Improvements to Antennas
Tue, 09 Nov 1999 20:51:24 -0500
Steve Rawlings wrote:
> Just a few brief notes that may be of interest . . .
> Following the pruning of two trees in my neighbour's garden, the feed
> impedance of my 12 m vertical (at the cold end of my loading coil) has
> reduced from 130 to 100 ohms, and reports on my signal indicate an
> improvement of between 2 and 4 dB (thanks to Dave G3YMC and John G4GVC
> for these comparative reports). When using the additional 8 m of
> balloon-supported wire, the feed impedance has changed from 100 ohms
> (before pruning) to 75 ohms.
> Initial results with a noise canceller on local sources of interference
> have been most encouraging. Based on an idea by VK5BR (RadCom, March
> 1993, p34), I constructed the phase-shift network and hybrid transformer
> using 3C85 toroids.
> - under test conditions, a sine wave can be nulled by greater than 50
> - my strongest noise source (possibly a light dimmer), which, since
> mid-1998, has prevented me operating in the evenings can now be reduced
> from S9 to less than S5;
> - a combination of phase/amplitude settings, which take about 40 seconds
> to optimise, can only null out one interfering signal at a time, for a
> given receive antenna;
> - the number of significant noise sources at my QTH has increased from 1
> to 3 over the past 4 months;
> - an untuned length of wire, about 10 m long, and located in the loft
> [attic] in proximity to mains wiring seems to be make a good noise sense
> antenna for nulling out interference received on my vertical antenna.
> - the 10 m wire does not always provide sufficient noise to null out
> interference heard while using my 60 m single-turn delta loop antenna.
> I regret to report that, during a recent telephone conversation with Stu
> GW3XYW, I learnt that Roy GW3ITD is now a silent key. Roy died in early
> October. Many in the LF Group will know of Roy's interest in the LF
> bands, either through QSOs on 136 kHz, or via the early LF nets.
> Indeed, it was his enthusiasm that sparked Stu's interest in 136 kHz.
> Regards to all,
> Steve GW4ALG