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Dipole antenna

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A simple half-wave dipole antenna that a shortwave listener might build.

A dipole antenna, developed by Heinrich Rudolph Hertz around 1886 [ citation needed ] , is an antenna with a center-fed driven element for transmitting or receiving radio frequency energy. These antennas are the simplest practical antennas from a theoretical point of view.

Contents

[hide] 1 Elementary doublet 2 Short dipole 3 Antenna gain 4 Half-wave dipole 5 Quarter-wave antenna 6 Dipole characteristics

o 6.1 Frequency versus length o 6.2 Radiation pattern and gain o 6.3 Feeder line

7 Common applications of dipole antennas o 7.1 Set-top TV antenna o 7.2 Folded dipole o 7.3 Shortwave antenna o 7.4 Whip antenna o 7.5 Dipoles vs. whip antennas o 7.6 Dipole towers o 7.7 Military

8 Collinear antenna systems based on dipoles o 8.1 Slim Jim or J-pole

9 Dipole types o 9.1 Ideal half-wavelength dipole o 9.2 Folded dipole o 9.3 Hertzian (i.e. short or infinitesimal)

dipole 10 Dipole as a reference standard 11 Dipole with baluns

o 11.1 Current balun o 11.2 Coax balun o 11.3 Sleeve balun

12 See also

13 References

[edit] Elementary doublet

An elementary doublet is a small length of conductor (small compared to the wavelength ) traversed by an alternating current:

Here is the pulsation (and the frequency). is, as usual . This writing using complex numbers is the same as the writing used with phasors or impedances.

Note that this dipole cannot be physically constructed. The circulating current needs somewhere to come from and somewhere to go through. In reality, this small length of conductor will be just one of the multiple bits in which we must divide a real antenna in order to calculate its proprieties. The interest of this imaginary elementary antenna is that we can easily calculate the far electrical field of the electromagnetic wave radiated by each elementary doublet. We give just the result:

Where,

is the far electric field of the electromagnetic wave radiated in the θ direction. is the permittivity of vacuum. is the speed of light in vacuum. is the distance from the doublet to the point where the electrical field is

evaluated.

is the wavenumber

The exponent of accounts for the phase dependence of the electrical field on time and the distance to the dipole.

The far electric field of the electromagnetic wave is coplanar with the conductor and perpendicular with the line joining the dipole to the point where the field is evaluated. If the dipole is placed in the center of a sphere in the axis south-north, the electric field would be parallel to geographic meridians and the magnetic field of the electromagnetic wave would be parallel to geographic parallels.

[edit] Short dipole

A short dipole is a physically feasible dipole formed by two conductors with a total length very small compared to the wavelength . The two conducting wires are fed at the center of the dipole. We assume the hypothesis that the current is maximal at the center (where the dipole is fed) and that it decreases linearly to be zero at the ends of the wires. Note that the direction of the current is the same in the both dipole branches. To the right in both or to the left in both. The far field of the electromagnetic wave radiated by this dipole is:

Emission is maximal in the plane perpendicular to the dipole and zero in the direction of wires, that is, the current direction. The emission diagram is circular section torus shaped (left image) with zero inner diameter. In the right image doublet is vertical in the torus center.

Knowing this electric field, we can compute the total emitted power and then compute the resistive part of the series impedance of this dipole:

ohms (for ).

[edit] Antenna gain

Antenna gain is the ratio of surface power radiated by the antenna and the surface power radiated by a hypothetical isotropic antenna:

The surface power carried by an electromagnetic wave is:

The surface power radiated by an isotropic antenna feed with the same power is:

Substituting values for the case of a short dipole, final result is:

= 1.5 = 1.76 dBi

dBi simply means decibels gain, relative to an isotropic antenna.

[edit] Half-wave dipole

A is an antenna formed by two conductors whose total length is half the wave length. Note that from the electric standpoint, this is not a noteworthy length. As we will see, at this length the impedance of the dipole is neither maximal nor minimal. Impedance is not real but it does becomes real for a length of about . The only outstanding property of this length is that mathematical formulas miraculously simplifies for this value.

In the case of this dipole, current is assumed to have a sinusoidal distribution with a maximum at the center (where the antenna is fed) and zero at the two ends:

It is easy to verify that for current is equal to and for the current is zero.

