Phasing error in Yagi array

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KVITAL, Jun 10, 2019.

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  1. KVITAL

    KVITAL QRZ Member

    Hello everyone. I have in mind a project to build home radiotelescope to detect radiopulsars. For this, I need to have a high gain antenna (a least 18 dB) at about 400-430 MHz. Single Yagi with these requirements will be more than 5m long, so I decide to use 4 Yagi with 11 elements (12.3 dB with less than 2m long boom), so the total gain will be about 18.3 dB.

    A problem, which stops me to build an array antenna is dephasing of individual Yagi in array and degradation of total antennas performance.
    What I understand, the phasing error is caused by 3 main reasons:
    1. Boom mount: the position of Yagi's dipoles are not in a plane
    2. Antenna: different impedance cause different responses to the current (signal) phase. Different impedance can be due to errors in positioning antenna elements and the impact of neighborhood objects (trees, buildings, other antennas in the array)
    3. Cable length

    For highly directive individual antennas (like in my case), the effect of dephasing is more strong.
    When the antenna is at a height lower than 10 wavelength, the effect of ground on the antenna impedance is high.

    The possible solutions are:
    1. Precise assembling of each antenna
    2. Install antenna high from the ground and far from obstacles (trees, buildings...)
    3. Usage of the dielectric mast, boom...
    4. Check each antenna with vector analyzer and fine-tuning of each antenna
    5. Simulation of the array in CST or similar software to get the error budget.

    What I saw at pictures of EME Yagi array, is that people use metallic mast, the mast is not so high. So probably this problem is not so big...

    I would like to hear the opinion of people, who construct Yagi arrays for EME communication. Did You care about this problem? Did You tune each antenna? What material for mast did You use?
     
    Last edited: Jun 10, 2019
  2. KA0HCP

    KA0HCP XML Subscriber QRZ Page

    Welcome!

    You have an interesting project in mind. First mention I recall of observing anything other than Jupiter.

    -In EME, yes, general care is made to assembly of antennas, adjustment for SWR, and phasing harness assembly. However, I don't think it requires critical accuracy. For observing objects that are light years away, I would think it would be even less critical.

    -One short cut that makes phasing harness assembly much easier is to use a manufactured phasing combiner/divider. With this, the length of the harnesses can be any convenient length as long as they are all the same length.
    https://www.m2inc.com/FG2222PRTPD

    -The spacing between antennas does not affect phasing.
    -Non-conductive cross-booms are used, typically fiberglass. Carbon fiber is conductive and is Not used.

    You should look for suitable amateur radio products under the key words: EME, Earth-Moon-Earth, and Moonbounce.

    M2 Incorporated is a high quality manufacturer of these sorts of products.
    https://www.m2inc.com/categories/amateur/antennas/vhf-uhf-microwave-antennas/eme-products.html

    Good luck, Bill.
     
  3. AA5MT

    AA5MT Ham Member QRZ Page

    For most yagis, the standard way to mount them is to place both booms on the same side of the crossboom, and mount each yagi on the same side of the boom, with the matching system on the same side of the boom.

    The stacking distance will also directly affect the total gain. I've seen all kinds of distances listed. I remember a forum discussion where .81 wavelength gave maximum gain. But .16, .25, .5 are also very popular numbers.

    The phasing network is easily done with something like a Cushcraft PD-2 power divider. It is a short phasing network, where you run equal lengths of 50 ohm coax to each antenna. Or you can run the odd quarter wavelength version with 75 ohms.

    Tom
     
  4. KVITAL

    KVITAL QRZ Member

    Thank You for Your quick reply!

    So basically, antennas in the array should be mounted symmetrically. SWR adjustment should be done for each antenna. Proper matching should be done to unite all antennas.

    The power dividers which You propose are basically Q-wave transformer?

    Regards,
    Vitalii
     
  5. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    The proper spacing between antennas isn’t any particular fraction or decimal of a wavelength, or any number of wavelengths.

    It’s based entirely on the antenna aperture, and aperture is related directly to antenna gain. Higher gain beams have larger apertures and require much wider spacing than lower-gain, shorter boom beams. This can all be very accurately modeled, although M2 and some other manufacturers of high performance Yagis do specify optimum stacking distances in their published data.
     
  6. VA3RR

    VA3RR Ham Member QRZ Page

  7. KL7AJ

    KL7AJ XML Subscriber QRZ Page

    Wider spaced arrays are in general a lot less "touchy" because there is less interaction/coupling between the elements. However, like everything else in life, NFL (no free lunch) reigns supreme. The price you pay for a high gain/wide space configuration is big sidelobes...which may or may not be a problem. HAARP, for instance, is a CLOSE SPACED array, with a lot of cross coupling (and associated hand-wringing) resulting during the initial test phase. However, HAARP has an EXTREMELY clean pattern, at considerable sacrifice of ultimate gain.
     

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