ZL1RS - Antennas

WSPR mode can be used as an alert for VHF band openings. Frequently these openings are via tropo-ducting and can be forseen by looking at weather maps and Hepburn maps ... but VHF band openings can also occur via sporadic E (Es) to small localised areas, so it is an advantage to have a beacon antenna with a wide azimuth radiation pattern as well as a reasonably high gain.

A vertical stack of short Yagis achieves this by compressing the vertical radiation pattern to improve gain while retaining a wide azimuth pattern, so I designed a 4-stack array of 3 element Yagis that provides 13dBi gain ... about the same as a 7 or 8 element Yagi ... but with wider azimuth coverage. The design was optimised for good F/B ratio and a 50 Ohm impedance so simple split-dipole driven elements could be used.



Antenna parameters as shown in a 4NEC2 computer software simulation


The Yagis were made on 900mm lengths of 25mm square aluminium tube with element mountings from Nuxcom (no longer trading). An element correction of 8mm was added to account for the metallic boom and element mounting effects. The rear 300mm section of the boom is used for the mast mounting clamp and has a double wall thickness for extra strength (insert of 22mm diameter tube). The driven elements are fed with LMR-195 coax phasing lines which have ferrite sleeves at each feed point as choke baluns. The four phasing lines are combined by a power divider made from LMR-400 coax cable.



Temporarily mounted on a 45mm diameter pipe with about 1m spacing, and pointed up at the sky during SWR tests





UPDATE April 2021: List of the 10 best spots sorted by distance as posted on VKspotter.com since Dec 2019 (distance is in km) ...





UPDATE 06 January 2020: the U3S is still transmitting at around 144.490550 MHz (144.489000 + 1550Hz offset), but was re-programmed to send 2 minutes of WSPR every 6 minutes. The WSPR is now followed by an FSK CW ident (ZL1SIX RF64VT) with the Morse code appearing as a 600Hz tone in a USB receiver tuned to the 144.489 MHz 2m WSPR 'dial frequency'.


UPDATE 27 November 2019: the 4 stack array was installed at ZL1SIX on the top of our hill ...







The ZL1SIX QRP-Labs U3S transmitter with QRP-Labs QLG1 GPS for timing. The U3S synth board has a TCXO fitted to improve stability at 2m.




The 20mW from the U3S is bosted to 12W by a TriQuint FH1-G FET driving a Class C PA board salvaged from a Tait T198 150MHz FM R/T (SD1012 driving a SRF1585)



Power output and SWR



The view from ZL1SIX to the west towards Australia