Secondary effects can be cross channel interference and intermodulation distortion in following circuits.

So for example, say there is a local high power transmitter and you want to look at a small signal at a different frequency. Any RF pre-amplifier has to handle the high power signal as well as presenting a low noise figure at the wanted signal frequency. Any non-linearity in the pre-amp will cause interference by mixing with other unwanted signals and many could be reflected back into the wanted signal band. Only front end filtering (or a resonant aerial) will prevent this. (post amp filtering cannot remove in-band signals) This is why often front end RF amps have to be designed as power amps. They have to handle the highest power signal whether the wanted one or (usually) not.

There are techniques for minimising these effects. If you give more detail of what you are trying to do I can look up an appropriate one.

Hope this helps

Dr. Paul

thank you sir but i want to know disadvantage  of nonresonant antenna  over resonant antenna.

My answer contains the disadvantages of non-resonant aerials (Aerial is another name for Antenna) Summarised as "cross channel interference and intermodulation distortion in following circuits".

Please explain a little more why you think this does not answer your question; we may be at cross purposes.

Regards

Dr. Paul

1) It has less efficiency and chances of internal losses are more due to back & forth of energy. 

2) The matching network between source (or receiver) requirement becomes critical from piece to piece due to reactive part.

3) The matching varies with feed cable length

4) In case of transmitting antenna, final amplifier (Especially for HP) requires strong protection (Otherwise cases of failure rate will be very high) 

why at receiving end  cross channel interference in travelling wave (nonresonant ) antenna is found  and at sending end high power transmitter is required?

Due to non resonance characteristic, an antenna is excited by & through a comparable energy level of different channels in a receiver. This creates channel interference at the receiving end.

Due to less or poor efficiency, high power is required at transmitter side

why strong protection in final amplifier?

It may help to define use and classes of aerial.

For single frequency operation, an aerial can be tuned and Omni directional. Tuning gives a better match between amplifier and free-space at the wanted (signal) frequency and rejection at other frequencies.

For a television receiver this is not useful as many frequencies are required, and the TV signal is not transmitted on a single frequency, but on a number of frequencies called a channel. (the carrier is one frequency, but when modulated occupies a wider channel.

On top of this one needs to have more than one TV channel, this is called a band. Modern TV aerials are wide-band but they are tuned. For example, in a Yagi, by increasing the width of the reflectors, rather than a single frequency the aerial can receive (or transmit) at a number of frequencies, or a wider band. It does still reject frequencies below and above the band, so is tuned. By definition it is non-resonant at one frequency. Another way to obtain wider bandwidth is by a log-periodic aerial. This time each reflector is a different length. Each reflector resonates at a single frequency, but because there are many different frequencies, the effect is an aerial tuned over a wide band, not a single frequency.

In the case of mobile phones, the aerial has to both transmit and receive. To obtain longer battery life the phone uses low power. However the mast can be high power (as it is mains driven). The design of the mast aerial is interesting. It has to operate in a band (so a single frequency tuned aerial is no good), however, it does use gain, because overall received power is proportional to gt x gr . (receiver gain times transmitter gain). The mast uses directional aerials. An aerial can be designed to transmit in one direction only but still over a band of frequencies. Theses are what you would call non-resonant designs. Each one has a different set of disadvantages. By putting a coil in the middle of the aerial, it can be made to have gain all around the aerial but not above or below. Typically 3dB to 6 dB gain can be achieved this way and it is called a collinear aerial. The disadvantage is that response into the sky gives a big loss, so aeroplanes cannot pick up the signal.

For satellite receivers, a reflector aerial is used which has a very high gain along one direction, and so can pick up signals from the sky with gains of maybe 20dB. However, the disadvantage is that is cannot pick up signals from any other direction, so the satellite has either to be stationary (and a long distance from the earth in geo-stationary orbit) so the receiving aerial is stationary. Or the receiving aerial has to be able to track a moving satellite adding complexity to receiver (as is used with GPS).

Most Mobile phone masts use a  sector aerial. This has high gain along the ground but only over (usually) 120  degrees. Typically 9 to 12 dB of gain can be obtained. The disadvantage here is that for full coverage around the mast, 3 aerials are needed (120 x 3  = 360 degrees).

Here is a good overview of different aerial types. http://en.wikipedia.org/wiki/Category:Radio_frequency_antenna_types , many are non-resonant but have gain.

