Antennas for 136kHz.pdf

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ON7YD, longwave, 136kHz, antennas
http://www.strobbe.eu/on7yd/136ant/
Antennas for 136kHz
About this page :
The main object of this page is to provide information. It has been deliberately kept simple, no fancy and flashy tricks, in order to
achieve maximum compatibility for the different browsers and to allow fast downloading.
Any comments and/or suggestions are welcome at :
on7yd@uba.be
A Japanese translation of this page, by Susumu Morioka (JH1GVY), is available
here
last updated on 30 January 2009
Index
1.
Introduction
2.
Short vertical antennas
1.
Vertical monopole antenna
2.
Short vertical monopole
3.
Vertical antenna with capacitive toploading
4.
Umbrella antenna
5.
Capacitive toploading of single-tower antennas
6.
Spiral toploaded antenna
7.
Vertical antenna with inductive toploading
8.
Vertical antenna with capacitive and inductive toploading
9.
Vertical antenna with tuned counterpoise
10.
Meander antenna
11.
Antenna with multiple vertical elements
12.
Using a non isolated antenna-tower as LF-antenna
13.
Antennas with a long horizontal section
14.
Helical antenna
15.
Short vertical dipole
16.
Why a horizontal dipole is a rather unefficient antenna on LF
17.
Safety precautions
18.
Bringing a short vertical monopole to resonance
1.
Loading coil
2.
Coil losses : the Q-factor
3.
Variometer
4.
Tapped coil
5.
Impedance matching
6.
Bandwidth considerations
3.
Efficiency of antenna systems on LF (short vertical antennas)
1.
Antenna system
2.
Efficiency
3.
Antenna system efficiency, antenna directivity, ERP, EIRP and EMRP
4.
Optimizing the antenna system efficiency
5.
Enviromental losses
6.
Ground loss
1.
Type (composition) of the soil
2.
Frequency
3.
Shape and dimensions of the antenna
4.
Radial system and ground rods
4.
Measuring ERP on LF
1.
Electric field / magnetic field & near field / far field
2.
Calculated ERP versus ERP measurements
3.
How to measure ERP
5.
Small loop antennas
1.
Single turn small loop as transmitting antenna
2.
Efficiency of a loop
3.
Enviromental losses of small loop antennas
4.
Single turn loop versus multi turn loop
5.
Directivity and polarization of a small loop antenna near ground
6.
Bringing a small loop antenna to resonance
1.
Resonance capacitor
2.
Impedance matching
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ON7YD, longwave, 136kHz, antennas
http://www.strobbe.eu/on7yd/136ant/
3.
Bandwidth considerations
6.
Other transmitting antennas
7.
Antennas for reception
8.
Software
9.
Appendices
1.
High power applications of toroidal core coils
1.
About toroidal cores
2.
Designing a ferrite cored transformer
(by Jim Moritz, M0BMU)
3.
Designing an iron powder cored coil
10.
Acknowledgements
1. Introduction
The main subject will be transmitting antennas for 136kHz as this often is the most important part of a longwave amateur radio
station. The aim of the transmitting antenna is to radiate the power coming from the transmitter.
The power radiated by any antenna is determined by 3 factors :
The radiation resistance of the antenna
The antenna current
The gain (directivity) of the antenna
Example :
Assume we have an antenna with a radiation resistance of 10 , an antenna current of 2A and a gain of 4 (6dB). This antenna will
radiate a power of 10 x 2
2
x 4 = 160 Watt.
The gain of an antenna is always given relative to a reference antenna. Most common references are the 1/2 wave dipole and the
isotropic radiator. This last is a virtual antenna that has no directivity at all, it radiates equally to all directions. In general the gain
of any antenna relative to a 1/2 wave dipole is given as dB
d
while the gain relative to an isotropic radiator is given as dB
i
. Due to
its directivity a 1/2 wave dipole has a gain 1.64 relative to a isotropic radiator (2.15dB
i
).
At first sight the radiation resistance of an antenna has no influence on the radiated power, as long as you match your transmitter
to this resistance. But unfortunately the radiation resistance is not the only resistance that is consuming the transmitter power,
there are also the loss resistances. These losses occur within the antenna (+ the antenna matching system) and in the environment
of the antenna (ground, objects near the antenna). On HF these loss resistances are often negligible as they are rather small
compared to the radiation resistance, but on longwave this is certainly not the case. For most longwave antennas used by amateurs
the radiation resistance of the antenna is in the range of 10 to a few hundred m while loss resistances are in the range of 30 to
150 . This means that, dependent on the antenna and its environment, about 99% to 99.99% of the transmitter power is not
radiated but absorbed in the loss resistances.
