Running the AH-4 Mobile

On this page, we'll discuss using the AH-4 for mobile operation. Note that most comments also apply to all tuners that are similar to the AH-4. Also please note that I have never actually operated mobile! I'd like to get around to that someday and as a result have been collecting information and documenting it here on this page. But until I get more time, I have had to rely on other hams for the practical experience that I lack.

One nice thing about running a web site is that I get a lot of E-mail from hams all around the world. Sometimes I get things wrong and they correct me. Sometimes they have information I was not aware of. Many of the times they ask questions. I hope that I will also get correspondence regarding this page. Consider this page to be a heavily moderated (by me) reference for mobile operation that has many, many contributors. In adding information to this page, I will be looking for facts, verifiable information, and good engineering practices. Rumors and unverifiable opinions will be so indicated.

Good Mobile Engineering Practices and Observations

  1. Get the power to the rig: Run large gauge wire (typically 8 gauge or larger) directly from the battery to the radio. Use a fuse at the battery. Run both positive and ground conductors. Why? Assuming you're using 100 watts, the radio may use as much as 20 amps. If there is a significant voltage drop to the radio, circuits within the radio may not operate correctly. For example, if the IC-706 input voltage falls below 10.5, the PLL may loose lock and the radio will start to change frequency. When the engine is off and you have been using the battery for a while, you can't afford much loss in the power connection!
  2. Think of the vehicle as being 1/2 of the antenna system. (The antenna is the other half.) Look at it this way, the vehicle is 1/2 of a dipole, the antenna is the other half. Where ever you make the electrical connection to the antenna, make sure you get a good ground to the vehicle. Also make sure that the metal you grounded to is grounded to the rest of the car!
  3. Grounding: Just because your car is made of bits of metal, don't assume that all metal bits are electrically connected together - at least from an RF point of view. You may need to electrically bond various components of your vehicle together.
  4. Get the antenna up in the air. If the base of the antenna on your van is mounted on the trailer hitch, that section of antenna that is near the van will interact with the metal of the vechile. Imagine you built a dipole, but at the center feed point instead of running the wires out away from each other, one side of the dipole ran parallel to the other for half of its length. How well would that dipole work? That's what you're doing when you run the antenna near the metal of your vehicle.
  5. Place the tuner's output connection within a few cm of the feed point of the antenna. And by that I don't mean 100 cm! Any wire you have running from the tuner to the antenna is part of the antenna system.
  6. Ground the tuner. The AH-4 mounting brackets are not grounded! Use the grounding terminal and make sure you have a good, solid ground.
  7. Make the antenna as long as possible. A 2-3 meter whip (7-9 feet) will almost always work well on the higher frequencies such as 20 through 6 meters. As the antenna gets electrically short (at lower frequencies) the design of the antenna system becomes more and more critical. It would seem to me to make sense to put a capacitive top hat on the antenna to improve tuning on the lower frequencies. Has anyone tried this?
  8. The impedance of the antenna can get real ugly as the frequency goes down. EZNEC tells me that a 3 meter whip on 10 meters over a perfect ground has an impedance of 60.41 + J 81.39 ohms. Not bad. The AH-4 can certainly live with that. At 20 meters the impedance becomes 9.637 - J 225.7 ohms. Getting ugly, but still well within the range of the AH-4. At 40 meters the impedance becomes 2.185 - J 609.8 ohms and 80 meters is 0.6297 - J 1189 ohms. Now that's real ugly! Icom suggests a minimum 7 meter antenna which would result in an 80 meter impedance of 3.577 - J 544.8 ohms. That gives us an idea regarding what the AH-4 will tolerate. I have to admit it would be impressive to see a mobile with a 7 meter (23 foot) antenna! All this says is that on 80, and perhaps 40 meters, it may be necessary to give the AH-4 a little help. Even if the AH-4 will tune a 3 meter whip on 80, the losses in the tuner may be large enough that you won't be radiating much.
  9. If you add a loading coil, use a large diameter conductor. Keep an eye on the spacing of the windings and look out for self resonance caused by interwinding capacitance. Your coil may look more like a trap at certain frequencies! Keep "stuff" like the metal body of the car, away from the coil.
  10. Get away from "stuff" when tuning your antenna. The mutual coupling of two mobile rigs parked near one another is impressive. At least get away from houses, buildings, antenna farms, etc. when tuning.
  11. Antennas do not have to be "resonant" to be efficient. Non-resonant antennas can be inefficient (as can resonant antennas), but that is because they were designed that way. An AH-4 installation can be as efficient as any other, but there are tradeoffs. A center loaded antenna can result in a more reasonable impedance to match, but then you have a big coil exposed to the environment. This may be the best solution if you want to operate on 80 and don't want to build a matching network between the tuner and the antenna. So here are the tradeoffs as I see them:
    AH-4 easily change frequency/band, works great 20-6 meters. Needs help matching 80-40 on short antennas, probably too much trouble.
    Center loaded whip antenna: works great (as far as mobile goes) on all bands, but not easily tuned to different band/frequencies. BUC (Big Ugly Coil) exposed to the environment.
  12. Don't expect the mobile installation to work as well as a base station. At best you'll be more than 10 dB below a well designed base installation. But that doesn't mean you won't make plenty of contacts! Folks running QRP work the world all of the time.