Getting out on HF by Jakey G3JKY

These are just a few thoughts in answer to the Chairman’s comments in the May edition of Vital Spark.

When the Pentium 133 gave the feed impedance of a centre-fed 20 metre wire aerial, had it been told what frequency was in use? From the figures given, 12.76-j1080, and the later comment about the DX100, I think we can assume that the frequency was about 3.7 MHz.
As the yokel said when asked the way, “if I were going there, I wouldn’t start from ‘ere.”  In more technical language, a quarter-wave centre-fed is a VERY poor choice of aerial.

If a straight half-wave cannot be accommodated in the space available, loading coils might be necessary to make the quarter-wave of wire look like a half-wave to the unsuspecting feeder.  If operation on more than one band is required, traps could be used to make the wire look like a half-wave on all of them.  An alternative approach might be to strap the feeders together on the lowest frequency band, to make the system work as a “tee,” against a decent earth or counterpoise.

In the reference to 450 Ohm ladder line there’s no mention of the length, but I assume the 3dB quoted is for a length of line terminated in the quarter-wave centre-fed.  The matched attenuation of the 300-Ohm ribbon, which is slightly more lossy than ladder line, is about 0.2dB/100ft at 3.5MHz.   The corresponding figure for open wire line is about 0.03dB/100ft.  Clearly, open wire is the best for mis-matched, tuned, feeders.

A single band aerial, matched to its feeder, will usually outperform a multiband system of similar size, but that’s not an advantage if the favoured band is totally devoid of signals!  Sometimes it’s necessary to trade efficiency for convenience.

And remember, a decibel was once defined as “the amount the kids turn down the stereo when you yell at them!”

73! Jakey – G3JKY. From VS June 2006.

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Get your amateur radio licence - Find out more about amateur radio licence training.
Month on the Air - G3MGQ's popular monthly DX contest/expedition list.
Wilf Gaye Memorial Cup - The clubs annual operating event in the memory of Wilf Gaye M0GYE.
St. Richard's College Buildathon/STEM/ARISS - HERC attends St. Richard's Catholic College for their various events surrounding the Tim Peake ARISS contact.
G3BDQ - John Hey's Rare QSL Cards.
Sussex Electronics Radio Fair - SERF Sussex Electronics Radio Fair 2016.
Vital Spark - A selection of articles re-published from the Vital Spark.
RSGB News - Find out how to get RSGB news on your mobile or PC.
Experimenters Corner - A selection of Proteus projects by Bob Gornal (G7DME)
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Radio Rallies 2016 - An up to date list of radio rallies scheduled for 2016.
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John Taplin - A bio of the late John Taplin.

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