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<H3 class=fw-title
style="MARGIN-BOTTOM: 1px; TEXT-DECORATION: ; COLOR: ; PADDING-BOTTOM: 1px; PADDING-LEFT: 8px"
align=left><FONT face="Times New Roman"><FONT style="FONT-SIZE: 15pt"
color=#39617b><FONT style="FONT-WEIGHT: normal">2m Mixed Mode
Helix</FONT></FONT></FONT></H3>
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style="MARGIN-BOTTOM: 10px; COLOR: ; PADDING-BOTTOM: 5px; PADDING-TOP: 0px; PADDING-LEFT: 10px; MARGIN-TOP: 0px; LINE-HEIGHT: 17px; PADDING-RIGHT: 10px"
align=justify>
<P class=MsoNormal
style="PADDING-LEFT: 5px; MARGIN: 0cm 0cm 10pt; PADDING-RIGHT: 5px"><SPAN
style="FONT-FAMILY: ; COLOR: ; LINE-HEIGHT: 15px"><FONT
style="face: arial"><FONT style="COLOR: black"></FONT></FONT></SPAN><FONT
color=#414141 face="Times New Roman"></FONT> </P>
<P class=MsoNormal
style="PADDING-LEFT: 5px; MARGIN: 0cm 0cm 10pt; PADDING-RIGHT: 5px"><SPAN
style="FONT-FAMILY: ; COLOR: ; LINE-HEIGHT: 15px"><FONT
style="face: arial"><FONT style="COLOR: black"><FONT style="COLOR: black"><FONT
style="FONT-SIZE: 10pt">I finally got some time to construct a 2m version of the
Mixed Mode Helix. The intention is to try this out on our microwave
</FONT></FONT><FONT style="FONT-SIZE: 10pt"><A
style="TEXT-DECORATION: ; COLOR: ; PADDING-BOTTOM: 1px; PADDING-TOP: 1px; PADDING-LEFT: 3px; PADDING-RIGHT: 3px"
href="http://websdr.suws.org.uk/" target=_blank><FONT color=#c13c50>WEB
SDR</FONT></A>.</FONT></FONT></FONT></SPAN></P><SPAN
style="FONT-FAMILY: ; COLOR: ; LINE-HEIGHT: 15px">
<P class=MsoNormal
style="PADDING-LEFT: 5px; MARGIN: 0cm 0cm 10pt; PADDING-RIGHT: 5px"><FONT
style="COLOR: black" face=arial><FONT style="FONT-SIZE: 10pt">The helix is about
1m long, 300mm in diameter with 300mm spacing between turns. The exact length of
helix has to be trimmed for resonance (zero reactance), and the gamma match is
then adjusted for lowest SWR.</FONT></FONT></P>
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style="PADDING-LEFT: 5px; MARGIN: 0cm 0cm 10pt; PADDING-RIGHT: 5px"><FONT
style="COLOR: black"></FONT><FONT face=Arial><FONT
style="FONT-SIZE: 10pt"></FONT></FONT> </P>
<P class=MsoNormal
style="PADDING-LEFT: 5px; MARGIN: 0cm 0cm 10pt; PADDING-RIGHT: 5px"><FONT
style="face: arial"><FONT style="COLOR: black"><IMG border=0
src="http://webzoom.freewebs.com/g8jnj/2m%20helix%20gamma%20and%20mast.png"
width=285 height=506></FONT></FONT></P>
<P class=MsoNormal
style="PADDING-LEFT: 5px; MARGIN: 0cm 0cm 10pt; PADDING-RIGHT: 5px"><FONT
style="COLOR: black"></FONT><FONT face=Arial><FONT
style="FONT-SIZE: 10pt"></FONT></FONT> </P>
<P class=MsoNormal
style="PADDING-LEFT: 5px; MARGIN: 0cm 0cm 10pt; PADDING-RIGHT: 5px"><FONT
style="COLOR: black"></FONT><FONT face=Arial><FONT
style="FONT-SIZE: 10pt"></FONT></FONT> </P></SPAN>
<P class=MsoNormal
style="PADDING-LEFT: 5px; MARGIN: 0cm 0cm 10pt; PADDING-RIGHT: 5px"><SPAN
style="FONT-FAMILY: ; COLOR: ; LINE-HEIGHT: 15px"><SPAN
