Properties of Vertical Hustler Whips with Multiple MO-3 Mast Sections

I've generally avoided using vertical antennas because of the difficulties associated with achieving a good ground, but I also wanted to make a good portable antenna for 17 meters. A recent ham friend by the name of Rick, mentioned that he was experimenting with verticals for similar reasons, and had constructed a vertical using two Hustler MO-3 sections and an additional resonator. He was also interested in having a portable antenna, as he travels extensively, and wanted to be able to operate from a number of locations. So I decided to see how these vertical perform. I used a photographic light stand as a main support and purchased another Radio Shack mirror mount to connect to the top of the light pole. However, I insulated the pole from the mount so it would not be a major player in the grounding. I began by making four quarter wave ground radials with 1/4" lugs at one end and egg insulators on the other end. The wire I selected was also from radio shack an was intended as flat speaker wire. I split the wire so that I got four radials form each roll with plenty left to spare. The lugs were connected to the quarter inch bolts on the mirror mount and stretched as guy wires to help support the lamp pole. I used nylon rope to connect to the egg insulators and stretched this to plastic tent stakes that can be purchased at any camping supply store. My first experiments with this antenna were not very encouraging. I called CQ for nearly an hour without an answer. Maybe the antenna is working but no one wants to talk to a K6 in Los Angeles. Before I insulated the lamp pole, the base impedance was 40 ohms at resonance, so it was not a bad match. After insulating the base pole, the impedance went up to 50 ohms for a perfect match, but how much power was being lost in the ground, how much of the 50 ohms was ground resistance? Surely some part of it was ground loss. The SWR meter actually indicated a 1.2: 1 reading at 18.140 MHz, so things were not exactly perfect, but not far from it. The next day, I added four more ground radials and rechecked the impedance. The impedance at the base was now 50 -j10 indicating that the antenna was capacitive or a little short. The best SWR was at 18.089 MHz, so I added 0.5" to the length of the stinger. This made the match poorer and gave an SWR of 1.3: 1. Despite this, I decided to see what I could work. I got a few quick signal reports form Denver, CO, Rainier, WA, These were between 57 and 59, so the antenna was working. I then proceeded to work AH6ASW/KH2 in Guam with a 52 report, JH7NNW in Japan with a 58 report. So I kept going for a while, and worked several other stations in WA. Then I heard ZL2ACE and ZL1AXB talking as the band was beginning to close, and I broke in on them. I got 55 reports from both of them, so I didn't feel so bad about this vertical. This still doesn't answer all the questions about the efficiency, but it does show that enough power is radiated to work a little DX when the conditions are good. So here are the dimensions and specifications for this antenna.

The lamp pole is set to be about 5.5 feet high. The two MO-3 mast parts and the coupler are 9' 1.5" long. I used a 15 meter resonator with the stinger set to a length of 21.5" as measured from the base of the loading coil. Each ground radial was cut to 13' 4", and the 4" were stripped and used to connect the egg insulators. The tie ropes were typically about 3 feet long, but some were tied to a fence and a few to trees, as my yard isn't really large enough to set this up perfectly. Most of the radials slope down at about 20 degrees, but those to the south side where nearly horizontal. So the base impedance should be somewhat higher than the 35 ohms expected for a perfect quarter wave vertical above a perfect ground plane. Obviously, this shortened vertical should be even lower in impedance, however, it really isn't that much shorter and is nearly top loaded.

Some further measurements are needed to fully understand this antenna, but progress stopped due to a few rainy and cold days in LA. I spent several mornings trying to work Cape Town, SA, at 16:00 Hrs UTC with no success. Well, I really couldn't hear anybody either, so I guess that should be expected. Still, the base impedance is not far off from the theoretical value for a vertical with radials sloping at 20 degrees.

I adjusted all of the radials to slope down at 20 degrees while it rained, and this raised the base impedance to 75 ohms and made the antenna slightly short. I re-adjusted the stinger to add another 1/2" which put it back in resonance at 18.110 MHz. The antenna now has a poorer SWR than before, with an SWR of at the transmitter of about 2.0: 1. It also matches better at lower frequencies where the impedance is lower and the reactance is capacitive. So it looks like the radials need to be nearly horizontal. More to come....

Detail of Vertical Mount and Antenna with Four Radials

17 Meter Portable Vertical Antenna System

Photo of the Full Antenna with Dual Hustler MO-3 Masts and 15 m Resonator