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Ultimate 3S with 5-band relay module in front, variable LM2596 power supply (with voltmeter) for the power amplifier behind left, a variable LM2596 supply set for 5 Volts for the Ultimate 3S in the middle, and the antenna switch to the right in the back.

WSPR – The system for Weak Signal Propagation Reporter makes it easy to compare antennas if your transmitter can easily switch antennas. The system shown here can send on antenna 1 for almost two minutes and then switch immediately to antenna 2 for the next transmission.

The Ultimate 3S already has software that supports that and application note 3 from QRPLabs (Controlling additional relays using the Ultimate3S “Aux”) describes how. I built mine following that note and the experience from EA1CDV.

The circuit is controlled from pin D7 and consists of a transistor, a relay, a resistor and an electrolytic capacitor. In addition I have two LEDs that indicate which antenna which is in use. In the first picture the green LED in the back right under the BNC antenna connector shows that antenna 1 is connected.

In the next picture, the whole layout is shown a little better. In this case LED 2 is lit, the faint yellow one. It sits right under the additional SMA antenna connector in the top left-hand corner that I had to fit.

I have used this setup for a few days now on 7, 10, 14, and 18 MHz with some crossed doublet antennas (somewhat like this setup, but not in the same location). I change the frequency between antennas, e.g. 50 Hz below the center frequency of the band for antenna 1 and 50 Hz above for antenna 2 in order to simplify discrimination between the transmissions.

The short 13 m antenna transmits best East-West, and the longer 26 m antenna North-South. The directivity is in general confirmed by the WSPR reports I see. Sometimes the difference can be more than 10 dB in SNR, but more often it is closer to 5 dB. But it also happens that only one of the transmissions is detected. This should make for some interesting analysis in the coming months.

The post “Comparing two antennas with WSPR” first appeared on the LA3ZA Radio & Electronics Blog.