There have been such tables since antennas began. Certain lengths of the
top and the feed are easier to feed being closer to resonance at a high
or low impedance point at the transmitter end of the feed. Often the
feed plus half the flat top length comes close to any multiple of a
quarter wave. When an even multiple the feed Z is high, when an odd
multiple the feed Z is low.
There is another school of thought that when those preferred lengths
don't fit the premises or the wire on hand, is that the preferred length
is the length that fits between the supports and the preferred feed line
length is the length that fits from the antenna to the tuner in the
shack. This can demand the tuner handle more obnoxious loads. This
school believes tough tuning and being on the air is better than being
quite bemoaning the preferred antenna won't fit the space available.
I notice in MFJ automatic tuner manuals that they don't like antennas
that present a high impedance resonant condition at the tuner. Which
means their tuners may be limited on voltage handling capability.
When a wire gets to be several wavelengths long or longer, the effects
of a resonant length are much smaller because the part near the feed
radiates and there is less current at the open end to be reflected. The
wire acts like a traveling wave antenna rather than a standing wave
antenna. Many computation techniques, from days of old and computers
neglect that change in current from radiation and give erroneous results
on long wire antennas. In feed Z and Z vs frequency and the radiation
pattern.
But one might need to be more careful about the length and orientation
of a long wire if one wishes to work in a certain direction. Like Europe
from the USA because there are more DXCC countries in that direction.
Then its important to use a NEC based antenna analyzer to learn the true
current distribution and radiation pattern.
Otherwise what you can put up will allow making contacts, which is often
a better situation that not being on the air for lack of an antenna.
73, Jerry, K0CQ
On 11/29/2010 7:29 AM, kf6e@mail.com wrote:
> Here's a link to an article listing the best lengths to use for a random wire
> antenna:
>
>
> http://www.hamuniverse.com/randomwireantennalengths.html
>
> I didn't measure mine; I just strung up some wire. I suspect you will have
> better results if you use one of the lengths suggested in the article.
>
>
>
> 73,
> Frank
> KF6E
>
>
>
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