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Receiver antennas, performance metrics

  1. Jul 26, 2011 #1
    Dear all,
    I am trying to understand what is the difference between a transmitter and receiver antenna.
    If I recall correctly an antenna is a dipole so you can always use it to either transmit or either receive.

    All the antennas that are used for transmitting purposes come with some performance metrics (gain pattern or SWR). How these two metrics are applied to an antenna working as a receiver antenna?

    The reason for this post is that I want to buy a receiver antenna that can work in the range of few Mhzs up to 3Ghz and I am trying to understand how all the performance metrics will affect the received-power measurement process.

    I would like to thank you in advance for your help

    Best Regards
    Alex
     
  2. jcsd
  3. Jul 26, 2011 #2

    berkeman

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    Staff: Mentor

    In general, the TX and RX characteristics are the same.

    For such a large frequency range, you would generally use multiple antennas, with each having a wide bandwidth. Log periodic antennas are an example of a wide bandwidth, directional antenna:

    http://www.google.com/search?tbm=is...ntenna&gbv=2&aq=0&aqi=g4&aql=&oq=log+periodic

    .
     
  4. Jul 26, 2011 #3
    Dear berkeman,
    I would like to thank you for your reply.
    I have found few antennas that can support wide frequency ranges from few khz to few Ghz.

    I have also in an spreadsheet file put the antennas with the prices, with the frequency ranges that support. Now I have also some how to qualify them so to make a decision which one to buy.

    They are all receiver antennas coming with some 'plots' either with SWR, gain patterns or dbi plots.

    Which one should I check for reception and how can I understand how qualitatively two antennas with the same frequency operating characteristics differ?

    I would like to thank you in advance for your help

    Best Regards
    Alex
     
  5. Jul 26, 2011 #4

    vk6kro

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    Science Advisor

    Antennas that have extreme bandwidth like this will have poor performance at all frequencies. This is because they have resistors in the antenna that reduce the normal resonances of the antenna.

    It is better to choose some frequencies of interest and then make or buy an antenna for those frequencies.

    If you make a dipole for 144 MHz, for example, it will give some reception over a wide range of VHF frequencies, but give best reception at 144 MHz.

    If you do have data on the performance of wide band antennas, look for dBi or dBd graphs or charts. This is transmit data, but it applies to receive as well.

    dBd refers to the performance compared with a dipole cut for the same frequency.
    A dedicated antenna cut for one frequency may have a gain of +5dB compared to a dipole and only in one direction, usually.
    This would be an excellent antenna, but it would only work over a frequency range of +/- 2% of the design frequency
    So, it might work well over a frequency range of 4 MHz centered on 100 MHz, for example.

    Antennas you make yourself are a lot cheaper than ones you buy and a lot more fun.

    You might like to read the following. although some of it may not make much sense to you at this stage. It includes a formula for making dipole antennas.
    http://en.wikipedia.org/wiki/Dipole_antenna
     
  6. Jul 27, 2011 #5
    Dear vk6kro,
    I would like to thank you for your reply. I understand that an antenna that does 'everything' is hard to find! But still I want to be able to read the plots that come with antennas so get a first feeling how good or bad can it be (a first estimation).

    I have understood about the dbi plots but how I should read the SWR plots? I think these only apply to transmitter antenna . Is that right?

    I have also searched in my university’s library and I have find many decent books (800 pages books about antennas). The problem though with this type of books are that they go too deep and I am easily lost. It would be good if there is something like a small guide-tutorial how to read the antenna's plots so to reach easier a decision.

    Could you please provide me feedback to my thoughts?

    I would like to thank you in advance for your help

    Best Regards
    Alex
     
  7. Jul 27, 2011 #6

    vk6kro

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    Science Advisor

    SWR (standing wave ratio) does not tell you much about how good an antenna is. It gives you information about matching the antenna to the feedline for transmission.

    To take an extreme example, a 50 ohm resistor is a perfect match for 50 ohm transmission line, and it will give a perfect 1:1 SWR. However, it will make a very poor antenna.

