RadCom April 2024, Vol. 100, No. 4

60 April 2024 Regulars Activity reports Back in February, John, G8IKP took advantage of the early spring sunshine at Hardy’s Monument in Dorset (IO80RQ44) to do some tests on 24GHz. Previously on 10GHz he had proved that he could bounce a signal from the Mendip TV Mast (IO81QF46) to his home location on the Blackdown Hills (IO80KX51). This time, testing with G4UVZ, they initially had a contact on 10GHz at just under S9 on SSB. John then set up on 24GHz and started the search for each other. Initially they were unable to find the ‘sweet spot’ but eventually G4UVZ was able to find a trace of a signal with which, after a number of adjustments, they achieved a Q5 SSB exchange, over a round trip distance of 106km. Another success with Wavelab transceivers! The Mendip mast is, according to G3YKI and the BBC drawing, 2.3m in diameter and 281m high – so quite a small target. G4UVZ to Mendip is line of sight, but the path from Hardy’s to Mendip, as can be seen in Figure 1 (the path profile and a contour map of the path), is not. On 8 February, Nick, G4OGI reported confirmed reception of the GM6BIG (IO85BU) personal beacon on 10368.500MHz via (he assumes) aircraft scatter at 606km. Plus, on the same morning, the HB9G and HB9BBD beacons were copied earlier in the day from his site in North Kent. He is waiting for some drier weather before he can run new power cables to allow him to transmit. It’s just a shame that there is not more home station activity to take advantage of this interesting 10GHz propagation. Phil, G0JBA recently re-installed a Diamond X5000 ‘white stick’ vertical antenna for use on the 2m, 70cm and 23cm bands. On 27 January, after seeing the prediction for enhanced Tropo Propagation on the F5LEN website [1] , from his location on the Kent Coast (JO01PG), he was monitoring the 23cm band FM segment for activity. During the afternoon, the GB3PS repeater/beacon on 1297.075MHz (locator IO92XA) was at increased signal strength peaking at 59+ around 1800UTC over a distance of 126km. Using an old Icom 2500E on low power he could access the repeater with 350mW to the new antenna. He worked PE2BZ through the repeater but could not hear him directly on the input. Around 2030UTC, GB3PS was back in the noise but he then heard GB3NO near Norwich (JO02PP) for the first time in two years. GB3NO had been QRT due to a fault. As per the tropo forecast, the event was moving north eastwards and the signal strength of GB3NO at 153km was building such that by 2045UTC it was peaking at 59+ and he could access the repeater/beacon with the Icom and low power. Sadly, there was no activity. Phil monitors the 23cm FM band on a regular basis and for about 100 days a year the band is open between his location and the Rotterdam and The Hague area, at around 227km. I also have GB3PS on in the background while I’m in the shack so if you hear it, call through. Thoughts on combining SSPAs I recently acquired a second solid-state power amplifier (SSPA) for the 10GHz band. It was apparently the same as the high-gain 16W device used in my terrestrial system. It looked the same, having come from the same source, so I decided to replace my existing EME PA with this ‘matched’ pair, removing the need for a driver amplifier after the transverter. I put the current 12W EME driver amp in my terrestrial system. When I powered up the new amplifier it seemed to need a different drive level to the existing one and only produced just under 14W Psat. As this is only 0.6dB less than 16W, I was undeterred as the new one had a good gain DC control so I could set both amps up to require the same drive to produce 5W out. The gains then tracked reasonably well up to full power (Psat). When I came to remove the huge heatsinks (designed for 24/7/365 at full power) and took the lids off, the device lineup was different, and it was made by a different manufacturer. A typical example of a large company buying from multiple sources for the same job. Moving on to my experiences combining them, there is an excellent treatise on PA combining at the Microwaves101 website [2] . It is worth a read but, in simple terms, to combine any pair of PAs you need to equally split the drive power, feed each amplifier with half, and combine the outputs again to get double the output. You need to ensure that in each half of the setup, the amplifier gains and phases track reasonably well over the operating band and it will work. You can use even-phase power splitters (or ‘hybrids’ as they are often called) and either 0° and 180° hybrids, (the amplifiers then being effectively in ‘push-pull’) or 0° and 90° hybrids where you have what is known as ‘quadrature combining’. The hybrids need to be of the same type at input and output but only the output one needs to be able to handle the full output power. The most common even phase hybrid is the three-port “Wilkinson” consisting of two transmission lines and a 100Ω balancing resistor [3] . 90° four-port quadrature hybrids usually consist of four lines in a rectangle with one port of the four terminated in a 50Ω load [4] . As I already had a pair of 7-12GHz packaged Wilkinsons with SMA connectors, I decided to go down this route. The SSPAs will be run close to saturation on digimode EME and the drive power GHz Bands FIGURE 1: Hardy’s Monument to Mendip Mast and the reflection path.