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This antenna project
should be considered experimental.
Ever wonder if you could make a 40 meter "home brew" wire
antenna that performs reasonably well and fits into into a space that
is 20' long, 1' wide, and 20' high? Impossible! That's
what my friend KI4PMI and I were thinking as we started looking at
options to satisfy these demanding requirements. We're both
fans of loop antennas so we decided to take a look at a vertically
mounted 1/8 wavelength 40 meter rectangular loop antenna fed at
the bottom with 300 ohm twin-lead.
We "fired up" EZNEC+ v.5 and cranked in the
numbers using 16.5' for the sides and 20' for the
height. EZNEC predicted an unacceptable SWR
curve over the 40 meter band.
EZNEC+ ver. 5.0
40 m 1/8 WL
loop
6/7/2011 6:51:48 AM
----------------SOURCE
DATA---------------
Frequency =
7.15 MHz
Source 1
Voltage = 1953 V at -89.59
deg.
Current = 7.113 A at 0.0
deg.
Impedance = 1.976 – J 274.6
ohms
Power = 100 watts
SWR (50 ohm
system) > 100 (300 ohm system) > 100
However, the predicted antenna radiation pattern looked
promising as seen below.
The EZNEC model can be downloaded from the link
below.
EZNEC
model 1/8 WL vertical 40 meter loop from
NC4FB.ORG
What the heck! We decided to raid the junk
bin, construct the antenna, and use an AIM-4170C analyzer to plot the
40 meter SWR curve. The antenna radiators (legs) were made
of #12 AWG stranded THNN coated wire. The transmission
line was made of 16.5' of Radio Shack 300 ohm twin lead connected at the
tuner end to a 1:1 current balun.
We hoisted the antenna and analyzed it.
Sure enough the SWR curve was totally unmanageable as
predicted by EZNEC.
Having nothing to lose at this point, we decided
to lower the antenna so the bottom leg rested on the earth and see
what happened. We were shocked to see that the SWR curve
improved dramatically. The length of the bottom leg was
incrementally increased by adding wire until a good SWR curve was
attained. We stopped adding wire when the bottom leg
reached 20.5'. Of course, the antenna was no longer symmetrical at
this point with the bottom leg being 4' longer than the other
three legs. The resulting SWR curve is shown
below.
AIM-4170C antenna analyzer SWR graph
(40 meter 1/8 WL vertical loop antenna)
Click Here for SWR
GRAPH
We decided to
connect the antenna to our test radio system to see if it would work
on 40 meters. The test radio system consists of an ICOM 706 MKIIG
transceiver and a Palstar AT-500 antenna tuner. We were able to
tune the antenna for maximum power transfer using the AT-500. Now
for the moment of truth! We quickly discovered that an Alabama
statewide QSO party was in progress. With no trouble at all, we
quickly made several solid contacts with stations across Alabama. A
couple of days later we tested the antenna again and were able to check in
with ECARS (East Coast Amateur Radio Service) net control located in
Pennsylvania. Our test QTH is located in central Piedmont North
Carolina (Lat: 35.685106; Long: -78.502348).
What can
be said about the antenna? It works reasonably well and has a very
small footprint. The SWR curve displayed by the AIM-4170C indicates
that most automatic antenna tuners will successfully tune the
antenna. Why does the antenna work? Good question!
Obviously lowering the antenna so the bottom leg touched the earth
improved the SWR curve dramatically. One
possible explanation is that the antenna's impedance
characteristics were favorably altered when the bottom
leg touched the earth. From our perspective,
the antenna falls into the category of ANTENNA ODDITIES
requiring a lot more research and experimentation at some future
date.
If you're interested in loop antennas, check out
Joe Carr's book (ISBN: 978-1882123285).
If you decide to build the antenna, let us know
how it worked for you. Click on the call
signs below for further information and to send an Email if you have
questions.
73.
NC4FB
Published Courtesy of
KI4PMI &NC4FB
Feedback and additional information
about this article. 12-31-2011
It
is discussed in the ARRL Antenna book, 14th Ed, 1984, Chapter 2.
pages 27 and 28, on half-wave loops.
A
closed loop will have the maximum current in the side opposite the feed
point.
A
closed loop will have a high impedance at the feed point due to it being a
low current point.
Thus your antenna had radiation at 90 degrees as a NVIS antenna due
to its vertical orientation.
Had you left the opposite side from the feed open, mounted the feed
point up at the top, you would have seen a low
impedance at the feed point closer to 50 ohms. The signal would also have
been NVIS if mounted vertical.
The feed point SWR was reduced by virtue of adding the additional 4
feet of wire.
This is understandable as a closed loop has no end effect and thus
requires a longer wire for resonance.
Therefore, your final length is closer to 493/Freq. than 468/Freq.
Mounted horizontally, you would have noticed a small front to back
ratio and signals slightly reduced from a full length half-wave
dipole.
The half-wave loops are very useful for those with small gardens
and limited space. Closed loops mounted high with balanced open feed
line to a tuner work very well.
Open loops also work well and can be fed with coax or balanced feed
line and multiband with balanced line and tuner.
Thank you for bringing these small loops to everyone's attention. I
hope the above helps answer your questions.
Dr
Don Sanders W4BWS
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