Even with this simplifying length, the formula obtained for the far electrical field of the radiated electromagnetic wave is rather displeasing:

But the fraction is not very different from .

The resulting emission diagram is a slightly flattened torus.

The image on the left shows the section of the emission pattern. We have drawn, in dotted lines, the emission pattern of a short dipole. We can see that the two patterns are very similar. The image at right shows the perspective view of the same emission pattern.

This time it is not possible to compute analytically the total power emitted by the antenna (the last formula does not allow), though a simple numerical integration or series expansion leads to the more precise, actual value of the half-wave resistance:

(In most cases 73.1296, or even 73.13, is adequate)

This leads to the gain of the half-wave dipole antenna, :

(Likewise, 1.64 and 2.15 dBi are usually the cited values)

The resistance, however, is not enough to characterize the dipole impedance, as there is also an imaginary part——it is better to measure the impedance. In the image below, the real and imaginary parts of a dipole's impedance are drawn for lengths going from to

, accompanied by a chart comparing the gains of dipole antennas of other lengths (note that gains are not in dBi):

Gain of dipole antennas

length in Gain

L l 1.50

0.5 1.64

1.0 1.80

1.5 2.00

2.00 2.30

3.0 2.80

4.0 3.50

8.0 7.10

[edit] Quarter-wave antenna

The antenna and its image form a dipole that radiates only upward.

The quarter wave antenna or quarter wave monopole is a whip antenna that behaves as a

dipole antenna. It is formed by a vertical wire in length. It is fed in the lower end, which is near a conductive surface which works as a reflector (see Effect of ground). The current in the reflected image has the same direction and phase that the current in the real antenna. The set quarter-wave plus image forms a half-wave dipole that radiates only in the upper half of space.

In this upper side of space the emitted field has the same amplitude of the field radiated by a half-wave dipole fed with the same current. Therefore, the total emitted power is one-half the emitted power of a half-wave dipole fed with the same current. As the current is the same, the radiation resistance (real part of series impedance) will be one-half of the series impedance of a half-wave dipole. As the reactive part is also divided by

2, the impedance of a quarter wave antenna is ohms. The gain is the same as that for a half-wave dipole ( ) that is 2,14 dBi.

The earth can be used as ground plane. However, the earth is not a good conductor. It is rather a dielectric. The reflected antenna image is good when seen at grazing angles, that is, far from the antenna, but not when seen near the antenna. Far from the antenna and near the ground, electromagnetic fields and radiation patterns are the same as for a half-

wave dipole.. The impedance is not the same a with a good conductor ground plane. Conductivity of earth surface can be improved with an expensive copper wire mesh.

When ground is not available, as in a vehicle, other metallic surfaces can serve a ground plane, for example the roof of the vehicle. In other situations, radial wires placed at the foot of the quarter-wave wire can simulate a ground plane.

[edit] Dipole characteristics

[edit] Frequency versus length

Dipoles that are much smaller than the wavelength of the signal are called Hertzian, short, or infinitesimal dipoles. These have a very low radiation resistance and a high reactance, making them inefficient, but they are often the only available antennas at very long wavelengths. Dipoles whose length is half the wavelength of the signal are called half-wave dipoles, and are more efficient. In general radio engineering, the term dipole usually means a half-wave dipole (center-fed).

A half-wave dipole is cut to length according to the formula [ft], where l is the length in feet and f is the center frequency in MHz [1]. The metric formula is

[m], where l is the length in meters. The length of the dipole antenna is about 95% of half a wavelength at the speed of light in free space. This is because the impedance of the dipole is resistive pure at about this length.

[edit] Radiation pattern and gain

Dipoles have a toroidal (doughnut-shaped) reception and radiation pattern where the axis of the toroid centers about the dipole. The theoretical maximum gain of a Hertzian dipole is 10 log 1.5 or 1.76 dBi. The maximum theoretical gain of a λ/2-dipole is 10 log 1.64 or 2.15 dBi.

Radiation pattern of a half-wave dipole antenna. The scale is linear.

Gain of a half-wave dipole (same as left). The scale is in dBi (decibels over isotropic).

