NVIS Tests of Three 40-Meter Antennas by Bill Savage, K3AN
The July, 2000 issue of QST Magazine contained an article by N6BT titled “Everything Works.” The author laid out his ideas on the relative performance and enjoyment factor of different antennas, from a light bulb on a 4-foot post to stacked Yagis on a tall tower. He related his own experience as a newby ham, thinking his first antenna was pretty good until he put up his second one, and so on. He also described his light bulb antenna, mounted on the wooden post, with a ferrite bead choke balun to minimize feedline radiation. With that light bulb and a 100-Watt transceiver he actually completed contacts to every continent on 10 Meters. “Everything Works” indeed!
I’ve had some epiphanies of my own concerning antennas. I still remember getting active once again in early 1980’s, with a used Knight R-100 receiver and T-150 transmitter. Upon bringing them home I was so anxious to test them that I stapled 33 feet of wire to the basement ceiling joists and ran a ground to the nearby copper water service entrance pipe. With the T-150’s pi-network I was able to load that wire and actually made contacts on 40-Meter CW (at night) as far as 500 miles away, much farther than expected. The antenna was near an outside wall, but it was no more than four feet above the surface of the ground outside.
Another epiphany occurred when I found out how poorly a 130-foot inverted L performs on 17-10 Meters compared to a smaller and slightly lower delta loop antenna. If I had never put up the delta loop I would have remained fat, dumb and happy with the L, which I had originally decided to use since it’s one of L. B. Cebik’s, W4RNL (SK) “five favorite backyard wire antennas.”
In some recent conversations and emails with local hams, I became interested in antennas for NVIS communications. Searching the web got me to some sites that attribute almost magical properties to very low antennas for short-range communication. Does a really low dipole outperform a higher antenna? There was only one way to find out. I borrowed some items to construct one new dipole, and pressed our Field Day 40-Meter dipole into service as well, so with my Daiwa 4-position coax switch I could quickly switch between them as well as my existing Inverted L. I present the three candidates below.
The Inverted L is a 130-foot insulated 18-gauge “stealth” wire running vertically up from near ground level about 60 feet into the top of a tall oak tree, and from there horizontally over to another tall tree. It is tuned at the base with an SGC-230 remote autotuner, with a ferrite choke balun on the feedline and the DC wires. There are seven insulated radials ranging in length from 25 to 50 feet, “buried” under a few inches of mulch (no grass on my heavily wooded lot). The total length of coax back to the coax switch is 62 feet. With the tuner, this antenna works on all bands 160-10 Meters. The SWR is 1.6:1 or less throughout the 40-Meter band, again thanks to the remote autotuner.
The traditional dipole is 66 feet of 14 gauge bare copper, stranded antenna wire, supported at the center by a tall pine tree. The feed point is 30 feet off the ground. The wires slope slightly downward a foot or so, toward support trees beyond either end. There’s a ferrite choke balun at the feed point. The total length of coax back to the coax switch is 94 feet. The SWR measures 1.3:1 at the bottom of the band, is 1.2:1 at 7.05 and 7.1 MHz, and rises to 1.8:1 at the top of the band.
The NVIS dipole is about 65 feet of insulated 16-gauge wire (one conductor of a zip cord pair) supported at the center and beyond each end by tree trunks, seven feet off the ground. This antenna also has a ferrite choke balun at the feed point. In accordance with the article that described this antenna, I placed three 70-foot runs of insulated 16-gauge wire on the ground in parallel with the antenna wires. One of the three wires is directly under the antenna and the others are eight feet either side of the first one. The total length of coax back to the coax switch is 113 feet. The SWR on this antenna was remarkably low, measuring 1.3:1 at the bottom of the band, 1:1 at 7.05 and 7.1 MHz, and rising to 2.4:1 at the top of the band. To be honest, I was both surprised and puzzled by the low SWR at resonance, but that’s what it was.
All coax is RG-8X. SWR measurements were made at the shack end of the cables. Even a hundred feet of RG-8X doesn’t have very much loss at 7 MHz so I wouldn’t expect the SWR measurements to be very different at the feedpoints. Only the coax run to the Inverted L is weathered (almost seven years now). The coax to the two dipoles has only been outside for a couple of Field Days. Over the course of several days I made 35 measurements of received signal strengths on stations ranging from 55 miles to as far away as 650 miles. I also contacted 14 of those 35 stations and asked them for reports from my end. In all cases the three antennas were identified to them as A, B and C, to try to prevent “confirmation bias” (Google it) from influencing the results.
Typical daytime 40-Meter QSB, while not as great as the fading on nighttime DX paths, still offers a challenge to making accurate comparison readings. Once I saw that the dipole at 30 feet was providing the strongest signals, I put its coax in the middle of three positions on the antenna switch. Then I would switch back and forth repeatedly between the two dipoles until I saw a pattern. This process was repeated between the dipole at 30 feet and the L. I used stations checking into South CARS for some of the measurements, and in some cases they transmitted for too short a time for me to accurately read the three antennas. Those stations were discarded from the results. By the way, I found that watching the bargraph S meter display on my Icom Pro 3 was a lot easier than watching the twitching D’Arsonval panel meter. Also, the Daiwa coax switch survived the several hundred switch position changes just fine.
So How’d They Do?
The dipole at 30 feet, which I will hereafter call the reference dipole, was the clear winner. Only three of the 35 stations had signal strengths on the L that were the same as on the reference dipole. The other 32 were less. None of the 35 received signals on the low dipole matched the signal strengths on the reference dipole. Also, all 14 respondents said the reference provided the strongest signal. Between the low dipole and the Inverted L, the received signal on the L was stronger 15 times, the two were equal 10 times, and the low dipole was stronger 10 times. Statistically that has to be considered a draw.
Compared to the reference dipole, the low dipole was always one to three S-units lower. The Inverted L was in that same range of one to three S-units most of the time, but there were three signals that were equal to the reference dipole and three signals that were four S-units down. The low dipole was as much as three S-units better than the L on the close-in stations (under 120 miles), but still not as good as the reference dipole. I have to conclude that, although it doesn’t put out the strongest signal, the low dipole works remarkably well for NVIS communication.
One of my pet peeves is seeing a review of a new antenna that uses mostly superlatives to describe it. OK, but what did you compare it to? A dipole at 30 feet? A triband yagi at the same height as your new Whiz-Bang Mark VI? You compared it to nothing else? Then how do you know it’s really performing well? After all, a light bulb was once used to complete the WAC award on 10 Meters. Yet no one would claim that’s a great antenna.
Oh, you used to have a different antenna and the new one seems to work better? Were the SFI and the A and K indices the same when the old antenna was up? How about time of day, and QRN and QSB conditions? My point is that unless you can instantly switch back and forth between the antennas being compared, your results should be taken with a big grain of salt. You also need to run a lot of tests, and record the results of each.
Lacking this capability, trying to work through a DX pileup is a pretty good way to determine how well your one-and-only antenna performs. Can you usually or often crack the pileups with just a few calls, or do you always have to wait in line, calling repeatedly and maybe never getting through?
You’re not a DXer? You can run the same test with any of the special event stations that pop up on the bands nearly every weekend. The recent Original 13 Colonies stations attracted some moderate pileups. How long did it take you to get through? Also, these stations usually give honest signal reports; not everyone is “five-nine.” How did your report compare to other reports the station gave out? Listen for a while and write them down.
But like me with my Inverted L, you still don’t know if something else would work better without putting up a second, different antenna, and testing the two. As the QST article says, “Everything Works,” even a dipole seven feet off the ground. It can be a lot of fun, as well as quite educational, to determine for yourself what works better.
K3AN