''ΕΡΑΣΙΤΕΧΝΙΚΟ ΔΥΚΤΙΟ ΕΛΛΑΔΑΣ ΝΟΣΤΑΛΓΙΑ''
H ΚΑΛΥΤΕΡΗ ΕΛΛΗΝΙΚΗ ΜΟΥΣΙΚΗ ΑΠΟ ΤΑ 1960 ΜΕΧΡΙ ΣΗΜΕΡΑ ΟΛΟ ΤΟ 24ΩΡΟ . Ο ΣΤΑΘΜΟΣ ΤΩΝ ΜΕΓΑΛΩΝ ΕΠΙΤΥΧΙΩΝ ΤΟΥ ΕΛΛΗΝΙΚΟΥ ΠΕΝΤΑΓΡΑΜΟΥ ΣΤΗΝ ΕΛΛΑΔΑ ΜΕ ΕΚΠΛΗΚΤΙΚΗ ΠΟΙΟΤΗΤΑ ΗΧΟΥ HD.
In this article LodeRunner explains
what an Inverted-L antenna is, how it works, and why you might strongly
consider building one for use on the lower bands. While explaining its
use with a high degree of technical information, he’s written it in a
manner that’s easy to follow and digest. The diagram data is sourced
from EZ-NEC, which a link to the software is provided in the sidebar.
The Inverted-L Antenna and NVIS
An
“Inverted-L” antenna is basically a wire antenna, typically ¼ to ½
wavelength long on the band it is designed for. The Inverted-L antenna
is a common antenna for the 160 meter and 80 meter amateur bands, where
typical ¼ wave verticals are impractically tall for most amateurs.
In
the Inverted-L configuration, the first portion of the wire rises
vertically from the feedpoint, and at some height is bent roughly 90
degrees, and then extends horizontally to the unterminated end. The
feedpoint is very close to ground level (typically not more than 3 feet
above ground), and the antenna is worked against a Ground consisting of
one or more ground-rods, and/or a counterpoise consisting of one or more
radial wires – which may be buried, laid directly on the ground, or
suspended above the ground at some low height.
Because
the input impedance of a typical Inverted-L antenna is low, and the
feedpoint is at or very close to ground level, where ground losses are
substantial, it is very important to establish a good ground to work the
antenna against.
Most
of the amateur literature regarding the Inverted-L antenna is focused
on optimizing performance of the Inverted-L antenna for “DX” operation –
that is, high efficiency in radiating its energy in a pattern that is
low in elevation (low Take-off Angle, or ToA) – typically below 30
degrees relative to the horizon – which maximizes the distance to which
communications may be achieved. Effective NVIS communications, on the
other hand, require an antenna which is optimized to produce a pattern
where the majority of the radiated energy has a high ToA pattern –
ideally between 60 and 90 degrees – to provide reliable communications
from zero to several hundred miles.
Fig. 1: DX dipole pattern on 80 Meters
First, lets take a look at the difference between a good “DX Antenna” pattern vs. a good “NVIS Antenna” pattern –
Figure
1 is a diagram of a dipole optimized for DX communications; Figure 2
represents the exact same dipole, but the height has been lowered by
approximately 1/3 wavelength to optimize the antenna pattern for NVIS
communications.
Fig. 2:NVIS Dipole Pattern on 80 Meters
These
diagrams make it easy to see the difference – first, the green line in
each diagram shows the direction of maximum radiated energy – the
Take-off Angle, which is commonly abbreviated “ToA”. For the DX dipole,
the ToA is at 30 deg above the horizon. For the NVIS dipole the ToA is
90 deg, i.e. straight upward. In the above figures, it is the signal
energy in the area between the two diagonal lines which produces NVIS
propagation. The purple lines show the angles which are called the “Half
Power” points. In simple terms, these are the ToAs where the signal
strength drops to half of what it is at the best ToA of the antenna’s
pattern. The angle between the two purple lines is called the
“Beamwidth” of the primary lobe. The beamwidth of the NVIS dipole
encompasses the entire NVIS “Sweet Spot” – which is roughly between 60
and 90 deg. ToA .
These
pattern plots show us that NVIS and DX communications are essentially
opposite objectives, so designs which optimize the antenna for DX will
produce very poor performance for NVIS, and vice versa. Because I have
not seen elsewhere any advice on optimizing an Inverted-L antenna for
NVIS on the 160 and 80 meter bands, and because I believe that effective
NVIS communications on these bands is essential for preparedness
communications, I have prepared this article as a practical guide to
achieving these objectives.