style="FONT-FAMILY: ; COLOR: ; LINE-HEIGHT: 15px"><FONT style="COLOR: black"
face=arial><FONT style="FONT-SIZE: 10pt">I built this version using 5mm copper
brake pipe and 22mm plastic water pipe with 90 degree bends ends and a
Tee.</FONT></FONT></SPAN></SPAN></P>
<P class=MsoNormal
style="PADDING-LEFT: 5px; MARGIN: 0cm 0cm 10pt; PADDING-RIGHT: 5px"><SPAN
style="FONT-FAMILY: ; COLOR: ; LINE-HEIGHT: 15px"><SPAN
style="FONT-FAMILY: ; COLOR: ; LINE-HEIGHT: 15px"><FONT style="COLOR: black"
face=Arial><FONT style="FONT-SIZE: 10pt">The pipe joints still need to be glued,
and the whole assembly waterproofed and
painted.</FONT></FONT></SPAN></SPAN></P><SPAN
style="FONT-FAMILY: ; COLOR: ; LINE-HEIGHT: 15px"><SPAN
style="FONT-FAMILY: ; COLOR: ; LINE-HEIGHT: 15px">
<P class=MsoNormal
style="PADDING-LEFT: 5px; MARGIN: 0cm 0cm 10pt; PADDING-RIGHT: 5px"><SPAN
style="FONT-FAMILY: ; COLOR: ; LINE-HEIGHT: 15px"><FONT style="COLOR: black"
face=arial><FONT style="FONT-SIZE: 10pt">The antenna has a fairly high Q and
moderately narrow bandwidth, so any flexing of the helix can change the
impedance match quite dramatically.</FONT></FONT></SPAN></P><SPAN
style="FONT-FAMILY: ; COLOR: ; LINE-HEIGHT: 15px">
<P class=MsoNormal
style="PADDING-LEFT: 5px; MARGIN: 0cm 0cm 10pt; PADDING-RIGHT: 5px"><SPAN
style="FONT-FAMILY: ; COLOR: ; LINE-HEIGHT: 15px"><FONT
style="face: arial"><SPAN
style="FONT-FAMILY: ; COLOR: ; LINE-HEIGHT: 15px"><FONT style="COLOR: black"
face=arial><FONT style="FONT-SIZE: 10pt">The 5mm brake pipe I used is really a
bit too soft for this size of helix, so I had to add an additional bracing
section to keep it in
shape.</FONT></FONT></SPAN></FONT></SPAN></P></SPAN></SPAN></SPAN>
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style="PADDING-LEFT: 5px; MARGIN: 0cm 0cm 10pt; PADDING-RIGHT: 5px"><SPAN
style="FONT-FAMILY: ; COLOR: ; LINE-HEIGHT: 15px"><SPAN
style="FONT-FAMILY: ; COLOR: ; LINE-HEIGHT: 15px"></SPAN></SPAN><FONT
color=#414141 face="Times New Roman"></FONT> </P>
<P class=MsoNormal
style="PADDING-LEFT: 5px; MARGIN: 0cm 0cm 10pt; PADDING-RIGHT: 5px"><SPAN
style="FONT-FAMILY: ; COLOR: ; LINE-HEIGHT: 15px"><FONT
style="face: arial"><FONT style="COLOR: black"><IMG border=0
src="http://webzoom.freewebs.com/g8jnj/2m%20proto%20Helix.png"></FONT></FONT></SPAN></P>
<P class=MsoNormal
style="PADDING-LEFT: 5px; MARGIN: 0cm 0cm 10pt; PADDING-RIGHT: 5px"><SPAN
style="FONT-FAMILY: ; COLOR: ; LINE-HEIGHT: 15px"><FONT
style="face: arial"><FONT style="COLOR: black"><SPAN
style="FONT-FAMILY: ; COLOR: ; LINE-HEIGHT: 15px"><FONT
style="face: arial"><FONT
style="COLOR: black"></FONT></FONT></SPAN></FONT></FONT></SPAN><FONT
color=#414141 face="Times New Roman"></FONT> </P>
<P class=MsoNormal
style="PADDING-LEFT: 5px; MARGIN: 0cm 0cm 10pt; PADDING-RIGHT: 5px"><SPAN
style="FONT-FAMILY: ; COLOR: ; LINE-HEIGHT: 15px"><SPAN
style="FONT-FAMILY: ; COLOR: ; LINE-HEIGHT: 15px"><SPAN
style="FONT-FAMILY: ; COLOR: ; LINE-HEIGHT: 15px"><FONT
