SADARS
SLIM JIM ANTENNA
discussion
ORIGINS of the SLIM JIM
• Invented by Fed Judd, G2BCX
• Original design was for a single band
• Overall length is nominally ¾ l
– 1.5m on 2m band so most suited to VHF and UHF bands
– 10m band versions have been made!
• Most usually vertical polarisation
• Suited to home construction
• Provides good Omni directional performance
Obtaining data about the Slim Jim
• Google ‘slim jim’ and you will find hundreds of hits
• M0UKD Slim Jim calculator is excellent
• Slim Jim antenna images provides hundreds of pictures
So, lots of information readily available, including
building instructions and drawings – mostly aimed at
2m band – to suit all tastes and all pockets.
Antenna – fundamental design
Original illustration as it
appeared in Practical Wireless
magazine.
Bottom section ¼ l long, used
to feed top section, does not
radiate (currents anti-phase, so
cancel each other out)
Top section, ½ l long - this is
the radiating section (currents
both in phase, so additive)
Development of J pole
antenna to Slim Jim antenna
Antenna gain 1
What do we mean by Antenna Gain?
Imagine a light bulb suspended in space with nothing around it.
Also imagine that this 2 dimension presentation is 3 dimension
The light will radiate
equally in all directions.
Line joining points
of equal intensity
Antenna gain 2
If we now place a mirror behind the light bulb, then the light
is reflected.
So there will be twice as much light in one direction, but no
light in the opposite direction
The area enclosed by the D shaped line is the
same as the area that was enclosed by the line
before the mirror was added [the light bulb’s
light output (power) is still the same]
So now we can say that there is gain, but only in
the direction that includes the reflections, and
that gain is twice as much as the original light
before the mirror was added
Antenna gain 3
The light bulb by itself is known as an ISOTROPIC radiator
The mirror adds gain (extra light) in one direction, at the
expense of light in the opposite direction. This gain that
includes the reflected/focussed light can be compared to
the light output from the bulb by itself its self as a ratio.
i.e. gain = `reflected’ light / `original’ light
This gain is `dimensionless’, i.e. it is just a number
without any units and is easily expressed in dB terms
Antenna gain 4
Now replace the light bulb by an antenna.
An ISOTROPIC antenna is a (theoretical only) small
antenna suspended in space which radiates RF energy
equally in all directions.
A real antenna has gain in some directions (but losses in
other directions)
This gain is expressed in dBi, i.e. the antenna’s gain in its
`desired direction’ when compared to the ISOTROPIC
case.
Antenna gain 5
Note that some antenna gains are expressed as compared to
a dipole antenna (which is a physical possibility, as opposed
to the physically impossible ISOTROPIC antenna) and these
comparisons are in dBd (dB referenced to a dipole) terms.
The gain of a dipole when compared to an isotropic antenna
is 2.15db, i.e. the gain of a dipole is 2.15dBi (db referenced
to an isotropic antenna)
When an antenna’s gain is quoted by a manufacturer, unless
it states dBd, assume the gain is dBi (bigger number).
i.e. an antenna with a gain of 5dB will actually be 2.85dB
more than that provided by a dipole antenna IN THE
DIRECTION OF MAXIMUM GAIN ONLY.
back to the Slim Jim
The plot below illustrates the gain of a Slim Jim antenna
compared to a 5/8 whip antenna in the vertical plane
Original illustration as it appeared in
Practical Wireless magazine.
Whilst the 5/8 antenna has a higher
gain, this is at about 30o to the
ground, so a lot of this energy is
directed into space.
The Slim Jim antenna’s maximum is
parallel to the ground, which is
where we want it to be, so the Slim
Jim is a more efficient antenna for
practical communications, especially
at VHF and UHF
plan view of the polar diagram
Again, this is an original illustration as
it appeared in Practical Wireless
magazine.
In the horizontal plane, the Slim Jim
has a 4dB gain advantage over the 5/8
whip antenna, so a 10W transmitter
with a 5/8 whip antenna will ‘look like’
a 25W transmitter when its antenna is
changed to a Slim Jim, with respect to
the signal arriving at a distant receiving
station (and vice-versa)
Just to explain the relationship between dB and Watts,
Gain (or loss) in dB terms = 10 x log of gain (for units of power)
= 20 x log of gain (for units of Voltage or Current).
Numerical to dB, 10W to 25W, gain = 25W/10W = 2.5 (no units)
Log 10 2.5 = 0.39794 = 0.4, so gain dB = 10 x 0.4 = 4dB
dB to numerical , divide dB value by 10 and then take anti-log.
For 4dB, 4/10 = 0.4 Antilog10 0.4 = 2.511886 = 2.5 times
So, 10W multiplied by 2.5 = 25W
Slim Jim construction
10m version
2m versions
Slim Jim construction
Slim Jim construction
A bit more detail
Dimensions for this version are in metres
.99m = 38.98”
0.495m = 19.49” (incorrect on drawing)
0.0445m = 1.75”
0.0254m = 1”
0.102m = 4”
Feeding the Slim Jim (it’s not really hungry)
Use 50W coaxial cable, preferably cut to the length that
will be used in the final installation
Connected about 4” from the bottom of the ¼ l section
Most published designs state;
connect the inner to the long section and the outer
to the short section
However, can get better results by;
connecting the inner to the short section and the
outer to the long section
Adjusting the Slim Jim
Simply move the coaxial cable connections up and
down from the 4” point to achieve the best VSWR
Dual band versions
see traps with white sleeving
(these were on sale at Dayton 2012)
Measurements of dual band version noted at Dayton
Ferrite sleeve
fitted over
cable about
1.5” below the
bottom of the
2m ¼ l section