23 cm antennas are a little more difficult to construct as the lengths of elements are quite critical.
To get round this problem I have experimented with broader band antenna systems rather than the usual Yagi.
I have a 45-element loop Yagi based on information from G3JVL, works OK, narrow beam width and fewer problems with element lengths than a conventional linear element Yagi. This antenna has a beam width of approximately 16° in both horizontal and vertical plane and a gain of 20 dBi, FB 25db.
The double quad, ideas from the German 'UHF-Compendium'. I have found that the so-called double double quad works quite well in a box of 4 with plane wire mesh reflector. In tests that I have made, this quad arrangement gives about the same gain as the 45-element loop Yagi but is much easier to construct, and the end result is a much more portable antenna.
I have a 4 way power splitter between the separate feed points to form a complete array that gives good forward gain with a fairly wide horizontal beam width. The above diagram is representative of the arrangement I have at the moment, but I think I will rearrange things by stacking all 4 antennas which will increase the horizontal beam width while retaining the gain of the array.
Look here for an animated 3D plot of X 4 double-double quad. And here is the polar plot of the antenna
Look here for a polar plot of 2 X double-double quad, half of that shown diagrammatically above
(Polar plots and antenna data made with K6STI AO software).
Below is a picture of the actual test antenna showing the arrangement of the wire forming the quad loops. The reflector
in this case is a piece of aluminium sheet, where the antenna I use has 1 cm square wire mesh as the reflector.
This reduces the wind loading and weight of the array. It measures 1 metre high and 50 cm wide. Each side of the
quad loop is 6 cm long and made from a wire gauge which is self-supporting. I am using 2 mm enamelled copper wire.
Double double Quad with aluminium plane reflector
The original article in the ' UHF Compendium ' claims the gain of the single double quad antenna with plane reflector to be 11 dB (no ref. is shown in the figures), with a comparable gain to a 10 element Yagi. This is the polar plot I managed to achieve with a double quad arrangement as shown below.
Unfortunately I have not been able to reproduce this result with the antennas I have made for both 23 cm and 70 cm. The results I have got seem closer to the computed data and measurements made in the field, which give about 8.5 dBd with the single double quad element as shown above.
The largest antenna I use for contest work is a section of plane parabola. I use 1 cm square wire mesh reflector 1 metre wide and 2 metres in the vertical section, like that shown in the diagram below at (d). The focal length of the parabola is 1 meter, and I use a pair of double quads bayed to illuminate the surface of the reflector. I think it is working quite well, F/B is greater than 25 dB. Gain is at least 6 dB better than the 45 element loop Yagi, horizontal beam width about 16° (which for an antenna with so much forward gain is quite good) and in the H plane about 8°.
Click here for graph showing comparisons of parabola size, gain, and frequency.
This is a diagram of the Plane parabola with feed at focal point 1m. (F/D 0.5). With close-up of feed shown on the right.
The two double quads that form the feed to the plane parabola have been optimized to illuminate the reflector fully with little over spill. They are fed with a two way power splitter, which couples directly to the individual double quad loops. Centre to centre spacing of the quad loops is l/2 so I have made a half-wave two way power splitter.
The principles of how to construct the power splitter are described in the general antenna page.
Here is a Band-pass filter for 23 cm.
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1999
