Antenna Oddities (small footprint 40 meter vertical loop)

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 1/2 wavelength 40 meter rectangular loop antenna (1/8 WL per side) fed at the bottom with 300 ohm  twin-lead.  See ARRL's  publication Small Antennas for Small Spaces (ISBN 97800-87259-839-3) page 3-15.

         1/2 WL 40 meter loop antenna view

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 1/2 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.

40 meter 1/8 wl vertical loop radiation pattern

The EZNEC model can be downloaded from the link below.

EZNEC model 1/2 WL vertical 40 meter loop

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 producing an isoceles trapezoid.  The resulting SWR curve is shown below.

AIM-4170C antenna analyzer SWR graph (40 meter 1/2 WL vertical loop antenna)

40m 1-8 WL loop swr curve

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 (K4IPV, SK) book (ISBN: 978-1882123285).

If you decide to build the antenna, let us know how it worked for you.  Click on our call signs below to send an Email.

73.

KI4PMI & NC4FB

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