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*=

How does a whole house surge protector work ?

What is a surge voltage ? How does it occur ?

Various types of surge voltages occur in electrical plants and electronic systems. They are differentiated mainly by their duration and power. Depending on the cause, a surge voltage can last a few hundred microseconds, hours or even days. The amplitude can range from a few millivolts to some ten thousand volts. The direct or indirect consequences of lightning strikes are one particular cause of surge voltages. Here, during the surge voltage, high surge currents with amplitudes of up to some ten thousand amperes can occur. In this case, the consequences are particularly serious. This is because the damaging effect first of all depends on the power of the respective surge voltage pulse.

The phenomenon of surge voltage 

Every electrical device has a specific dielectric strength. If the level of a surge voltage exceeds this strength, malfunctions or damage can occur. Surge voltages in the high or kilovolt range are generally transient overvoltages of comparatively short duration. They generally last from a few hundred microseconds to a few milliseconds. As the maximum amplitude of such transients can amount to several kilovolts, steep voltage increases and differences are often the consequence. Surge protection is the only thing that helps. Indeed, the operator of an electrical system generally replaces the material damage to the system with corresponding protection. However, the difference in time between failure of the system to maintenance represents a risk in itself. This failure is often not covered by insurance and, within a short period of time, can become a heavy financial burden – especially in comparison to the cost of a lightning and surge protection concept.

This is how surge protection works

Surge protection should ensure that surge voltages cannot cause damage to installations, equipment or end devices. As such, surge protective devices (SPDs) chiefly fulfil two tasks: • Limit the surge voltage in terms of amplitude so that the dielectric strength of the device is not exceeded. • Discharge the surge currents associated with surge voltages. The way in which the surge protection works can be easily explained by means of the equipment's power supply diagram (Fig. 7). As described in Section 1.4, a surge voltage can arise either between the active conductors as normalmode voltage (Fig. 8) or between active conductors and the protective conductor or ground potential as common mode voltage (Fig. 9).

With this in mind, surge protective devices are installed either in parallel to the equipment, between the active conductors themselves (Fig. 10) or between the active conductors and the protective conductor (Fig. 11). A surge protective device functions in the same way as a switch that turns off the surge voltage for a brief time. By doing so, a sort of short circuit occurs; surge currents can flow to ground or to the supply network. The voltage difference is thereby restricted (Fig. 12 and 13). This short circuit of sorts only lasts for the duration of the surge voltage event, typically a few microseconds. The equipment to be protected is thereby safeguarded and continues to work unaffected.

Lightning and surge protection standards 

National and international standards provide a guide to establishing a lightning and surge protection concept as well as the design of the individual protective devices. A distinction is made between the following protective measures: • Protective measures against lightning strike events: lightning protection standard IEC 62305 deals with this. A key component of this is an extensive risk assessment regarding the requirement, scope, and cost-effectiveness of a protection concept. • Protective measures against atmospheric influences or switching operations: IEC 60364-4-44 deals with this. In comparison with IEC 62305, it is based on a shortened risk analysis and uses this as the basis for deriving corresponding measures. In addition to the standards mentioned, if applicable, other legal and country- specific stipulations are also to be considered.

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