A question on transmitter and receiver antennas

[atmaca]

I know that for a transmitter antenna, the length of antenna is important.
And the length of antenna must be at least Lambda/2.

For receiver antennas there is no such kind of restrictions.

What are the reasons of these two issues ?

Thanıks.

 

[papyaki]

Hi,

Lambda/2 is not a minimal requirement for a transmitter. You can use lambda/4 with ground radiants. The more important is the impedance adaptation between the output stage of the transmitter and the antenna.

If your transmitter has an output impedance of 50ohms, the antenna must have an impedance of 50ohms. If it is not the case you can connect an T or PI impedance adapter (also known as matcher) which will adapt transmitter to antenna. With this kind of material, you can connect any antenna to your transmitter.

This is done to avoid reflective energy not absorbed by the antenna to return to the transmitter and destroy the output stage, specialy when the output power is high.

In the receiver case, the length of the antenna is less critical, because the ernergies are too small. But if you try a lambda/4 without radiant and your compare with the same antenna by adding some radiants, you will note a best signal strength with the radiants. That's because the transfert of the received energy from the antenna to the receiver is better due to the impedance matching.

Regards

Papyaki

 

[mehta_jp]

MAY I ADD A BIG DIFFERENCE? IT IS POWER HANDELING OF EACH & DUE TO THAT RETURNLOSS OF TX ANTENNA MUST BE HIGH. SO THAT POWER RETURENED TO THE TRANSMITTER IS LESS.
FOR THE RECEIVER IN SAME MANNER SENSIVITY SUFFERS. HERE, POWER IS NOT AN ISSUE BUT GAIN & SENSIVITY IS.
IN BOTH CASES LAMBADA PLAYS MAJOR ROLE.

 

[sergio mariotti]

no no dear atmaca, the reciprocity theorem is sill valid!
The importance of a good matching is THE SAME! in both the cases TX and RX.
The power reflected, (so lost) is the same.
In the receiving case since the power is exremally lower, usually people don't experience the lost of power because AGC circuits, but this lost exist!.
A little help became from the fact that the radiated EM filed is often "generous" (strong enough) to leave the user free to receive also with a very short antenna.

 

[scoppulal]

HI,
I think that your statement is not right:

- An antenna can be shorter: for example the lambda/4 verticals used by CB radio.

- Another example is if you try to receive with a 10 cm wire the emission of a HF broadcast you will fail.

What you probably meant is that the physical lenght of an antenna could be shorter than the electrical lenght (loaded antenna), but this way the efficiency will be a lot lower.
So, if you want your system to perform well, you have to consider proper dimension for both TX and RX antennas.

Regards