Why is antenna size/design less critical for receivers then transmitters? - Page 1
Forum section: electronics / beginners

author: joeyjoejoe
author_rank: Frequent Contributor
timestamp: October 17, 2018, 05:51:46 pm
content: I think I've noticed that in general, antennas for RF Transmitters are much more challenging then receivers.For instance, evaluation boards for two ISM IC's, TX and RX - the TX is very specific and has application notes on the antenna on the design and constraints. The RX is just a big copper rectangle. I've also seen this pattern elsewhere - 433MHZ ISM breakouts I have, the antennas are different. SDR sites saying that generally for receiving, the antenna isn't as important until you get into transmitting...Is this accurate, and if so why is it the case?

author: Benta
author_rank: Super Contributor
timestamp: October 17, 2018, 06:06:14 pm
content: There are several answers to this.For low power ISM transmitters, it's a question of efficiency. You want to translate as much supply power as possible to "air power". The receivers do not have an efficiency issue, they'll consume as much power as needed, regardless of antenna design.For higher power transmitters an additional problem arises. If the antenna is not correctly matched, you get what's called a bad standing wave ratio (SWR). This means, that RF power not radiated by the antenna is reflected back into the transmitter power stage, worst case causing it to fail or even blow up.

author: joeyjoejoe
author_rank: Frequent Contributor
timestamp: October 17, 2018, 06:13:14 pm
content: So is it more a question of "good enough" for RX? Sure, they could spend more hours designing an antenna, but the gain would be a few percent, without any real negative effects. TX the time has to be spent to minimize the negative effects, and as a result we also get a nicely tuned antenna?

author: Benta
author_rank: Super Contributor
timestamp: October 17, 2018, 06:24:26 pm
content: Yes, it's a question of "good enough" for Rx. Most ISM networks are not range limited. Same with, eg, FM receivers, where the network of transmitters is normally dense enough.In certain applications like long range WLAN you'll see small parabolic antennas on both ends.

author: tautech
author_rank: Super Contributor
timestamp: October 17, 2018, 06:45:49 pm
content: It ain't rocket science to achieve good results with the right tools and a little bit of knowledge.The type of antenna (and good design) used is key so to the use of all of the TX power available so particularly SWR is minimized.This will give you the basics of what to aim for:https://www.eevblog.com/forum/rf-microwave/antenna-project-log/

author: joeyjoejoe
author_rank: Frequent Contributor
timestamp: October 17, 2018, 06:51:13 pm
content: Quote from: tautech on October 17, 2018, 06:45:49 pmThose right tools sure cost a lot

author: xaxaxa
author_rank: Regular Contributor
timestamp: October 17, 2018, 06:54:13 pm
content: There is a more specific reason receive antennas are less critical which is the receive sensitivity bottleneck is usually environment noise. Suppose you have an antenna that loses 10dB of power; when you transmit this translates directly to 10dB loss of link budget, but for receive if the environment noise is 15dB above thermal noise, then your sensitivity isn't even affected and you will notice no difference compared to a perfect antenna.The environment noise is roughly proportional to 1/f. From my experience the noise averages >30dB above thermal noise in the FM broadcast band, so a fm receive antenna can lose up to 30dB before it becomes a problem.This is also the reason why the "best transmit antenna is also the best receive antenna" theory doesn't hold: when you introduce additive noise the system is no longer linear.

author: tautech
author_rank: Super Contributor
timestamp: October 17, 2018, 07:06:06 pm
content: Quote from: xaxaxa on October 17, 2018, 06:54:13 pmCan we explore this further ?Yes environmental factors do effect performance but overcoming them is the challenge as with a different frequency or a point to point link could do.Still, the foundingantenna reciprocitylaws need guide us in antenna design.

author: xaxaxa
author_rank: Regular Contributor
timestamp: October 17, 2018, 07:30:47 pm
content: I read this a while ago: (on sm0vpo.com website)http://213.114.131.21/antennas/anten2.htmQuote

author: xaxaxa
author_rank: Regular Contributor
timestamp: October 17, 2018, 07:35:42 pm
content: 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.

author: tautech
author_rank: Super Contributor
timestamp: October 17, 2018, 08:00:01 pm
content: Quote from: xaxaxa on October 17, 2018, 07:30:47 pmQuote from: xaxaxa on October 17, 2018, 07:35:42 pmStill 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.

author: GeoffreyF
author_rank: Regular Contributor
timestamp: October 17, 2018, 08:25:32 pm
content: 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.

author: rfeecs
author_rank: Frequent Contributor
timestamp: October 17, 2018, 08:40:02 pm
content: Maybe the original paper on antenna reciprocity:"The Principle of Reciprocity in Antenna Theory"https://ieeexplore.ieee.org/document/1694633Derives 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

author: dmills
author_rank: Super Contributor
timestamp: October 17, 2018, 11:25:54 pm
content: 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.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 18, 2018, 12:29:20 am
content: 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