Front-end saver for LB antennas

Hello,

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 specially 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) great 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 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.

Update 17July2017

There is not necessary to use the 2N2222A in metal TO-18 package! It is possible to use plastic TO-92 package as the most of parameters are same and there is not impact for that application. Most important is to identify the correct pin-out of your device!

There are several manufacturers producing several different versions like 2N2222A, P2N2222A, PN2222A etc. and the pin-out is NOT the same or standardized!

Check carefully your purchased version of transistors and identify the correct pin-out before you will assembly in order to avoid FES malfunction!

 

On picture above is an example of correct orientation of KSP2222A-E19 transistors TO-92 package on PCB...

After the final assembly I highly recommending testing the device for its correct functionality. It is much better to spend few minutes more instead of damaging your transceiver front-end stage 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. 


Update 18Sep2017
Some of the people asked me how to implement this FES into the Icom radios setup. So here is the layout example for wiring it together with Icom transceivers. (just quick pencil drawing...)

The "PTT output" port on the drawing means port on your transceiver where hot pin goes to ground while transmitting to control external equipment. In our case the FES. On the Icom radios this port is labeled as "SEND"!

See port 10 marked as SEND. This is your PTT for wiring it with FES.

The "Ext RX ant IN" on the drawing means port dedicated to connecting an external receiving antenna to your transceiver. On the Icom radios this port is marked as "RX ANT"! See port 1 marked as RX ANT on the Icom rear panel picture above.

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

Building document done by P. Ourednik, 20-Dec-2017. Document 12-17, rev.4.0