Jerry writes:
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.
Depending upon the situation, I agree with that school. A practical
example:
A couple of years ago, my wife and I rented a second home in the Texas hill
country. It was in a no-antenna subdivision, but had 8 acres of property
with quite a few trees. I used an Outbacker for 6-30 meters, but it wasn't
very effective on the lower bands, even with the radials I put down. So,
with the help of a friend, I strung up an end-fed random wire.
Unfortunately, the trees weren't very tall, so it was only 15-20 feet high.
It was
about 90 feet long because that's how much wire was on the spool I had, and
the orientation was "as far much from the house as possible."
An MFJ tuner (962D) loaded it up to a 1:1 SWR at the transmitter, and with
100 watts it worked the world on 40 and 80 CW. At that low height, the
free-space pattern didn't matter. Most of the RF went straight up! Yes, the
trees absorbed some energy, but they also kept the neighbors from
bothering me about either antenna because they weren't very visible from the
road.
A good trade-off, IMHO.
73 Ray W2RS
In a message dated 11/29/2010 5:29:11 P.M. GMT Standard Time,
geraldj@weather.net writes:
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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