Yet Another Front End Saver

The KD9SV Front End Saver in a new fashion.

If you use a separate receive antenna, you need a Front End Saver to eliminate the possibility of blowing out your receiver's front end while using an auxiliary receive antenna. Unfortunately, all the radios produced by Icom, Kenwood, Yaesu & TenTec have the same problem - they do not turn off the receiver during transmit. The protective circuit is 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.

Basically, there are two ways how to protect the front end of your transceiver. Many users suggested installing 1N914 diodes back-to-back across the input of the aux receiver. No auxiliary control signals, no power supply, no chance to forget to switch on a small device. The main disadvantage is a possible degradation of receiver strong signal performance and harmonics which can affect the SO2R capabilities.
 
Another option uses fast reed relays which should do the following:

1. Disconnect the antenna
2. Short the receiver antenna input to ground
3. Provide keying relay to prevent damage the T/R circuits in the transceiver

A good solution is the Front End saver designed by Gary R. Nichols, described in CQ, Feb 97 - p. 32, which uses fast reed relays to open the feedline and short the receiver input to ground. This article can be hard to obtain (at least here) and if you look on the design you find an uncommon, rather unspecified type of SPST 5 Volt reed relay. Because here are the 'dry reed DIP packaged relays' more widely available I decided to redesign Gary's Front End Saver to accomodate such small but powerful devices.

A PRMA reed relay has the same domensions as the 14 pin DIP package, its expected lifetime is 500 million operations, is fast enough (operate time including contactbounce at nominal coil voltage is less than 1.2 ms, release time less than 0.8 ms) and its capacitances (less than 3 pF both across open contact and contact to coil] are negligible for 160 and 80m band. They are available for 5, 12 and 24 Volts (download the data sheet here). What do we need more?

The schematic of my redesigned Front End Saver is here. Don't be confused with the rather complex drawing showing the relay internals, the basics are just the same as the initial KD9SV's design (see here).

Front End Saver schematic
(click the image to enlarge)

Here is the pin layout of the PRMA relay.

Pin layout of the PRMA relay.

 The PRMA relay has also molded rugged construction and an internal protection diode. They are cheap (~ $1.20) and are manufactured by Comus, Clare, SRC and many others. Because my Orion provides 12 Volts, I used PRMA 1A12 type.

The PCB is very small (43.2 x 47 mm), therefore cheap.


The Front End Saver PCB.
Click here to download a high resolution, TIFF image (zipped, 32 kB).

The parts placement layout is here:


The Front End Saver parts placement.

Three short coax cables with RCA type plugs on each end are the only other things required to hook this unit up to my Orion (+12V, RX ANT and TX output] plus another cable to provde amp keying. This small device solved all my previous problems including fried RX input of a modified TS-850 which I counted to usual disasters after any contest.