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How to evaluate an antenna's performance

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by K7HUT, Feb 26, 2021.

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  1. K7HUT

    K7HUT Ham Member QRZ Page

    I would like to understand how well different antenna turn electrical power from my battery into clean radiation to a far off receiver. Testing SWR only tells me something about impedance matching, but how much does it tell me about antenna efficiency or ability send RF out.

    For example, I compared two dual band antenna in UHF mode, using all the same equipment to drive the antenna, the same location, etc., the only variable is the antenna. Remotely, let's say 25 yards away, I had a field strength meter with it's dipole rabbit ear antenna tuned to roughly half wavelength. I verified with my handheld that the meter needle responds to a TX push from my handheld, and that response drops off with frequency adjustments up and down from the 'center frequency'. So I'd say the meter needle is not an absolute dBm measurement, but a decent relative comparison.

    I go back to the truck and swap out antenna, and I see that one antenna barely moves the needle, while the other moves the needle about 20x that amount. Again all else remains exactly the same.

    Question 1:
    Is this a fair test of these two antenna radiating efficiency/capability? Is there anything I should do differently?

    Question 2:
    The antenna that fared worse on transmit does seem to also not receive weak UHF stations as well as the antenna that fared better. This is anecdotal, and not measured carefully at all. In this case, the two antenna are mounted in different locations in the vehicle (though the poor performing antenna enjoys a higher mount, with excellent ground plane, while the better performing antenna is lower and poor ground plane, for this test).
    Is it true that if an antenna doesn't transmit as well as another, all things being equal, that it also won't receive as well either? Some rule of reciprocity?

    Thanks for any insight. Basically, after having drilled holes in my roof, sprung for good cables, mountes, radios, etc... I want to find an antenna that pumps out max RF power to the intended receivers. And SWR is fine, it's one data point, but don't we really want to see how much power is going into the air, at the end of the day?
     
  2. SM0AOM

    SM0AOM Ham Member QRZ Page

    You have understood this correctly.

    To start with, SWR is a very "blunt instrument" in evaluating antenna performance.
    A good SWR by itself does not tell very much about the performance of an antenna, and somewhat paradoxically, the flatter SWR curve, the more losses unless the antenna is specifically designed as a broad-band structure.

    Answering your questions;

    Yes, a substitution method has been applied, and by comparing the field-strength values it may be seen that one antenna gives significantly lower fields at a distance.
    This comparison does not give an actual value however.
    One way to get an estimate is to reduce the RF power of the transmitter when connected to the "good" antenna.

    Say that there are power steps of 25 and 5 W, and this reduction of power brings down the field-strenght indication to the value of the "bad" antenna at full power, it points in the direction that it is a factor of 5 worse, or - 7 dB.

    Passive antennas are reciprocal devices, and a signal loss affects the performance equally in both directions at a given frequency.

    The performance of an antenna in the transmit direction is called "directive gain" and is the ratio between the field-strengths at a distant location in the maximum direction between the antenna and a reference, usually a dipole, when fed by equal amounts of RF power.

    Due to the reciprocity, the directive gains in both directions are usually seen as equal, which is a somewhat simplified picture.
    The performance of a receiving antenna is more stringent expressed by the concept of "aperture" or "absorption area",which is a way of expressing the ability of the antenna to intercept the electromagnetic waves in its vicinity.

    An antenna with a high "gain" also has a large absorption area, and this is related to a gain figure with the expression:

    A = G*(wavelength)^2/(4*pi) where A = absorption area and G = directive gain.

    It can be seen that a lowered gain figure results in a smaller absorption area and for this reason a lower intercepted power in the receive direction from the surrounding waves.

    73/
    Karl-Arne
    SM0AOM
     
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  3. KM3F

    KM3F Ham Member QRZ Page

    A beginner can't be educated here that has no back ground understanding.
    The testing he thinks he did had no controls to make valid testing of any real value.
    Example; both antennas can have the same efficency but different patterns depending on mouting, placement and design. Bottom line is too many vairables unaccounted for. Hand held radio is not a fair testing of either antenna at 25 feet if it is held in the hand.
    SWR is not the end all for any testing.
    What would be the dfference between a 1/4 wave and a 5/8 wave design for radiation angle if 25 feet were to be used for that test?
    What is the design demensions for both dual band antennas etc.
    Both could have the same good SWR and not register the same signal strength at the signal meter 25 feet away and yet the 5/8 technically has a bit of gain farther out at a higher angle. the signal meter would not show, and both can have the same efficiency.

