Monday, 20 March 2017

A Full Explanation

Dr Bruce Gillies Taylor has supplied a detailed account which now fully explains the purpose of the Boghall installation:

The antenna was used for Riometer (Relative Ionospheric Opacity Meter) measurements at 30 MHz during the IQSY. 1 The horizontal wires strung between the vertical poles formed one plane of a corner reflector, while the other plane was formed by the sloping terrain. The centreline of the antenna was directed at the north celestial pole, so that the antenna continued to receive radiation from the same area as the earth rotated on its axis. I wasn't involved in the construction of the antenna, but during the summer of 1964 I did go out there at Dr Muggleton's request to make impedance measurements at the receiver end of the feedline, and I installed a stub to obtain a correct match. The 30 MHz receiver, by the way, was built by the electrical engineering lab technician, the late Harry Matthews.

The reason that I remember something about the antenna is that during 1963/1964 I researched different noise sources for calibrating the Riometer, and built an automatic calibrator using a semiconductor noise source that was installed with the receiver in the hut at Boghall. I've attached a photo of the calibrator. This work was the "Honours Special Project" that was part of my final year undergraduate study, and there is more information about the antenna, the galactic noise sources that it received, etc, in the electrical engineering department report "Calibration of a 30 Mc/s Riometer", Ref. H.S.P./22, 25 May 1964.

Unfortunately I don't have a copy of this report, because only one typed and one carbon copy of the text were made. I submitted the typed one to Prof Farvis, and gave the carbon copies to the department's photographer, who had kindly taken many photos for me and wished to submit a copy of the report to a photographic body for the award of some kind of certificate. These HSP reports were not held in the library, but were kept personally by Prof Farvis in a cupboard in his top floor office.

The measurements at Boghall were recorded on punched paper tape, and at a later stage I wrote an ATLAS computer programme to reformat them. Dr Muggleton did indeed carry out ionospheric research on the Monte Carlo - Edinburgh path, but this work was done in the 40-metre broadcast band and not at Boghall. 

Yes, the antenna was erected specifically for the IQSY Riometer, and wasn't used for any military or HF communication applications.

I don't recall whether the dipole within the corner reflector was supported by two additional wooden poles, or just suspended between the guy wires. The antenna had a 3 dB beamwidth of about 30 deg in both E and H planes and a 10 dB beamwidth of about 65 deg. The H plane sidelobes were 20 dB down at +-70 deg, and other sidelobes were more than 30 dB down.

The sidelobes could be ignored in the calculation of the background level, since at no part of the sky in the northern hemisphere is the level more than 3 dB greater than at the celestial north pole, but had to be considered when evaluating the contribution due to the intense discrete sources, such as the sun. From about April to August, the sun passed through the upper H plane sidelobe daily around noon. (This lobe pointed south at an elevation of about 50 deg). However, during the quiet sun period 1964-65 I calculated that the contribution from the sun would be negligible.

The steelwork just downhill from the masts supported the wooden hut that contained the receiver and the noise generator calibrator, and it was supplied with AC mains power by the overhead lines. The noise generator ran from a regulated 180v DC supply. A separate 36v DC supply powered a Londex coax antenna switch, and a Ledex solenoid driving the rotary attenuator, via a control box containing dropping resistors. During a calibration cycle, the input to the receiver was automatically switched from the antenna to the noise generator, and the attenuator then rotated in steps to provide signals at +10dB, +5dB, 0dB, -5dB and -10dB relative to the expected quiet sun level.

The clock signals that triggered the calibration cycles at intervals were transmitted to the receiver hut by overhead lines. After the system had been installed at Boghall, I was asked to go to the site because it was reported that the calibration switching was not functioning reliably. I discovered that the cause was that Harry Matthews hadn't been aware that these lines were carrying pulses rather than just DC, and he had connected large capacitors from the lines to ground as precautionary surge protection!

 (Information supplied in  two e-mails.  Bruce was contacted at the suggestion of Bill Hume)

1. IQSY =
International Quiet Sun Year, the name given to a series of coordinated Sun-related observational programs performed in 1964 and 1965

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