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Helix Match



HI:

I have noted with interest the discussion that has appeared re matching
the nominal 140 ohm helix impedance to a 50 ohm line or low noise preamp.

The fin match is a rather crude attempt at a 1/4 wave matching section.
To match 140 ohms to 50 ohms a 1/4 wave matching section must have
an impedance of 84 ohms.  i.e.. the square root of 140 x 50,

A circular conductor above a ground plane has an impedance defined by
the formula Z=138 log 4h/d where h is the height of the conductor above
the ground plane and d is the diameter of the conductor.

An 84 ohm matching section can be made by placing a 1/4 wavelength
of grounded metal fin under the last quarter turn of the helix.
The width of the fin is not critical as long as it is 10 or more times the
diameter of the helix conductor.

For practical construction I use brass hobby stock. A semi circle notch
is cut into the end of the fin and two holes are drilled on each side of the
notch to match the type N female connector against which it is fitted.
The far end is bent up towards the conductor. For 3.1 mm diameter
aluminum conductor the spacing should be about 4 mm.

To get the method to work the fin must also be well grounded at the far
end as well.  I have used a U shaped piece of copper braid  or another
piece of thin brass sheet.  One end of the U is soldered to the fin and
the other bolted to the back plate. Adjustment can be made with a long
screw which is run through a threaded hole in the back plate,  It is
advisable to have an insulated cap over the adjusting screw because
intermittent contact of the screw can result in variations in VSWR
The reflector surface must have a uniformly conductive surface throughout
the circle behind the helix.  I have used a small piece of aluminum
centered in a reflector of 1/2 hardware mesh for 70cm with excellent
results.  But I ran into trouble when I used a piece of aluminum strip
from the aluminum center piece out to the N connector to support it.
When I replaced the aluminum with a plastic strip I had no trouble getting
a 1:1 VSWR across the entire 70 cm band.

I have constructed several 20 to 30 turn helix ants for 2400 MHz. I use the
same method but using a solid reflector.  I have no means of accurately
measuring the VSWR but have adjusted it for max signal from a week
signal source.  The adjustment is very broad for receiving in any case.
I have been able to copy the LHC beacon of UO-11 with my 28 turn RHC
ant so do not expect any problem with P3D

I have also made a 28 turn helix for 1269 MHz and got a 1:1 match with
the same feed method.

On the matter of polarity my experience with Mode S or for that matter
Mode B on Oscar 13 was that for all practical purposes right hand circular
was all that was required as opposed to FO-20, FO-29 and AO-10 as it
goes through parigee, where the polarity often changes.

Clare   VE3NPC






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