The objectives which will be satisfied are –
High efficiency NVIS radiation pattern for both 160 and 80 Meters
Feed impedance directly compatible with typical auto-tuners
Build a simple matching device which provides an efficient, low-loss match to the feedline
Only a VSWR meter is required to adjust the antenna and matching unit for best performance
To
meet these criteria, the antenna wire will need to be between 175 and
215 feet in length, and the horizontal section must have a height of 50
to 65 feet at each end. This means that the horizontal span of the
antenna wire will be between 110 feet and 165 feet. A small amount of
slope [ +/- 10 feet ] in the horizontal section is not a problem, and
some sag in the middle of the horizontal wire unavoidable, but no part
of the horizontal wire should be below 40 feet Above Ground Level (AGL)
or above 65 feet AGL for optimized NVIS performance.
t’s
important to note here that, even if antenna supports of greater than
65 feet are available, the antenna should not be hung at a height
greater than 65 feet, unless the length is also increased (which will
increase the challenge of matching the antenna to the feedline). If we
hang a 200 foot wire at a height above 65 feet, then the Current Loop
(point of maximum radiation) will be in the vertical section of the
wire, and the NVIS radiation pattern will be substantially degraded (it
will become a good DX antenna, but that’s not what we want in this
case).
The
antenna wire should not be closer to any non-conducting materials (such
as tree branches) than 6 feet, and ideally the minimum separation from
branches/support poles/building roofs/etc. should be 20 feet or more, in
order to minimize the RF losses in the near-field of the antenna.
Separation
from metallic objects should be at least as far from the wire as the
antenna is above ground, i.e. 50 feet or more – this includes flag
poles, utility poles (they have a ground conductor running down their
entire length), metal roofing/siding of buildings, etc. This is
particularly important because metallic materials in the near-field of
the antenna may have substantial effects on its radiation pattern and
efficiency.
DEFINITELY
ENSURE THAT NO PART OF THE ANTENNA CAN COME IN CONTACT WITH ANY UTILITY
LINE, EVEN IF ONE OF THE SUPPORT ROPES FAIL WHILE SUBJECT TO STRONG
WINDS. YOUR LIFE, AND THE CONTINUING OPERABILITY OF YOUR EQUIPMENT
DEPEND ON MEETING THIS REQUIREMENT.
First,
let’s take a look at the typical (¼ wavelength) Inverted-L antenna’s
pattern, and why it favors DX over NVIS communications –
Fig. 3: Elevation Pattern of a 1/4 Wavelength Inverted L Antenna
Looking
at this elevation plot and the associated data, we see that the maximum
gain occurs at 30 deg., and that this gain is roughly equivalent to a
dipole above typical earth, i.e. ~3dBi (although this antenna, unlike
the dipole, produces mostly vertically polarized energy).
We
can see that the radiated energy in the NVIS sweet spot from 60 to 90
deg. is down 3dB to 5dB from the peak. Remember that -3dB = 50% power,
a.k.a. The “Half Power Point”, so the NVIS signals are at least 50%
weaker than those at the 30 deg ToA peak. This is obviously a DX antenna
on 160 - 200 Meters, and would be very difficult to match on 80 Meters, so I
won’t spend the time to go through the 80M plots.
Another
important consideration with Inverted-L antenna is their losses. With a
¼ wavelength Inverted-L, the point of maximum radiation is right at the
feedpoint, just above the earth. With typical soil conditions, a large
portion of the signal radiated into the ground is absorbed, lost.
Fig. 4: Point of Maximum Radiation and Consequent Losses
In
the above diagram of a ¼ wavelength Inverted-L antenna, the red circle
indicates the area where the strongest field-strengths occur. Notice
that essentially half of our radiated energy will interact closely with
the ground. Depending on soil characteristics and the quality of the
radial field under the antenna, somewhere between 30% and 70% of this
energy will be lost.
Now, lets look at what happens when we change the antenna wire from ¼ wavelength to 3/8 wavelength, again at 1850Khz –
Fig. 5: Elevation Pattern of 3/8 Wavelength Inverted L Antenna
Pictured
right is an Elevation Plot of an Inverted-L antenna which is 50 feet in
height, and has a horizontal span of 150 feet – so the radiating
element is 200 feet in length – this is roughly 3/8 wavelength at
1850Khz.
