See the original and rare "2BCX Slim Jim" article published by him here!
'‘Slim Jim'’ comes from the acronym ‘J-type Integrated Match’. Such a feed system is employed in the J-pole aerial and has been employed in one form or another for several decades.
The real origin of the Slim Jim is, at best, uncertain, but it is not: - a development of the J-pole, - anything to do with the ‘Zepp’ (only one reference used the Zepp to illustrate the concept), - nor was it ‘invented’ by a few amateurs looking for a simple aerial made up from twin feeder. Leading opinion (where there was any interest), had it that it is a development of a ‘grounded J’ (see “Antennas” by Jasic). Interested readers should also see “IEEE Transactions on Broadcasting”, vol BC32-1, March 1986, where the Slim Jim was analyzed with a view to using it for Broadcast purposes; the authors called it the MSJ (Modified Slim Jim).
Dimensions are in Metres.
Base Frequency 101.8MHz
It can thus be seen that re-scaling can be achieved for almost any frequency of operation. (See the "formulas" on the Slim Jim project here)
Some adjustment of the feed point may be necessary.
Made with care, the Slim
Jim is capable of a performance superior to the J-pole and the radiation
angle is reduced to about ten degrees (which is often enough for a couple
of S points at times). For best results, to say nothing of the radiation
angle, it is best fed balanced; as for that matter is the J-pole. Fed
thus, the impedance at the top of the matching section will be equal, and
Construction: (taken from original writing)
6 or 8mm aluminum tube, stiff galvanized wire (coat hanger!), or 300 ohm ribbon.
The spacing is not critical.
The connections should be protected against the weather.
The insulator (used as much for rigidity as anything else) may be PTFE or Perspex. The wire ends must not touch.
The support insulator may be necessary when aluminum tube is used.
If supported from the bottom, leave at least a quarter-wave space before metal work.
As long as the matching
section is an electrical quarter-wave, everything else will fall
If the materials for construction are limited to simple wire (almost any gauge), the following might be of help:-
If the wire used is about 1/8 inch diameter, some support may be needed along the length.
Using RG58 coax, the choke can be wound as shown (6 turns for 2m band).
The whole assembly fits
in a 40mm white plastic waste
The original author did not seem to consider the idea of a balanced feed. He recommended testing it with the full length of coax; tricky at best and found to cause more confusion than necessary.
Both aerial designs require some sort of balanced feed, although it can be fed without (the radiation pattern may be distorted, matching not easy and bandwidth a bit strange).
This can sometimes be
achieved by use of a ‘choke’ balun, the easiest of which to construct is
about 7 turns of (RG58) coax feeder on a piece of 20mm plastic water pipe.
The choke prevents radiation from the feed coax by presenting a high
impedance to ‘screen currents’ which tend to upset the radiation pattern
and make it easier to match. There are other methods (such as Ferrite
materials) which have their own advantages.
Six or seven turns of UR43 or RG58 coax cable (for VHF). UHF will require fewer; low-band VHF will require more. It is important that the adjacent turns touch. Seal with heat-shrink. Wind on air core plastic or PVC pipe.
The relatively recent adoption of Ferrites in the reduction of EMC on AV equipment has widened the range available (particularly in the UK). Almost ever monitor and projector has several.
The number of these ‘beads’ required on an aerial depends upon the intended power to be used. According to experts to whom I’ve spoken, about six are required for up to 100w.
They can be secured to the cable by heat-shrink tubing, The 10.7mm bore types work well on RG8 or UR67.
Here is how to figure out an electrical 1/4 wave length of coax:
246 x (Velocity Factor) / Frequency (MHz) = Length in Feet
In Inches:2952 x (Velocity Factor) / Frequency (MHz) = Length in Inches
Metric formulas for Centimeters:
7500 X (Velocity Factor) / Frequency (Mhz) = Centimeters
After the stub assembly is complete, seal all ends and shields well!
Silicone Grease (AMbersil MS4) is good for sealing the ends.
Sometimes this is easier
to use; it taps further up the matching section and can be grounded for
Seal all coax ends and connections well against weather!
As long as the cables
have the same characteristics (VPF), it is possible to use
a different type for the loop. For example, RG8 (UR67)
feeder, RG58 for the loop).
Aerials can be strange beasts. They work one minute and then not another. Adjusting the tapping point can be awkward if the thing is hanging in mid air at the end of a bit of string. I’ve found it easier to put the whole assembly on a plastic garden chair (on top of the plastic table). It’s far enough from the ground to make it a little easier. A sliding short can also help get all the variables fairly close. Get the short, the tap and the length right and you can expect really good response.
One useful bit of kit is
a simple Field Strength Meter; it can save hours of messing
Components for above:
Components for above:
Note: The larger the
meter, the easier it is to see. A digital meter is not ideal for this
It’s also possible to use almost any meter up to 1mA.
Reliable operation at 70cms is not too difficult if the connections are kept short.
Set VR2 to read zero on the meter. Anything received thereafter must be RF !
On the other hand, MFJ do make quite a good one !.
A large diameter radiator will give an increase in bandwidth, although at the cost of increased weight. The screen on a piece of RG8 works quite well.
The J-match section may be used to drive a co-linear aerial, as suggested by Franklyn.
As usual with these things, it is up to the individual Ham to experiment and get what performance he needs.
Thanks are due to all those who have helped me with this document.
Please email questions to email@example.com
Antennas, by Jasic: Published by McGraw
Original Slim Jim
article by Fred Judd,
G2BCX from "Out of Thin Air" published by PW
Slim Jim antenna project on Hamuniverse.com
Other Slim Jim projects listed on the Antenna Projects page of Hamuniverse
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