Best tools for analysing Receiving (Active) Antennas? - Page 1
Forum section: electronics / testgear

author: vinlove
author_rank: Frequent Contributor
timestamp: January 26, 2023, 10:43:59 am
content: For transmitting antennas, we use SWR readings for checking the efficiency of the antennas with SWR meter.But for receiving antennas, what do you use for the checking and analysing?Especially for active rx antennas, what are the factors for checking and testing?What tools are the best for the tests, checking and analysis of receiving and especially the active rx antennas?

author: wasedadoc
author_rank: Super Contributor
timestamp: January 26, 2023, 11:14:51 am
content: A SWR meter reveals nothing about the gain of an antenna (transmit or receive) which is achieved through its directivity. What parameters of receiving antennas are you wanting to analyse?

author: Berni
author_rank: Super Contributor
timestamp: January 26, 2023, 11:18:41 am
content: Id say a directional coupler connected to a spectrum analyzer.That way you can see how much of a signal is coming in and how much noise/interference there is around it.

author: vinlove
author_rank: Frequent Contributor
timestamp: January 26, 2023, 11:27:53 am
content: Quote from: wasedadoc on January 26, 2023, 11:14:51 amResonance for the receiving frequency.

author: vinlove
author_rank: Frequent Contributor
timestamp: January 26, 2023, 11:31:18 am
content: Quote from: Berni on January 26, 2023, 11:18:41 amCould it not be possible also just by listening to the signal on the radio?

author: tszaboo
author_rank: Super Contributor
timestamp: January 26, 2023, 11:55:17 am
content: You can do S21 metering with a known good antenna. And repeat the test rotating the DUT.

author: Berni
author_rank: Super Contributor
timestamp: January 26, 2023, 11:58:58 am
content: Quote from: vinlove on January 26, 2023, 11:31:18 amYep you can.But you ware asking for instrumentation to monitor the antenna. You didn't give any context as to how and why you need to do that. It is possible your receiving radio doesn't give sufficient information about the incoming signal quality. Especially since you said you want to test it while in operation. Or perhaps you have your radio somewhere far away on a remote location and need to monitor it remotely or get alerts about issues with the radio link.

author: vinlove
author_rank: Frequent Contributor
timestamp: January 26, 2023, 12:30:03 pm
content: Quote from: Berni on January 26, 2023, 11:58:58 amDon't other factors which are out with the antennas affect the how much of a signal is coming in and how much noise/interference there is around it, such as the environment interference or EMF in the adjacent space where the antenna is and testing is held.Using measuring equipment or tools for testing antenna would perhaps only deal the antenna itself for the frequency for efficiency and resonance of the reception? Hence wondering if tools such as NanoVNA or Tiny Spectrum Analyser could be used for the best analysis?