[edit] Feeder line

Ideally, a half-wave (λ/2) dipole should be fed with a balanced line matching the theoretical 73 ohm impedance of the antenna. A folded dipole uses a 300 ohm balanced feeder line.

Many people have had success in feeding a dipole directly with a coaxial cable feed rather than a ladder-line. However, coax is not symmetrical and thus not a balanced feeder. It is unbalanced, because the outer shield is connected to earth potential at the other end. [2] When a balanced antenna such as a dipole is fed with an unbalanced feeder, common mode currents can cause the coax line to radiate in addition to the antenna itself, and the radiation pattern may be asymmetrically distorted. [3] This can be remedied with the use of a balun.

[edit] Common applications of dipole antennas

[edit] Set-top TV antenna

The most common dipole antenna is the "rabbit ears" type used with televisions. While theoretically the dipole elements should be along the same line, "rabbit ears" are adjustable in length and angle. Larger dipoles are sometimes hung in a V shape with the center near the radio equipment on the ground or the ends on the ground with the center supported. Shorter dipoles can be hung vertically. Some have a dial also used to clarify the picture.

[edit] Folded dipole

Another common place one can see dipoles is as antennas for the FM band - these are folded dipoles. The tips of the antenna are folded back until they almost meet at the feedpoint, such that the antenna comprises one entire wavelength. The main advantage of this arrangement is an improved bandwidth over a standard half-wave dipole.

[edit] Shortwave antenna

Dipoles for longer wavelengths are made from solid or stranded wire. Portable dipole antennas are made from wire that can be rolled up when not in use. Ropes with weights on the ends can be thrown over supports such as tree branches and then used to hoist up the antenna. The center and the connecting cable can be hoisted up with the ends on the ground or the ends hoisted up between two supports in a V shape. While permanent antennas can be trimmed to the proper length, it is helpful if portable antennas are adjustable to allow for local conditions when moved. One easy way is to fold the ends of the elements to form loops and use adjustable clamps. The loops can then be used as attachment points.

It is important to fit a good insulator at the ends of the dipole, as failure to do so can lead to a flashover if the dipole is used with a transmitter. One cheap insulator is the plastic carrier that holds a pack of beer cans together. This beer can insulator is an example of how a household object can be used in place of an expensive object sold for use as an item of radio equipment. Other objects that can be used as insulators include buttons from old clothing.

[edit] Whip antenna

The whip antenna, is probably the most common and simplest-looking antenna. These are monopoles, and the most common and practical is the quarter-wave monopole which could be considered as half of a dipole using a ground plane as the image of the other half. The commonly referred-to end-fed dipole is actually just a half-wave monopole whip antenna.

[edit] Dipoles vs. whip antennas

Dipoles are generally more efficient than whip antennas (quarter-wave monopoles). The total radiated power and the radiation resistance are twice that of a quarter-wave monopole. Thus, if a whip antenna were used with an infinite perfectly conducting ground plane, then it would be as efficient in half-space as a dipole in free space an infinite distance from any conductive surfaces such as the earth's surface.

[edit] Dipole towers

Large constructed half-wavelength dipole towers include the Warsaw radio mast and Blaw-Knox Towers.

[edit] Military

The US Military occasionally uses a doublet antenna, especially during dismounted unconventional warfare. A radio operator may choose to bring several doublet antennae for different frequencies, such as an antenna cut to length for the set MEDEVAC (medical evacuation) frequency, NCS (net control station) frequency, and tactical frequency (the frequency used by troops in the field). This approach may not be acceptable depending on the mission. Note that a doublet antenna will not work with the standard SINCGARS radio when using FH (frequency hop) but is effective for SC (single channel). A doublet antenna is more practical for radios not intended for FH, such as the AN/PRC-117F or AN/PRC-150.

The following information is not official US Military procedure. Special Forces Communications Sergeants have unofficially claimed to have used the cable from an M18A1 Claymore Antipersonnel Mine to construct a dipole antenna. The makeshift antennae is easily concealed among trees. Performance varies. These jerryrigged antennae are inexpensive and can be left in the field when stealth is not important.

[edit] Collinear antenna systems based on dipoles

J-Pole Antenna

Dipoles can be stacked end to end in phased arrays to make collinear antenna arrays, which exhibit more gain in certain directions—the toroidal radiation pattern is flattened out, giving maximum gain at right angles to the axis of the collinear array.

