Several years ago, I had the good fortune to be introduced to Gene Bowman, WB4MSG, at the Winston-Salem Hamfest. I recognized Gene’s callsign because of his top place finishes in many contests. I was particularly interested in what kind of antenna he used on Top Band.
Gene told me that if I had a few minutes, he would be glad to have me come by his house so I could see his antenna farm. I was fascinated to learn that Gene had placed 1st Place QRP World during the recent CQWW160M SSB Contest with his 160M loop. Gene mentioned that a full write up of his 160M antenna appeared in Jan 2010 QST (160 Meter Inverted Delta Loop by Charles Kluttz, W4TMR, pp.40-41).
I built my own version of Charlie’s 160M loop shortly thereafter and was quite pleased with its performance. The vertically polarized loop showed noticeable gain over my high 160M dipole for stations beyond my local area, and after a short while I found myself no longer using the dipole.
As a result of a severe lightning storm in August 2011, all my antennas were damaged to the extent that I had to start all over from scratch. Fast forward to Jan 2016, it was high time to get back in the game. With the help of two local QRP friends Chris N4PBQ, and Dick N4HAY, plans were hatched to resurrect the Excalibur Antenna Farm, beginning with a Top Band antenna. It was decided to rebuild a version of the W4TMR Vertical Loop.
Chris, N4PBQ digging the hole for the support post
Recent logging on the property made it a bit challenging to find two tall trees that were suitably spaced from one another to support the proposed loop. I was initially disappointed to find that the two ideal trees our team had selected were not spaced far enough apart to accommodate the original loop. After spending a couple hours with EZNEC, a revised loop model with a suitable fit was found. Fortuitously, the feedpoint impedance of the new model was very close to 50 ohms.
The new Excalibur 160M Vertical Loop is rectangular in shape, as can be seen in the EZNEC antenna view below. The top horizontal section and bottom horizontal section are each 80′ long. Each vertical section is 60′ long. The bottom horizontal loop wire is cut in half in order to feed the antenna. One end of the loop wire attaches to the elevated counterpoise and coax feedline shield, and the other end of the loop wire connects to the coax feedline center conductor.
As can be seen in the elevation plot below, this loop provides good low angle radiation, which is ideal for working DX. It is also not an issue to work local stations with this antenna. Most local stations (NC, SC, VA) are S9 with this loop, so I no longer find it necessary to augment the loop with a 160M dipole.
The following azimuthal plot shows that the radiation pattern is omnidirectional within 1dB.
The counterpoise consists of 25 wires, with each wire being 85′ long. The average height of each wire is 8′. Each counterpoise wire is attached to a common ring, as can be seen in the photo below. One end of the vertical loop also connects to this common ring. The coax feedline shield also connects to this ring.
The elevated counterpoise contains over 2,000′ of 14 gauge aluminum fence wire. The far end of each counterpoise wire is tied to a length of paracord, which is in turn tied off to a suitable tree.
Dick, N4HAY tying off the far end of a counterpoise wire
The white PVC pipe above the counterpoise ring allows the other (hot) side of the loop to be supported. This loop wire is fed through this pipe and attaches to a small inductor.
The inductor’s job is to resonate the antenna system. The orange wire nut was used to allow the proper inductance tap to be selected to resonate the antenna system at the desired frequency. The inductor will be housed in a suitable weatherproof box as time permits.
The loop was self resonant at 1885 kHz. By selecting various taps on the series inductor (FT125K material), tuning the system to resonance was straightforward, resulting in a final resonant frequency of 1835 kHz. The loop’s feedpoint impedance was so close to 50 ohms that it was easy to achieve low SWR and ideal bandwidth (2:1 SWR approx 85 kHz), so no antenna tuner will be necessary to work the entire CW portion of the band.
This plot from the AIM4170c Antenna Analyzer provides a wealth of information and closely coincides with the EZNEC model in terms of feedpoint impedance and bandwidth:
(click on image to enlarge)
The feedline from the shack is attached to a homebrew choke balun. This choke’s job is to ensure that no common mode current flows on the outside of the feedline to the shack. The measured reactance of this choke to common mode current is > 5,000 ohms at 1.8 MHz! This choke uses a 2.4″ O.D. Type 31 ferrite toroid. As can be seen from the following photo, homebrewing a suitable choke balun is not that difficult:
N4PBQ, N4HAY, and I were well on our way toward completing the 160M antenna project with a bit of time to spare before the CQWW160 CW Contest two weeks hence. Then along came Icemageddon 2016, which left the vertical loop touching the ground and the carefully tensioned counterpoise looking like this:
The ice loading was breaking tree limbs all around and adding significant weight to the counterpoise wires and the supporting paracord:
Fortunately, the heavy duty 14 ga. aluminum fence wire and the paracord were up to the task. All but two of the counterpoise wires retensioned themselves to their original tension after the sun melted the ice three days later.
Despite all this antenna work, we had not had time to actually hook the antenna feedline to one of our rigs for a test drive. We were most curious to see how it would play!
We will address that very interesting story in our next blog sometime next week. Stay tuned for Excalibur vs CQWW160 CW.