Electronics > Beginners
Why is antenna size/design less critical for receivers then transmitters?
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tautech:

--- Quote from: xaxaxa on October 17, 2018, 07:30:47 pm ---I read this a while ago: (on sm0vpo.com website) http://213.114.131.21/antennas/anten2.htm

--- Quote ---The best transmitting antenna is also the best receiver antenna!

Ah! That old chestnut! Yes, and no. Transmitters and receivers have very different properties. The best transmitter antenna is one that radiates all the power you shove up there. It must also radiate it in the right direction and be sited so that other nearby objects (including the ground) do not absorb the power. Let us talk for a moment about volts. A transmitter will give out a signal of typically 100 volts (into 50 ohms). We want as much of this signal to be received at the other end as possible.

A receiver antenna, on the other hand should ideally catch as much of the signal as is available, but the signal voltage is often as low as 0.000001 volt, frequently smaller. At these sort of signal levels there are, unfortunately, loads of other signals we do not want, such as solar noise, atmospheric noise, and a huge variety of man-made noise. The best transmitting antenna will, by definition, receive everything.

If on the other hand I had an HF loop antenna made from coaxial cable and earthed the braid, I would receive nothing. Break the braid half-way round the loop and it will receive again. The wanted signals may be only 30% of the level using an unscreened loop, but static and other such signals will be eliminated. The effects of assymetric capacity to nearby objects will also be eliminated. You could get an interference reduction of 80%. In this example your signal has gone down to 30% but the interference has reduced to 20%. Clearly the difference between the wanted signal and the other rubbish has improved.

Another difference could be the use of loop or frame antennas using their directional properties to "notch out" on-frequency signals coming from other directions. Again there is a large improvement in received inteligibility, but not necessarily an improvement in signal strength.

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--- Quote from: xaxaxa on October 17, 2018, 07:35:42 pm ---However I think antenna reciprocity mostly holds at UHF and above since you can keep objects and interferers out of the near field and the noise floor is much lower.

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Still correct design has everything to do with performance and while a TX and RX antenna might be each vastly different designs the laws of antenna reciprocity still apply. Obviously a TX must be able to physically capable of handling TX voltages and power whereas RX can be scaled down to uV, nV or pV levels however reciprocity still applies.
Optimize an antenna for it's intended use and the results will show it's worth it. No amount of reading or application of antenna formulae will make it work 100%, it must be proven with the correct gear.
GeoffreyF:
If the receiver is far from the transmitter - it is a lot more critical.

The transmitter may be intended for a certain impedance out to the antenna. If the antenna is grossly different from that, the final stage of the transmitter could fry, in some circuits anyway.
rfeecs:
Maybe the original paper on antenna reciprocity:
"The Principle of Reciprocity in Antenna Theory"
https://ieeexplore.ieee.org/document/1694633

Derives the equivalence between receiving and transmitting antenna parameters and discusses some practical differences between receiving and transmitting antennas, most already mentioned in this thread.

From the abstract:

--- Quote --- It is shown that, in the case of strong interference, (1) the highest possible directivity is of importance both in the transmitting and in the receiving antennas and (2) the efficiency and the coefficient of exploitation of the receiving antenna are of no importance.

In the case of low interference, it was found that, (1) the directivities of both the receiving and transmitting antennas are of equal importance and (2) the efficiency and coefficient of exploitation of the receiving antenna are just as important as the efficiency of the transmitting antenna.
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dmills:
That looks about right, at HF sky noise almost always dominates, so I can hear pretty much everything there is to hear on an inefficient antenna, and may in fact trade off quite a lot of gain for a notch pointing at somewhere annoying (Italy!), and rejection of near field interference.

By the time you get to 70cms the sky noise is MUCH less of an issue and high efficiency low noise temperature designs (sometimes with masthead HEMTs or such) make a real difference on receive.

In both cases for transmit you want to put as much power in the direction of interest as you can.

73 Dan.
T3sl4co1l:
Two factors: asymmetry and noise.

In a great many applications, there is one common transmitter, serving many small receivers.  The receivers must be compact and cheap.  The transmitter must be static, and usually high powered in order to cover a commercially viable area with sufficient power to overcome background noise.

Electrically small antennas have low radiation resistance: on the one hand, the received signal is very weak; on the other, the noise due to the antenna's own resistance is small, so the signal you do receive, though weak, is still largely received signals, and atmospheric and ambient noise.  Low noise amplifiers aren't terribly hard to make (a few modestly sized JFETs will do fine for AM/FM BCB purposes, or fancier kinds in the GHz), so again the detected signal is largely from the airwaves rather than antenna and front-end noise.

There are also some special cases that kind of flip this around, but not in such a way that symmetry results.  Take deep space probes for instance: by necessity, they are equipped with a fair sized (low ~meters) antenna, and not very much power (tens of watts).  The ground station, on the other hand, is made up of a worldwide network of massive radiotelescopes (10s of meters or more), synchronized so that gain by synthetic aperture is possible.  (Hmm, honestly I don't know if they go quite that far for the DSN; it's possible, as it's done regularly for VLB astronomy.)  Bidirectional communication is necessary, often with greater bandwidth requirements transmitting from the probe (since it's the thing collecting data; I don't know how large the uplink payloads usually are, say for command or software updates if applicable, but I've got to imagine they just aren't as much overall?).  But as it's rather impractical to fly a 25m+ antenna to space, of course this is the compromise made.  Not that it's much of a compromise, anyway: on the physical level, antennas are necessarily symmetrical, no matter who's transmitting or receiving, and the total link gain is the product of both antenna gains, period.

Tim
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