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Ideal receive antenna

Discussion in 'Radio and Wireless' started by SparksFly, May 5, 2021.

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  1. SparksFly

    SparksFly

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    Jan 7, 2019
    Hello,

    This isn't a project specific question, but hopefully someone can point me in the right direction anyway. I'm struggling with the concept of receiving signals on an antenna, more specifically how to 'improve' the ability for an antenna to receive. All of my reading to date seems to focus on the Tx (measuring SWR etc), and assumes if Tx is good then Rx will also be adequate.

    Ignoring Tx, how could I design an antenna to capture as much mV/m RF signal out of the air? A huge copper sphere? Or an antenna made to match the impedance of air (~400ohms?) for best power transfer between free space and the antenna? Essentially, how would I visualise an Rx version of an isotropic antenna - is it affected by wavelength?

    Following from that, I acknowledge that it's rare that you'd want to receive everything and would usually focus on the SNR on a specific frequency/direction. I understand directivity, but is frequency filtering part of Rx design, or is that a job for separate filters?

    For a basic test I set up a VHF and HF antenna to receive the same HF signal. If what I've been told about "the best Rx antenna is also the perfect Tx antenna" was true then I'd have expected a very big difference, but there wasn't much difference at all.

    Thanks if you can help me in any way at all!
     
  2. bertus

    bertus Moderator

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    davenn likes this.
  3. SparksFly

    SparksFly

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    Jan 7, 2019
    Hello Bertus,

    Thank you for that book, it was very interesting. It did help, particularly where it says typically the SNR doesn't change as receive level increases. Unfortunately the book is very much focussed on Tx and the Rx bits always start with "generally..." or "typically..." or suggests "this would be good HF receive antenna" ... ok that's great to see as one line but I really want the detail!

    Maybe I need to read more books instead of trying to learn online all of the time.

    Thanks
     
  4. bertus

    bertus Moderator

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  5. Nanren888

    Nanren888

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    Nov 8, 2015
    Not really my area, but perhaps some comments. Other may be able to point out any errors that I make.

    Are you energy harvesting? Otherwise not sure of the aim.

    Despite the comments in books such as those referenced about wave or Beverage antennas being preferred for receive, in general reciprocity does provide some insight. That is, the match seen transmitting is the match seen receiving. So good energy transfer works both way.
    For power efficiency on transmit, sometimes inclusing loss components to affect match are not a good idea, as power transfer frm the PA, but not to the ether and Johnson-Nyquist noise on receive.
    .
    It's not clear what you plan to receive. If harvesting, then that noise that limits SNR in some bands is signal for you too.
    .
    There are many styles that claim to be wide band in various ways. All are compromises.
    .
    There is no omni antenna. Often not a proble, as we tend to live in a 2D world and tend to mean omni in the horizontal plane.
    Otherwise if we know where we want to communicate, direectivity is available to us.
    Directivity gives gain.
    You can loose, by including loss, but in general directivity only squeezes the sensitivity, or radiation, into one region, direction preferrentially to others. So all omni antennas have the same theoretical maximum "gain".
    .
    If we think of the EM field in space, it is a combination of many waves, each having a particular pattern of regularly changing electric and magnetic fields, propagating in some direction, with some polarisation. When we put an antenna in the field, we are collecting one resulting current in the antenna, combining the field components into one signal determined by the projection of the field at that point (including propagation directions in some cases) onto the antenna in some spatial + polarisation space.
    .
    So in simple example terms, an electrical dipole does not see an orthogonal electric field, nor a field propagating end-on: zero resulting projection onto that vector. An electrical dipole sees only half of a purely circular polarised wave otherwise aligned with its orientation.
    .
    People combine antennas, to get coverage of different bands. Directivity can be viewed similarly combining multile elements as in a log-periodic, or in similar senses a Yagi-Uda. Combining dipoles at a point, of course just creates one new vector as they collect coherently and signals add in a phased sense.
    .
    So, depends on what you want to achieve, but as a first guide, reciprocity means TX approximately the same as RX.
    Mismatch looses you power.
    Directivity to the source gains you: we often know where to point or look for the best direction.
    .
    There was a fuss a while back about the "crossed field antenna" which I think was an attempt to collect more energy.
    .
    Sorry, random things. The books suggested by bertus are really good guides.


