Kind Words and Interesting Problems


hfdOne of the great pleasures of having written a book is hearing feedback from readers. I’m particularly interested in readers who have applied ultrawideband antenna ideas and concepts to solve problems with which I was completely unfamiliar. Martin Judd of High Frequency Diagnostics in Glasgow, Scotland has graciously agreed to allow me to share his comments:

Hello Hans,

I’ll keep this as brief as I can, since it looks like your life is pretty full on all fronts! I’ve had reason to dip into your “Ultrawideband Antennas” text a few times recently (sadly not been able to read it from cover to cover) and I wanted to congratulate you, not just on the technical quality and practical relevance of the book, but even more so on the sense of history and exploration that is conveyed by the engaging (and un-pompous) style of writing.

I also wanted to mention why ultrawideband technology has relevance to someone who has spent most of his career working in a high voltage lab using 50 Hz sinusoids: Partial electrical breakdown of insulation (so called partial discharge, such as might occur in an air bubble in an epoxy resin casting) is probably about the closest thing to a true ‘current filament’ radiator of an impulsive signal. There’s no feeder cable to distort the fields – what was once insulation momentarily becomes a path of rapidly accelerating charge, radiating a pulse and then reverting to insulation, leaving behind an electrostatic dipole. For some kinds of partial discharge, the pulse width can be less than 100 ps. Effective detection requires a wideband UHF sensor. Applications include power transformers (where defect location by time-difference-of-arrival at the 2c/3 velocity in oil is used), gas-insulated substations (where the coaxial lines formed by the busbars are so large you can measure intricate aspects of impulse propagation in waveguides in the time-domain) and overhead power lines (where arc faults can be located by a modified version of a lightning tracker system).

Sensors need to be wideband because it is important to detect the signal arrival time with an accuracy of better than 1 ns, a requirement that forces the antenna to have a bandwidth in excess of 500 MHz. Fortunately, our designs don’t have to be too intricate because we don’t care about phase or signal fidelity – just detecting the leading edge as clearly as possible. Monopoles are quite effective. However, for metal-clad medium voltage switchgear, it seems to us there is considerable scope to build slot antennas into the metal panelwork so that the equipment has built-in partial discharge sensors. This was the primary motivation that led me to your book. We’ve got some good results with small slots, which are both wideband and sensitive. The next step is to persuade manufacturers to include them!

Kind regards

Martin

Martin Judd BSc (Hons) PhD CEng MIET SMIEEE
Technical Director
HFDE Ltd
Suite 116
Baltic Chambers
50 Wellington Street
Glasgow  G2 6HJ
Scotland, UK
web
: www.hfde.co.uk

If you have a high voltage diagnostic problem, Martin Judd of High Frequency Diagnostics would be a great engineer to contact.

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