Posted by
David Woolley (E.L) on
Jan 05, 2008; 12:08am
URL: http://elecraft.85.s1.nabble.com/dipole-antenna-efficiency-tp458071p458118.html
WILLIS COOKE wrote:
> Shaun, it will decrease the efficiency drastically. A
> two meter long 80 meter dipole will be about 10% as
> efficient as one 40 meters long at the same height.
That's only conditionally true, although the conditions are probably
going to apply in this case. An antenna that is a long way from the
ground (or over a low loss ground) will have a low radiation resistance
if it is short, but could be efficient if the loss resistances in the
antenna and ATU are small. (I suspect that an antenna that is short
compared with its height may also have lower ground losses, but I
haven't researched this.)
As far as I know, a helically loaded antenna will have a higher
radiation resistance than one with all the inductance lumped in the ATU,
otherwise they wouldn't be so popular on hand-helds. On the other hand,
it may well have rather higher ohmic losses. If
<memo.cgu.edu.tw/jui-ching/antenna3.pdf> is correct, one would expect an
80m dipole, constructed from normal mode helices, with a span of 2
metres, to have a radiation resistance of about 200 milli-ohms, whereas
<
http://memo.cgu.edu.tw/jui-ching/antenna1.pdf> seems to suggest 123
milliohms for lumped loading in the ATU, The radiation resistances
scales as the square of the length, providing the antennas are short.
I'm also fairly sure that the amount of wire to wind on the helix is
significantly different from the amount that would be used in a straight
line. My gut feeling is that it should be less, but I haven't
researched that, either, although another URL pulled in searching for
the above ones suggests one gets the length by pruning back from a
quarter wave, although it didn't say whether that allowed for end effects.
If this is being used indoors, coupling to the building structures and
services may well increase the losses - or they could act as the real
radiator!
Note that these radiation resistances are over an order of magnitude
outside the specification range for the KAT2.
I do sympathize with the problem though, as I'm antenna restricted, and
haven't really solved the problem yet.
--
David Woolley
Emails are not formal business letters, whatever businesses may want.
RFC1855 says there should be an address here, but, in a world of spam,
that is no longer good advice, as archive address hiding may not work.
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