[edit] Slim Jim or J-pole

A Slim Jim or J-pole is a form of end-fed dipole connected to a quarter-wave monopole used as a stub matching section.

[edit] Dipole types

[edit] Ideal half-wavelength dipole

This type of antenna is a special case where each wire is exactly one-quarter of the wavelength, for a total of a half wavelength. The radiation resistance is about 73 ohms if

wire diameter is ignored, making it easily matched to a coaxial transmission line. The directivity is a constant 1.64, or 2.15 dB. Actual gain will be a little less due to ohmic losses.

If the dipole is not driven at the centre then the feed point resistance will be higher. If the feed point is distance x from one end of a half wave (λ/2) dipole, the resistance will be described by the following equation.

If taken to the extreme then the feed point resistance of a λ/2 long rod is infinite, but it is possible to use a λ/2 pole as an aerial; the right way to drive it is to connect it to one terminal of a parallel LC resonant circuit. The other side of the circuit must be connected to the braid of a coaxial cable lead and the core of the coaxial cable can be connected part way up the coil from the RF ground side. An alternative means of feeding this system is to use a second coil which is magnetically coupled to the coil attached to the aerial.

[edit] Folded dipole

Folded Dipole Antenna

A folded dipole is a dipole where an additional wire (λ/2) links the two ends of the (λ/2) half wave dipole. The folded dipole works in the same way as a normal dipole, but the radiation resistance is about 300 ohms rather than the 75 ohms which is expected for a normal dipole. The increase in radiation resistance allows the antenna to be driven from a 300 ohm balanced line.

[edit] Hertzian (i.e. short or infinitesimal) dipole

The Hertzian dipole is a theoretical dipole antenna that consists of an infinitessimally small current source acting in free-space. Although a true Hertzian dipole can not physically exist, very short dipole antennas can make for a reasonable approximation.

The length of this antenna is significantly smaller than the wavelength:

The radiation resistance is given by:

The radiation resistance is typically a fraction of an ohm, making the infinitesimal dipole an inefficient radiator. The directivity D, which is the theoretical gain of the antenna assuming no ohmic losses (not real-world), is a constant of 1.5, which corresponds to 1.76 dB. Actual gain will be much less due to the ohmic losses and the loss inherent in connecting a transmission line to the antenna, which is very hard to do efficiently considering the incredibly low radiation resistance. The maximum effective aperture is:

A surprising result is that even though the Hertzian dipole is minute, its effective aperture is comparable to antennas many times its size!

[edit] Dipole as a reference standard

Antenna gain is sometimes measured as "x dB above a dipole", which means that the antenna in question is being compared to a dipole, and has x dB more gain (has more directivity) than the dipole tuned to the same operating frequency. In this case one says the antenna has a gain of "x dBd" (see decibel). More often, gains are expressed relative to an isotropic radiator, which is an imaginary aerial that radiates equally in all directions. In this case one uses dBi instead of dBd (see decibel). As it is impossible to build an isotropic radiator, gain measurements expressed relative to a dipole are more practical when a reference dipole aerial is used for experimental measurements. 0 dBd is often considered equal to 2.15 dBi.

A dipole antenna cut from an infinitely large sheet of metal, with sufficient thickness, is complementary to the slot antenna, both giving the same radiation pattern.

[edit] Dipole with baluns

Coax acting as a radiator instead of the antenna.

When a dipole is used both to transmit and to receive, the characteristics of the feedline become much more important. Specifically, the antenna must be balanced with the feedline. Failure to do this causes the feedline, in addition to the antenna itself, to radiate. RF can be induced into other electronic equipment near the radiating feedline, causing RF interference. Furthermore, the antenna is not as efficient as it could be because it is radiating closer to the ground and its radiation (and reception) pattern may be distorted asymmetrically. At higher frequencies, where the length of the dipole becomes significantly shorter than the diameter of the feeder coax, this becomes a more significant problem. One solution to this problem is to use a balun.

Several type of baluns are commonly used to transmit on a dipole: current baluns and coax baluns.

[edit] Current balun

Dipole with a current balun.

A current balun is a bit more expensive but has the characteristic of being more broadband.[4]

[edit] Coax balun

Here is a dipole using a coax balun.