This
pattern is much better for NVIS: the ToA for peak power is at 80 deg,
and at no point in the 60 – 90 deg “NVIS Sweet Spot” is the signal
strength down more than 1dB from that peak. You should also notice that
the Max Gain is 5.69dBi. This is 2.6dB more gain than the ¼ wavelength
Inverted-L, and the Beamwidth of this antenna includes the entire NVIS
Sweet Spot. In round numbers, this antenna puts about four times as
much RF energy into the NVIS Sweet Spot as the typical Inverted-L
designed for DX work.
With
regard to losses, we can see that moving the point of maximum radiation
up into the horizontal wire has substantially reduced the portion of
the radiating field which must interact closely with the earth. In
Figure 6 below, the small blue circle indicates the (approximate) point
where maximum radiation occurs on 160M. Increasing the height of the
point of maximum radiation (in this case from 0 feet to 50 feet) reduces
earth losses by approximately 1 to 3dB – again, depending on soil
conditions and the quality of the antenna’s ground and/or counterpoise.
Fig. 6: Elevating the Point of Maximum Radiation
Another
effect of moving the point of maximum radiation up into the horizontal
section is this – the feedpoint has a much higher [real] resistance.
Since the efficiency of an antenna is determined by the ratio of Rrad (radiation resistance) to the sum of Rrad + Rloss {Efficiency = Rrad / Rrad + Rloss}, we have two ways to improve the efficiency – increase Rrad and/or decrease Rloss. In the case of an Inverted-L antenna, it is much easier to increase Rrad than decrease Rloss,and
increasing the efficiency of the antenna accounts for a good portion of
the increased gain of the 3/8 wavelength versus the ¼ wavelength
Inverted-L.
So how does this 50ft/150ft antenna perform on 80 meters? Lets take a look –
Fig. 7: 80M Elevation Plot, in line with horizontal wire, e.g. North-South
I’ve
included both elevation plots (Fig. 7 & Fig. 8) because I want to
make obvious that the orientation of the horizontal wire is very
relevant to the coverage provided by the antenna on 80M. So if you
desire broader coverage East-to-West, then the antenna wire should be
hung North-South, just as you would for a dipole.
Notice
in Figure 8 that there is a vertically polarized radiation pattern from
the antenna (in red) which will provide some East-West coverage beyond
NVIS range when band conditions are favorable.
Fig. 8: 80M Elevation Plot, perpendicular to horizontal wire, e.g. East-West
All
of the preceding assumes that the antenna is worked against a “good”
ground. At a very minimum, this antenna requires three radials of at
least 100 feet in length, and the efficiency and pattern will be much
better when the antenna is worked against a set of 6 or more radials
(ideally) of at least 150 feet in length each.
Using the
minimum of 3 radials, the layout should look something like Figure 9,
where the red line indicates the antenna wire, and the black lines
represent the ground radials. The center radial is directly below the
antenna wire, and the two outer wires are “fanned out” to run parallel
to the center wire at a distance of 15~25 feet to either side. If the
three radials can be extended to the same length as the horizontal
section of the antenna wire, this will improve the antenna’s efficiency
and VSWR at best match.
Fig. 9
A
better arrangement of ground radials is shown in Figure 10, above. The
three radials going to the right of the antenna feed point are the most
important, and the optimal length for these is ~150 feet each. The
radial going to the left of the feedpoint is least essential, and can be
bent, shortened, or omitted if space constraints require. The outer
half of the radials going vertically in Figure 10 can be bent to suit
the available space without substantial effect on the efficiency or
pattern of the antenna; if these radials are shortened, this may affect
the efficiency and/or matching requirements, but not to such a degree as
to make the design of the antenna matching unit described below
unworkable.
At
least one ground rod should be installed at the feedpoint of the
antenna as a lightening ground, and a suitable surge
protector/lightening arrestor device installed at the feedpoint.
~~~
Since
construction of the 3/8 wavelength Inverted-L antenna was thoroughly
covered in a previous post by JohnnyMac, I’ll move on to describe how to
obtain a simple and high efficiency match from this antenna to your
feedline.
On
160 Meters, a simple series capacitance will suffice to resonate the
antenna on 160 Meters. Depending upon the details of wire length,
height, and soil conditions, a total capacitance of 80pF to 180pF will
resonate the antenna on any desired frequency within the 160 Meter band.
The VSWR curve will look like this –
At the bottom of the 160M band, a capacitance of 145pF (0-j600 ohms) gives us this:
While
not perfect, this is a very workable match for the auto-tuners built
into many modern transceivers, as well as external tuners such as
produced by LDG and other companies.
