Re-edited by N4UJW from an
original article by David Younker KA8OGD
(callsign no longer active) 73 Magazine April
1989
While recently going thru some of my old ham radio
magazines, I ran across this inexpensive and easy to build antenna project
for 2 meters. I have not seen it on the internet so here it is for you to
try!
I personally have not tried this antenna but it should work
fine if you follow the very simple directions!
It can be built as
is for 2 meters, or you can try it on other bands or frequencies with the
formulas provided by me below.
THE SLINGSHOT ANTENNA NOT DRAWN TO SCALE Please note
in drawing that elements are bent 90 degrees. Make your bends as needed
depending on material used for elements. If you use copper tubing, a 90
degree elbow on each should work fine. If you are comfortable with
bending, great, if not, get the hardware store to do it for
you!
The completed antenna is
bi-directional with a rough figure 8 pattern and is composed of 2, 3/4
wavelength sections of electrical conduit bent and cut to the
lengths in the drawing and supported as shown on any type of
insulating material attached to the mast with whatever arrangement of
bolts, nuts, clamps, etc. You should note that the bottom
(horizontal element portion) is 1/4 wavelength long and the top (vertical
element section) is 1/2 wavelength long. The element mounting plate (in
yellow in the drawing) can be plexiglass, painted wood or whatever you
happen to have that is NON CONDUCTIVE. You can use copper or
aluminum for the active 3/4 wave elements, but aluminum would be prefered
due to less weight. Although electrical conduit comes in various sizes,
the size was not stated in the original article but I would suggest 1/2
inch or larger in diameter. (The larger, the greater the
bandwidth.)
The total length of each element is 60 inches + - and
they are attached about 4 inches apart on the mounting plate with enough
bolts and nuts as needed. The coax attachment points are in red on the
picture, and I would suggest that you use spade lugs on the ends of the
coax to attach it to the bottom end of each element (the ends nearest the
bend) with bolts, nuts and lock washers all the way thru the element and
plate. There must be a good electrical connection between the coax center
conductor and shield braid and each element. Keep the connections lengths
from the end of the coax as short as possible. They become part of the
radiating element lengths.
It does not matter which conductor from
the coax is attached to which element. SEAL ALL CONNECTIONS AND THE
END OF THE COAX! When attaching the elements to the mounting plate,
drill enough holes all the way thru the elements and plate for good
mechanical stability and attach with bolts and nuts. The elements and coax
connections must not touch the support mast at any point if the mast is
made of metal of any kind! You could use a pvc pipe or length of lumber of
the required length instead of metal to get the antenna up as high as
possible and a half wave or more is preferred!
"This design,
untrimmed, up a half wave, presented an SWR of 1.5:1 across the top
2 MHZ of the band (146-148mhz)".......KA8OGD
A note or two more
about experimenting with this antenna: MAXIMUM SIGNAL IS OFF BOTH ENDS
(TO THE RIGHT AND LEFT AS DRAWN NOT BROADSIDE. Point the boom at your
target!) ANTENNA SHOULD BE ROTATED IN DIRECTION NEEDED!
The
formulas for calculating the lengths for this project seem to be
aproximately the following. There is a more complicated formula first
and then a simple version....take your choice...they both yeild the same
result:
Simple
version
formulas: 8856 / freqmhz = 3/4 wavelength
section in inches (total element length) 5904 / freqmhz = 1/2
wavelength section in inches 2952 / freqmhz = 1/4 wavelength section in
inches
Lets
do a calculation for 144.200Mhz ssb using the more complicated version
formula: 11808/144.200 = 81.88 inches 3/4 wavelength = .75 x
81.88 = 61.4 inches total element length per side 1/4 wavelength would
be = 81.88 / 4 = 20.47inches or 1/3 of 61.4 inches. (The vertical section
takes 2/3rds of the total length of one side of the antenna
element) The 90 degree bend will be at the 1/4 wave point on the total
length.
Footnote to construction:
It is
advisable to add about 5 or 6 turns of coax at the base of the antenna as
an air choke to help keep rf off the feedline. Some builder do
this....some don't.
