Saturday, January 4, 2014

Simple 70cm GP-antenna

70cm Ground Plane Antenna

This little aerial was intended to make it easier for me to connect to the local packet-radio digipeater and to work the various phone and multimedia repeaters. Again the design was choosen as portable as possible. Especially in combination with the glass-fibre mast, known from the other projects on this page.

Materials used

  • about 90cm of 1mm silver steel rod
  • one luster terminal with at least two

  • lead guides beeing able to carry 2mm leads
  • 50Ohms coax cable
  • glass fibre mast

Construction & Design

Sketches or drawings are IMHO not necessesary because of the extreme simplicity of the "design". Cut 19cm off the rod, and bend 2cm at one end by an angle of 90°. This will be the radiator, 17cm straight up in the air. It will directly go in one whole of the luster terminal. Watch out, that the terminal´s skrews are pointing downwards, elsewhise you will later have problems with water and corrosion. The radials are made of two rods with a length of 34cm. Stick both through the other whole of the luster terminal, centered. You might now bend each rod to an angle of 90°, with the luster terminal in it´s centre. To give it the correct feed-point impedance the radials should be twisted slighly downwards.
Connect the coax cable to your little aerial, using again the abilities of the terminal. BTW, I used RG174 (lossy!) for the last meter, to reduce the weight.
In the middle of modern luster terminals there are wholes, to mount the terminal wherever it should go to. This whole (3mm) is perfect to stick the mast through. At the very top mine has a diameter of 2.8mm. This makes the ground plane to rest only 12cm below the top.

Alignment

In principle exactly the same way as for any other antenna, you have built up to know. Find it´s resonance, and cut off as much as needed to obtain the desired resonance frequency. The SWR can be improved by variing the downward angles of the radials.

Experience

I am happy! Whenever the reflection I have to use with my indoor HB9CV-antenna does not provide nice signals, I can errect the glass-fibre mast with the ground plane, and establish a stable link to my local digipeater (running the transceiver´s lowest power setting). Besides that I could hear a lot more phone repeaters than before. The only pitty is, that I do only posses one such mast... so, no shortwave during packet, or vica versa :-(

Photographs

Taken at 31.12.2000, the last day of the second millenium... Various angles and views...
 


 
 
 

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