In the top half of the band, 102pF (0-j800 ohms) obtains a nearly perfect match to a 50 ohm coaxial feeder all by itself:
Looking
at possible matching solutions on 80 Meters – again, a simple
capacitance of 560pF (0-j75 ohms) gives us a very usable mid-band VSWR
plot as follows –
Towards
the bottom of 80M, no capacitance at all is required for an excellent
match. This is the point at which the antenna wire is ¾ wavelength long.
This VSWR plot is with a direct connection between the feedline and the
antenna wire –
What
this tells us is that, to obtain a good match at the bottom of 80M with
a 200 foot (50’v/150’h) radiating element, an inductive element (coil)
will be needed at the feedpoint to match the antenna, which would
complicate our matching arrangements – but should be within the ‘reach’
of most commercial matching units.
However,
if we increase the length of the wire a bit, we can shift all the
needed tuning reactances into the negative (capacitive) range across
both bands, and keep our matching system simple and efficient for both
160M and 80M.
If
we extend the wire another 10 feet horizontally (50’v/160’h) then 93pF
(0-j900 ohms) obtains a near-perfect match at 1900Khz.
And 121pF (0-j725 ohms) will give us a very workable match at the bottom of 160M –
Adding 10 feet to the horizontal section shifts our 80M matching points as follows —
At 3600Khz, 590pF (0-j75 ohms) obtains a near-perfect match which looks like this:
Obtaining
a 1:1 VSWR match at 3500Khz would require over 1000pF of capacitance
(0-j40 ohms) so the lowest frequency with a ~2:1 or better VSWR match
using this method will be approximately 3550Khz. This limitation only
applies if you are building the simple capacitive matching unit
described below – with a commercial tuner a 1:1 VSWR can be achieved
anywhere in the 80 Meter band.
In the middle of the 80M band, 236pF (0-j180 ohms) obtains this very good VSWR curve –
And at the top of 80M , 115pF (0-j350 ohms) obtains a very workable match, also –
With
all this data, we can confidently conclude that an Inverted-L antenna
of 200~215 feet in total wire length, with a vertical portion 50 to 65
feet in height, can be effectively matched to a 50 Ohm coaxial feeder on
160M with nothing more than an adjustable capacitance in series between
the feedline and the beginning of the antenna wire. Assuming we can
install the antenna with a total length of 210~215 feet, then a series
capacitance is all that is required to resonate the antenna across most
of the 80 Meter band, also.
If
your primary interest is towards the bottom of both bands (CW and Data
segment) then a total radiator length of 210~215 feet is optimal
(depending on soil conditions); if your interests incline more towards
the middle/upper portions of the band for ‘phone operation, then 200~205
feet will suffice.
Assuming
that you are able to place a good antenna tuner right at the base of
this antenna, then no series capacitors (outside the tuner itself)
should be required to obtain a good match across most of both bands, and
you are ready to get on the air.
If
you wish to “roll your own” tuner, either as a stand-alone, or to bring
the VSWR to a workable value right at the feedpoint (and then finish
matching with an auto-tuner built into your rig, or located away from
the feedpoint of the antenna) then read on.
Assuming a 210 foot total wire length (50’v / 160’h) over a good counterpoise for all of the below calculations, then:
At 1900Khz
we simply adjust the series capacitor for the best match, which will be
very near the calculated value of 93pF(some value between 90 and 100pF
will obtain the best match), and a variable capacitor covering the range
of 85 to 125pF will provide a match of better than 1.5:1 across the
entire 160M band. If this is not the case, then the length of the
antenna wire should be adjusted to achieve the best match at 1900Khz
with 90~95pF of capacitance. This will have to be determined
experimentally once the antenna wire is installed in it’s final
location. If you can get a “perfect” 1.0:1 VSWR at 1900Khz with a
capacitor setting very near 93pF, then the antenna is optimized for
160M, and you are set to operate across all of 160 meters – only the
variable capacitor will need adjustment.
To
obtain a 1:1 match at the bottom of 80M (~3600Khz) only the series
capacitance is required, and this will be very close to 590pF. Since
this is a larger value than commonly available in a transmitter-rated
variable capacitor, a combination of a fixed capacitor (e.g. 250pF,
400pF, or 500pF) and the variable cap in parallel can easily be used.
