KB6NU's Ham Radio Blog

///////////////////////////////////////////
VGKids makes cool stickers

Posted: 29 Feb 2016 07:11 AM PST
http://feedproxy.google.com/~r/kb6nu...m_medium=email


This is kind of off-topic, but you might find it amusing.

I found these stickers when cleaning off my desk yesterday. These are
stickers fromÂ*VG Kids, the company that prints my Hams Obey Ohms Law
stickers and make my Im a CW Geek buttons. They threwÂ*these in with my last
order. Theyre very creative and a great company to work with, and I just
wanted to give them a plug.



The post VGKids makes cool stickers appeared first on KB6NUs Ham Radio Blog.


///////////////////////////////////////////
2016 Extra Class Study Guide: E9C - Wire and phased-array antennas

Posted: 28 Feb 2016 12:20 PM PST
http://feedproxy.google.com/~r/kb6nu...m_medium=email


E9C Wire and phased array antennas: rhombic antennas; effects of ground
reflections; take-off angles; Practical wire antennas: Zepps, OCFD, loops

There are many ways to put up antennas that are directional. Yagis are
directional antennas, but they require a structure, such as a tower, to get
them high in the air. One way to get directionality without a tower is to
use phased vertical arrays.

In general, the phased vertical array consists of two or more quarter-wave
vertical antennas. The radiation pattern that the array will have depends
on how you feed the vertical antennas.

So, for example, the radiation pattern of two 1/4-wavelength vertical
antennas spaced 1/2-wavelength apart and fed 180 degrees out of phase is a
figure-8 oriented along the axis of the array. (E9C01) The radiation
pattern of two 1/4-wavelength vertical antennas spaced 1/4-wavelength apart
and fed 90 degrees out of phase is a cardioid. (E9C02) The radiation
pattern of two 1/4-wavelength vertical antennas spaced 1/2-wavelength apart
and fed in phase is a Figure-8 broadside to the axis of the array. (E9C03)

How and where you install an antenna affects its radiation pattern. For
example, the far-field elevation pattern of a vertically polarized antenna
is affected when it is mounted over seawater versus rocky ground. What
happens is that the low-angle radiation increases. (E9C11) The main effect
of placing a vertical antenna over an imperfect ground is that it reduces
low-angle radiation. (E9C13)

Placement also affects horizontally-polarized antennas. If you compare the
performance of a horizontally polarized antenna mounted on the side of a
hill with the same antenna mounted on flat ground, you will find that the
main lobe takeoff angle decreases in the downhill direction. (E9C14) The
radiation pattern of a horizontally polarized 3-element beam antenna will
also vary as you change its height above ground. What happens is that the
main lobe takeoff angle decreases with increasing height. (E9C15)

Wire antennas

There are many different kinds of wire antennas. Some of the more popular
are the long-wire antenna, the folded dipole antenna, the off-center fed
dipole antenna, the Zepp antenna, and the G5RV antenna.

The long-wire antenna, as the name implies, is just a long piece of wire,
as long as you can make it, strung up as high as you can string it. An
antenna tuner is almost always required to match the impedance at the end
of the long wire to the 50 Ω output of the transmitter. The radiation
pattern of a long-wire antenna is very unpredictable. What happens to the
radiation pattern of an unterminated long wire antenna as the wire length
is increased is that the lobes align more in the direction of the wire.
(E9C04)

The folded dipole is a variation on the basic half-wave dipole antenna. A
folded dipole antenna is a dipole consisting of one wavelength of wire
forming a very thin loop. (E9C08) The approximate feed point impedance at
the center of a two-wire folded dipole antenna is 300 ohms. (E9C07) This
antenna was very popular when 300 Ω twinlead was used for TV antennas. The
reason for this is that 300 Ω was widely available and you could use is for
both the antenna and the feedline.

Another popular variation on the half-wave dipole is the off-center fed
dipole, or OFCD. An OCFD antenna is a dipole feed approximately 1/3 the way
from one end with a 4:1 balun to provide multiband operation. (E9C05) The
reason this antenna has become so popular is that not only provides a good
match on its fundamental frequency, but on harmonically-related frequencies
as well. An OFCD antenna cut for the 80 m band, for example, will also
present a good match on 40 m, 15 m, and 10 m bands.

Zepp antennas are also half-wavelength antennas. A Zepp antenna is an end
fed dipole antenna. (E9C10) Because the impedance is very high at the
feedpoint, the Zepp antenna uses a quarter-wave length of transmission line
to transform the impedance to something closer to the 50 Ω transmitter
output impedance.

An example of a wire antenna that is not a half-wavelenght long is the G5RV
antenna. A G5RV antenna is a multi-band dipole antenna fed with coax and a
balun through a selected length of open wire transmission line. (E9C09) The
G5RV was originally designed as a single-band, 20m antenna that provided
some directionality. Amateurs soon found that they could easily tune this
antenna on other frequency bands, and this multi-band capability has made
it very popular.

Rhombic antennas are large wire antennas that have four sections, each one
or more wavelengths long, arranged in a rhombic shape, and fed at one end
of the rhomboid. When the end opposite the feedpoint is open, the antenna
is bidirectional. Putting a terminating resistor on a rhombic antenna,
however, changes the radiation pattern from bidirectional to
unidirectional. (E9C06)



The post 2016 Extra Class Study Guide: E9C Wire and phased-array antennas
appeared first on KB6NUs Ham Radio Blog.