The two most common transmitting antennas on longwave are the short vertical monopole (Marconi antenna) and the small loop
antenna. The short vertical monopole is an electric antenna, it creates an electric field
'on the spot'
(near the antenna) while the
magnetic field is created
'on the fly'.
Opposite to this the small loop is a magnetic antenna, it creates a magnetic field
'on the spot'
while the electric field is created
'on the fly'.
As a result of this the main source of losses for a short vertical monopole is in the environment (ground, trees, buildings etc.)
while for a small loop the major losses are within the antenna. Therefore a small loop is less dependent on the environment for its
functionality.
But for both types of antennas the goal is to get the ratio of radiation resistance versus loss resistances as large as possible. In
practice most amateurs achieve better results with short vertical monopoles, only when environment losses are extremely high a
small loop will be superior.
Remark
: Throughout these pages the terms
ERP, EIRP, dB
i
and dB
d
will be used frequently. If you are not familiar with these
terms I would recommend to read
this
first.
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2. Short vertical antennas
2.1. Vertical monopole antenna
Most radioamateurs are familiar with the quarter-wave vertical
monopole antenna, often also called a "Marconi
antenna".
It is a
quarter wave long, is fed against ground (eventually improved by
a radial system) and has a radiation resistance of 36 . The
dimensions of a quarter wave vertical antenna might be suitable
from the 40m band upward, some brave hams might even have
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ON7YD, longwave, 136kHz, antennas
http://www.strobbe.eu/on7yd/136ant/
this antenna for 80m and 160m. But for 136kHz it would be over
500m (1500ft) high, without doubt beyond the range of any ham.
Thus at longwave there is no other way than using a vertical
monopole that is (very) much shorter than a quarter wave.
When a vertical monopole is less than a quarter wave (it's natural
resonance) a few things change :
the radiation resistance will drop as the antenna becomes
shorter
the antenna gain (directivity) will slightly drop, but this effect is can be neglected (less than 0.5dB)
the antenna impedance will have a capacitive component
the ground loss will increase as the antenna becomes shorter (see
footprint theory)
The effect of the antenna length on the radiation resistance and
antenna gain can be seen on the first picture at the right. So, in
contradiction to what many believe, the antenna gain of a short
vertical monopole is only 0.4dB less that that of a fullsize quarter
wave vertical (even if the short monolope is only a fraction of the
wavelength). Nevertheless the performance of a short vertical
monopole is -20dB to -40dB below that of a quarter wave
vertical, because the efficiency (ratio of radiation restistance and
loss resistances) rapidly decreases as the antenna becomes
shorter.
Example :
1. A Quarter wave vertical has a radiation resistance of 36 and
a loss resistance (groundloss) of 10 . The efficiency of this
antenna is :
(36 / (36 + 10)) * 100% = 78.3% (or -1.1dB)
2. A short vertical monopole of 1% of the wavelength has a radiation resistance of 0.04 , while there is a groundloss of 50 and
a loss in the loading coil of 20 . The efficiency of this antenna is :
(0.04 / (0.04 + 50 + 20)) * 100% = 0.057% (or -32.4dB)
As a result the quarter wave vertical will outrange the short vertical monopole by 31.7dB (31.3dB efficiency + 0.4dB antenna
gain).
The second picture shows "overall gain" (efficiency + antenna gain) of an average antenna as a function of its length.
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2.2. Short vertical monopole
Assume we have a
short vertical monopole
with a
height H
and fed against
ground.
If H is
small compared to the wavelength then :
The antenna will act as a capacitance (C
V
) in series with the radiation resistance (R
A
)
and the loss resistance (R
G
)
The
antenna current (I)
will decrease linear from the feedingpoint to the top of the
antenna, where it will reach 0
The voltage over the entire antenna will be the same
The current distribution, that is different from the sinusoidal distribution we are used to, can
be explained as follows :
The antenna capitance is not located at one single point on the antenna, but is distributed
equally over the antenna. As the antenna current flows into the antenna it gradually
'disappears' via the distributed antenna capacitance, resulting in a linear decrease.
Another - and maybe more correct - way to look at it is to compare a short vertical with
a full size (quarter wave) vertical. The full size vertical has a sinusoidal current and
voltage distribution whith a 90 degrees phase shift between U and I. The short vertical
can been seen as just the end of a fullsize vertical, where the voltage distribution is
(almost) constant and the current distribution decreases (almost) linear.
