Transcript Wire Antennas - South Canadian Amateur Radio Society

The Wonderful World of
Antennas.
Or, If you transmit in the
forest with something
other than a Yagi with a 34
ft boom, will anyone hear
you?
All there is to know about antennas.
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The one thing I am absolutely sure about, is that any antenna is
better than no antenna at all.
And, It should be as high and as long as possible.
Unless……You want reliable, close in coverage.
Then 7ft or so will do. We’ll save NVIS for another day.
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But Wait! What if I don’t live on a big piece of real estate?
Getting started at your house.
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Make a drawing of your lot showing house,
trees, tower (if you have one), where you
might put a push-up pole etc.
Take measurements between locations
where you might string wire.
Note height of tower, pole or trees.
This should give you an idea of the
possibilities for antennas.
Measuring your lot.
43
Start with longest of
the possibilities.
62
54
75
Take trees etc into
consideration.
Then add the other
possibilities.
Types of antennas.
(From here on, each slide could be material for a program.)
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The most basic antenna is the ¼ wave vertical.
It is ¼ wave long, omnidirectional and pretty much
the standard other antennas are compared to.
It relies on an artificial ground. Radials or a car body
etc.
Most multiband verticals require a radial system. The
recommended lengths are: 80M 65-70 ft 40M 31.5ft
20M 17.3ft 15M 11.6ft 10M 8.6ft.
Types of antennas. Continued
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A longwire is effective for the listener who wants to
cover multiple or all the bands.
A dipole is basically a longwire with an insulator in
the middle (or somewhere)
Dipoles come in many different varieties.
You got your doublet, you got your off-center fed or
end fed, You got your fan, your coax, your folded and
trapped. Then there are some other varieties. i.e.
Loop, Sturba Curtain, Bobtail Curtain, Inverted L, VBeam, Rhombic, Beverage & so on.
Antenna Characteristics
The ratio at any given point on the antenna of Voltage
to current is the IMPEDANCE. It will vary based on
Height, surrounding objects etc. but a ¼ wave with near
perfect ground = 36 ohms. With radials = 50 ohms.
A ½ wave dipole = 75 ohms
A ½ wave folded dipole = 300 ohms
A coax fed ½ wave dipole does not provide a low SWR
over the whole band. Several fixes exist. 1.X-mission
line match or resonator 2. Parallel tuned circuit or
matching xfmr. 3.Fat dipole.
OR-
The Easy Way! J.M. Haerle
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If room is available, a Dipole 135 feet long
and fed with 450 ohm “window line” into a
good tuner will allow you to work 10 – 80M.
This can be shortened somewhat and still
give good results. Put up as much as you
can, use a tuner and enjoy.
Why Haerle says what he does.
Life after 50 ohm coax.
You have an 80 M dipole, resonant at 3750 khz. Using a transmatch
or tuner, feeding with 50/75 ohm coax you can cover most all of
the band…If you tried to use this antenna on 40 M it would not be
unusual for it to present an impedance of 4000 ohms. Using coax,
the SWR would then be 4000/50 or 80:1 The transmatch couldn’t
handle this nor could the coax. It would be subjected to
abnormally high voltages and attenuation losses.
Take the same antenna and feed it with balanced twin lead, say 450
ohm ladderline. Now the SWR is 4000/450 or less than 9:1
Most any good transmatch can handle this, attenuation losses
are negligible and voltage breakdown is no longer a problem. You
can use this dipole now on 10-80M. The only minus is as you go
higher in frequency the radiation pattern will become more
directional off the ends.
Experiment and have fun.
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Double Windom or Delta Match fed dipole
40-80 X configuration
G5RV (102 ft dipole fed with 34 ft of window
line, then >65ft coax to shack)
Off Center fed dipole if your support is not
located centrally in your available area.
A “Sloper” can give some directivity to signal.
NVIS 40M Dipole 7ft off the ground.
80/40 X Dipole
NVIS 40M antenna
Using the time honored forumla: and
one trick when it doesn’t work.
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To construct a dipole you know the old
formula of 468/freq. = total length or
234/freq.=each side length. So you whip out
a dipole for 18.130 by cutting each wire to
12.9’, put it up and it’s resonant at 17.80
RATS! Not to worry. 17.80/18.130 x original
length (12.9) =12.7’
So cut 2” off each side and it should be near
your freq.
Left Overs…
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This still leaves room for discussing:
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Antennas of non-resonant length (most all broadcast
stations use non-resonant antennas. How do they do that?
Line Attenuation, Standing Waves, Reflected Power
(Probably more myths about these than any other topic)
Transmission Line length, odd multiples
The Transmatch
The Balun
Ground Radials
Modeling Antennas..Get a glimpse of this now from N5KUK
using the EZNEC software….
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