SERIES
Rt = Total Resistance
Rt = R1 + R2 + R3
PARALLEL
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Rt = Total Resistance
Rt = [R1 x R2]/[R1 + R2]
PARALLEL (2 or more)
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Rtotal (Rt) = 1/ [1/R1 + 1/R2 + 1/Rn]
VOLTAGE DIVIDER
Vout = Vin x [ R1/(R1 + R2)]
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An Active antenna for 160 to 4 meters
This active antenna has been doing its job in my loft for well over 10 years. l’'ve seen anumber of designs over the years, and this is the simplest, deriving from experiment. When | recently saw a highly sophisticated-looking commercial unit, | felt it was time to ‘go public’.
Active antennas rely on a combination of an antenna element (Such as a dipole, monopole, orloop) and anamplifier, which is the ‘active’ part. The antennaelementis non-resonant, and tends to be physically small. They have broad operating bandwidths, so don’t need to be tuned. In comparison, a resonant antenna would need tuner adjustments to cover the whole HF and lower VHF spectrum. So, the attraction of active antennas is convenience.
It is only fair to point out that some people dislike them, and there are pitfalls, which | shall point out. If you want a really excellent receiving antenna for all the HF amateur and broadcast bands, and have masses of space, why not put up a Beverage or rhombic antenna? If, as in my case, that’s out of the question, then consider an active antenna and, better still, try building your own! This one can be put together in a few hours and covers 160 to 4 metres.
Active antennas rely on a combination of an antenna element (Such as a dipole, monopole, orloop) and anamplifier, which is the ‘active’ part. The antennaelementis non-resonant, and tends to be physically small. They have broad operating bandwidths, so don’t need to be tuned. In comparison, a resonant antenna would need tuner adjustments to cover the whole HF and lower VHF spectrum. So, the attraction of active antennas is convenience.
It is only fair to point out that some people dislike them, and there are pitfalls, which | shall point out. If you want a really excellent receiving antenna for all the HF amateur and broadcast bands, and have masses of space, why not put up a Beverage or rhombic antenna? If, as in my case, that’s out of the question, then consider an active antenna and, better still, try building your own! This one can be put together in a few hours and covers 160 to 4 metres.
DESIGN CONSIDERATIONS
THE CHOICE of a small antenna element (less than a tenth of a wavelength or so) is between the dipole and monopole, which respond to the electric field component of the radio wave; and the loop, which responds to the magnetic field component. A broadband active loop antenna is still on my list of things to try.
My first homebrew active antenna was a dipole, and was quite successful. The main thing it taught me was thatit’s nota good idea to have too much gain. It is natural to conclude that, as a short antenna picks up a smaller signal than a resonant dipole, the gain must be made up in the amplifier. Being a broadband device, the amplifier is subjected to the entire HF radio spectrum including powerful broadcast transmitters. What tends to happen in practice is that it distorts, generating intermodulation products. These appear to the receiver as additional signals and, though giving the impression of a ‘lively’ receiving system, are entirely unwanted. An attenuator between the active antenna and receiver is of no use at all, if the distortion has already happened in the active antenna.
It occurred to me to try asingle wire monopole, which made for a simpler amplifier. This worked and has been in use ever since.
My first homebrew active antenna was a dipole, and was quite successful. The main thing it taught me was thatit’s nota good idea to have too much gain. It is natural to conclude that, as a short antenna picks up a smaller signal than a resonant dipole, the gain must be made up in the amplifier. Being a broadband device, the amplifier is subjected to the entire HF radio spectrum including powerful broadcast transmitters. What tends to happen in practice is that it distorts, generating intermodulation products. These appear to the receiver as additional signals and, though giving the impression of a ‘lively’ receiving system, are entirely unwanted. An attenuator between the active antenna and receiver is of no use at all, if the distortion has already happened in the active antenna.
It occurred to me to try asingle wire monopole, which made for a simpler amplifier. This worked and has been in use ever since.
