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RE: 5 1/2 turn helix for offset dish



Hello Mike

> Can anyone help..who has made a helix for offset dish...I have gotten info
> off of several web sites...one refers to flattening the 1st 1/2 inch of
the
> helix..and soldering  coax foil sheild to it?? making matching section...I
do
> not  understand what to do here...I know you dont want the helix to touch
the
> reflector....

It depends upon the individual design as to how the matching section works,
and whether you need any 'bending'.

A G3RUH-based design uses a 'tapered microstrip' transmission line for the
quarter turn, and this quarter turn is not designed to radiate. By
thickening the quarter turn with flat copper foil or brass shim, and varying
the distance (this is the 'tapering') from the ground plane reflector, you
achieve a 50 ohm characteristic impedance at the beginning (closest to the
reflector) and 140 ohms at the end. 140 ohms is the (ballpark)
characteristic impedance of the axial mode helix.

The foil is on the side of the helix closest to the reflector.

The coax foil I speak of at http://www.g6lvb.com/60cm.htm is used because
raw brass stock is often difficult to get hold of, certainly in the UK.

When making the tapered microstrip matching section, the thickness of the
foil or shim is largely insignificant in the impedance transformation, but
the distance of the foil from the ground plane, and its width, is.

See http://www.g6lvb.com/radcomclarifications.htm#MatchingSection. My
variation on the G3RUH dimensions achieves the impedance transformation
without any bending.

Some other designs use a quarter wave matching transformer instead of the
tapered microstrip technique, where, for 1.25", a certain thickness of wire
(or using shim as above to thicken it), possibly with some dielectric
material such as the PVC insulation, achieves matching.

I've seen other methods too.

However I'm not sure what "flattening the 1st 1/2 inch" achieves. If you
want to let me know where this is perhaps I can explain this one.

One point: don't bust a gut on this matching section if it's stopping you
getting on the bird! Especially if your downconverter is at the feed point,
you can certainly hear the bird without any matching at all.

When I described how to make this helix, I discovered it's strangely quite
difficult to describe exactly what you're trying to achieve.

73, Howard G6LVB


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