WICEN Vic: WICEN Members: CEMR Meeting Report - Antennas

[John and Bronwyn Morrissey]

Hi Ian

 

This is a great topic for a WICEN discussion! 

 

It is also one where there are many possibilities with many different
tradeoffs. 

 

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. 

 

So:  some discussion follows. 

 

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. 

 

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. 

 

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. 

 

A 5/8 wave is also relatively poor, also due to its relatively high
feedpoint current.  

 

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.  

 

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.  

 

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. 

 

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.  

 

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. 

 

Collinears can be useful to improve gain - with two caveats.  

 

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.  

 

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!). 

 

 

Hope this helps - and keep the discussion going!   Antennas is one area we
can really make a significant difference to communications effectiveness!  

 

 

Cheers,   John VK3ZRX

 

 

 

 

 

 

  _____  

From: Members [mailto:members-bounces at vic.wicen.org.au] On Behalf Of
vk3ifm at gmail.com
Sent: Friday, 6 March 2015 3:24 PM
To: members at vic.wicen.org.au
Subject: WICEN Members: CEMR Meeting Report

 

Folks,

 

A short report on the meeting held 4 Mar 15

 

Apologies from :- Don 3SP, Ross 3LYN, Andy 3ADG and Rik 3EQ

 

Present  Derek 3XY, Peter 3MV, David 3VP, Ross 3SF, Paul 3YYF, Fred 3JM,
Dirk 3FPAJ,  Neil 3NM, Gerard 3GER and Ian 3IFM

 

Activity for the night was to discuss 2 M antennas for WICEN field work.
Members brought a number of antennas which were presented.

 

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.

 

These were selected because of their easy of construction, robustness and
easy of transporting them in vehicles.

 

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.

-- 
Regards
Ian Morris VK3IFM
0407805210
WICEN --- Regional Coordinator  --- Central/Eastern Metropolitan Region:
Boroondara, Knox, Manningham, Maroondah, Monash, Whitehorse, Yarra Ranges

 

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