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<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'>Hi Ian<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'>This is a great topic for a WICEN
discussion! <o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'>It is also one where there are many
possibilities with many different tradeoffs. <o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'>I have found there is little discussion or
recognition of a very important factor in vertical antenna performance - the
problem of mast / feedline currents. These are especially important
when we try to press an antenna into service which is really intended for
vehicle mounting. <o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'>So: some discussion follows. <o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'>If there is significant mast current (and just
about all vertical antennas have some, just some have more than others) then
there will be pattern distortion – which invariably raises the angle of
radiation towards the sky. So, much of our hard-generated RF power goes
skyward. The issue is not whether there will be pattern distortion –
just how much. <o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'>As a class, end fed half wave antennas
tend to have less mast current, simply because the end feedpoint impedance is
higher and so there is less “mirror” current flowing down the mast
to match the current going upwards into the antenna. Even so, I have
found large differences in performance – for example the RFI antennas are
not as good as a half wave SuperGainer. <o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'>A quarter wave ground plane is probably
the worst – there is NEC modelling available on the Net which shows the
radials are actually not very effective in choking off the mast currents. A
ground plane antenna is NOT the same as a “half a dipole” and does
not radiate very much towards the horizon. And, as with all verticals,
achieving a good SWR match does not necessarily mean there is a good pattern,
unfortunately. <o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'>A 5/8 wave is also relatively poor, also due
to its relatively high feedpoint current. <o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'>A coax dipole is better but still creates
significant mast currents due to coupling between the sleeve and the mast. The
sleeve is not as good a choke as might be thought, and the sleeve / mast
diameter ratio is important. There have been commercial antennas using
cones (the AEA Isopole) to provide more effective decoupling and much better
control of mast currents and pattern. <o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'>Another common half wave end fed antenna
is the venerable J-pole. Depending on the design, these can work quite well;
the difficulty is that the stub length and feedpoint impedance adjustment can
compensate for the radiator length not being “quite right”. The
result is that the half wave feedpoint impedance (at the top of the stub) is
lower and mast currents will be higher – leading to pattern distortion.
And there is no indication this has happened apart from a pattern test. <o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'>A further indication of mast currents is
my experience with a 6m J-pole whose performance is sensitive to mast
length. Hence it is clear that mast currents are significant with this
antenna. <o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'>Some antennas with higher gain are
actually worse. For example a well-known 6/2/70 antenna works poorly when
on a mast, because it has a low feedpoint impedance on 6 and 2m. Again,
currents injected into the mast are the problem. Initially, this
shows up as poor SWR which can be improved by adding radials. This helps
the pattern too, although some mast current remains. <o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'>But, radials are a nuisance to transport
and in the field. In particular the joint to the mast requires some
ingenuity to achieve the requirements of mechanical strength, easy to assemble
/ dismantle, and good RF performance. <o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'>Collinears can be useful to improve gain –
with two caveats. <o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'>One is, the feedpoint impedance of a
collinear array is always lower than a single half wave, so there is more mast
current. Ergo – some pattern distortion. <o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'>The other trap for home-brewers is
achieving the correct phase relationship between the array elements. A
phase error of only a few degrees will significantly affect the vertical
pattern. (Many repeater antennas have deliberate downtilt of a few
degrees). It is quite difficult to test for this. And, the tilt with
a collinear is frequency sensitive. (You may have noticed I sometimes use
a folded dipole array on the 2m VK3RWP repeater, although it is much less
convenient than a collinear and has less height on the same mast. Now you
know why!). <o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'>Hope this helps – and keep the
discussion going! Antennas is one area we can really make a
significant difference to communications effectiveness! <o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'>Cheers, John VK3ZRX<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'><o:p> </o:p></span></font></p>
