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Building Phasing Harnesses, Make you own phasing harness for DB antennas.

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  • Building Phasing Harnesses, Make you own phasing harness for DB antennas.

    Make you own phasing harness for DB antennas
    Building Phasing Harnesses
    Phasing harnesses for multiple dipole or folded dipole antennas are simple. They mainly use 50, 75, or35 ohm coax paralleled for impedance matching. Folded dipoles are fed balanced and have an impedance of 300 ohms or so, depending on surrounding objects that can lower the resonant frequency (and impedance) by adding capacitance. The impedance also depends on the diameter of the material used and the spacing.
    Decibel Products uses folded dipole elements that are fed unbalanced. In my opinion, they are actually "folded mono-poles" with a counterpoise. Second, the elements are spaced from the mast in such a manner that the pattern is somewhat directional and the impedance of each element is 100 ohms. This means that there must be some matching done in order to achieve the desired 50ohms that most two way systems need.
    The basics of matching are done with odd electrical quarter wavelength multiples using different impedance's of coaxial cable. By electrical wavelengths, this means you must measure the cable physically but then calculate the velocity factor of the cable and cut accordingly. By compensating for the velocity factor, you are taking into effect that RF slows down as it passes though dielectric of the cable. Therefore, the electrical wavelength is somewhat shorter than the physical wavelength.
    Most coaxial cables with solid dielectric (RG-213 etc) have a VF of around 66%. Other cables with foam dielectric have a VF of 82% or thereabouts. If in question, consult the technical specs of the cable you are using to create a very accurate measurement when building phasing harnesses.

    To calculate an electrical quarter wavelength of RG-11 A/U cable with a VF of 66%, first find the physical length by dividing the frequency into
    2952 / 445 = 6.63 inches.
    Now, take 6.63 inches and multiply by the velocity factor of 66% or 0.66
    6.63 x 0.66 = 4.38 inches.
    So, an electrical quarter wavelength at 445 MHz using coax with a VF of 66% equals 4.38 inches. You may multiply this number by 3, 5, 7, 9 or any odd number and the effect as a matching transformer will be the same. The same formula above holds true with 75 Ohm and 35 Ohm coax, or any coax for that matter and at any frequency.
    When using odd quarter wavelengths to match impedance's, you will usually deal with several numbers, mainly 100, 75, 50 and 25 ohms. As listed below, are some typical matches you may deal with when building antennas phasing harnesses.

    100 Ohm element through odd 75 Ohm coax will produce 50 Ohms
    50 Ohm load through odd 75 Ohm coax will produce 100 Ohms
    25 Ohm load through odd 35 Ohm coax will produce 50 Ohms
    100 Ohm load through even ½ wave on any Ohm coax will produce 100 Ohms


    Another factor to consider is the effect that EVEN ELECTRICAL HALF-WAVES can play in matching. Any time a load is connected through this length of coax, the impedance of the load will exactly repeat itself at the other end of the cable. For example, if an antenna element is 100 ohms, and an even half wave or even multiple of a half wave piece of any impedance coax is placed in line, the impedance at the other end of the coax will be 100 ohms. See the diagram below which depicts how to match two and four elements considering that each element is 100 ohms.

    A = odd ¼ 75 Ohm coax
    B = odd ¼ 35 Ohm coax
    OR
    A = even ½ coax any Ohm coax

    B = 50 Ohm coax any length
    -------------------------------------------------------------------------------
    C = odd ¼ 75 Ohm coax
    D = odd ¼ 35 Ohm coax
    E = odd ¼ 35 Ohm coax
    OR
    C = even ½ coax any Ohm
    D = odd ¼ coax 75 Ohm
    E = 50 Ohm coax any length
    Click image for larger version