A coax balun is a cost effective method to eliminate feeder radiation, but is limited to a narrow set of operating frequencies.

[edit] Sleeve balun

Here is a dipole using a sleeve balun.

At VHF frequencies, a sleeve balun can also be built to remove feeder radiation.[5]

What are the basics of antennas?

Antennas, to quote a friend, are one of life's eternal mysteries. "All I'm totally certain of is that any antenna is better than no antenna and the antenna should preferably erected as high and be as long as is possible or desirable". Here we will discuss the very basics of antennas. Remember that thought: these are just some introductory antenna basics. Each type of antenna will eventually have its own page. In particular I would commend everyone to read my page on earth dangers. I think it ought to be compulsory reading.

The basic antenna

The most basic antenna is called "a quarter wave vertical", it is a quarter wavelength long and is a vertical radiator. Typical examples of this type would be seen installed on motor vehicles for two way communications. Technically the most basic antenna is an "isotropic radiator". This is a mythical antenna which radiates in all directions as does the light from a lamp bulb. It is the standard against which we sometimes compare other antennas.

This type of antenna relies upon an "artificial ground" of either drooping radials or a car body to act as ground. Sometimes the antenna is worked against an actual ground - see later.

Antenna Polarisation

Depending upon how the antenna is orientated physically determines it's polarisation. An antenna erected vertically is said to be "vertically polarised" while an antenna erected horizontally is said (not so surprising) to be "horizontally polarised". Other specialised antennas exist with "cross polarisation", having both vertical and horizontal components and we can have "circular polarisation".

Note that when a signal is transmitted at one polarisation but received at a different polarisation there exists a great many decibels of loss.

This is quite significant and is often taken advantage of when TV channels and other services are allocated. If there is a chance of co-channel interference then the license will stipulate a different polarisation. Have you ever noticed vertical and horizontal TV antennas in some areas. Now you know why.

Antenna Impedance

Technically, antenna impedance is the ratio at any given point in the antenna of voltage to current at that point. Depending upon height above ground, the influence of surrounding objects and other factors, our quarter wave antenna with a near perfect ground exhibits a nominal input impedance of around 36 ohms. A half wave dipole antenna is nominally 75 ohms while a half wave folded dipole antenna is nominally 300 ohms. The two previous examples indicate why we have 75 ohm coaxial cable and 300 ohm ribbon line for TV antennas.

A quarter wave antenna with drooping quarter wave radials exhibits a nominal 50 ohms impedance, one reason for the existence of 50 ohm coaxial cable.

The quarter wave vertical antenna

The quarter wave vertical antenna is usually the simplest to construct and erect although I know a great many people who would dispute that statement. In this context I am speaking of people (the majority) who have limited space to erect an antenna.

Figure 1. - a quarter wave vertical antenna with drooping radials

In figure 1 we have depicted a quarter wave vertical antenna with drooping radials which would be about 45 degrees from horizontal. These 45 degree drooping radials simulate an artificial ground and lead to an antenna impedance of about 50 ohms.

A quarter wave vertical antenna could also be erected directly on the ground and indeed many AM radio transmitting towers accomplish this especially where there is suitable marshy ground noted for good conductivity. An AM radio transmitting tower of a quarter wave length erected for say 810 Khz in the AM band would have a length of nearly 88 metres (288') in height.

The formula for quarter wave is  L =  71.25 metres / freq (mhz) and in feet L = 234 / freq (mhz). Note the variance from the standard wavelength formula of 300 / freq. This is because we allow for "velocity factor" of 5% and our wavelength formula becomes 285 / freq.

When a quarter wave antenna is erected and "worked" against a good rf ground (called a Marconi Antenna) the earth provides a "mirror" image of the missing half of the desired half wave antenna.

Figure 2. - a marconi antenna

In figure 2 above where I have depicted the Marconi Antenna imagine a duplicate of the quarter wave antenna being in existence from the top of the ground and extending down the page. This is the mirror image.

Half wave dipole antenna

The half wave dipole antenna becomes quite common where space permits. It can be erected vertically but is more often than not erected horizontally for practical reasons. I gave quite a good example of its use in my paper on radio telescopes from my original site. I have reproduced it in figure 3 below.