<div>
<div class=MsoNormal align=center style='text-align:center'><font size=3
face="Times New Roman"><span lang=EN-US style='font-size:12.0pt'>
<hr size=2 width="100%" align=center tabindex=-1>
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<p class=MsoNormal><b><font size=2 face=Tahoma><span lang=EN-US
style='font-size:10.0pt;font-family:Tahoma;font-weight:bold'>From:</span></font></b><font
size=2 face=Tahoma><span lang=EN-US style='font-size:10.0pt;font-family:Tahoma'>
Members [mailto:members-bounces@vic.wicen.org.au] <b><span style='font-weight:
bold'>On Behalf Of </span></b>vk3ifm@gmail.com<br>
<b><span style='font-weight:bold'>Sent:</span></b> Friday, 6 March 2015 3:24 PM<br>
<b><span style='font-weight:bold'>To:</span></b> members@vic.wicen.org.au<br>
<b><span style='font-weight:bold'>Subject:</span></b> WICEN Members: CEMR
Meeting Report</span></font><span lang=EN-US><o:p></o:p></span></p>
</div>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'><o:p> </o:p></span></font></p>
<div data-externalstyle=false>
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<p class=MsoNormal><font size=3 face=Calibri><span style='font-size:12.0pt;
font-family:Calibri'>Folks,<o:p></o:p></span></font></p>
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<p class=MsoNormal><font size=3 face=Calibri><span style='font-size:12.0pt;
font-family:Calibri'><o:p> </o:p></span></font></p>
</div>
<div>
<p class=MsoNormal><font size=3 face=Calibri><span style='font-size:12.0pt;
font-family:Calibri'>A short report on the meeting held 4 Mar 15<o:p></o:p></span></font></p>
</div>
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<p class=MsoNormal><font size=3 face=Calibri><span style='font-size:12.0pt;
font-family:Calibri'><o:p> </o:p></span></font></p>
</div>
<div>
<p class=MsoNormal><font size=3 face=Calibri><span style='font-size:12.0pt;
font-family:Calibri'>Apologies from :- Don 3SP, Ross 3LYN, Andy 3ADG and Rik
3EQ<o:p></o:p></span></font></p>
</div>
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<p class=MsoNormal><font size=3 face=Calibri><span style='font-size:12.0pt;
font-family:Calibri'><o:p> </o:p></span></font></p>
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<div>
<p class=MsoNormal><font size=3 face=Calibri><span style='font-size:12.0pt;
font-family:Calibri'>Present Derek 3XY, Peter 3MV, David 3VP, Ross 3SF,
Paul 3YYF, Fred 3JM, Dirk 3FPAJ, Neil 3NM, Gerard 3GER and Ian 3IFM<o:p></o:p></span></font></p>
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<p class=MsoNormal><font size=3 face=Calibri><span style='font-size:12.0pt;
font-family:Calibri'><o:p> </o:p></span></font></p>
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<p class=MsoNormal><font size=3 face=Calibri><span style='font-size:12.0pt;
font-family:Calibri'>Activity for the night was to discuss 2 M antennas for
WICEN field work. Members brought a number of antennas which
were presented.<o:p></o:p></span></font></p>
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<p class=MsoNormal><font size=3 face=Calibri><span style='font-size:12.0pt;
font-family:Calibri'><o:p> </o:p></span></font></p>
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<p class=MsoNormal><font size=3 face=Calibri><span style='font-size:12.0pt;
font-family:Calibri'>The meeting agreed that the antennas most suitable for
WICEN field work was a Coax dipole for vertical applications and a HB9CV
2 element beam where a little more gain is required.<o:p></o:p></span></font></p>
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<p class=MsoNormal><font size=3 face=Calibri><span style='font-size:12.0pt;
font-family:Calibri'><o:p> </o:p></span></font></p>
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<p class=MsoNormal><font size=3 face=Calibri><span style='font-size:12.0pt;
font-family:Calibri'>These were selected because of their easy of
construction, robustness and easy of transporting them in vehicles.<o:p></o:p></span></font></p>
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<p class=MsoNormal><font size=3 face=Calibri><span style='font-size:12.0pt;
font-family:Calibri'><o:p> </o:p></span></font></p>
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<div>
<p class=MsoNormal><font size=3 face=Calibri><span style='font-size:12.0pt;
font-family:Calibri'>The information and notes will be available at the
next. The following meeting these antennas will be constructed and
tested by those wanting sign off on this component of training.<o:p></o:p></span></font></p>
</div>
<div data-signatureblock=true>
<div>
<p class=MsoNormal><font size=3 face=Calibri><span style='font-size:12.0pt;
font-family:Calibri'>-- <br>
Regards<br>
Ian Morris VK3IFM<br>
0407805210<br>
WICEN --- Regional Coordinator --- Central/Eastern Metropolitan Region:<br>
Boroondara, Knox, Manningham, Maroondah, Monash, <st1:City w:st="on">Whitehorse</st1:City>,
<st1:place w:st="on"><st1:PlaceName w:st="on">Yarra</st1:PlaceName> <st1:PlaceType
w:st="on">Ranges</st1:PlaceType></st1:place><o:p></o:p></span></font></p>
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<p class=MsoNormal><font size=3 face=Calibri><span style='font-size:12.0pt;
font-family:Calibri'><o:p> </o:p></span></font></p>
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