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
1 Voltage = 1953 V at -89.59
However, the predicted antenna radiation pattern looked promising as seen below.
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)
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 ofANTENNA 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.
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 haveseen 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
Hamuniverse.com uses Green Geeks Web Hosting!