CIRCUIT DESCRIPTION
THE AMPLIFIER, shown in Fig 1, is a source-follower circuit designed around Tr1, aJ310 FET (field-effect transistor). This has to present a high impedance to the small monopole, otherwise signal voltage is lost, and then deliver the signal to the receiver input, commonly a 50 Ohm impedance.
The FET has an output impedance in
the region of 50 to 100 Ohm , which means
that, if the FET source fed the receiver
50 Ohm input, more than half the signal voltage would appear across the FET, and
less than half would be delivered to the
receiver. That's where the transformer
T1, in the source circuit, comes in. I used
a quadrifilarwinding to give a 4:1 voltage
step down ratio. This gives the source
follower an overall gain of almost 1/5 in
voltage (-14dB when the ratio of gate
voltage to output voltage is expressed in
decibels).
The benefit of doing this is that the FET
has much less work to do. The action of
the transformer makes the impedance
presented to the FET source bigger by a
factor of 42 = 16 times, which is 800 Ohm.
The result is improved linearity. Locations differ, but I have never known the
active antenna produce unwanted signals. You may be concerned that this low
gain would produce a rather 'deaf' receiving system but, from experience, comparing it to a transmitting dipole and tuner,
you won't miss much, if anything. The
internally-generated noise is very low,
and the background noise in most of the
HF spectrum is high.
Power to the active antenna is fed via
the coaxial cable, and the supply is
injected via choke RFC1 housed in the
power-feed unit near the receiver. R4 is
included to limit the current in the event
of an accidental short-circuit. An LED in
series with the supply indicates that current is being
drawn, and protects against inadvertent supply polarity reversal.
Shown on the circuit diagram is a
power feed for a receiver. This is for
the case where the
receiver and antenna can share
the same power
supply. You may
choose to omit it.
The frequency
response is shown
in Fig 2, and is
nominally flat to
within 1dB to
60MHz and within
2dB to 100MHz
CONSTRUCTION
TRANSFORMER T1 requires some care
in construction, and is described in some
detail, starting with the quadrifilar wire
itself. This would probably be a labourintensive and expensive item to produce
commercially, and is where the amateur's craft skills come into their own.
Take four strands of 0.2mm diameter
(35/36 SWG) enamelled copper wire,
length approximately 300mm for each
strand. Placing the wires side-by-side,
clamp one end and, pulling the wires
taut, fix the free end in the chuck of a
hand drill. Turn the drill to twist the
strands together. There is no need to
twist too tightly, a few twists per centimetre being adequate.
The core should be a high-permeability (greater than 100) ferrite toroid, 10 to
15mm in diameter. The purpose of the
core is to produce a sufficiently high
inductance to avoid gain roll-off at low
frequencies, and given a high enough
permeability, a wide variety of types, still
to be found at rallies, should be suitable.
If you are buying new, Table 1 shows the
types that should be suitable. Supplier
contact details are given at the end of the
article. Between them, they should be
able to source all items needed for construction.
Wind seven or eight turns of the
quadrifilar wire on the core. (Each time
the wire passes through the core counts
as one turn.) The photograph shows
how this has been done on a T37 -61
core. To secure the winding, the core
has been dipped in polyurethane varnish
and left to dry.
The individual wires need to be separated and the windings identified. Each
wire end should be stripped of its insula tion. An easy way to do this is to hold the
wire end in a blob of solder on the end of
a soldering iron for a few seconds. Make
sure you do this in a well-ventilated area
and avoid inhaling the fumes or getting
them in your eyes.
The ends of each winding can then be
identified with a multimeter or continuity
tester. I found it useful to markthewindings
with short strips of insulation stripped
from ribbon cable, and slid overthe wires
as shown in the same photograph. If you
do this with three windings, the fourth can
be left plain.
Naming the windings arbitrarily 1 to 4,
take the end of winding 1, and twist
together with the start of winding 2. The
end of 2 is then twisted with the start of
3, and so on. Twist fairly close to the
toroid, and make electrical connection
using the soldering iron, as described
above, observing the precautions. The
transformer is now complete. Check for
electrical continuity through the whole
transformer by measuring across the
un-paired wires.
