What is it that makes antenna length so important?

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Why is it that an antenna length must be somehow proportional to the wavelength of the frequency
 

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  • #2
radiation of Electromagnetic energy form the antenna depends on the voltage and current distribution over it. so in order to make the radiation constructive you have to keep the length of the antenna half to its operating wave length and has to tilt the structure from the end of transmission line. Its a detail understanding with help of images so if you want you can download US NAVY TRAINING SERIES FOR ELECTRICAL ENGINEERING and consult Antenna portion there. it is also available at www.tpub.com
 
  • #3
sophiecentaur
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Why is it that an antenna length must be somehow proportional to the wavelength of the frequency

Any length of antenna (or even the proverbial dead sheep) can be made to radiate. The clever thing is to make it radiate efficiently. If you think of a dipole, fed at the centre and the current that will flow into it from the feeder. The ends of the wires don't 'lead anywhere' so there must be no current flowing at those points. For a short dippole, this causes a standing wave of current on the antenna because power is reflected from the ends, back to the feeder, presenting a high impedance to the power coming in from the feeder. Some of the power, sloshing around will be radiated but, only when the dipole is about a half wavelength, will the wave pattern allow maximum current to flow into the feed point - and hence all the power you wanted to radiate. A dipole of this length is 'tuned', naturally as the 'reactive components' cancel (jargon but it has to be said!) and you get resonance. With the appropriate matching network of other components at the feed point, you can get more power out of any length but the losses are greater and it can be inconvenient to achieve. You also find that longer dipoles of 3/2, 5/2 wavelengths will be tuned because they also resonate at the wanted frequency because a standing wave can also exist on them.

There is a mechanical analogue with vibrating strings. If you excite them at a natural resonant frequency (where there are integral numbers of half waves on them) then the vibrations are biggest and the sound radiated is a maximum.
 
  • #4
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So then those progressively larger rods on yagi-udas on houses match different wavelengths, so as to get the best reception of the certain signals?
 
  • #5
berkeman
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So then those progressively larger rods on yagi-udas on houses match different wavelengths, so as to get the best reception of the certain signals?

Correct. And their arrangement in a line like that gives the antenna some directional gain (in the direction that the antenna is pointing).
 
  • #6
sophiecentaur
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So then those progressively larger rods on yagi-udas on houses match different wavelengths, so as to get the best reception of the certain signals?

It would be better to say that the 'mis-matches' of the rods at the design wavelength induce currents in different phases in each element. Remember, only one element is fed - the others are all 'parasitics'. The effect of all the parasitics is to make the antenna directional.
It's easier to describe the way a Yagi works as a transmitter and then rely on the reversibility argument to say that the receive pattern is the same as the transmit pattern.
The driven element is chosen to be 1/2 wave long at the design frequency. The longer, 'reflector' element is slightly inducitve (>λ/2) and picks up the signal from the driven element and re-radiates it with a phase which is advanced. The spacing is chosen to make the signal it re-radiates cancel the main signal - giving a null / minimum in the rear direction (hence the name 'reflector'). The familiar 'H' antenna works like this. It has a broad pattern with a single null to the rear. You can make the array more directional by putting 'directors', which are shorter than the driven element (<λ/2). These are slightly capacitive and their re-radiated signals add up in the forward direction. Again, the spacings have to be right.
This arrangement of a single driven and several parasitics has a max to the front and a min to the rear but, to the sides, the pattern may be very ragged (high sidelobes). The more directors, the narrower the beam of the array (the higher the gain). It is a very elegant and cheap design. Arrays which involve many fed elements can have better 'tailored' patterns but they are costly and need power splitters and complex cabling. A Yagi can be 'stamped out' with a crude assembly process and can be DIY connected up. Ideal for your chimney.

Bandwidth: a Yagi antenna can be made very good at a single frequency but, at other frequencies, its pattern may get terrible - like a fried egg - (the elements and spacings are not ideal fractions of a wavelength). Most domestic receiving antennae need to operate over at least 20% bandwidth so the commercial designs are a compromise between bandwidth and performance.
Spot the 'Log Periodic' antenna on some rooftops. That has two 'booms' with elements attached to them. Every element is fed from the signal travelling along the boom. They have a big range of element lengths (much more tapered) and give a good 'front-to-back' performance and a wide bandwidth (but, alas) a lower gain. But you can't have everything.
 
  • #7
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I still don't get it. If the pulse (current wave), reflects at the end of a dipole, then it will form a stationary wave in the dipole. Is this what's considered resonating? Resonance, in my understanding happens when more than one waves interfere with same phase, which increases the effect and you get an amplified wave. Can any body help? Thanks in advance.
 
  • #8
vk6kro
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A transmitter is producing sinewaves and these are being fed into the dipole.

The first sinewave travels to the end of the dipole and is reflected back towards the center of the dipole. This takes time and the sinewave is still traveling when the next sinewave arrives.

The signal being reflected interacts with the next part of the sinewave coming from the transmitter.

If the wavelength of the dipole is right, then there will be standing wave produced.

Typically, this will be a pattern with a current peak at the center of the dipole and voltage peaks at the ends of the dipole.

This results in the center of the dipole being a low impedance of 72 ohms in free space and this is very suitable for matching the dipole to common transmission lines from the transmitter.
 
  • #9
sophiecentaur
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@tgnana
Rather than talking in terms of a pulse, which could be likened to plucking a guitar string, it is better to use a wind instrument as an analogy. The antenna only 'settles down' to a resonance after many cycles have passed.
As has been said already, one point of resonating an antenna is to make it an efficient radiator (on transmit). When used for reception, this may be less important. A very short whip can be used to receive mf transmissions but would be very poor for transmission.
The point is that anything 'can' be made to radiate by appropriate matching. Half wave dipoles happen to be easy to feed and suit the yagi arrangement very well.
 
  • #10
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The best analogy is a paint pan ( shallow long pan) - if you rock the paint pan too slow or to fast - you just get waves in the pan - but if you rock the pan at just the right rate ( frequency) you will have one single wave going back and forth - and it will be difficult not to spill the water ( don't use paint (;-) - If you double the frequency - you will actually generate 2 waves - out of phase with one another. - Note - where these two waves come together - ideally in the middle - the peak height of the wave is double at that point.

This is not an exact analogy - but a good physical model for showing a small amount of energy can be controlled and sustained.
 
  • #11
sophiecentaur
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The problem with the paint pan analogy is that a surface wave on the paint is not sinusoidal and not continuous. In an antenna, the wave goes on for 'ever' and the power is constantly being supplied by the transmitter and dissipated into space. It's not pulses as it is in the paint pan.

With even a short dipole, there is always a standing wave, in fact. There is always a current node at the ends. The point about going for resonance is that the series reactive component becomes zero - giving a good match and minimal resistive loss; maximal power will be radiated. Furthermore, when matched perfectly to the transmission feeder, there is no reflection, which minimised the voltages at the transmitter (VSWR is near unity) and the frequency response is also not affected and echos are minimised (particularly relevant to colour TV).

Actually, the significant radiation resistance of a dipole loads the LC resonance greatly. This means that the Q of the resonance is not very high so there is not a huge peak in the frequency response.
 

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