|
|
|
|
Some Notes on the Slim Jim
Antenna Dave Coomber
M0UXB, UK
Introduction. 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. 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:
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 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:-
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. Ferrites. 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 4:1 Balun:
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:
DC AMPLIFIER Note:
The larger the meter, the easier it is to see. A digital meter is not
ideal for this job. On the other hand, MFJ do make quite a good one !. Conclusion. 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 Sources: Antennas, by Jasic: Published by McGraw Hill Original Slim Jim article by Fred Judd, G2BCX from "Out of Thin Air"
published by PW Magazine
|
|
