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300 Ohm Twinlead 6
Meter Slim Jim Antenna Project
Hi there, my name is
Geoff from Bridgetown in Western Australia.
I am an amateur
radio operator, my callsign is VK6HOG. I am new to 6 metre
operations and am in the process of designing a wire based antenna
to use on six metres. A colleague of mine told me that the Slim
works well, I have seen your web site and note that you have a
project for a 2 metre slim jim version.
I wonder, can
the 300 Ohm twin lead specifications be used for a (6) six metre
version of the Slim Jim
Here is the answer to my question:
My experimentation in creating a 6 metre 300-ohm Slim Jim
antenna is here for you to try...
I have finally made a
prototype of the antenna, and it appears to work! I have attached
some pictures for you to look at. Essentially the first versions I
made were unstable. Using standard very light weight 300-ohm TV
cable I had problems managing the very fine wire, both exposing it
to make the connections and soldering the wire to the feedline of 50
Ohm coax. However, in the end I used a couple of short lengths of
dowel to strengthen the feedpoint and top of the antenna to make it
a bit more robust. I think ultimately the antenna would benefit from
being mounted inside some PVC tube to help with weather proofing and
rigidity. The final measurements (METRIC) I used are as follows:
(The 6 metre Slim Jim antenna is centered on 52 Mhz as I am
only allowed to Tx in this region of 6 metres !!) The
formulae will obviously work at which ever frequency you
Copy these lengths and convert to inches,
feet, or any other conversion here! We have done the conversons
for you to inches and metric.
4155mm (163.58 inches)
Short element (from base):
1338mm (52.67 inches)
Top element (from top):
2740mm (107.87 inches)
Leaving a gap of 77mm. (3.03
The feedpoint is 209mm up from the base. (8.22
At the design frequency (52mhz) the SWR of the
Slim Jim is almost flat, with it being around 1.5 to 1 at each end
of the band. I have it 'inserted' about 15 metres up in a pine tree
at the moment.... suspended by a cord - using the top mounting
configuration (dowel with a hole through it and the ribbon cable) to
suspend it. Whilst I haven't made any Dx contacts with it, I will
let you know whether it is a 'radiator' or a very long dummy load...
The cut out section is wrapped in tape at this stage to help with
See pictures and diagram below. I am currently using
an Icom 706 MKIIG radio rated at 100 Watts on SSB.
The fact that
having it suspended in a pine tree (RF vacuum) is not ideal, but my
QTH here has no large masts at this stage. Given that the
antenna is 4 metres plus (in excess of 12 feet) you need space and
if I had the opportunity to suspend it from another non-conducting
structure, I would !
Once I rack up some DX (local or
otherwise) I will let you know, but given that I have only a local 6
metre operator a few blocks away, and no other operators close by,
that may be a while.
Maybe someone else might be crazy
enough to try it like me. When I pull it down I will silicone
up the ends of the 300 ohm line to weather proof it as much as
possible, as well as the feedpoint.
Hopefully it doesn't
change anything, but as stated, it has almost flat SWR at
52.300mhz... I am sure this would travel up and down depending
on how you made it. At the feed point, I only exposed a short
amount of wire to connect to the coax, because everytime I exposed a
lot of wire, it became brittle and I had major hassles with
connection. The draw back of a small exposure of wire is that
if I need to tune, I basically can't !! I would have to expose more
wire, bit by bit.
I have seen other 300 ohm twinlead with
thicker wire... That may be worth a shot and would be much more
workable than the thin, cheap stuff I have used in this project.
Anyway, all the best.
Have a go at this 6 metre
version of the Slim Jim for yourself and have a G'day on 6
73 de Geoff
Editor's notes follow:
These are the formulas that you can
use for designing the antenna for 6 meters or any other band for
that matter. They follow:
Lengths in inches
3/4 wave (longest section) = 8415/MHZ
wave section =
wave section = 2805/MHz
* 1/4 wave freespace = 2953/MHz
distance that antenna should be from mounting boom, mast, or tower.
89.5MHZ = 8415/89.5 = 94.02"
5610/89.5 = 62.68"
89.5MHz = 2805/89.5 =
89.5Mhz = 2953/89.5 = 32.99"
Metric Formulas: for mm:
220.5 / mhz X 980 = Overall mm length
11.1 / mhz X 980 = Feedpoint up from bottom in mm
71 / mhz X 980 = Lower Wire Breakpoint(Length from wire
cut point to base of
Example: 220.5 / 50.0mhz
x 980 = 4321.8 mm
11.1 / 50.0mhz x 980 = 217.56 mm
71 / 50.0mhz x 980 = 1391.6 mm
Either of the formulas
above should get you very close to the proper lengths for the
antenna with a bit of experimentation as usual with any project.
They may yeild lengths that are longer than
needed for the final lengths and should allow room for
As far as the gap spacing, I have not found
any formula in my research for calculating it. As with any antenna,
scaling can probably be used.
For instance if the 2 meter gap is
1 inch as an example, then since 6 meters is 3 times as big, then
the gap should be 3 times as large. This seems to hold true with
Geoff's experiment with his 6 meter Slim Jim antenna.
Another example, if the gap for a 4
meter antenna is 1/2 inch, then the gap for a 2 meter would be about
2 times smaller, or 2 x .5 = 1 inch.
You should have no trouble
running 100 watts with it. It should also be noted that when I built
the Slim Jim antenna from 1/2 inch diameter aluminum tubing, that
the free space distance was accidentally not
used in the installation by error.
No change in swr was
noted either with or without using the freespace distance from
surrounding metal. Since I have no way of checking the difference in
the "8 degree" pattern other than over the air, with or without the
freespace distance, only further experimentation will tell.
Email Geoff here with your