    SWR is valuable information about the resonant frequency of the antenna and this applies to receiving as well as transmitting.

    Gain graphs (dBi or dBd) give the actual performance of the antenna compared with a known antenna, so you can decide if the extra complexity of the antenna is worthwhile.
     
  8. Jul 27, 2011 #7
    If I understand it right the low SWR is the worst the performance is. I found a SWR diagram but instead of the double dot : (1:1) it only had a signle dot . (1.1) . I assume that they are the same equal.

    What would be good SWR values?
    Best Regards
     
  9. Jul 27, 2011 #8
    Ok I will try to make my questions more concrete by giving a real example.
    I have uploaded 3 pictures (I have hidden anything regarding model and brand) from the same antenna that is sold as receiver and transmitter antenna!

    First picture
    http://imageshack.us/photo/my-images/688/swrt.jpg/
    (y-axis SWR)
    two swr plot for different frequency spans. How should I understand these two plots?

    One more question

    two db diagrams for
    145Mhz
    http://img16.imageshack.us/content_round.php?page=done&l=img16/9754/rad145mhz.jpg [Broken]

    and for
    437Mhz
    http://imageshack.us/photo/my-images/508/rad437mhz.jpg/

    What I do not understand in the last two db plots. Does it give extra 30db to a received signal or a 30db loss? (which is huge!). It also seems that the antenna for 0 and 180 degrees is getting almost 'blind'.

    I hope my examples make things easier for you and I have not confused u.

    B.R
    Alex
     
    Last edited by a moderator: May 5, 2017
  10. Jul 27, 2011 #9

    vk6kro

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    Science Advisor

    Low SWR is good. So, the antennas would give a good match to a 50 ohm transmission line where the SWR dips towards 1:1. Anything below 2:1 is OK.

    You can read about it here:
    http://en.wikipedia.org/wiki/Standing_wave_ratio

    This is usually also a resonant frequency of the antenna. This can be altered by cutting wire from a dipole or by adding wire. So, it gives you information you need to optimise and antenna for a particular frequency.

    The polar plot diagrams do not mean much unless you know what they refer to.
    Typically,the outer ring might be equal to the output of a dipole in its best direction. Rings closer to the center are areas of reduced gain compared to the outer rings.

    The polar diagrams show the radiation (or reception) in different directions around the antenna.
     
  11. Jul 28, 2011 #10
    Thank you very much for your answer!
    The polar diagram for the reception how should be interpreted? I can see in the polar plor these 10,20,30db values. What do they really mean? +10,+20,+30 db in reception? Does this apply that the antennas has a gain in these directions?
     
  12. Jul 28, 2011 #11
    Can you explain that? Why it will give a poor antenna? In your last post you said that 1:1 is very good ratio.
     
  13. Jul 28, 2011 #12

    vk6kro

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    Science Advisor

    The polar diagram shows the radiation in each direction around the antenna. This is shown by the distance of the trace from the center of the circle.

    So, if the trace is near the outer edge of the circle, the signal will be stronger in that direction compared with directions where the trace is nearer to the center of the circle.

    The outer circle should be defined, but it may be the signal you would get from a dipole in its best direction. The inner circles mean that a trace at these points will be 10 dB, 20 dB etc weaker than at the outer edge.

    It is possible to draw such diagrams for antennas that don't work well in any direction, so you need to know the meaning of the outer circle.

    An antenna needs to be comparable in size to the wavelength of the signal it is trying to receive. A 50 ohm resistor is a very small object and it will not work well as an antenna at any frequency.
    However you could connect a transmitter to it and the transmitter would see a perfect load because none of the power would be reflected.
    All the power would be dissipated as heat and none would be radiated.

    This is why SWR does not tell you much about the effectiveness of an antenna.
     
  14. Aug 3, 2011 #13
    I would like to thank you for your reply.

    Best Regards
    Alex
     
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