Once the transformer is done, the rest
of the construction is straightforward.
Start with R2 as 47 Ohm or 68 Ohm . It may need
to be changed on test. The photograph shows my loft unit built into a
diecast box, with a couple of solder tags
for earthing to the box. Alternatively, the
circuit can be built above a small piece
of plain copper-clad board, which can
then be fitted inside a weatherproof enclosure if outdoor mounting is required.
The enclosure itself can be plastic - it is
an antenna after all!
Make sure you select the correct tap on
transformer T1 for the output, and take
care to prevent the unused taps from
shorting to any other part of the circuit.
THE POWER FEED
AS THE PHOTOGRAPH (right) illustrates,
I built my power feed unit in a small plastic
box. The choke isa single winding of around
20 turns on another high permeability toroid,
which can be the same type as that used
for the transformer. A metal enclosure would
make sense for the power feed unit, since
it will be near the receiver and possibly also
domestic interference sources. If using a
plastic housing, link the coaxial sockets
with coaxial cable: I used some RG178.
Keep the braid 'tails' short to avoid unwanted pickup.
TESTING AND
COMMISSIONING
CHECK CAREFULLY forwiring errors. For
bench testing, the powerfeed and antenna
units can be linked with a short coaxial
cable. Having ensured that its voltage and
polarity are correct, connect the power
supply and check that the LED is lit. Measure the voltage across R2 and divide this by
its resistance to find the current, or measure the supply current directly. This should
be in the region of 10 to 20mA. I selected R2
for a current of around 15mA.
Ifthe power feed output is now connected
to a receiver, a small amount of additional
hiss should be heard. Nothing should be
heard until a short wire (1 metre or less) is
placed on the antenna input. Signals should
be heard on the HF bands, given suitable
propagation conditions, or perhaps television or PC monitor timebase harmonics.
The antenna unit should be installed as
high and as far away from local sources of
interference as practicable. Mine is at the
apex of the loft, with an antenna wire of
around 1 metre length, suspended from a
hook in the highest beam. Avoid the temptation to increase the wire length excessively in order to increase the signal. This
brings the risk of distortion, and departure
from a flat gain with frequency.
from a flat gain with frequency.
POSITIVE - EARTH VERSION
WITH A NEGATIVE supply and positive
earth, a couple of components can be omitted. This is shown in Fig 3, below.
However, note that, while this is fine on its
own, it must not be connected to a receiver with a negative earth, because this
shorts out the supply
=* by Ian Braithwaite, G4COL *=
Build an Active Antenna
You have heard many times the term “active antenna.” Perhaps you have wondered “exactly what is this antenna and how might it be used?”
Let’s start by defining the word “active.” This does not suggest physical activity on the part of an electronic device. Rather, it tells us that the circuit is active in terms of voltage and current. A passive device, on the other hand, is a circuit that requires no operating voltage. It will exhibit some power loss as a signal is passed through it. Examples of passive devices are diode mixers, filters that use inductance and capacitance (LC filters) and all manner of wire antennas, etc.
An active mixer, on the other hand, uses a transistor or an IC, and operating voltage is applied to it. The mixer draws current and can cause a signal increase from the input to the output terminals. This is known as “conversion gain.” Active antennas contain RF amplifiers that require an operating voltage. Some active circuits may be designed to provide gain, while others may have unity gain (1) or a negative gain (signal loss). The nature of the active circuit depends upon its particular application.
Filters may be made active or passive. An active filter is often used to increase receiver selectivity at audio frequencies. This type of filter has no coils or inductors. Instead, it uses resistors, capacitors and ICs. An active filter may be designed for unity gain, or it may have a gain of 2 or 3, typically.
Active Antennas
What is an active antenna and why might we wish to build one? Active antennas are physically short, and they cover a wide spectrum of frequency. For example, an active antenna may perform uniformly from, say, 550 kHz to 50 MHz if it is designed well. This means that no antenna tuning or matching circuits are needed.
