3 Meter Slim-Jim Antenna (JIM = J Intergrated
Match)
| | <-- spacing: 3" at 72MHz
_________ ____ _________
^ 90 degree --> / \ ^
| copper elbows | | |
| | | <-- 1/2" copper pipe elements |
| | | |
| | | |
| ^ | | ^ |
| | | | | arrows indicate | 1/2
| | | | | current direction | wave
| | | | | |
| | | |
| | | |
| 7.8' (89.5MHz) | | |
| | v --> copper sleeve for tuning |
| | ---> 3" space air gap _______V
| | | ^
| | | |
| | | | ^ |
| | | | | |
| | | | | | 1/4
| v | | | | wave
| | | |
| * X <-- tap * = center conductor |
| | | X = shielding |
v_________ \____/ _______v
NOTE: Adjust 1/4 wave and 1/2 wave
section lengths as follows:
1/2 wave section = 5610/MHz
Example: 89.5MHz = 5610/89.5 = 62.68"
1/4 wave section = 2805/MHz 89.5MHz = 2805/89.5 = 31.34"
* 1/4 wave freespace = 2953/MHz 89.5MHz = 2953/89.5 = 32.99"
* distance that antenna should
be from mounting boom, mast, or tower
Description:
This is a vertically
polarized omnidirectional free space antenna which offers approximately 1.8dB of gain. It has a radiation efficiency 50% better
than a ground-plane antenna due to its low radiation angle, it is unobtrusive, and has no ground-plane radials - therefore
low wind resistance.
Why Slim Jim? Well this
stems from its slender construction and the use of a 'J' type matching stub (J integrated matching = JIM) that facilitates
feeding the antenna at the base thus overcoming and problems of interaction between feeder and antenna. The feed impedance
is 50 ohms.
Why is the Slim Jim so
much more efficient than the popular 5/8 wave or other ground plane antennas, despite the latters claimed 3dB over a dipole?
The Slim Jim vertical angle of radiation is almost parallel to ground so maximum radiation is where it is needed: straight
out and all round. (The included diagram illustrates that the vertical angle from the SJ is 8 degrees, while the common 5/8th
wave ground plane antenna is about 32 degrees. -Steve) With all ground planes, including those with radials an entire wavelength
long, the vertical angle radiation is tilted upwards at an angle of 30 degrees or more. This gives the Slim Jim a gain over
a 5/8th wave of 6dB when measured parallel to the ground!
Operation:
Basically it is an end-fed
folded dipole operated vertically. The matching stub provides a low impedance feed point (50 ohms) at the base and couples
to the antenna section at high impedance at one end. As with all folded dipoles, the currents in each leg are in phase, whereas
in the matching stub they in phase opposition, so little or no radiation occurs from this. (See diagram for current direction
arrows.) Correctly matched, the VSWR (Voltage Standing Wave Ratio -Steve) will be much less than 1.5:1, and remains so across
the band.
Construction:
The Slim Jim should be
constructed from 1/2" copper pipe. The bends are made with soldered 90 degree copper elbows. A slip sleave made from copper
can be added to the element above the gap for tuning purposes, although the average length of the gap and spacing between
the elements is 3" at 72MHz and 1" at 220MHz. No part of the antenna should be grounded to the tower or mast. The recommended
mount is the use of PVC pipe and PVC pipe "T's." Make sure the space between the tower or mast and the antenna is one "freespace"
1/4 wavelen.
Testing:
Stand upright (on a railing
or something, but clear of metal water tanks, drainpipes, etc.) and fit the coaxial cable to the antenna with some crocodile
clips. Attach about 2 inches up from the bottom and check the VSWR. Adjust the clips up or down to get the best match (mine
managed 1.2:1), mark where they are to go, remove the clips, and solder the coax directly. Use the copper sleeve, if added,
for any necessary tuning.