    Didn't understand where you were going with this explanation. If you used an antenna not designed for HF and one designed for HF, then you would generally expect the HF one to perform better on HF, both directions. Sounds as if something went wrong. Out of interest what is your means to measure?
     
  6. SparksFly

    SparksFly

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    Jan 7, 2019
    Bertus, thank you again for the links. I haven't had a chance to read the next book yet, but will make sure I do.

    Nanren, thanks for getting involved! It's given me a lot to think about, and over the past day or so I've been letting your comments sink in. Below are my random thoughts in response as I try to work it out in my mind.

    I'm not energy harvesting and there is no real project it's just something I'm struggling to understand. I found it easy to find resources for Tx, and less for Rx, so I assumed it must be because it's simple. When I asked a few amateur radio guys they all gave different (and mostly wrong) answers. I've now gone down the rabbit hole trying to learn more, but I've only discovered I'm probably not clever enough to understand it!

    My question about capturing mV/m out of the air was because when transmitting ideally you'd match, say, 50ohm transmitter, 50ohm feeder to a 50ohm antenna for full power transfer. I understand that the reverse is true and that you'd want to minimise losses in the system, however presumably there is the possibility for a loss between the RF in free space and the antenna. I guess what I'm asking is when the RF passes the antenna it is magnetically coupled to induce a current. Is this transfer of energy perfectly efficient, or can it be improved, or is it a non-issue?

    On to the antenna; As Rx is all about the SNR and increases in gain (assume flat gain profile) do not affect the SNR, I take that to mean all an antenna needs to do is raise the signal floor of the device it's plugged in to by a small amount. Assume for a second we were in a point in space - say in the middle of a field floating 60m in the air. We can take a tiny cheap telescopic antenna and plug it into a spectrum analyzer and extend it until we see the noise floor raise slightly. At this point, are we receiving "everything" there is to receive, noise and all? So technically it'd be receiving waves from everything from radio & TV stations to the faulty washing machine motor in our neighbours house. If we were magically able to subtract the noise, we'd be able to reclaim any signal we wanted, to see telemetry from Voyager 1&2 for example.

    Now we know we can receive everything, the trick is obviously to design an antenna that receives only what we want to see, ie just the signal and no noise. In my experiment, you expected my VHF antenna to receive HF & VHF, but for the VHF to have a better SNR? And the same the other way around. I ran my test with an SDRPlay Duo, a 2 receiver SDR and viewed the output on the spectrum analyser display, looking at Rx levels of the signals and noise floor. As you point out there are many things that I could have done wrong - there's probably an AGC that makes everything look the same too. I'll borrow a spectrum analyser from work and try it again under more controlled circumstances.

    I understand how directivity will improve the situation (ie tradeoffs). The one question left (I'm on the verge of an "aahhh" moment) that I don't fully understand is how antenna length/frequency tuning comes to help. Is it AFTER the wave has coupled onto the antenna, and once the signal is 'inside' the antenna? I.E. if it is at the resonant frequency then there is less attenuation than if the signal was not at the resonant frequency? I think it's this, but could do with it being confirmed! My original theory was that a wave of, say, 100MHz found it 'easier' to couple to a 100MHz antenna than a 10MHz antenna, but I now think I may have been wrong on this.

    Finally, if all of that stands true it raises another problem; SWR is used to check a transmit antenna and ensure a good match at a given frequency. If we're saying that a good receive system has an on-frequency resonance and matched impedance components, doesn't this mean the best test would be to calculate SWR? My understanding was that SWR was not entirely relevant to Rx, and that insertion loss was the most important thing to test - but it's looking like Rx&Tx antennas are fundamentally the same.

    Thank you for taking the time to read all that!
     
  7. bertus

    bertus Moderator

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    Nov 8, 2019
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