The RX switch for contest operation on 144 MHz

Even I don't like "multi-beaming" contest style with more of power amplifiers and many antennas (however each fixed in different direction) due to a huge EMC troubles, requests from other operators pushed me to build such a simple and operators friendly RX antennas switch for such a kind of contest operation. The result of my effort is described below.

The mental stress of VHF operator on 144 MHz is enormous, because of very different signal levels (compare HF) of incoming signals. So if operator should control, except another duties, some additional manual switch (or even more switch knobs) for choosing the right RX path from specific antenna several times for every CQ, it would makes OP exhaustion within short time period. Based on our experience with choosing propper receiving antenna I decided to make the RX control as simple as possible. I built it on premise that operator isn't so interested in which antenna he actually use, however he requires pretty fast choosing of the antenna with upcoming response of station after his CQ (with the best signal readibility).

The result of RX control is a small plastic box lying on the table by the side of transceiver and telegraphy paddle, with installed low-lift microswitch. The switching over RX antennas is pretty fast as a response on repeated click on that microswitch indeed. On the control box are as well as LED indicators shows activated RX antenna.

Coaxial switch is made for receiver switching in between of 3 receiving arrays only, because the higher number of antennas systems from portable QTH is very difficult to build up & dismantle. (Contest stations which have larger antenna systems will solve it anyway, hi.) The controller box is made from iron-plate box with size 90x65x20mm available in GM Electronic shops (CZ) and for this size is designed printed circuit board as well. 3 input and one output HF connectors are BNC 50 Ohm type. As a switch over relay are used 3 ordinary small plastic relays for printed circuit with 2 switching contacts and you can buy them for example under label M4-12H. Even this (and similar) relays aren't primary made for switching of HF signals, in this case they make a good job. The relay is connected with only one pin for signal input, signals from just non selected antennas are grounded and actually selected antenna is connected to output connector.

This kind of construction gives satisfactory results, transit attenuation is less then 0,2dB and antennas separation is over 40dB, return attenuation over 20dB, which is for contest operation on 144 MHz more then enough. The relay of RX path is controlled by switching FETs BS170, which are step by step switching over the CMOS CD4017 decoder. Printed circuit should be constructed on double sided clad laminate FR4 with thickness 1,5mm. The construction isn't difficult and all is working after first switch on.

Data for home made RX switch construction follow:


Scheme      Printed circuit of switch (Side A)      (Side B)     Inserting scheme      View on final switch without cover

More detailed notes: if you haven't possibility to make metalized holes printed circuit board, use on the marked places hollow brass rivet which you'll connect with minimal impedance ground on one side of printed circuit and the right pin of switching relay. Pins of components which aren't connected through the isolated circles should be solder up on both sides of the circuit. The value of blocking capacitors isn't critical, you can use ceramic capacitors from 4,7nF to 100nF. The value of inductiveness of 4 used suppressors isn't critical as well, during manufacturing of (several) samples were used old suppressors with cca 10nH. This switch isn't suitable for RX switching on 432 MHz because of insufficient character of such a kind of relay. The three way switch is galvanic passaged and allow the power supply of LNA.

In case that you'll try to build such a 2m switch I wish you enjoyment during manufacturing and then during the followed operation.

  Used components:

IO CD4017 DIL ........... 1x (pcs)
IO 7812 ................. 1x
transistor BS170 ........ 3x
LED 3mm green ........... 2x
LED 3mm yellow .......... 1x
LED 3mm blue ............ 1x
ceramic C 100nF......... 10x
tantalum C 10uF/35V...... 1x
resistor 0,6W 10k ....... 1x
resistor 0,6W 100k ...... 1x
suppressor min. 8,2uH ... 4x
relay M4-12H ............ 3x
button P-DTH2SW .,,...... 1x
connector BNC panel Z50 . 4x
female plug DIN panel 5PIN.. 1x
female plug DIN panel 7PIN.. 1x
male plug DIN cable 5PIN.... 1x
male plug DIN cable 7PIN.... 1x
box U-KM22 ABS ............. 1x
metal box U-AH102 ....... .. 1x

After your clicking on the pictures you'll see them with full resolution. The boards of printed circuits for this construction you can order here.

CU on VHF!