author: eloso
author_rank: Regular Contributor
timestamp: January 26, 2023, 01:11:44 pm
content: Its an interesting question and as has ben pointed out already - it really all depends on what you are wanting to achieve, if you can express this in more detail.I have done a lot of practical work with active antennas as a hobbyist. Ultimately what I have been interested in has been real world comparisons of wanted signals versus noise/intermod in comparison with reference antennas. This has given me most information about the success or otherwise of particular designs.But that is not very rigorous. To actually make meaningful measurements you would want to consider that there are two principal parts to an active antenna - the antenna itself, and the active bit - i.e. an amplifier. The antenna should ideally be matched to the amplifier although in return loss terms this is nowhere near as critical, nor at times, even important as with antennas used for transmitting. When transmitting you are usually interesting in maximum power transfer, for receiving this is often not as important as other considerations.The antenna part is like any other antenna and can be analysed with a VNA. This can tell you about its complex impedance at given frequencies so you can ensure that your amplifier design is appropriate for it. You will have the problem that for many types of antennas the impedance may be way beyond what your test gear can sensibly cope with.You can check amplifier design by making S11 measurements with a VNA for complex impedance, and gain by using S21. Again, a complicating factor is whether your VNA can cope with the range of complex impedance your amplifier is presenting.My own experience is with active antennas for lower than a few hundred Mhz. I have never actually gone so far as to provide accurate conjugate match been the antenna and the amplifier - normally I am only concerned about the order of magnitude - i.e. if the antenna is a voltager probe type, is the amplifier a high input impedance. Some folk might need to do all of this in a less adhoc way and my hat is off to them - they have more knowledge than I do.Another measurement is of noise. factor of the amplifier. Again, for me, it usually isn't too hard to ensure that the noise factor is a magnitude better than the noise presented by the antenna part so I don't see much need for measurement (can you hear the antenna noise when you connect it ? If yes, your amp has low enough noise factor). But if you are working at the limits, then sophisticated noise analysis and measurement may be required. If you were working at the limits I guess you wouldn't be asking the question so I don't think this is an area you will get much benefit from following up.To me the key measurement for active antennas is their inter modulation performance. Poor IMD can make an active antenna less than useless. Measure this with a spectrum analyser and two RF signal sources of adjustable and known level. Or do this somewhat empirically with an SDR connected to the antenna and watching/listening for IMD products to appear/disappear as you vary the input to the active part by means of a stepped attenuator.CheersEloso

author: Berni
author_rank: Super Contributor
timestamp: January 26, 2023, 01:44:55 pm
content: Ah right you mean active RX antennas as in an antenna with a built in LNA. (Mistaken it for an antenna that is actively in use for reception)The case is still the same tho. You can just watch the output of the antenna while it is receiving a signal. For testing purposes you can use a 2 port VNA to feed a regular passive antenna with the stimulus signal, then point your RX antenna at that antenna and look at the signal you get back.Tho this also hinges on the fact that the test transmitter antenna has to be known. This can be taken care of by building 2 identical transmitter antennas, then pointing them at each other as the tx and rx pair. The response you get is the response of both antenna together, so if you subtract away half of that response you get roughly the response of that antenna. This lets you correct your measurements to be flat. Tho this can be problematic if you are testing low frequency antennas (under 100MHz) as simple predicable designs like a dipole get too big to be practical.Something like a NanoVNA is plenty good enough for doing these sort of measurements.

author: nctnico
author_rank: Super Contributor
timestamp: January 26, 2023, 01:51:07 pm
content: Are you sure that using two identical antennas will work? From what I've read, antennas may have different RX & TX gains so splitting the result in half won't work. A while ago I found a paper from R&S on this subject and they proposed a method where you use 2 different antennas in 4 different combinations in order to get to the RX & TX gain of each antenna.

author: Berni
author_rank: Super Contributor
timestamp: January 26, 2023, 02:05:48 pm
content: Quote from: nctnico on January 26, 2023, 01:51:07 pmYep i agree.Antennas can have loss inside them that eats up some of the energy and it could even be non linear with power. They can also have horizontal vs vertical polarization or even circular polarization and a lot more RF black magic i will likely never understand.However if you just do a simple dipole out of metal rods, the response of it is going to act pretty predictable, and there is no worries about directionality as long as you stay away the two null points on the ends.Ideally you would also be doing this in an anechoic RF room and use special calibrated antennas, but most of the time you don't need super accurate results within single digit dBs. Taking the simple dipole antenna outside in an open area of the yard is close enough to get a good idea of how well your antenna is performing.

author: switchabl
author_rank: Frequent Contributor
timestamp: January 26, 2023, 07:08:57 pm
content: If the objective is to characterize antenna gain, the simplest approach is probably the substitution method where you measure the same signal with a reference antenna (with known gain, probably a dipole) and then mount your AUT in the same location and compare power levels. The advantage is that the transmitter is relatively uncritical, as long as it provides a stable signal and is reasonably far away and you don't need perfectly predictable propagation conditions. You could even use something like a commercial broadcast.Quote from: nctnico on January 26, 2023, 01:51:07 pmThere isn't really anything wrong with the two antenna method (based on the Friis formula P2/ P1= G1G2(λ / 4πd)2, solve for G1= G2). But there are a number of underlying assumption and these may not always be practical:- the antennas are actually identical- the antennas are reciprocal (usually true for passive antennas)- the distance is sufficient for far-field approximation to hold- free space propagation, no reflections- polarization is known a priori

