SADARSSLIM JIM ANTENNAdiscussion

ORIGINS of the SLIM JIMInvented by Fed Judd, G2BCXOriginal design was for a single bandOverall length is nominally l 1.5m on 2m band so most suited to VHF and UHF bands 10m band versions have been made!Most usually vertical polarisationSuited to home constructionProvides good Omni directional performance

Obtaining data about the Slim JimGoogle slim jim and you will find hundreds of hitsM0UKD Slim Jim calculator is excellentSlim 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 designOriginal illustration as it appeared in Practical Wireless magazine.

Bottom section l long, used to feed top section, does not radiate (currents anti-phase, so cancel each other out)

Top section, l long - this is the radiating section (currents both in phase, so additive)

Development of J pole antenna to Slim Jim antenna

Antenna gain 1What 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 dimensionThe light will radiate equally in all directions.Line joining points of equal intensity

Antenna gain 2If 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 directionThe 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 bulbs 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 3The light bulb by itself is known as an ISOTROPIC radiatorThe 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 lightThis gain is `dimensionless, i.e. it is just a number without any units and is easily expressed in dB terms

Antenna gain 4Now 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 antennas gain in its `desired direction when compared to the ISOTROPIC case.

Antenna gain 5Note 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 antennas 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 JimThe plot below illustrates the gain of a Slim Jim antenna compared to a 5/8 whip antenna in the vertical planeOriginal 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 antennas 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 diagramAgain, 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 construction10m version2m versions

Slim Jim construction

Slim Jim construction

A bit more detailDimensions 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 (its not really hungry)Use 50W coaxial cable, preferably cut to the length thatwill be used in the final installationConnected about 4 from the bottom of the l sectionMost published designs state; connect the inner to the long section and the outer to the short sectionHowever, can get better results by;connecting the inner to the short section and the outer to the long section

Adjusting the Slim JimSimply 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 DaytonFerrite sleeve fitted over cable about 1.5 below the bottom of the 2m l section