    Hint; he is a new Tech.
    Sorry to be a nay sayer but it is what it is, >>at this point.
    5 miles out to another radio at least would have offered a more accurate compairison, but still not of antenna efficiency.
    Don't take this wrong, but a good place to start by posting the question and begin learning it's not that simple.
    Good luck..
     
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  4. EI7KS

    EI7KS Ham Member QRZ Page

    A very good way to compare the performance of various antennas is to monitor your signal on a remote SD
     
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  5. EI7KS

    EI7KS Ham Member QRZ Page

     
  6. AI3V

    AI3V Ham Member QRZ Page

    There is the fly in the ointment.

    Radio waves interact with the environment, and this causes the field strength to vary, often significantly, in a more or less repeating fashion every 1/2 wavelength in space.

    You probably noticed this while driving and listening to a radio and hearing the signal fade in and out.

    Also, for different antennas mounted in the same location, the "phase center" of the antennas may not be in the same physical space.

    Repeat the test, and move each antenba in turn left, right, forward,back,up and down and compare peake readings.

    But otherwise your understanding is correct.

    Rege

    P.S. see also my thread "Diversity"
     
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  7. W1VT

    W1VT Ham Member QRZ Page

    Yes, there is a large measurement uncertainty. One way to reduce it is to place UHF antennas in an room lined with absorption material, so there are no reflected signals.
    A much cheaper alternative that I've seen is to place hang the absorption material below the ceiling and shoot the a signal from a beam antenna straight up into the material.

    It is also possible to do experiments that have a huge measurement uncertainty. The Millikan milk drop experiment to measure electric charge has a lot of uncertainty. One has to repeat the experiment many times to reduce that.
    It has so much uncertainty that this experiment can be used to do human experiments on Confirmation Bias.

    With computers and automation is now quite practical to do a huge number of measurements to reduce measurement uncertainty.

    Zak W1VT
     
  8. K0UO

    K0UO Platinum Subscriber Platinum Subscriber QRZ Page

    Good info by others above, and a great question for a newer amateur, others should be asking this question also/ even old timers.

    See my call on QRZ
    That's a lot of information on that page so just go clear down to the bottom to Far field testing and modeling.

    I know the theory and I have done all the testing and modeling, however propagation, antenna radiation and efficiency still fascinates me after all these years. Amateur radio is still an interesting science.
    Weather questions you're asking "we" can tell that you're going to make an excellent operator and Elder for others
    73
     
    Last edited: Feb 26, 2021
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  9. KX8C

    KX8C QRZ Lifetime Member #153 Platinum Subscriber Life Member QRZ Page

    I did a DOE a few years ago. [Inv-Vee vs Loop, 1W vs 5W, 20m vs 40m] with WSPR as the medium. Ran the experiment for 24/7 for 3 months. Picked out of the data several stations that were OTA 24/7 on both 20m and 40m, one in Texas and one in Germany. Ran the WSPR reports, solar data, and setups through Mini-Tab (statistical analysis software package). Learned lots of interesting things when I sliced and diced the data.

    Ham radio provides a fascinating means for me to keep my technical skills sharp.

    What did I learn? At 40m the antenna did not matter. At 20m, though, the loop was equal to the vee except to Europe where the loop was far superior. I changed my antenna as a result. More power, more and better contacts. Strong quiet solar conditions provided more and better contacts. While all of this might be obvious, learning it from actual test data and analysis that I did was very satisfying.

    Regards, Jim KX8C
     
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  10. K7HUT

    K7HUT Ham Member QRZ Page

    Thanks for the great answers, good insights.
    I do have some background, am an EE that has taken basic electromagnetics, etc. But there is basic theory and there are the nuances that can add up in real life situations.
    Just to repeat, this is not a test of antenna on a handheld. The antennas under test are being swapped onto a vehicle roof NMO mount with a decent ground plane (presumably).
    The remote field strength meter is not 25 feet, but 25 yards away, if that makes any difference in the field being "organized" by that distance.
    I set the height of the field strength meter to be about same height as the roof antenna, both aligned vertically.

    Trying to think of geographically where I can set up, and how to set up, a practical test just seeing how one receiver at a truly far distance can copy my truck transmitter, with the different antennae, but am wondering if that test wil reveal any new information on the antenna comparison.

    Thanks again for the links and info, will sift through KOUO's page, which I found.
     
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