To cover all of 160M, and the majority of 80M the wiring of the “match box” looks like this –
Fig. 10: Simple Tuner for 160 and 80 Meter Inverted L of 210~215 Feet total length
For a transmitter of up to 150 watts, suitable components are as follows:
C*Tune needs to be a variable capacitor with a Max value of at least 130pF and be rated for at least 500 volts RF. 250PF is the ideal Max. value for this variable capacitor.
250pF and 150pF transmitting variable caps are a very common find at
hamfests, and can often be purchased for as little as $10~$20 dollars.
Ideally you want a 250pF unit, to give you the broadest possible tuning
range on 80 Meters.
C*Fixed is a typical
“doorknob” transmitting capacitor rated for at least 500 volts RF, and
having a value of 500pF (if your C*Tune is a 150pF unit) or 400pF (if
your C*Tune is a 250pF unit). Two fixed capacitors of lesser value may
be used in parallel, e.g. two 200pF units to obtain 400pF total, or two
250pF units to obtain 500pF total. The objective here is to obtain a
maximum combine value of 600pF ~ 650pF, so that there will be sufficient
tuning range to reach the bottom of 80 Meters, and roughly 2/3 of the
total capacitance should be in the Fixed capacitance so that adjustment
of C*Tune on 160 Meters will not be too difficult (fine) to find the
best match easily.
SW1
is a DPDT manual switch or relay rated for 250VAC/5Amps or more. If
using a relay, the coil voltage can be whatever is available and
convenient – relays with a 12VDC coil are common, and can be powered
from the same power supply as your transceiver. 16Ga. or 18Ga. “Zip
Cord”, or “Thermostat Wire” are inexpensive, and can be used to carry
the switching power from the control position to the matching unit at
the feedpoint of the antenna. The entire match-box assembly should be
built in a good weatherproof enclosure.
If
you plan to operate at power levels >150 and <=1000 watts, then
the voltage ratings of the capacitors should be higher – 2000 Volts for
up to 1000 watts of transmitter power, and SW1 must be rated for at
least at least 1000VAC/10Amps per set of contacts. Otherwise, flashover
of the relay and/or capcitors will occur and your transmitter may be
damaged.
Other
configurations of antenna wire and/or matching unit are certainly able
to produce a good NVIS pattern on 160 and 80 Meters, but for all
possible configurations the key elements for optimized NVIS performance
of the antenna are:
The horizontal portion of the
wire should be no more than 65 feet AGL to maintain an NVIS radiation
pattern on both 160 and 80 Meters;
The Current Loop (where the
majority of signal is radiated from) must be in the horizontal section
of the wire on both bands, and
You must work the Inverted-L
antenna against a good ground or efficiency will be reduced and matching
will be more difficult to achieve
I
hope this article has given you all the info you need to put a strong
NVIS signal on the air. I will answer questions posed in the comments
section.
73, LodeRunner
~~~References and Annex Materials~~~
ARRL Antenna Compendium Vol. 7 (2002) ISBN: 0-87259-860-8,
“Horizontally Extended Inverted-L and Flattop Vertical Antennas”, Pp.
17-21
https://www.everythingrf.com/rf-calculators/whip-antenna-calculator
– useful for calculating STARTING POINT resonant lengths and matching
components for Inverted-L as well as vertical “whip” antennas. Treat
this as a “quick and dirty” way to get into the ballpark, then use EZNEC
to work out the fine details.
http://toroids.info/
Not just for calculating turns on toroids. This tool also allows you
to calculate inductance and capacitance needed to give you a particular
reactance at a given frequency, or vice-versa.
EZ-NEC file data for 210ft antenna:
50ft height / 150 ft span. 200 feet total wire length.
1850Khz – 3/8 wavelength Inverted L at this frequency
Max current is in Wire2/Segment2 @1.44 times the feed current.
VSWR@1850Khz = 1.39:1 for a 50 ohm Zfeed / 1.44:1 for 25 ohm Zfeed Zfeed is 35.9 -j0.73 ohms
For full-band coverage of 160M with an auto-tuner, a 150pF capacitor placed in series with the feedpoint is more than adequate.
80M coverage —
0+j135 ohms (6.14uH coil) resonates this antenna at 3500Khz
0-j200 ohms (200pF) resonates this antenna at 3950Khz.
Therefore, full 80M band coverage is well within the range of
any competent auto-tuner placed at the feedpoint. A relay should be
employed to shunt across the 150pF capacitor for 80/75 Meter coverage.
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