This type of antenna would be quite lossy if it did not include an RF amplifier section. In other words, if you connected a 6-foot whip antenna to your SW receiver and measured a 6.7MHzsignal at S3, that same signal might register 10 dB over S9 on your S meter if you switched to a full size dipole that was cut for 6.7 MHz. However, if we add an RF amplifier to the 6-foot whip before the signal is routed to the receiver, the S meter will indicate a similar reading to that when the dipole is used.
Why Use an Active Antenna?
Active antennas provide an alternative to no antenna at all if you are an apartment dweller or live in an urban area where external antennas are prohibited. These small active antennas are desirable for those who conduct business travel and find it necessary to stay in hotels or motels while on the road. The SWL need not be without an antenna if he is willing to build an active one
Figure 1: Schematic diagram of the active antenna amplifier. Capacitors without polarity marked are disc ceramic, 50 volts or greater. Resistors are 1/4 watt carbon composition or carbon film. RFC 1 and RFC 2 are miniature iron-core RF chokes (see text). JI and J2 are jacks of the builder's choice. Tl has 12 turns of no. 26 enam. wire (primary winding) on an Amidon Assoc. or FairRite FT-50-43 ferrite toroid core (850 mu). The secondary winding has six turns of no. 26 enam. wire wound uniformly over the primary winding. Overall amplifier gain is approx. 30 dB.
A Simple but Practical Active Antenna
Figure 1 contains a schematic diagram for an active antenna. The parts are inexpensive and easy to obtain. You can tack this circuit together in an evening. It may be constructed on a piece of perf board or a breadboard of your choice. The leads should be kept as short as practicable in order to ensure wide frequency coverage and the prevention of unwanted self-oscillations.
Qt is ajunction field-effect transistor (JFET). It has an input impedance of 1 megohm when wired as shown. This is an ideal situation when we attach a short antenna at JI]. You may use a long telescoping whip antenna, or a short hank of wire may be used. Any length from 6 to 10 feet is okay. Longer pieces of wire may be desirable for reception below 20 MHz. Don’t be afraid to experiment.
Q2 further amplifies the incoming signal (10 dB) and Q3 performs the same function, adding another 10 dB of gain. The gain of Q2 and Q3 may be as great as 15 dB per stage, depending upon the beta of the particular transistor plugged into the circuit. Q2 and Q3 operate as linear broadband amplifiers that use shunt and degenerative feedback. These two stages can be replaced by a single CA3028A or MC1350P IC, should you wish to do your own thing.
The output of Q3 is approximately 200 ohms. A 4:1 broadband step-down transformer (Tl) converts the 200-ohm output to 50 ohms. This makes it suitable for use with most shortwave and amateur receivers.
Although the circuit calls for a 12-V power supply, it will work well at 9V, should you wish to use a battery. Total current drain is on the order of 13 mA at 12 V, and it drops to 8 mA when the supply voltage is lowered to 9.
This circuit works well from 1.6 to 35 MHz. Operation at lower frequencies may be had by changing RCF1 and RFC2 to 10-mH units.
Using the Active Antenna
Connect a short antenna at J]. Vertical polarization will result if the wire or whip is vertical. Moving the antenna to a horizontal position will favor horizontally polarized signals. Be sure to experiment with the orientation of the antenna when monitoring different bands.
In an ideal situation the active antenna and its electronics would be located out of doors (on a balcony, deck or whatever). This will keep it away from electrical house wiring and steel frameworks if you live in an apartment. These man-made objects not only absorb signals but they may radiate noise. You may use RG-58 coaxial cable between the active antenna (T1) and your receiver. Any convenient length is suitable.
If you live near a powerful commercial broadcast station, a CBer with illegal power or an amateur radio station, you may find that the active antenna will overload and cause spurious signals across the tuning range of your receiver. This is a price that must be paid when a broadband circuit is used. Tuned circuits create needed selectivity for eliminating interference from nearby stations with strong signals. Active antennas do not contain tuned circuits.
Build the circuit in a metal box so that it is shielded. You should route the circuit ground to the metal box and ground the box to acold water pipe or an earth ground. This is not an essential action on your part, but it will help to improve the active antenna’s overall performance.
