When you want to transmit on different frequencies but only have the option to set up a vertical antenna, there are quite a few options. For instance, there are multiband vertical antennas with electrical adjustments using coils and capacitors. These tune the antenna to specific frequencies. A less common approach is to use an automatic tuner to adjust the antenna at the feed point.
Tuning at the Feed Point
When you tune an antenna from your shack, losses in the coaxial cable can significantly increase. This loss can be avoided by tuning the antenna at the feed point, where the antenna radiates. However, using a manual tuner at the antenna is not very practical. Automatic tuners designed for shack use are not built to operate outdoors. The solution is a so-called remote autotuner.
Often Used Professionally
Remote autotuners are frequently used professionally for HF. For example, in the maritime industry, long vertical antennas are used to tune across various shortwave frequencies. Military units also use automatic tuners to quickly adjust (wire) antennas.
A Series of Coils and Capacitors
Such an autotuner essentially consists of a series of coils and capacitors, switched using a series of relays. A microprocessor ensures optimal tuning (usually SWR < 2:1). When the tuner detects a signal from the transmitter, it adjusts the antenna to the operating frequency. The tuner saves the tuning points in its internal memory, allowing frequency changes to occur in a fraction of a second.
These tuner usually operates on 12–15 volts. Depending on the model, it may receive power via a bias tee over the coaxial cable or directly through a two-wire cable.
Popular Remote Autotuners
There are several manufacturers and models of remote autotuners. The most commonly used models for ham radio are:
- CG Antenna CG-3000
- ICOM AH-730
- Yaesu FC-40
- SGC SG-230
- MFJ-926B (discontinued)
These tuners are largely similar in specifications. Some can handle higher power levels, while others offer a wider tuning range.
Easy Installation
Installing such a tuner is very straightforward. You simply mount the tuner to a mast, wall, fence, or tree. Then, attach the radiator (a wire or long metal pipe) to the tuner’s output. Next, connect the coaxial cable and, depending on the model, the control or power cable.
The aluminium plate protects the tuner against solar heat and offers some counterpoise and shielding.
Counterpoise
Like any antenna, an antenna with a remote autotuner also needs a counterpoise, such as a metal surface—the larger, the better. A network of radials spread across the ground works well too. For example, 12 radials, each 4 meters long. If the antenna is elevated, you can use radials that are a quarter wavelength of the longest operating frequency. Do you have a metal roof? Then you’re in luck! It’s an almost ideal counterpoise. While a single wire can suffice for the tuner to work, it won’t contribute to optimal radiation. Connect the counterpoise to the ground terminal on the tuner.
Decoupling the Coax and Power Cable
What many remote autotuner manuals fail to mention is that the tuner also uses the coaxial shielding and the negative wire of the power cable as a counterpoise. This often results in RF feedback in the shack, as these cables (and everything connected to them) become an active part of your antenna. Decouple these cables using ferrite.
Common Mode Choke for Decoupling
For the coaxial cable, you can purchase a ready-made common mode choke, which should be placed close to the tuner. For the power cable, you can use an FT240-31 or FT240-43 ferrite core and wind the cable around it 12 or 16 times, respectively. A series of clamp-on ferrites (material 31) also works on most cases.
Avoid Certain Lengths
Most remote autotuners can tune a radiator as short as 2.4 meters on 80 meters. However, for these longer wavelengths, the rule applies: the longer, the better. Avoid half-wavelengths, as they have high impedance, leading to high voltages in the tuner that can eventually damage it. Commonly used lengths are 4.5m, 6.4m, 9.4m, and 13.2m, which avoid being a half-wavelength on any amateur band.
Will It Tune?
The fun of an automatic tuner is that it can tune almost anything. In the absence of proper radiator material, it can even tune a metal gutter (if electrically isolated), a ladder, a clothesline, a bicycle frame, or a bed spring—with surprisingly good results at times.
Real-Life Experience
Since 2014, I’ve been using the CG-3000. Initially, I used it with an elevated vertical radiator, 9.4m long, on a flat roof 6m high. As a counterpoise, I used 14 radials of various lengths angled downwards. The CG-3000 could tune this setup from the 10-meter band to the 160-meter band. Tuning is quick, often within a second, although it takes longer on 160 meters.
The performance on 160 and 80 meters wasn’t great. Later, while exploring the 60-meter band, I extended the radiator to over 13m and used quarter-wave radials for that band and 80 meters. This resulted in my first DX contacts on those bands.
Minimal Counterpoise Effort
Currently, the CG-3000 is mounted against a wooden shed in my garden. From there, a 17m wire runs to the top of a mast, with the last 2m of the mast also serving as a radiator. I haven’t put much effort into creating a robust counterpoise, which impacts radiation efficiency. Achieving DX on the lower bands remains elusive. Improving the counterpoise in my small garden is a project for 2025.
Lightning Strikes
A tuner is a delicate piece of electronics, as I’ve experienced twice in quick succession. A lightning strike near my home (20–50m away) caused the electronics to fail both times. In both instances, the tuner was electrically grounded via the coaxial cable and power cable. The accumulated electrical charge flowed to the ground through the tuner, damaging it. If you plan to use a remote autotuner, disconnect the coax and power cables to prevent damage to the electronics.