Adapter for Antenna Switching
I like to make DX QSOs especially on 3,5 MHz over winters. Everybody who operates there knows that city/town noise (man-made noise) can be very problematic for successfully making contacts (1). Now something about the situation in my QTH a few years ago: I was using an almost vertical LW antenna (39 meters) for 3,5 and 1,8 MHz for transmitting and also for receiving. The noise probably came from next two towns (about 10 km /cca 6 miles/ far from my QTH). My S-meter showed S3 to S6 of the one. The noise signals came with vertical polarization - ground wave. Because of those facts I saw that I should have used some separate receiving antenna. The one should have had the horizontal polarization of its receiving characteristic for suppression of the noise. I thought about various antennas including for example ferrite rod but finally I used horizontally oriented LW antenna (end feeding with an unun /1:9/ – transformation to 50 Ohm coax cable). The noise then was decreased almost to 0. I have used two antennas (the almost vertical LW for transmitting and the horizontal LW /about 18 meters long/ for receiving on 80 and 160 meters) since that time. I operate TRX ICOM IC 7410 (and an Ameritron solid state amplifier) in my HAM shack. This TRX (and similar transceivers of various suppliers) does not have an extra input connector for receiving antenna. Because of that I must have used the simple Adapter for Antenna Switching (my own design) for my better operation on 80 and 160 meters. The scheme of it is presented below (drawn by ProfiCad free software for homes - European standard / but I hope it can be understandable anywhere/ ). I used old components of former Czechoslovak company TESLA. Electrical parameters of the components are not critical and it would be surely possible to use modern components (I will describe the parameters below). You can see the box of the Adapter on the photograph. Remark: three switches in low line (with a fuse) were not finally used because no preamplifier was finally necessary – those had been planed for attenuators before.
Scheme of Adapter:
Now something about the parameters: Resistor 3k3 (3,3 kilo Ohms) is small (0,025W). Capacitor 10k (10 nF) is ceramic and small (15 V). IND. 12V (LED diode with an integrated resistor) indicates 12V DC after switching on 12V DC supply (switch – in high right corner of the box). The transistor is PNP (maximum collector current about 500 mA, maximum voltage between collector- emitter about 30V). The relay is DC (50 mA) for switching AC (230V, 10A). The impedance of all antennas is 50 Ohms (used coax cable RG58 – remark: good enough for the frequencies /3,5 and 1,8 MHz/ and about 300 W power). The blocking capacitor 150k (150 nF) is ceramic and small (15 V). The used fuse is 0,5A DC. Diode KA501 is a Si diode (maximum frequency about 10 MHz, maximum current about 50 mA).
I have been operating the Adapter without any problems for longer than a year (local and DX communication). I have used BK-in mode (about 350 ms delay) for CW and VOX for SSB.
References: (1) http://w4rnl.net46.net/ant16.html