style="COLOR: black"></FONT></SPAN></SPAN><FONT face=Arial><FONT
style="FONT-SIZE: 10pt"></FONT></FONT></SPAN> </P>
<P class=MsoNormal
style="PADDING-LEFT: 5px; MARGIN: 0cm 0cm 10pt; PADDING-RIGHT: 5px"><FONT
style="COLOR: black"><FONT style="face: arial">The turns of copper pipe are held
in place by a single turn of thin copper wire wrapped around the pipe and
soldered close to the plastic pipe. The antenna is quite high Q so requires some
fine tuning once built. I made the last turns slightly longer than required and
then cut them to resonance. Painting the antenna will lower the resonant
frequency slightly. Very fine tuning can be achieved by bending the gamma match
wire.</FONT></FONT></P>
<P class=MsoNormal
style="PADDING-LEFT: 5px; MARGIN: 0cm 0cm 10pt; PADDING-RIGHT: 5px"><FONT
style="COLOR: black"><FONT style="face: arial">Two or three suitable ferrite
beads also need to be placed on the coax inside the boom arm near the feed
point, in order to choke off common mode currents and create a balanced
feed.</FONT></FONT></P>
<P class=MsoNormal
style="PADDING-LEFT: 5px; MARGIN: 0cm 0cm 10pt; PADDING-RIGHT: 5px"><FONT
style="COLOR: black" face=arial>The support boom was made with an inner of 22mm
water pipe and an outer of 25mm plastic electrical conduit. A plastic conduit
straight coupler can be persuaded to fit over the 22mm Tee by warming it with a
heat gun.</FONT></P>
<P class=MsoNormal
style="PADDING-LEFT: 5px; MARGIN: 0cm 0cm 10pt; PADDING-RIGHT: 5px"><FONT
style="COLOR: black" face=arial>The whole assembly is not waterproof, so the
coax connection to the feed point needs to be protected with something like
epoxy resin, hot melt glue or silicone sealant. A small hole also needs to be
drilled in the bottom of each length of plastic tube, to allow any water that
does collect inside is able to drain away.</FONT></P>
<P class=MsoNormal
style="PADDING-LEFT: 5px; MARGIN: 0cm 0cm 10pt; PADDING-RIGHT: 5px"><FONT
style="COLOR: black" face=arial>Although I made the first antenna with 5mm
copper car brake pipe, I suggest that 8mm copper central heating pipe may be
more suitable for the 2m version and provide a slightly wider operational
bandwidth.</FONT></P>
<P class=MsoNormal
style="PADDING-LEFT: 5px; MARGIN: 0cm 0cm 10pt; PADDING-RIGHT: 5px"><FONT
style="COLOR: black" face=arial>I also had to use a short length of rope from
the top corner of the antenna to the support mast, in order to help support the
weight of the antenna and prevent the 1.5m long boom from sagging. A length of
aluminium tubing placed over the boom arm may help improve the stability in high
winds.</FONT></P>
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style="PADDING-LEFT: 5px; MARGIN: 0cm 0cm 10pt; PADDING-RIGHT: 5px"><FONT
style="COLOR: black" face=arial></FONT> </P>
<P class=MsoNormal
style="PADDING-LEFT: 5px; MARGIN: 0cm 0cm 10pt; PADDING-RIGHT: 5px"><SPAN
style="FONT-FAMILY: ; COLOR: ; LINE-HEIGHT: 15px"><FONT
