When you’re trying to receive an RF signal while on the move, you need an antenna with an omnidirectional beam pattern in the horizontal plane. Usually, you use some sort of whip/vertical antenna for this task. Problem is, antennas like these are for vertically polarized signals. When you’re doing weak-signal work, for various reasons, you want to use horizontal polarization.
Enter the halo antenna. The halo antenna provides both, and at VHF and higher frequencies, the size is quite reasonable. While some commercial halo antennas exist, hams seem to typically build their own.
The basic design for this Halo antenna draws heavily from KR1ST’s design (http://www.kr1st.com/70cmstack.htm). I have adapted the design in several key ways.
One halo instead of four. To make sure this antenna can handle the wind load of being on a car at 75-85mph, sticking to one halo element keeps the antenna assembly from getting too tall and suffering under too much wind load.
Gamma match/center conductor mount. The connections for the feed-points are attached using small hose clamps instead of being soldered. This allows for easily tuning of the antenna by moving the position of the connections.
Halo mount. Halo element is attached to the PVC pipe by inserting the ends inside of a PVC pipe “T” connector. The halo is then secured using duct mastic. Also supporting the halo element is the coax itself, which is attached to the main “mast” using three nylon zip ties.
Building the Antenna
Halo antennas are reasonably easy to build yourself. The first step is to cut a piece of copper tubing at the appropriate length and bend it into a circle. I had the most success getting a good, clean bend by heating the tubing using a propane torch. Next, build the gamma match by bending an appropriate length of #6 bare copper wire. Attach both to the halo using small hose clamps. Solder the center conductor to the end of the gamma match.
- 10′ RG-8x coaxial cable
- 2x small hose clamps
- #6 copper wire for gamma match
- 1/4″ copper refrigerator tube
- Duct mastic
- 10′ 3/4″ PVC pipe
- 4 3/4″ PVC T connectors
Tuning the Antenna
Since I don’t have an antenna analyzer or a network analyzer for 70cm, I tuned the antenna using an SWR meter (specifically the Signstek Professional UV Dual Band SWR and Power Meter) a spare Baofeng F8HP HT. To be cautious, I would recommend using an HT you’re willing to part with, since there’s a small risk that you’ll burn out its power amplifier if the SWR is too high.
After some painstaking tuning, which included both adjusting positions of the two feedpoints as well as the circumference of the halo and the separation distance between the two open ends, I attained an SWR in the range of 1.6-1.7:1. While not ideal (generally, people strive for SWR’s below 1.5:1), it’s good enough for these experiments. Using the coax/SWR loss calculator at (http://www.qsl.net/co8tw/Coax_Calculator.htm), a 1.7:1 SWR loses approximately 0.1dB versus an ideal 1:1 SWR.
To test out this mobile antenna, I put on top of the roof of my car. I then drove south along I-25 in order to listen to incoming signals transmitted from a 15-element 70cm Yagi antenna mounted on the roof of my house (I’ll discuss this antenna setup in a future post).
The weather was…rough. Down around Los Lunas through Belen, there was series of major storms. These storms included not only rain, but a significant amount of hail. Well, the antenna’s durability will definitely get tested now!
Overall, the antenna did great. It continued to receive while moving at highway speeds, in the rain, and even while hail was pouring down. It survived serious hail and rain without any damage or even having any of the antenna elements thrown out of alignment.
My car, however, did not do so great. When I pulled over during one of the heavy downpours, I heard thump-thump-thump from the rear passenger-side wheel area. I pulled over, and checked the tire. It looked fine. I started to drive again, and heard a horrible grinding sound. I pulled over again, and looked more carefully. I discovered that the brake caliper had detached itself from the knuckle. At point, I had a good friend rescue me, and I left the car behind. I returned the following day, and had a tow truck tow the car back to the house.