Figure 3. - half wave dipole antenna

This particular antenna was dimensioned for use at 30 Mhz. You will note that the left and right hand halves are merely quarter wave sections determined by the formula given earlier. The input impedance (affected by many factors) is nominally 50 ohms.

As with all antennas, the height above ground and proximity to other objects such as buildings, trees, guttering etc. play an important part. However, reality says we must live with what we can achieve in the real world notwithstanding what theory may say.

People erect half wave dipoles in attics constructed of fine gauge wire - far from ideal BUT they get reasonable results by living with less than the "ideal". A lesson in life we should always remember in more ways than one.

The folded dipole antenna

The folded dipole antenna is probably only ever seen as a TV antenna. It exhibits an impedance of 300 ohms whereas a half wave dipole is 75 ohms and I'm certain someone will be alert enough to ask "why 75 ohms, if figure 3 above is 50 ohms?".

Within the limits of my artistic skills I have depicted a folded dipole antenna below.

Figure 4. - half wave folded dipole

One powerful advantage of a folded dipole antenna is that is has a wide bandwidth, in fact a one octave bandwidth. This is the reason it was often used as a TV antenna for multi channel use. Folded dipole antennas were mainly used in conjuction with Yagi antennas.

The Yagi antenna

The Yagi antenna or more correctly, the Yagi - Uda antenna was developed by Japanese scientists in the 1930's. It consists of a half wave dipole (sometimes a folded one, sometimes not), a rear "reflector" and may or may not have one or more forward "directors". These are collectively referred to as the "elements".

Figure 5. - the Yagi antenna

In figure 5 above I have reprinted a UHF Yagi antenna array from my radio telescopes page. You will note, not altogther clearly.

However in figure 6 below, which happens to be a photograph of a neighbour's TV antenna, I can clearly point out details of a practical Yagi antenna.

This particular antenna has been optimised for dual band operation. It is designed to pick up both VHF and UHF transmissions. Because I live in a regional of NSW in Australia, TV antennas tend to be single channel types designed either for higher gain or better directivity. Different examples will be presented later.

Figure 6. - a practical Yagi TV antenna

Looking from left to right on this dual band Yagi we have six UHF "director" elements which improve gain and directivity. Next is the UHF half wave dipole which could have easily been a folded dipole but is in fact a plain half wave dipole.

The next three much longer elements form a "phased array" for the VHF band. I am unsure of the function of the three remaining smaller elements, information is quite scant here but one would certainly be a UHF "reflector". Likely the other two also fulfill this function also.

Note: This is a horizontally polarised antenna and is orientated roughly NNW, 315 degrees.

You will notice the effect of very strong storms from the sea have had in bending the second larger elements. In my locality storms are a problem but not as much as roosting parrots such as large sulphur crested cockatoos.

UHF Yagi antenna

In the photograph in figure 7 below you can see a classic UHF Yagi antenna. It has a total of nineteen "elements" comprising seventeen "directors", a fancy folded dipole with a "low-noise mast head amplifier" and a "reflector".

Figure 7. - a vertically polarised UHF Yagi antenna

This is a a vertically polarised UHF Yagi antenna and it is orientated WSW or 225 degrees. It does in fact pick up signals about 100 Km or 60 mile distant from Sydney.

This is the very same antenna I was suggesting to be used in the radio telescope array I depicted in figure 5 above.

Stacked half wave dipoles or a collinear array

The majority of TV antennas in my retirement village are stacked half wave dipoles. These consist of four sets of a half wave dipole and a reflector only, but mounted one above another. These antennas owe their origin to the days we only had VHF TV in the area. Surprising with the introduction of UHF they continued to function quite well in picking up UHF as well. This particular antenna is my one and I've never had the need to go to a UHF antenna. The top two elements normally are home to roosting "top knot" pidgeons, a pigeon native to Australia.

Figure 8. - four stacked half wave dipoles collinear antenna

To the left of the photograph are the "reflectors" and to the right are the four vertically stacked half wave dipoles. The wires connecting each half wave dipole are done in a "phased way". This comprises a collinear antenna array and is so arranged for improved gain.

Note this antenna is horizontally polarised.