Regards

Dr. Paul

thank you sir

The protection in final stage PA is required because in case of non resonance, chances of high power reflection is more and on the collector of final amplifier transistor if amplified output of incoming signal and reflected signal added constructively, there will be very high energy accumulated on collector which can have as high voltage as doubling of output. This can be above breakdown voltage of collector junction capacity and lead to amplifier failure. In order to save the power transistor (It is very costly), strong protection is required 

thank you sir .i wnat to know that when we consider wave nature of current and voltage  then there is a reflected current and voltage due to mismatching of transmission line load but  how current  and voltage is produced in far end of transmission line when mismatching occur? is it induced voltage in far end because of that induced current ? 

in other words i want to know that how reflected current and voltage is produced  in far end when wave nature is not taken into consideration. 

In case of wired network (or physical media case),  it is not wave nature but physical existence of current and voltage. Current has nature of flow and direction. if the incident current is flowing in say +Z direction, reflected current will have flow in -Z direction. This will create reflected voltage in network which will have opposite nature (in terms of direction or phase) with reference to the incident voltage. This is how reflected current and voltage are produced

There is confusion in some of the discussions, and someone is not using the correct researchgate etiquette. You should only mark down an answer if it is offensive or manifestly wrong.. It should not be marked down if it does not answer the question. It is better to mark up answers that are correct and add to the discussion if you require clarification. You can correct the marking if the person that marked down, then marks up the question.

There is a difference between a mismatched transmission line (TL) and load, and resonance.

If a TL is matched with its characteristic impedance (the load) a travelling wave will progress from amplifier to load.

If the load is not matched then there will be a reflected wave that travels from load to amplifier. If you combined two waves in opposite directions, one gets a standing wave, or a combination of travelling and standing waves. here is a demonstration in excel that only uses sine functions. https://dl.dropboxusercontent.com/u/102255847/wavesanimation.xls change the turquoise coloured cell between 0 and 1 to see the effect of varying levels of reflections.

In the case of a PA connected to an aerial, the aerial can present a real or imaginary impedance or a mixture of both. and impedance mismatch will cause reflection, real or imaginary. When the load matches the TL impedance there is no resonance, because there is no reflected wave and hence no standing wave.

Only travelling waves transmit power. So a badly terminated TL that has reflections will have a VSWR > 1 (=1 is pure travelling wave). high VSWR means that at certain parts along the TL length voltages will add and at other parts they will subtract. If the voltages add at the PA, its collector (or drain in the case of a FET) will exceed the supply voltage and as described above may damage the transistor. This is an effect of a badly terminated TL that is effectively resonating. (a standing wave is a resonance). the opposite of what was described above.

If the TL is short compared with the transmitting bandwidth the system can be considered to be made up of lumped parameters, i.e. just L C and R. If the energy transferred into the R each cycle is low compared with the energy in L and C then the system will resonate. at 1/sqrt(LC).

In the case of a PA connected to a TL which has an aerial as a load, the system is a little more complex. The impedance at the PA can be complex, and the aerial can present a complex load to the TL. A correctly tuned system will have a low VSWR ~ 1. This is usually achieved by adding a tuning element somewhere along the TL, (for example a variable capacitor) . The intent is to match load to the PA at one frequency to obtain a travelling wave. The aerial may match free space at one frequency, and hence be resonant, but the TL is not resonant when transmitting maximum power.

Regards

if you not taking wave nature of voltage and current  then how can be current flow in both direction at same time? from where reflected current has come?

Dear Gautaum,

You ask an important question. At each point along the TL, there is a single voltage and single curent that varies with time. The term forward and reflected wave refer to how the voltage and current varies with time and how they change along the TL.

Voltage and current are values in the time domain, waves are frequency domain descriotions. Both are valid and can be transposed using Fourier transforms. When discussing transmitters or aerials, the frequency domain description is easier to use and visualise or predict circuit behaviour.

If you look at the spreadsheet I included above, it just shows one parameter, either voltage or current. If you press F9 you can see how the profile along the  TL changes with time. Each point at one point in time has a single value, but this value changes along the length of the TL and with time.

Does this make more sense now?

Paul

Please note that in the spreadsheet, the y axis is normalised to 1, so the voltage amplification effect of a standing wave is not apparent. 

sir please clear it more.in book given that incident and reflected both flow in same time like wave . also clear that some point voltage will add and some point will subtract

The terms incident and reflected refer to waves, not voltages or  currents. Try thinking of an analogy. A vibrating violin string has points along its length each point has a displacement away from the centre line. Make this displacement analagous to a voltage. The string has only one place for each point, but when you look at the whole string, you can see a wave.