80M Grounded Half Loop
Hams often ask me to describe my 80 meter grounded half loop. The physical arrangement of the antenna is difficult to describe, especially on CW. But the idea for the antenna is relatively simple and came from an article in a 1984 QST by Jan Gunmar, SM0AQW1. In that article, Jan described the physical layout of his property and the difficulties he had finding a functional, multi-band HF antenna that would fit on his property. Since his yard was similar in dimensions to mine, I was interested in his solution and how I could adapt it to my situation. Below is a description of my installation.
A sketch of my antenna is shown below. Basically, it's a half wave length wire on 80 Meters, 132 feet. The wire is grounded at the far end and fed against ground at the feed point (see below). Although the end of the antenna and the feed point are both grounded and therefore "connected", I ran a wire from the far end ground point back to the feed point and included that wire in the antenna ground system. Several radials were placed at the far end and at the feed point. Since my yard is relatively small, the radials are not long. I have six radials on the far end, two at quarter wave lengths for 80, 40 and 20 (66, 33, and 16 feet, respectively). At the feed point, I have 17 radials of various lengths, going as far as I could toward the house and the property lines.
The sketch shown is an idealized version of the antenna. In reality, my installation is not in the clear. The initial 33 feet from the feed point is supported by a 36 foot push-up pole. Although I took pains to keep the wire separated from the pole by at least 3 feet using a PVC spacer, the wind undoubtedly moves the wire closer at times. In addition, immediately behind the push-up pole, about 3 to 4 feet, is the back wall of my house. The 59 foot horizontal section angles up about 10 degrees and goes to an egg insulator hanging in a Sycamore tree. The wire then runs down though some of the branches of the tree to an 8 feet ground rod, which sticks up from the ground about 14 inches. My point is this: Don't be discouraged if your yard situation is less than ideal. You may be surprised at the performance you get from such a compromised installation (see below).
One point on safety: The far end of this antenna extends down to the ground. If you're running any more than QRP power, please take care to protect people and pets from any part of the antenna that they can reach.
The Feed Point
This antenna will present an acceptable SWR over a relatively small portion of the 80M band (see below). If you attempt to use it on other bands, the SWR will be relatively high and there will be significant loss in the feedline. There are several ways to solve this problem. One is to use an open wire feeder, something like 450 ohm twin lead. Another solution, and the one I chose, is to tune the antenna remotely at the base. I used the SGC-237 remote antenna coupler. In my case, the antenna coupler is my feed point. I made the SGC-237 ground continuous with the radials at the "feed point", which in turn are continuous with the ground radials at the far end. Coax extends from the SGC-237 to the shack, a distance of approximately 85 feet. I'm able to tune all bands from 80 to 10 Meters, including 60M, with an SWR of less than 1.4:1.
The table below will give you an idea of the electrical characteristics of the antenna at my location. They differ considerably from those published by SM0AQW for several reasons. First, Jan used a HP8407A network analyzer, while I used a MFJ-259B (enough said on that point). In addition, the physical length and materials of Jan's installation differ from mine. I couldn't fit an electrical half wave length of wire in my yard and had to use about 127 feet of wire, an electrically shortened length. Finally, the surrounding terrain almost certainly differs at our locations. My antenna was in close proximity to my house and large Sycamore tree in my yard. There are probably other reasons, but suffice it to say, our measurements differ.
80 and 40 Meter Bands2
30, 20, 17 and 15 Meter Bands3
What is apparent from the numbers is that the antenna is slightly too short. Remember, I had to use 127 feet of wire, rather than 134 or 135 feet. The shortfall is seen in the impedance measurements. Had the antenna been slightly longer, the better match, shown on the top end of 80 and 40 meters, would have been lower in frequency, closer to the CW portions of the bands. The lowest SWR occurs at about 4.0 Mhz.
A full wave loop in free space is approximately 120 ohms. A half way loop would be expected to be approximately 60 ohms. Notice that the present antenna ranges from 33 to 108 ohms resistive on 80 and 40 meters. Resistances are higher on 15, 17, 20 and 30 meters. Providing a good match to 50 ohm coax on all these bands would ordinarily be a challenge, but remarkably, the SGC-237 antenna coupler succeeds.
My main ham activity is rag chewing on CW. I do almost no contesting or DX'ing. I work most 80, 40 and 30 Meters. I live in a densely populated area of town, surrounded by malls and two hospitals. I've been using this antenna for nearly 9 years and I must say that it's probably the best all around antenna I ever used at this location. I wish I had some way to prove that statement using numerical data, but I don't. I feel subjectively though that I've gotten more good reports with this antenna than any of my previous wire antennas. My last antenna, a 102 foot doublet antenna fed with 450 ohm ladder line, was a good performer, but I believe this grounded half loop is better. I notice that I get better signal reports and hear stations better when they are using a vertically polarized antenna. I've worked more DX in the past 9 years than I have in the past and not because I tried. I hear a DX pile up and usually move the opposite end of the band. If by chance I hear a DX station calling on my frequency without a pileup, I will often give a causal call. The DX station will usually respond.
My only question regarding the grounded half loop has to do with noise. Loops are generally quiet antennas, and although the grounded half loop is a closed antenna system, it is fed against ground like a vertical. I have no data on how this antenna compares with the doublet or full wave loop fed with open wire as far as noise is concerned. Since noise is so prevalent at my location, the relative noise of the antenna becomes an important consideration. I have plans to compare the noise on this antenna with a doublet. Results will be reported here.
I hope that the information provided above helps someone contemplating a ground half loop antenna.
1Jan Gunmar SM0AQW, "The Half Delta Loop Goes Rectangular!" QST, p. 26, 1984.
2Measurements taken with a MFJ-259b Antenna Analyzer. QRG = frequency in Mhz, SWR = Standing Wave Ratio, Real R = resistive component of impedance, imaginary (jX)= -, capacitive or +, inductive reactance.
3Same as note 2 above.