23cm 1/4 wave Ground Plane Antenna

Saturday 24 June 2017.  I’m constructing a 1296 MHz 12el 23 Yagi, I got little distracted from the yagi when I decided to take a break and build a 23cm 1296 MHz 1/4 wave ground plane antenna.  Here it is..

Materials:

  • N Type flange socket
  • 70 mm length of 2 mm diameter copper wire
  • 300 mm length of solid 2.2 mm aluminium wire or soft copper wire
  • 4 * 3 mm screws and nuts
  • 4 * 3 mm washers

Construction:

  • Solder the length of 2 mm copper wire to the N Type socket center pin and trim to 58 mm above the face of the socket.  Take care when trimming.
  • Ground plane elements. With a pair of needle nose pliers place a small loop at one end of the 2.2 mm aluminium wire and secure it to one corner of the flange with a 3 mm screw, washer and nut.
  • From the edge of the flange measure 61 mm and cut the remaining wire off.  Repeat steps 2 and 3 for the other three ground plane elements.
  • Bend the ground plane elements down to an angle of ~45 degrees.

Test and adjust for a 50 ohm impedance

  • Connect a short length of coax to the antenna socket. The coax should be suitable for use at 1.2 GHz
  • Connect the other end of the coax cable to a 50 ohm 1.2 GHz SWR meter antenna socket
  • Connect your RF source to the SWR meter Tx socket
  • Before applying power to the antenna check the operating frequency isn’t in use.  Use your radio amateur protocols for antenna testing.  If the frequency is clear apply a short burst of RF power using a FM or AM carrier, check the forward and reflected power readings.
  • If required trim the vertical radiator by very small amounts taking off 0.5 mm at a time.  For 1296 MHz the finished vertical radiator is 56 mm long.  Starting with an angle of 45 degrees, move each of the ground plane elements up or down to obtain a low SWR.  Repeat this process each time you trim the radiator.  πŸ™‚

Photos: Β© Copyright 2017 Andrew VK1AD

    1296 MHz 23cm 1/4 wave ground plane antenna complete and ready for portable operations

    Using a SWR Meter

    Connect the antenna to a 1.2 GHz SWR meter via a short length of coax, I used as short length of RG316.  Set the ground plane elements to about 45 degrees below the imaginary horizontal line of the flange.

    23 cm antenna connected to the SWR meter

    Apply power to the antenna and check the forward reading.  In this case the forward power from my 23 cm transverter is 2.4 watts.

    Forward power 2.4 watts using a FM carrier. SWR meter on loan from Andrew VK1DA

    Read the reflected power.  Adjust the antenna radiator length by very small amounts. Each time you adjust the element length recheck the reflected power reading.  With each small trim of the radiator element move the ground plane elements up or down to obtain a low reflected power reading.  Be patient this process takes time.

    If you take too much of the radiator element you can always start over!   πŸ˜‰

    Reflected power almost zero, fine tuned for a 50 ohm impedance

    If you own a 1300 MHz antenna analyser your lucky!

    That’s it, you now have a homebrew 23 cm ground plane antenna ready for use in the field or perhaps in the shack.  I think the N Type socket and matching plug should drop nicely into a short length of plastic conduit.   Oh, yes I am still constructing the 23 cm 12el Yagi.  Blog post to follow.  πŸ˜‰

    A big thanks to Andrew VK1DA/VK2UH for the loan of his microwave SWR meter.  πŸ™‚

    For the VK1 SOTA group, if you are interested in using aluminium wire for a VHF or UHF GP I have a surplus of 2.2 mm wire. I’m happy to share lengths of the wire with you.

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    3 thoughts on “23cm 1/4 wave Ground Plane Antenna

    1. Oh N connectors – you sort of need a pretty powerful soldering iron to get those babies together. We used to use what we called the Mother of All Soldering Irons to do ours. Since then a solder station that you can crank up to 427C. That does nicely.

    2. Hi Andew,
      A most impressive and neat little ground plane antenna. I notice you have a BNC connector as the last bit of the base for connecting to cables etc. Do you use BNCs for this task rather than an N connector?
      I will be interested in your reasons.
      Cheers
      John D
      VK5BJE/VK5PF

      • Hi John and thanks for the great question. At the time it was convenient to use a BNC adapter. I don’t have a N Type male connector fitted to a short length of RG316 but I do have a 1 meter length of RG316 with BNCs at each end. I do have a 6 meter length of LMR-400 with N Types on each end but I found the LMR-400 a little too rigid, on my work bench, to carry out the procedure. In the field I do plan to use a N Type plugs for the 23cm 1/4 wave and the 23cm 12el yagi.
        73, Andrew VK1AD

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