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Some Notes on the Slim Jim Antenna
Dave Coomber
Since it’s publication in 1978, the Slim Jim aerial has been built by many UK amateurs. It was described by the late Fred Judd, G2BCX, in the UK magazine Practical Wireless and subsequently in other journals and books.
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).

The MSJ dimensions:
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 very high.

The original aerial for the ham band was made from 1/4 or 3/8 inch aluminum tube and designed to mount on a vertical mast. It’s design centre frequency is 145MHz (at the centre of the UK 2m band).

The Original Slim Jim

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.

Scaling for other frequencies.

As long as the matching section is an electrical quarter-wave, everything else will fall into place.

Suggestions for practical construction.

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 water-pipe.

Practical Feeding.

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.

The Coax Choke balun:-


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.

Experiments have show that the ‘proper balun’ does the job better:-
Here are a couple of examples:
1:1:- Pawsey Stub



Here is how to figure out an electrical 1/4 wave length of coax:

In Feet:

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.

4:1 Balun:

Sometimes this is easier to use; it taps further up the matching section and can be grounded for increased safety,

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).

Setting up.

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 about.

Simple Field Strength Meter.
Note; I make no claims for originality. It’s just the one I use.

Components for above:
L1 -- 1mH--50 ohm carbon restistor works better



D1--OA90 SMALL Signal diode:matched pair

D2-- Same as D1

R1--20K? Depends on meter (50 -- 100uA)


Note: The larger the meter, the easier it is to see. A digital meter is not ideal for this job.
The amplifier can be put into a separate case and connected by twin wires.
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 dave@m0uxb.wanadoo.co.uk


Antennas, by Jasic: Published by McGraw Hill
VHF/UHF Manual, 4th Edn., Ed. G.R. Jessop, G6JP, published by the RSGB
Technical Topics scrapbook (85-89), P. Hawker, G3VA, RSGB.
The Amateur Radio Handbook, 2nd Edition: Published by RSGB
Practical Wireless magazine, April 1978. (and later books)
Out of Thin Air, published by PW magazine. (out of print and rare!)
Two-metre Antenna Handbook, by F C Judd, G2BCX, published by Newnes Technical, 1980.
Antennas & Antenna Systems, TM11-314, 1943 (US War Department).

Links and further information for the Slim Jim antenna:

Original Slim Jim article by Fred Judd, G2BCX from "Out of Thin Air" published by PW Magazine

Slim Jim antenna project on Hamuniverse.com
Other Slim Jim projects listed on the Antenna Projects page of Hamuniverse




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