ARLA/CLUSTER: Transformador 49:1 para as Antenas "End Fed Half Wave"

[João Costa > CT1FBF]

Steve Ellington N4LQ started up a group on Facebook called End Fed Half
Wave Antennas. The purpose of this group is to explore the use of an end
fed half wave antenna. An end fed half wave antenna is one of a specific
length depending on the frequency that it is to be used on, for example, an
end fed half wave antenna for 80 meters is about 130′ long. The length of a
half wave antenna is calculated using the following formula: L (ft) = 468 /
f (freq in MHz) or (L (m) = 300 /  f (freq in MHz ). The end fed half wave
antenna presents a large impedance to your rig, so this impedance must be
stepped down to a more useful impedance that is closer to 50 ohms. Most
radios today expect to see an antenna with an impedance of 50 ohms at the
end of it.

End-fed Half Wave antennas (or EFHWs) cover multiple bands without traps,
stubs, or resonators. End-fed wires resonate on their 1/2-wave fundamental
frequency plus all odd and even harmonics above. By adding a broad-band
matching network, the wire’s high impedance feed point is transformed down
to 50 ohms across a wide frequency range and, in most cases, you don’t need
a tuner to operate. Note that a single-wire radiator may be installed using
only one high center or end support, making it fast and easy to set up at
home, on the road, or as a “grab-and-go� emergency antenna.

There are many different ways to transform this impedance to 50 ohms. The
one that Steve promotes is to use a 49:1 unun and information about this
method follows.

The following is a diagram which explains how to construct a 49:1
transformer.

Wound with #14 enameled wire using FT240-43 toroids:

Steve made a video in which he showed the results of antenna tests using
various setups.
Video: EFHW Installation Summary or Is your feed line part of the antenna?
<https://www.youtube.com/watch?v=MzfQybYMlUc>

Steve’s best test results were with the transformer grounded close to the
ground and fed with enough coax to reach the transmitter also grounded.
Details are shown in the next two images:

Steve Ellington’s antenna:
133 ft. inverted L with 51 feet vertical. Transformer grounded, very well.
Coax on ground 100+ feet to shack, no chokes. Home brew transformer 2
primary 13 sec. mix 52 three stacked cores. 14 ga wire .. swr acceptable
…below 2.5 and much lower

Modification for 75 meters:
Here is how to raise the resonant frequency on your myAntennas EFHW-8010
for 75 meter phone operation without affecting the other bands. Simply cut
the antenna at the half way point and insert a capacitor. 500pf will put
you at around 3700khz. Lower values i.e. 300pf will raise it even higher. I
suggest a ceramic disc capacitor rated for at least several KV just to be
safe. I have tested and verified this and it works great.
-------------- próxima parte ----------
Um anexo em HTML foi limpo...
URL: http://radio-amador.net/pipermail/cluster/attachments/20200529/637bfe94/attachment.html