The radiation resistance of a short vertical monopole with a height H and at a
wavelength is :
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ON7YD, longwave, 136kHz, antennas
http://www.strobbe.eu/on7yd/136ant/
[1a]
For 136kHz this becomes :
[1b]
(R
A
in m
The capacitance of a vertical wire of a height H and diameter d is :
and H in m)
[2a]
(C
V
in pF, H and d in m)
In most cases the simplified formula
C
V
= 6pF/m
[2b]
is accurate enough.
In order to get a maximum radiated power we need a maximal current through the antenna. This can be done by compensating the
capacitive component with an inductive component (loading coil), or otherwise said : bringing the antenna to resonance. Based on
the formula for resonance (Thomson formula) we can calculate the inductance we need (see the chapter "Loading
coil"
for
details).
Example :
Assume we have a 10m long vertical wire (3mm diameter) with and an enviromental loss of 60 .
Based on formula 1a the radiation resistance is calculated as 8.2m , the antenna capacitance, based on formula 2a is 67pF. To
bring the antenna to resonace on 136kHz we will need a loading coil of 20.2mH. The reactance of the coil is 17.4k , so if we
assume a Q of 300 then the coil-loss will be 58 . This brings the total loss resistance to 118 .
If we put a power of 100W into the antenna we will have an antena current of 0.92A, resulting in 6.95mW radiated power and a
voltage of 16kV over the loading coil.
In the above example we calculated a radiated power of 6.95mW (0.92A into 8.2m ). To get the ERP (Effective Radiated Power)
we have to take the gain of the antenna into account, for a short vertical monopole this is 2.6dB
d
. So the calculated power in this
case will be 12.6mW ERP.
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2.3. Vertical antenna with capacitive toploading
The efficiency of a short vertical antenna can be improved by increasing the
radiation resistance. This is done my improving the current distribution over the
antenna, as the radiation resistance is proportional to the square of the average
current through the vertical section. For a short vertical monopole, as described
above, the average current is 50% of the current at the feeding point. One way to
improve the current distribution is to add capacitive toploading to the vertical
antenna.
The current distribution over the antenna has still a linear decrease, but due to the
fact that the minimum now is at the end of the horizontal section the average
current in the vertical part is higher.
The capacitance of a horizontal wire with a length L, a diameter d and at a height
H is given by :
[3a]
( C
H
in pF, H, L and d in m)
In most cases the simplified formula
C
H
= 5pF/m
[3b]
is accurate enough.
The total antenna capacitance C
A
= C
V
+ C
H
. The antenna current at the top of the vertical section is determined by the ratio of
C
H
and C
V
(assuming that the same amount of current 'disappears' via every pF) :
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ON7YD, longwave, 136kHz, antennas
http://www.strobbe.eu/on7yd/136ant/
[4a]
Thus the average current through the vertical section is :
[4b]
And the radiation resistance is proportional to the square of the average current through the vertical section, relative to the
average current in a vertical monopole (I
A
= I
0
/2) :
[5a]
for 136kHz this is :
[5b]
(R
A
in m
and H in m)
This means that the radiation resistance can be quadrupled by adequate capacitive toploading.
An additional benefit of capacitive toploading is that the antenna capacitance can increase significantly. Therefore the inductance
(loading coil) needed will decrease, resulting in lower losses in and lower voltages over the loading coil.
Example :
Assume we still have the 10m long vertical wire (3mm diameter) and the
enviromental loss of 60 of the previous example, but now we extend the
antenna with a 20m long horizontal topload wire (at 10m height).
The capacitance of the vertical section will be 67pF (formula 2a) while
the capacitance of the topload will be 116pF (formula 4a), resulting in a
total antenna capacitance of 183pF. The radiation resistance will be
21.9m (formula 5a). The loading coil must be 7.4mH, at a Q of 300 the
loss in the coil will be 21 and the total loss will be 81 .
If we put a power of 100W into the antenna we will have an antenna
current of 1.11A, resulting in 27mW radiated power and a voltage of 7kV
over the loading coil. Taking into account the gain of 2.6dB
d
the ERP will
be 49mW, this an overall 5.5dB improvement compared to the same
antenna without capacitive topload.
The gain that can be achieved by having a better current distribution is 6dB, but due to the increased capacitance (and thus a
smaller loading coil needed) some dB extra gain can be won, as you can see in the graph.
A vertical antenna with capacitive toploading can be constructed in various configurations, besides the 'inverted-L' configuration
there are also the 'T' and 'umbrella' configurations that are frequently used. In general any shape of capcitive topload will work,
the goal should be to get an many wire as possible as high in the air as possible. The topload wires can be sloping (umbrella
antenna), but this will cause a decrease in the radiation resistance. As a rule of thumb can be said that sloping topload wires
should never come lower than 50% of the antenna height.
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