Simple Receiver Front End Protection Ciruit Idea

AD5X diagrammed a T/R relay setup here (link).
It uses a DPDT relay. One pole switches the antenna between the TX and RX. The other pole shorts the RX antenna input to ground when in TX mode, and pulls a RX mute line low when in RX mode:



Bryan WA7PRC

 

Reference the original post; use a relay to short the Scanner antenna port to ground when you have your transmit radio powered on.
Do it from the radio's power supply or some other DC supply.
When you turn it on, the Scanner relay is connected to shunt the Scanner input.
Can't be any simpler.
Stubs need to be cut to specific length to work as a shunt.
Diodes will not shunt the path until about .3 to .7 volt is sensed.
That is way to high for protection.
Do the relay way and be sure.
Good luck.

 

Yes, solder a wire between the center pin of a PL-259 and the shell.

Glen, K9STH

 

Bryne,

Lot of good information and I will speak to a point I did not see covered in other posts.

The degree of heat and time to make a solder connection to the flange may damage the diode. I would use an internal tooth lock washer with solder tab and then machine screw with nut thru the flange hole.

 

I remember while working at Motorola, the Field Engineers discovered a problem with the top of the line HTs and Mobiles, essentially they both used the same engineering architecture for the RF Front End design. A couple of radio were brought in to make some changes by the top RF engineers. They all sat down to evaluate the problem that customer was experiencing and they decided to add a diode to the Receiver's Front End of these two transceivers. The diode was added and it solved the problem except it then created two unexpected problems that were much worst than the original one. Scratch that idea. In the end, the original problem couldn't be solved.

 

The use of diodes for front end protection is popular with many home brewers. The first time I saw a circuit with diodes used this way was in W7EL's Optimized QRP Transceiver back in the 80's. One major benefit is that it is simple.

http://www.arrl.org/files/file/Technology/tis/info/pdf/93hb3037.pdf

I use the circuit in a 20W homebrew 40M transceiver. In my circuit when transmitting the diodes ground the 0-75pF trimmer and the trimmer becomes part of the low pass filter when paralleled with the 430pF capacitor to equal ~470pF.

During receive the 10uh inductor and the 0-75pF capacitor form an approx. 50ohm series tuned circuit to my receiver. It works very well to protect my RX front end and provides nice QSK operation.

It is not likely that the OP will be considering a 20W input to his front end. The signal will be strong but not that strong.

Inexpensive 1N914 diodes work well.

K5UOS