Using MMANA-GAL – Free Antenna modelling software
(Copy of article given to Bangor and District Amateur Radio society for their members magazine "CONTACT" )
Antenna modelling software can be complex but you can use
it in a very simple way to get useful results. The free package MMANA-GAL is
well supported with Youtube videos listed at the end of this article but in
practice you only need 4 or 5 commands to use it effectively. This article makes more sense if you download
and install the software and run it whilst reading!
As an example of its utility, what if you were thinking
of putting up a dipole for 40m, what's the difference between having it at 7m
or 9m. What if it was made of thicker wire. What happens if you droop the ends
to make an inverted V? You can "test" all this in software. You can
specify the metal the antenna is made of (e.g Copper or Aluminium) and what
thickness. You cannot specify a velocity factor in MMANA which means insulated
wire ends up a bit longer (95-97%) but that is no real hardship. You can
specify different types of earth, although I have only ever used the default
"normal earth". The results
are useful for comparison purposes, don’t assume they are 100% right but they
are a reasonable estimate of the truth.
Of course once you start fiddling (experimenting) with
MMANA you might not find time to actually make antennas - so you do need a bit
of discipline to avoid wasting time (having fun).
To use MMANA quickly note that the opening screen has 4
main tabs - you only need the "Geometry", "Calculate" and
"Far Field Plots" tab. You also need to access the wire editor
To draw a dipole you need to run the "Wire
edit" tool using the 6th icon in from the right or hit Control-W. You also need to know that the view you will
see has XYZ directions where Z is the height. You can think of X as pointing East
and Y pointing North as the default directions. Once in the wire edit window
click on the button marked XY. The diagram below is a screen dump of the Wire
edit window.
The wire edit window has an icon tool for editing a wire
and another one for creating a wire. These are the only two you need. You could
instead enter all data into a table - I usually create a simple wire model,
with approximate lengths, then nip into the table and adjust the lengths to
what I want. Depends how good you are with a mouse!
A couple of tricks; you can specify height, either when you draw the antenna, or later when you go to calculate its performance. I usually leave it at zero when drawing.
Example: A dipole, one wire 20 m long and fed in the
middle; in the Wire edit window, (XY
view) draw one line that crosses the corner where the blue Y and X axes cross.
It should turn red. Click OK which takes you back to the main geometry window,
listed below
Edit the X1 and X2 values above to the lengths you want
and click on the box marked PULSE under the Sources 1 window (bottom LHS) and
enter the text w1c – this puts the source at the centre of wire 1
Switch to the calculate tab and Set 4 things; the frequency,
the type of ground (“Real”), the height and the material of the wires e.g
copper. You are then ready to click the “Start” button at the bottom LHS. Your
window should look like that below;
Don’t worry about the SWR if it is below 2. – in real
life it varies with your type of earth and antenna height. You can also look at where the best
directions are and the relative signal strengths at various angles. Click “Far
Field Plots” up at the top, beside the Calculate tab. (if you click on the
“Plots” button at the bottom you get a static display. If you click your mouse on the antenna plots
you can get various gain values in different directions. The left hand plot is
a bird’s eye view – but you are best to specify a particular angle of
elevation. Antennas send signals out at the take off angle to bounce off the
Ionosphere – google TOA MUF Calculator for a useful tool. (I found mine at http://www.kolumbus.fi/pekka.ketonen/TOA
MUF calculator.xls ) Energy leaving your antenna at 5 degrees will go 2000
km (DX!), an elevation of 15 degrees will give ranges of 1000 km (Inter-Europe)
and 30 degrees for 500 km for UK use.
– click the “Elevation” button at the bottom and
set the value to 5 degrees.
So at 5 degrees you get a “gain” of -10.7dBi. At 15
degrees you get -1.6dBi and at 30 degrees you get 3.3 dBi. When using dB (or dBi) you need to think in
terms of ratios and factors, 3dB is a doubling, 6dB is a factor of 4 and 10 dB
is a factor of 10. You add the dBs but multiply the factors. Anyway – for this
article I just want to show the relative “gains” at different
heights. At 7m this antenna puts 20 times more power into UK stations as DX
ones. From -10.7 to +3.3 is 14dB – 13dB is (10dB +3 dB so 13dB is a factor of
20 to 1 (10:1 times 2:1)
If we go back to the calculate tab and change the height
to 9m, click start and use the far field plots we can get the relative “gains”
at various angles, and we can repeat this for 12m to get the table below.
|
Elevation
angle/likely distance reachable
Height of antenna
|
5°
(2000km)
Best/Worst
Directions
|
15°
(1000km)
|
30°
(500km)
|
|
7m (SWR=1.07)
|
-10.7/ -16.3
|
-1.6/ -9.7
|
3.3/ -3.8
|
|
9m (SWR=1.45)
|
-10.1/ -17.8
|
-1.2/ -10.9
|
3.5/ -4.3
|
|
12m (SWR=1.91)
|
-8.9/ -18.8
|
-0.1/ -11.7
|
4.2/ -4.4
|
Note I also show
the relative gain in the X direction (the worst direction) to the right of the
‘/’
Note also the SWR
changes as we are close to the ground – if your coax is poor quaility you will
see better values in the shack (try https://www.qsl.net/co8tw/Coax_Calculator.htm to see how SWR changes.
The summary is,
height matters! Unless your antenna is a vertical (which can pick up local noise).
All antennas work and all real antennas are compromises.
I expect to
demonstrate MMANA-Gal at the BDARs March talk. It comes with many, many
examples of quite sophisticated antennas – you can have traps and feeders other
than the default 50 Ohm coax. There are examples of cobwebs, hex beams and
more.
Good YouTube
videos on MMANA are available from Callum, M0MCX – who makes the DX Commander
Verticals – all his videos are good! – his channel is at https://www.youtube.com/user/m0mcx/videos and if you search for titles such as
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