ARLA/CLUSTER: Antenas para NVIS - Antenas incomuns instaladas a poucos pés do solo

[João Costa > CT1FBF]

Ham Radio, an unusual stealth aerial only feet above the ground

Ham radio is without doubt a great hobby to get interested in, however I
hear many say that aerials and their location is a big problem because of
their small gardens or local planning rules. Don't worry too much about
placing your aerial high in the sky, you can still put out a signal on the
high frequency bands which allows you to be heard.

Have you ever wondered how well an aerial works when only a few feet off
the ground, you may be surprised to learn that it works very well indeed
and large portions of your country can be worked with this type of
arrangement it's called a near vertical incident sky wave, you may be
surprised to learn that 300 to 400 miles can be successfully covered with
an N.V.I.S. aerial, (Near Incident Vertical Sky-wave) when placed only one
twentieth above ground level.

A forty metre dipole measuring about 66 foot and fed in the middle like a
conventional dipole works very well and is virtually undetectable, the
signal from the aerial is transmitted straight up to the ionosphere where
it is reflected back to earth where it can cover many hundreds of miles.

Running an additional earth wire slightly longer under the dipole seems to
pay dividends and I have noticed an increase in the received signal by as
much as a signal point when placing the earth wire just a few inches above
ground, the earth helps reflect your signal into the sky and reduces the
absorption or radio by the ground.

Connecting dipoles pruned for other ham radio bands and configured like a
nest of dipoles or fan aerials will give you additional frequency
allocations but my experiments have suggested that bands from 1.8 Megahertz
up to about 10 Megahertz work best I have found twenty metres to be
disappointing and ten metres was severely attenuated using this method
however you may have completely different results if you live in an area
where the ground has a high water content.

Using an N.V.I.S. aerial especially on the forty metre band is very
affective if your interest is to work all Britain and apply for their
awards, the aerial eliminates signal from continental Europe and
experiments show that attenuation using the N.V.I.S. aerial is substantial
when compared to a simple vertical aerial cut for the same band.

Feeding the aerial with twin 75 ohm cable works best, I have tried 50 ohm
coaxial cable which works, however twin gives a much better reduction in
noise.

This aerial works very well on the medium and long wave bands, at these
frequencies propagation tends to follow the curvature of the earth.
A simple matching unit can be added to the feed point to bring the aerial
into resonance.

By
*John Allsopp G4YDM*

https://g4ydm.blogspot.co.uk/

Article Source:
http://EzineArticles.com/expert/John_Allsopp/1925417
<http://ezinearticles.com/expert/John_Allsopp/1925417>

Article Source:
http://EzineArticles.com/9615576 <http://ezinearticles.com/9615576>
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