A Mobile, Horizontally-Polarized 70cm Antenna

I built a mobile halo antenna for 70cm. Here’s how I did it, and how well it worked.

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.

Component List:

  1. 10′ RG-8x coaxial cable
  2. 2x small hose clamps
  3. #6 copper wire for gamma match
  4. 1/4″ copper refrigerator tube
  5. Duct mastic
  6. 10′ 3/4″ PVC pipe
  7. 4 3/4″ PVC T connectors

 

wp-1528317578039.jpg
Close-up of the halo, showing the gamma match and mounting.

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.

Tuning Setup
Overview of the tuning setup. The HT provides a signal to drive the antenna, while the SWR/power meter measures the SWR.

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.

SWR
Results of the SWR testing. At 433MHz, the SWR is roughly 1.6-1.7:1. The topmost SWR line is the one to read, since the output power of the Baofeng F8HP power level is below 15W.

Field Testing

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).

Enter a caAntenna mounted on the roof of my car. I secure the antenna to my Yakima removable roof rack with zip ties.

 

 

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!

IMG_20180603_153528628.jpg
I-25 Southbound near Belen. That’s hail, not snow, on the ground.

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.

IMG_20180605_204609907_LL.jpg
Antenna post-hailstorm. Still looking good.

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.

Surveying the damage after having the car towed back home. Brake hose is cut (it got torn apart after being taken off the tow truck), caliper is cut into, and backing plate is bent.

RS-485: A Long Distance Serial Port

When you need serial communications over a long distance, it’s time to give a good look at RS-485 over TTL-UART or RS-232. You gain a lot of benefits. First, RS-485 can give you a very long distance (distances of a mile or more are possible if you keep the datarates low). Second, the cabling is simpler — RS-485 requires three wires, with two of them being a signal pair and one being a ground. Cheap Category 5 data cable is more than enough for this use, and you still have 5 conductors for other things, like supplying power.

The long cable length enabled by RS-485 enables allows for interacting with an RF transceiver at some remote location (like indoors) while keeping the transceiver near the antenna. Putting the transceiver near the antenna minimizes loss in the coax cable while keeping costs down.

I used a USB-to-RS485 adapter purchased from AliExpress (https://m.aliexpress.com/item/32714895507.html?); Price was $2.53 with shipping at time of purchase. This adapter claims to use an FTDI USB UART chip, and no issues were found when using it in either Windows (Windows 10) or Linux (Mint 18.3). Note that you need to supply your own USB Type A-to-B cable.

Top side of the board. For normal use, leave the jumpers in place, as they appear to be used to connect to the RS485 transceiver chip. Also note that there is a Tx and an Rx LED.
Backside of the adapter. Note the flux that hasn’t been cleaned off. (You get what you pay for!)

Introduction: Where Amateur Radio and the Maker Movement Meet

I decided to start this blog to discuss some of my amateur radio/Maker experiments. My goal is to show others what I’m doing, and to bounce ideas/discuss ideas with other people. Ever since I gained my Technician license in September 2017 (I upgraded to Extra in February 2018) I’ve found fascinating the intersection point where amateur radio and the Maker movement meet. While ham radio is typically stereotyped as being about making long-distance contacts over shortwave frequencies (typically at frequencies between 3MHz-30MHz), an amateur radio license actually allows its license holders to use frequencies throughout the radio frequency spectrum with far fewer technical restrictions (transmit power, antenna design, time on air, etc.) than Makers have in other unlicensed (or licensed) frequencies.

Maker culture and ham radio enjoy a lot of overlap. Amateur radio has a long tradition of experimentation, including with homebrew radio equipment as well as re-purposing existing radio equipment (both commercial and even some military-surplus radio equipment) for use on amateur frequency bands. Amateur radio can allow Makers e.g. longer range communications over clearer channels (the amateur bands, except for the HF bands, are typically less busy than the 915MHz and 2.4GHz ISM bands), and with higher datarates (e.g. Wi-Fi communications has legal limits on power levels and antenna gain, limiting its effective range).

The overlap between amateur radio is becoming more visible — many FPV (First-Person Video) drone systems require an amateur radio license to legally operate them, as they use higher transmit power levels to enable longer range. High-Altitude Ballooning endeavors use APRS (Automated Packet Reporting System) to convey telemetry information. Amateur radio clubs are appearing more and more at Maker Faires, and Makers are going to ham radio conventions to learn how to use amateur radio to help them make more awesome things.

Where do I fit in? I’m personally interested in both. I enjoy doing a great variety of DIY projects that range from electronics, home improvements, and wrenching on cars. The idea of being able to “play with” RF and do novel things with it has always fascinated me, ever since I had one of those electronics experimenter kits from Radio Shack and I could transmit at low power levels to a nearby AM radio. Having an amateur radio license now allows me to do much more sophisticated tinkering with RF than I ever could in the past (at least without incurring the wrath of the FCC).