author: tautech
author_rank: Super Contributor
timestamp: January 26, 2023, 07:32:15 pm
content: Quote from: vinlove on January 26, 2023, 11:27:53 amWhat frequency are we looking at ?There are a few options depending on what info you need from the antenna and I have found the actual resonant frequency the most important to have correct.Rather than work with any live signal source with an SA or VNA I prefer to use the antenna reciprocity principles and use a VNA to provide the stimulus to initially get the resonance/emissivity tuned in with Log Mag and then engage SWR and Smith charts to further tune it to near perfection.I have a antenna project I did a few years back documented here with a couple of screenshots using a SA and reflection bridge but the real grunt work was all done with a VNA.https://www.eevblog.com/forum/rf-microwave/antenna-project-log/There I only needed excellent sensitivity but right on frequency as I was working with a small 200m range remote.The results were better than expected and I again thank the members that pointed me in the right direction with their pearls of wisdom.

author: vinlove
author_rank: Frequent Contributor
timestamp: January 26, 2023, 09:18:51 pm
content: To reiterate the OP,1. Is it possible to analyse Rx antennas especially the active antennas for the resonance and efficiency for the given band or frequency using measuring equipment or tools such as NanoVna or Tiny Spectrum Analyser?2. If it is, then what would be the parameters? - What are we measuring and against what etc?3. Which tester / measuring tool would be best for the job?

author: tautech
author_rank: Super Contributor
timestamp: January 26, 2023, 09:31:29 pm
content: Quote from: vinlove on January 26, 2023, 09:18:51 pmWhich could be frequency dependant as AFAIK these cheap devices use a square wave for their stimulus.Best tool = proper VNA.

author: Bud
author_rank: Super Contributor
timestamp: January 26, 2023, 10:41:58 pm
content: Quote from: nctnico on January 26, 2023, 01:51:07 pmThis is weird. Antennas are reciprosal devices, that is - have exactly same charactristics in transmit vs receive. Any pointer to the article?

author: tautech
author_rank: Super Contributor
timestamp: January 26, 2023, 10:58:43 pm
content: Quote from: vinlove on January 26, 2023, 09:18:51 pmWe'd use just 2 related measurements to initially quantify an antenna, Mag Loss and SWR.From there we might use a Smith chart measurements and adjustments to get the best 50impedance match for the passive part of the antenna. Any active parts would be analysed differently which for a 2 way active device would require S11 and S21 measurements.Again you've given us no idea of the frequency you need to cover or anything in the future you might want to work with, say 900 MHz LoRa or 2.4, 5 GHz WiFi ?

author: Bud
author_rank: Super Contributor
timestamp: January 26, 2023, 11:03:58 pm
content: Quote from: vinlove on January 26, 2023, 09:18:51 pm1. I do not see why not, if those toys you mentioned have sufficiently small frequency step size and output not exceeding the antenna's max input level (also see #3)2. Just like with a passive antenna, you sweep the given frequency range with frequency increment and amplitude from #13. This is where the problem is. With a passive antenna you just connect a VNA to the antenna's terminals. With an active antenna you can't because there is the amplifier in between. In general, you'd connect the VNA to a separate transmitting antenna with a wider bandwidth, and locate your active antenna some distance away. The testing tool here is a generic VNA with step size small enough to capture the tested antenna resonance and with output power (taking in account the transmitting antenna's gain and distance to the receiving antenna) not exceeding the receiving antenna's max input power.

author: nctnico
author_rank: Super Contributor
timestamp: January 27, 2023, 07:03:06 pm
content: Quote from: Bud on January 26, 2023, 10:41:58 pmLook at discone antennas.