics shop is used in order to save building time and it does not look so bad.

After the final assembly I am highly recommending to test the device for its correction functionality. It is much better to spend few minutes more instead of damaging you transceiver front-end because of non-working Front-end saver...

The short theory of relays operation in this circuit is here:

State nr. 1
(QSK T/R is floating)
Re1 - closed
Re2 - open
Re3 - open

In this case ANT port is connected to RCVR input port and AMP KEY port is floating.

State nr. 2

(QSK T/R is grounded)
Re1 - open
Re2 - closed
Re3 - closed

In this case ANT port is disconnected/floating, RCVR input port is grounded and AMP KEY port is grounded too.

According to the theory of operation described above you can make easy and quick test using DMM before you will connect it to your radio and antenna.

1. Apply the 12V DC power to the circuit.

2. Check with DMM that ANT port hot pin is connected to RCVR IN port hot pin and that AMP KEY port hot pin is floating.

3. Using the tweezers or any similar tool short the hot pin of the QSK T/R port to ground.

4. Check that you are able to hear switching clicks of the relays when you are shorting the hot pin of the QSK T/R port to ground. Be very attentive as the chattering of relays is very quiet and fast so it is hard to hear in noisy environment. 

5. Check with DMM that in that state the ANT port hot pin is disconnected from the RCVR IN port (refer to point 2.) and it is floating. Then check that RCVR IN port hot pin is grounded. Finally check that AMP KEY port hot pin is grounded too. 

Do not hesitate to ask for details if You need but all is clear from the pictures and circuit is pretty simple. Also let me know if you want to help with PCB or even with PRMA 1A12 relays for building your own FES...

With courtesy permission from Martin, OK1RR

73 - Petr, OK1RP


  1. This comment has been removed by a blog administrator.

  2. Yes, PCB are still available as same as the PRMA relays.

    1. This comment has been removed by the author.

  3. New batch of fast PRMA relays and PCBs arrived. Kits are available again...

    1. Hi
      Can you please send me details of the kit.


    2. Hi David,
      thank you for reply. I have sent the detailed information to your email already. 73 - Petr, OK1RP

  4. Hi Petr. Great to hear a new patch just came in. I am interested in a Kit. Thanks, Matt K3WAZ

  5. Hi Matt. Thanks for note. I have sent you an email with detailed informations already. 73 Petr OK1RP

  6. Hi Petr,
    Please send me the ordering information for the kit. My email address oh6xi@sral.fi
    Vy 73 de Ulf

  7. Hi Ulf,
    Tnx. Yes I will send you more details Monday morning as I am out of home until Sunday nite. Best 73 Petr

  8. Hi
    Can you please send me details of the kit.

    73 Akis SV1CKZ

    1. Dear Akis, details emailed...73 - Petr, OK1RP

  9. Hello:
    Pls send ordering info for U.S.A.
    Thank you & 73, NN7DJ

    1. Dear Harvey, details emailed...73 - Petr, OK1RP

  10. Hi, Could you please send me information on the kit too? Thank you!

  11. Hi, upon several emails there is short intro for used relays. The PRMA 1A12 relays can not be replaced with any other cheap and slow relays! These relays have around 0.5ms switching/release time and it is the key parameter of this design! Do not try to replace them with standard slow relays without checking their switching time first. Remember that the FES must be fastest in the system to protect your RX...