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    Let's take a look at what's happening to the dual element array on the right. First, the elements operate at
    100 ohms. Section "A" is an odd quarter wavelength multiple (probably around 5 odd quarter wave lengths) of 75 Ohm coax which makes the 100 Ohm load turn into 50 Ohms at the end of the cable. The bottom antenna element goes through the same thing, making its impedance 50 Ohms also. When these are connected together, or paralleled, the impedance at the TEE connector is 25 Ohms. You may now convert this into 50 ohms by using an odd quarter wavelength of 35 Ohm coaxial cable, perhaps 7or 9 multiples long shown in section "B". This creates a 50 Ohm impedance at the connector which goes into your transmission line.
    If you do not have access to 35 Ohm cable, you may wish to try a different method. You may use even half wavelengths of 50 Ohm cable in section "A" which makes each element impedance repeat at the TEE connector, 100 Ohms. Then parallel the second element of 100 ohms also, and get 50 Ohms. Then, section "B" may be any length of 50 Ohm coax.

    The antenna on the left is the exact same thing as the one on the right, except twice as many elements. Remember in our first example the impedance at the end of section "B" was 50 Ohms. Now, just add another antenna array identical to this with a TEE connector. Parallel two 50 Ohms and get 25 ohms. Now, match these 25 Ohms to 50 by using yet another section of 35 Ohm cable as shown in section "E" and get 50 Ohms at the end. This configuration is used by the popular Decibel DB-224 and DB-411antennas.
    IC-9100, IC-7100, AT-180, Yeasu G-5500, GS-232B, JT222M, HRI-200 Wires-X, Astron RM-60M, DR-1X, SCOM 7330, Wouxun KG-UV950P, TYT TH-9000D, Motorola CDM 1250, Stepper IR DB11 Antenna

  • #2
    Here is an example of the above using this formula from above centered on 146 MHz:
    C = even ½ coax any Ohm
    D = odd ¼ coax 75 Ohm
    E = 50 Ohm coax any length


    I have:
    LMR-400 UF coax which is 50 Ohm and has a velocity factor of 85% or .85 and
    RG11/U which is 75 Ohm and has a velocity factor of 66%

    This is a 4 bay antenna with 100 Ohm elements tuned for 146 MHz

    The phasing harness is:
    C = 34.372 inches of theLMR-400 UF coax (if an antenna element is 100 ohms, and an even half wave or even multiple of a half wave piece of any impedance coax is placed in line, the impedance at the other end of the coax will be 100 ohms. Two 100 Ohm loads tied together at a tee will produce a 50 Ohm load.)

    D = 66.723 inches of the RG11/U coax. (50 Ohm load through odd 75 Ohm coax will produce 100 Ohms. Two 100 Ohm loads tied together at a tee will produce a 50 Ohm load.)

    As you can see now at the last tee we have a 50 Ohm load to feed the transceiver with.
    IC-9100, IC-7100, AT-180, Yeasu G-5500, GS-232B, JT222M, HRI-200 Wires-X, Astron RM-60M, DR-1X, SCOM 7330, Wouxun KG-UV950P, TYT TH-9000D, Motorola CDM 1250, Stepper IR DB11 Antenna

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    • #3
      Here is an example of the above using this formula from above centered on 146 MHz:

      C = odd ¼ 75 Ohm coax
      D = odd ¼ 35 Ohm coax
      E = odd ¼ 35 Ohm coax


      I have:
      RG-83 coax which is 35 Ohm and has a velocity factor of 66% and
      RG11/U which is 75 Ohm and has a velocity factor of 66%

      This is a 4 bay antenna with 100 Ohm elements tuned for 146 MHz

      The phasing harness is:
      C = 40 1/8 inches of the RG11/U coax

      D = 66 3/4 inches of the RG-83 coax.

      E = 40 1/8 or 66 3/4 of the RG-83 coax.

      As you can see now at the end of E we have a 50 Ohm load to feed the transceiver with.
      IC-9100, IC-7100, AT-180, Yeasu G-5500, GS-232B, JT222M, HRI-200 Wires-X, Astron RM-60M, DR-1X, SCOM 7330, Wouxun KG-UV950P, TYT TH-9000D, Motorola CDM 1250, Stepper IR DB11 Antenna

      Comment


      • #4
        Can you show how to do an 8 bay dipole phasing harness for 147.855 ? Please.

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