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.
The example shows the 1/2 wave plus the 1/4 wave elements (62.68″ + 31.34″ = 94.92″) as being longer than the left element (7.8′ = 93.6″) leaving no room at all for the air gap. Is there an error here or am I misunderstanding something?
Sorry, typo, that should have been: 62.68? + 31.34? = 94.02? but that doesn’t change the question.