style="face: arial"><SPAN style="FONT-FAMILY: ; COLOR: ; LINE-HEIGHT: 15px"><IMG
style="HEIGHT: 1551px; WIDTH: 757px" border=0
src="http://webzoom.freewebs.com/g8jnj/2m%20Helix%20Dimensions.png" width=1074
height=2713></SPAN></FONT></SPAN></P>
<P class=MsoNormal
style="PADDING-LEFT: 5px; MARGIN: 0cm 0cm 10pt; PADDING-RIGHT: 5px"><SPAN
style="FONT-FAMILY: ; COLOR: ; LINE-HEIGHT: 15px"><FONT
style="face: arial"><SPAN
style="FONT-FAMILY: ; COLOR: ; LINE-HEIGHT: 15px"></SPAN></FONT></SPAN><FONT
color=#414141 face="Times New Roman"></FONT> </P>
<P class=MsoNormal
style="PADDING-LEFT: 5px; MARGIN: 0cm 0cm 10pt; PADDING-RIGHT: 5px"><SPAN
style="FONT-FAMILY: ; COLOR: ; LINE-HEIGHT: 15px"><FONT
style="face: arial"><SPAN
style="FONT-FAMILY: ; COLOR: ; LINE-HEIGHT: 15px"></SPAN></FONT></SPAN><FONT
color=#414141 face="Times New Roman"></FONT> </P>
<P class=MsoNormal
style="PADDING-LEFT: 5px; MARGIN: 0cm 0cm 10pt; PADDING-RIGHT: 5px"><SPAN
style="FONT-FAMILY: ; COLOR: ; LINE-HEIGHT: 15px"><FONT
style="face: arial"><FONT style="COLOR: black"><SPAN
style="FONT-FAMILY: ; COLOR: ; LINE-HEIGHT: 15px"><FONT
style="face: arial"><FONT
style="COLOR: black"></FONT></FONT></SPAN></FONT></FONT></SPAN><FONT
color=#414141 face="Times New Roman"></FONT> </P>
<P class=MsoNormal
style="PADDING-LEFT: 5px; MARGIN: 0cm 0cm 10pt; PADDING-RIGHT: 5px"><SPAN
style="FONT-FAMILY: ; COLOR: ; LINE-HEIGHT: 15px"><FONT
style="face: arial"><FONT style="COLOR: black"><IMG border=0
src="http://webzoom.freewebs.com/g8jnj/2m%20Helix%20SWR.png"></FONT></FONT></SPAN></P>
<P class=MsoNormal
style="PADDING-LEFT: 5px; MARGIN: 0cm 0cm 10pt; PADDING-RIGHT: 5px"><SPAN
style="FONT-FAMILY: ; COLOR: ; LINE-HEIGHT: 15px"><FONT
style="face: arial"><FONT style="COLOR: black"><SPAN
style="FONT-FAMILY: ; COLOR: ; LINE-HEIGHT: 15px"><FONT
style="face: arial"><FONT
style="COLOR: black"></FONT></FONT></SPAN></FONT></FONT></SPAN><FONT
color=#414141 face="Times New Roman"></FONT> </P>
<P class=MsoNormal
style="PADDING-LEFT: 5px; MARGIN: 0cm 0cm 10pt; PADDING-RIGHT: 5px"><SPAN
style="FONT-FAMILY: ; COLOR: ; LINE-HEIGHT: 15px"><SPAN
style="FONT-FAMILY: ; COLOR: ; LINE-HEIGHT: 15px"><FONT style="COLOR: black"
face=arial><FONT style="FONT-SIZE: 10pt">Modelling produced some good plots,
which seem to be representative of the measured
performance.</FONT></FONT></SPAN></SPAN></P>
<P class=MsoNormal
style="PADDING-LEFT: 5px; MARGIN: 0cm 0cm 10pt; PADDING-RIGHT: 5px"><SPAN
style="FONT-FAMILY: ; COLOR: ; LINE-HEIGHT: 15px"><SPAN
style="FONT-FAMILY: ; COLOR: ; LINE-HEIGHT: 15px"></SPAN><FONT
style="COLOR: black" face=arial><FONT style="FONT-SIZE: 10pt">When constructed
as shown, the radiation pattern is predominantly Right Hand Circularly polarised
at low angles of elevation.</FONT></FONT></SPAN></P>
<P class=MsoNormal
style="PADDING-LEFT: 5px; MARGIN: 0cm 0cm 10pt; PADDING-RIGHT: 5px"><SPAN
style="FONT-FAMILY: ; COLOR: ; LINE-HEIGHT: 15px"><FONT
style="face: arial"><FONT style="COLOR: black"></FONT></FONT></SPAN><FONT