According to the article, 15 meters is
about as low in frequency as it can be used before it becomes very
difficult to keep it up due to size and weight! (one element would be
about 34.5 feet long according to my Texas Instruments model TI-7140
handheld calculator and the above formulas!) HI!
MODIFICATIONS, COMMENTS AND
UPDATES! Experimentation performed by LA2PJ of Norway
taken from his email: (January 23,
2003)
Tonight I have tried a construction from your
webpages, The Slingshot Antenna. Just soldered two wires to the end
of a short length of coax and pinned it to the wall in my shack with small
needles to get the correct shape.
The results were amazing! The direction of the
wall is in the right direction to a distant repeater here on the west
coast of Norway. Using an Alinco handheld with approx
1W, I was able to work through the repeater with full
quieting. The distance is 94 kilometers (approx. 55 miles. The reports
indicated that they could not notice the difference when I switched
between this indoor antenna and a Diamond X-510 vertical on the roof. The
SWR was 1.4:1 at 144MHz rising to 1.7:1 at 146 MHz, indicating that the
antenna is a bit long. But then the elements are made of 1mm stranded
copper wire. Am thinking of a way to produce the antenna to be used
outdoors.
If your offer is still valid,
I would like to present this antenna in Norwegian at our web
site.
Best 73's Egil - LA2PJ
(I said yes to his request
for adding the project to his site in Norway. Stay tuned here for the link
when he gets it up and running for our fellow Norwegian Ham friends to
enjoy.) Editors note: Egil, LA2PJ, is the
former Webmaster for the NRRL, the Norway counterpart to our ARRL!
Update from KC2GOA: The two meter
slingshot works now that I made some changes. I had to change the spacing
between the two elements to 1/2 inch and cut the short lengths to 19 1/4
inches and the long ones down to 39 1/4 inches and I came up with a 1.2
swr at 146.000mhz. 73's KC2GOA.........
[Editors note: The diameter of the elements and the
spacing at the center insulator will play an important part in getting the
antenna to resonate at your frequency of operation for lowest SWR. Some
experimentation may be needed with your particular construction
techniques!]
More updates: January 2004
440mhz scaled version by N9YBP CLICK
HERE
From the editor: I hope you try this antenna project as is and if you are
pleased with the results, please let me know, and if you have tried any
modifications to it and they worked a lot better in performance, please
email mewith them. I will be glad to add them
to this project with full credit going to you!
EXPERIMENT!
EXPERIMENT! EXPERIMENT! (See latest experiments
with this antenna and input from builders next below.
MORE MODS FEBRUARY,
2005 BY ROY: I constructed this antenna as per KC2GOA’s latest
dimensions. However I added an S0 239 connector between the two elements
for direct connection with coax with a PL259 connector. See drawing
below: For the antenna itself. I used the ½” copper tubing AS STATED
ABOVE, but for connecting the two sides together, I used a 12” piece of ½”
PVC which the copper tubing fit snuggly inside of, and attached an SO 239
connector in the middle.
I mounted
the 239 connector on the PVC with a self tapping screw into the GAP
between the antenna sides. I then bolted thru both PVC and copper
tubing with brass machine screws and nuts to secure the tubing to the
PVC.
To make the connections to the 239 connector, I
bore oversized holes into the PVC so that the screw head and nuts would
contact the copper tubing. I bought the brass screws long enough - 1 ¼” -
so that I could put two nuts on them; one to hold it to the tubing and the
other to act as a lug to connect a short piece of bare #14 copper wire to.
I then soldered the other end to the 239 connector. I repeated that for
the other side. This makes it a neat and clean antenna. I hung my
antenna from the rafters in the attic - upside
down! I use it as my east/west antenna as I already had built the
Hentenna antenna that you have on your
website for the north/south coverage! They both work great! If someone
wanted to mount this antenna right side up, they would need to put a PVC
tee between the sides and point it downward! I used 12” of PVC pipe, but
you could use it much longer and add more screws for better support in the
vertical position! 73 N9AGT,
Roy.
Editors note:
Great work
Roy! I'm sure there are other ways to attach the coax to the
antenna........ an MFJ ANTENNA ANALYZER would help in tuning the
antenna, but use what you have....... keep experimenting and have
fun!.......N4UJW
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