OSCAR100 smalband [ 14: Werking matching section Helical ]

Deze tekst wil ik jullie toch niet onthouden .
Het is weliswaar in het Engels maar geeft een beter inzicht wat het afregelvaantje doet.
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g6lvb




Can you explain the matching section in more detail?

The matching section is the first quarter turn (not a quarter wavelength) of the helix. It is really a tapered microstrip transmission line, and not meant to radiate.

The theory goes like this: an 8mm wide piece of thin conductor, or ‘shim', mounted 1.6mm over a ground plane (the reflector in this case) with air dielectric has a 50 ohm impedance, and is frequency independent. A quarter turn later, it is 8mm higher (each turn is 32mm from its adjacent turn), giving a total distance from the ground plane of 9.6mm. The 8mm wide shim has an impedance of about 140 ohms at 9.6mm, which is the characteristic impedance of an axial mode helix.

For want of a better description, it is fabricated like a helter-skelter, soldered to the underside of the copper wire.

For an accurate match, the 1.6mm distance (use some standard 1.6mm PCB as a gauge) and the 8mm shim width are the most critical dimensions, followed by the 9.6mm distance which you can easily be +/- 1mm or so out without making much practical difference to the matching.

The dimensions of the helix after the first quarter turn matching section are less critical.

The reality is that you can pick up AO-40 without any matching section at all. However the aim of the design was to present a reproducible design which would present an accurate match without having to resort to any test equipment or adjustment of any kind. I have built about a dozen helices based on this design, and all have presented VSWRs between 1.1:1 and 1.2:1. I have had a report from a Belgian reader who attached the helix to a network analyser and achieved a return loss of 26dB, equivalent to a 1.1:1 VSWR.

There's some more pictures of the feed and matching section here.

Mijn afregelvaantje voldoet niet volledig aan deze beschrijving . Nochtans was de afregeling vrij goed en de resultaten in de parabool bevredigend . Ik laat het voorlopig zo tot ik er wat meer aandacht kan aan besteden.