You may substitute 2N4416 FETs for the MPF102 shown at QI of Figure 1. Similarly, you may use 2N4400, 2N4401 or 2N5179 transistors at Q2 and Q3. The 1-mH RF chokes are available from your local store or other store.
=* by Doug Demaw, W1FB *=
Directional Antennas
It has already been mentioned that ideal omnidirectional antennas cannot be produced
in reality. Nonetheless only antennas that focus their radiated power in a particular
spatial direction can properly be called directional antennas. At an equivalent transmit
power, they significantly improve the signal-to-noise ratio, but must be aligned on the
distant station so that in many cases a rotation facility has to be used. For directional
antennas the parameters gain, directivity and all values associated with the radiation
pattern, like front-to-back ratio, side-lobe suppression or half-power beamwidth as
already discussed give an overview about how much the radiated energy is
focused into a certain direction.
The simplest form of a directional antenna is a setup of two monopole antennas at a
predefined distance, which are fed with different phase
 |
| Principle of a directional antenna |
In the example a distance of a quarter wavelength and a phase difference of 90° have
been chosen, resulting in a cardioid shaped radiation pattern when the
far field strengths generated by the two individual antennas are added.
 |
| Cardiold shape radiation pattern |
Even though this configuration does not produce strongly focused radiation, it however
exhibits a sharply defined null towards the backside which can effectively be used to
suppress interfering signals.
By superimposing the diagrams obtained by combining two or several radiators
arranged at defined distances and with defined phase shifts, directional patterns can
be generated whose directivity is limited mainly by the available space to setup the
number of required radiators.
Instead of feeding the radiators via cables , the principle of
radiation coupling is mostly applied in practice, with only one radiator being fed from
the cable and the remaining elements activated by this radiator. Yagi-Uda antennas,
which are commonly used for the reception of TV and VHF sound broadcast signals,
have typically between 4 and 30 elements and yield gain values of 10 dB and more.
The possibility of changing the direction of the main beam of a highly directive antenna
by purely electronic means is utilized to an increasing extent also with antenna arrays
for very high frequencies (e.g. for satellite radio services). The antennas used are
referred to as planar antennas and mostly consist of a dipole curtain which, in
contrast to curtain antennas, is installed in front of a conductive plane. This array can
also be implemented by etching the radiators as tracks into a printed circuit board
(microstrip antenna). In this way, even large arrays of antennas can be implemented
for the microwave frequency range with high precision and efficiency.
=* by Rohde & Schwarz*=
What is the basic principle of antenna?
An antenna is defined by Webster‘s Dictionary as ―a usually metallic device (as a rod or wire) for radiating or receiving radio waves.‖ The IEEE Standard Definitions of Terms for Antennas (IEEE Std 145–1983) defines the antenna or aerial as ―a means for radiating or receiving radio waves.‖ In other words the antenna is the transitional structure between free-space and a guiding device. The guiding device or transmission line may take the form of a coaxial line or a hollow pipe (waveguide), and it is used to transport electromagnetic energy from the transmitting source to the antenna or from the antenna to the receiver. In the former case, we have a transmitting antenna and in the latter a receiving antenna.
An antenna is basically a transducer. It converts radio frequency (RF) signal into an electromagnetic (EM) wave of the same frequency. It forms a part of transmitter as well as the receiver circuits. Its equivalent circuit is characterized by the presence of resistance, inductance, and capacitance. The current produces a magnetic field and a charge produces an electrostatic field. These two in turn create an induction field.
Definition of antenna
An antenna can be defined in the following different ways:
1. An antenna may be a piece of conducting material in the form of a wire, rod or any other shape with excitation.
2. An antenna is a source or radiator of electromagnetic waves.
3. An antenna is a sensor of electromagnetic waves.
4. An antenna is a transducer.
5. An antenna is an impedance matching device.
6. An antenna is a coupler between a generator and space or vice-versa.
source : https://www.sathyabama.ac.in/sites/default/files/course-material/2020-10/SEC1301.pdf
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