Loop Antennas

The loop antenna comes in an amazing number of configurations. It is a "small space" antenna and although extremely inefficient is capable of surprising results. In receiving applications the loop antenna works on the principle of the "differences" in voltages induced by the current flowing in the sides of the antenna. As you might imagine these difference voltages can be extremely minute in amplitude and any loop antenna usually requires an associated amplifier capable of at least 25 dB power gain following it.

One example of a shielded loop antenna is taken from my tutorial on mobius winding techniques is shown in figure 9 below.

Figure 9. - mobius winding of a loop antenna

This is the general loop antenna which has one interesting characteristic. It responds well to signals arriving in one direction, either from the left hand side of your computer screen or the right hand side of your computer screen for the loop shown in figure 9 (b) above. Signals from either your face or from behind your monitor would produce equal signal currents from both sides of the loop and consequently produce no difference voltage output.

Technically speaking, a loop antenna responds to the magnetic field rather than the electric field.

Rather than being omnidirectional (as a whip antenna would be) the loop antenna responds to the cosine of the angle between its face and the direction of arrival of the electromagnetic wave. This actually produces a figure eight pattern, which for receiving presents no probems. The addition of a small whip antenna in conjuction with proper phasing allows the direction ambiguity to be resolved and we have an antenna relatively ideal for direction finding.

The most common loop antenna you will encounter is the loopstick antenna [in the U.K. it is referred to as a "ferrite rod antenna"] built into portable receivers. In figure 10 below is the AM and shortwave loopstick antenna in a Sanyo model RP2127 MW / SW receiver (it's old).

Figure 10. - AM and shortwave loopstick antenna

The AM and shortwave loopstick antenna is located in the upper half under the words "loopstick antenna". For greater efficiency and size reduction, a loopstick antenna is wound on a "ferrite" rod. This particular one happens to be circular but you may encounter ones which are rectangular.

As an experiment you might, if you have a loopstick antenna radio available, tune to a weak station and rotate the radio around 360 degrees. You should notice two points 180 degrees apart where the signals seem to be the strongest and similarly notice two other points 180 degrees apart where the signals seem to be the weakest - these are called "nulls". This is the aid to "Radio Direction Finding - RDF"

Terminated Tilted Folded Dipole

Now here is a little gem. The terminated tilted folded dipole is bound to give a "rush of blood to the head" of any avid DX'er (that means long distance -dx- receive / transmit enthusiast).

The terminated tilted folded dipole is somewhat similar to the half wave folded dipole in figure 4 above yet the claims for its performance are quite astonishing. The terminated tilted folded dipole is claimed to have a bandwidth of something like 5 or 6 to one, been extensively tested and adopted by the US Navy, easy to construct from readily available materials and, has a feedpoint impedance of around 300 ohms.

Figure 11. - Terminated Tilted Folded Dipole

The dimensions "A" and "B" for a terminated tilted folded dipole are as follows:

Each leg "A" = [ 2 X pi ( 15.25 / Fo )] and;

Distance "B" = [ 2 X pi ( 0.915 / Fo )]

where in both instances 2 X pi = 6.28 and Fo is in Mhz.

There seems to be some debate about the exact formula, my friend L. B. Cebik (see next) says:

"The "Wide-Long" version coincides with standard construction formulations, since the antenna is about 300/F(MHz) long and 10/F(MHz) wide. (Excessively fussy cutting formulas for this antenna are largely superfluous, since strict resonance is not in question)."

My friend L. B. Cebik (see later) has modeled this antenna. Modeling the T2FD

Further comprehensive details on the claims for the amazing terminated tilted folded dipole antenna and its construction can be found at: http://www.hard-core-dx.com/nordicdx/antenna/wire/t2fd.html

Conclusion on antenna basics

The reason there has been emphasis on TV antennas is simply because nearly everyone can look at examples in their own locality for comparison. At TV frequencies the physical dimensions are such I can offer practical examples with photographs.

The same basic principles apply at HF and LF although physical sizes tend to be totally impractical.

As time permits I will flesh out more and more in depth articles on all these antennas and even more types not even mentioned here. This page alone comprises well over 2,000 words so you can imagine the job ahead with competing demands on my time. Meanwhile consider this important publication on antennas.

Meanwhile I would also suggest that you take a good look at L.B. Cebik 's W4RNL great web site. My good friend LB is "THE antenna guru".