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Re: Looking for Helix info



Hello again Mike,

When you said you had the original article by James Miller I didn't include any details of
the matching strip. The original article was all about that matching device. Perhaps the
article you have is not the original. It may be a reprint.

The matching strip is made from thin shim brass and is cut to the shape of the first
quarter turn of the helix. For 2.4 GHz the helix element is wound from 1/8" wire or tubing
so the matching strip is made 1/4" wide and this allows you to solder it on to the first
1/4 turn with a bit sticking out each side of the wire to allow for the solder. It is
soldered on to the "reflector" side of the wire. When you have done that you can bend the
first 1/4 turn so that it is almost parallel with the reflector. The helix can then be
mounted and the end soldered on to the "N" connector. You can then do the final
adjustments. From memory, James' measurements were 1 mm away from the reflector where it
contacts the "N" connector pin and 6 mm away from the reflector at the end of the 1/4
turn. The next 3/4 of the first turn has to be bent to smoothly go from that point to the
point it would normally be if there was no matching turn. That is to say, ALL turns
including the first turn have the same "pitch". This will require bending the last 3/4
turn of the first turn slightly wider than the others to make it cover the same pitch as
all the rest of the turns.

If the helix will only be used for receiving, (AO-40 S-band) those dimensions are
sufficiently accurate. You may have to cut back the centre pin of the "N" connector to get
it down to 1 or 2 mm and you may have to use some shims behind the connector body if the
body of the connector sticks out too far behind the reflector. Try to get it flush with
the reflector on the helix side. If your reflector is very thin you will have to shim it
out a bit to make it flush on the helix side. The helix element can be supported by using
a Teflon or Delrin rod with holes drilled to the pitch of the helix. You can 'wind' the
helix through the holes and fix the rod to the reflector with a short screw.

Be sure to get the "hand" of the helix right. A right hand helix will produce right hand
polarisation by itself but if used to feed a dish it will produce left hand polarity off
the dish surface (and vice versa). The polarity changes when reflected off a dish. Wrong
polarity will mean a huge loss of signal, some 10s of dB which is impossible to get back.

Good luck,

Bill...vk3jt

Milawa
Australia



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