|
|
|
Tune Around! Topics For
Technicians! |
If you are reading this you most
likely have built the Slim Jim antenna for the 2 meter band using either
copper or aluminum tubing. After your final tuning, you may not have seen
that "perfect" swr reading of 1:1 that many look for and could not get any
lower to reach "perfection".
Here is a method
that was sent to us recently that jogged our memory from research done
years ago before we put the Aluminum Slim Jim antenna project on the
site.
By the addition of a
homebrew tuning capacitor across the air gap, you can now get it tuned
much better than you may have before doing this simple modification to the
air gap.
So with all that said,
read on for the details! N4UJW
Brian Kent, G6EXX has
been kind enough to share some ideas and comments about the Slim Jim
antenna that is used widely in the UK on their version of the Slim
Jim antenna....below are a couple of edited emails he sent
us:
"Hi
there from G6EXX
I live
in the UK and have ALWAYS used a slim jim antenna for both 2m and 70cms.
The primary reason is very simple, why blow 80% of your radiated power up
into the sky?
A
simple 1/4 wave antenna which most operators start off with is great for
just setting up and trying to get things going. But then you realise that
a good proportion of your radiated power is firing off at 30 degrees into
the clouds. Hence the Slim Jim antenna. This baby has such a low angle of radiation that you are no
longer throwing lots of power at an antenna to
gain good coverage. From my QTH in Hertfordshire which is around 475
ASI, the Slim Jim antenna made R5 contacts into London, Bristol, and
Wales with the fartherest at least 200 miles
away, using just 5 Watts. Forget 'J' beams and 5/8th wave
antennas, etc, go slim jim!
Now, as an added
advantage, I fit a capacity plate to mine which
covers the 'gap' at the lower end of the radiator.
Short
description:
All you do is intert an insulator, plastic or nylon between
the gap of the radiator and the short stub. On the outside of the nylon,
fit an oblong section of 1/16th inch aluminium plate allowing about 2mm of
air spacing between it and the metal antenna elements that will just cover
1/4 inch of both the top and bottom sections. Put a screw in the middle to
hold it and allow it to rotate. Now start by turning it at 90 degrees to
the radiator, check VSWR, turn it gradually checking VSWR all the time and
eventually you will tune this baby to perfection." See drawing
below.
Long description and very simple to
do...refere to drawing above. "You know the
format of a slim jim, its a big oval loop (UK version) with a small
gap. In that gap, usually around 10-12mm, fit a piece of square section
nylon, essentially so you can put a small screw into the center of it.
Next cut a piece of flat aluminium, 1/16 inch is fine, to about 10 x
25mm and put a small hole in the centre. Screw this into the centre of the
nylon and turn it to 90 degrees. Set up the antenna and tune for lowest
VSWR with normal tuning via the matching stub feed points. All slim jims
will come down to at least 1.3:1 even in the worst case.
Now start adjusting the 'butterfly' capacitor and lo'n'behold, like magic, your VSWR will come sweetly down to 1.2:1 or better. An old one I made years ago just did not even budge the meter, it was brilliant. I would like to add a point to watch here, if anyone ever buys a shop manufactured slim jim, do not use it until you have checked it. Just about every one I have ever seen has the feeder connected to the short rising stub, it should be to the long section so that it matches the impedance to the radiator. These things will never tune anyway, thats why my friend asked for help tuning his. Since then we have found all of them are the same. Made by "Donuts". You
simply cannot beat a slim jim, and when it is tuned to perfection you
will reap the rewards.
I am
currently about 580 ASI and to be honest, I have never used more than 5 W
transmit power, I cannot go any lower but I can go up to 50W, transmitter
runs cool, and gets the job done! My antenna is only about roof
height of 30 feet, as neighbours and XYL or 'her indoors', as she is
known , doesn't like antenna's, one of those 'I hate wires'
women."
Happy Days.
73
Brian
G6EXX Editors Note:
Since the UK Slim Jim antenna
design is slightly different than most used in the U.S.A., some adjustment
of the gap space and or the addition of the gap capacitor may or may
not be needed in your installation. There is certainly no harm in
using the technique described above if you get frustrated with tuning and
if it helps.
If you have difficulty finding
or making an insulator for the gap to support the capacitor, try fitting
one inside the tubing rather than on the outside as shown in the drawing.
The important thing is that the gap between the
plate of the capacitor and the metal antenna elements is no more than
about 2 mm spacing, (5/64 inch), and that the capacitor is supported well
by the insulator. Use your "ham engineering"!
When you are tuning the
capacitor, tune and test in very small increments until the VSWR
lowers to a minimum and just
starts to rise. STOP THERE and go in the opposite direction if desired.
After you have finished tuning and to prevent the capacitor from turning
due to wind vibration and water or dirt intrusion, seal it well with some
liquid weather tight material that is non-conductive like Liquid
Electrician Tape so it can not turn. Once the sealer is dry, you are ready
to get it on the air.
UPDATE: Download the original G2BCX Slim Jim "Ultra" antenna article detailing the Slim Jim antenna with this modification here in pdf form kindly submitted to us by Peter Kay.
|