color=#414141 face="Times New Roman"></FONT> </P>
<P class=MsoNormal
style="PADDING-LEFT: 5px; MARGIN: 0cm 0cm 10pt; PADDING-RIGHT: 5px"><SPAN
style="FONT-FAMILY: ; COLOR: ; LINE-HEIGHT: 15px"><FONT
style="face: arial"><FONT style="COLOR: black"><IMG border=0
src="http://webzoom.freewebs.com/g8jnj/2m%20Helix%20polarisation.png"></FONT></FONT></SPAN></P>
<P class=MsoNormal
style="PADDING-LEFT: 5px; MARGIN: 0cm 0cm 10pt; PADDING-RIGHT: 5px"><SPAN
style="FONT-FAMILY: ; COLOR: ; LINE-HEIGHT: 15px"><FONT
style="face: arial"><FONT style="COLOR: black"></FONT></FONT></SPAN><FONT
color=#414141 face="Times New Roman"></FONT> </P>
<P class=MsoNormal
style="PADDING-LEFT: 5px; MARGIN: 0cm 0cm 10pt; PADDING-RIGHT: 5px"><SPAN
style="FONT-FAMILY: ; COLOR: ; LINE-HEIGHT: 15px"><FONT
style="face: arial"><FONT style="COLOR: black"><FONT
style="FONT-SIZE: 10pt">Pattern is Omni-directional with very slight asymmetry
due to the gamma match and interaction with the mast and feed
coax.</FONT></FONT></FONT></SPAN></P>
<P class=MsoNormal
style="PADDING-LEFT: 5px; MARGIN: 0cm 0cm 10pt; PADDING-RIGHT: 5px"><SPAN
style="FONT-FAMILY: ; COLOR: ; LINE-HEIGHT: 15px"></SPAN><FONT color=#414141
face="Times New Roman"></FONT> </P>
<P class=MsoNormal
style="PADDING-LEFT: 5px; MARGIN: 0cm 0cm 10pt; PADDING-RIGHT: 5px"><SPAN
style="FONT-FAMILY: ; COLOR: ; LINE-HEIGHT: 15px"><FONT
style="face: arial"><FONT style="COLOR: black"><IMG border=0
src="http://webzoom.freewebs.com/g8jnj/2m%20helix%20gamma%20and%20mast%20plot.png"></FONT></FONT></SPAN></P>
<P class=MsoNormal
style="PADDING-LEFT: 5px; MARGIN: 0cm 0cm 10pt; PADDING-RIGHT: 5px"><SPAN
style="FONT-FAMILY: ; COLOR: ; LINE-HEIGHT: 15px"><FONT style="COLOR: black"
face=arial><FONT style="FONT-SIZE: 10pt">This antenna is now in use on the<SPAN
class=Apple-converted-space> </SPAN></FONT><A
style="TEXT-DECORATION: ; COLOR: ; PADDING-BOTTOM: 1px; PADDING-TOP: 1px; PADDING-LEFT: 3px; PADDING-RIGHT: 3px"
href="http://websdr.suws.org.uk/" target=_blank><FONT style="FONT-SIZE: 10pt"
color=#c13c50>Southampton University Wireless Society uWave WEB
SDR</FONT></A></FONT></SPAN></P>
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style="PADDING-LEFT: 5px; MARGIN: 0cm 0cm 10pt; PADDING-RIGHT: 5px"><SPAN
style="FONT-FAMILY: ; COLOR: ; LINE-HEIGHT: 15px"><FONT style="COLOR: black"
face=arial></FONT></SPAN><FONT color=#414141
face="Times New Roman"></FONT> </P>
<P class=MsoNormal
style="PADDING-LEFT: 5px; MARGIN: 0cm 0cm 10pt; PADDING-RIGHT: 5px"><SPAN
style="FONT-FAMILY: ; COLOR: ; LINE-HEIGHT: 15px"><FONT style="COLOR: black"
face=arial><IMG border=0
src="http://webzoom.freewebs.com/g8jnj/2m%20Helix.png"></FONT></SPAN></P></DIV>
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<DIV style="FONT-SIZE: 12pt; FONT-FAMILY: 'Calibri'; COLOR: #000000">73's de
Carlos CT1GFQ<BR>SKCC#466C www.skccgroup.com<BR>REP#1406
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