Mobile Antenna Notes

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The majority of HF mobile antennas are electrically short. For example, an 80 M mobile antenna 8 feet long is electrically 11° in length as opposed to 90° of a full-sized 1/4 wave vertical. Thus, the input impedance is around 15 to 25 ohms depending upon loading coil position and Q factor, and ground and resistive losses. The formula is as follows:

Rt=Rr+Rc+Rg

Where Rt=total or input resistance (more correctly input impedance), Rr the Radiation resistance, Rc the coil resistance, and Rg the ground loss resistance.

We have some control over the radiation resistance, but for our example above, the Rr is under 1 ohm! Since it is a factor of the electrical length of the antenna (loading coil position is also a factor), we must lengthen the antenna to increase it. Obviously, there's a limit here. After all, who could drive around with a full 1/4 wave, 61 foot, 80 meter vertical on their vehicle?

The Rc is the resistive component of our loading coil, which cancels the high capacitive reactance of our short vertical. On 80 meters, the coil will have between 75 to 200 uh of inductance depending upon where in the antenna it is located. The higher up the mast it is located, the higher the radiation resistance, but the larger the coil (inductance) needs to be. There is a trade-off limit, however, because the larger the inductance, the greater the resistive losses of the coil. The reactive resistance versus the resistive losses determines the "Q". The higher Q, the less loss, and the more efficient the antenna will be. On 80 meters it is difficult to obtain Qs much over 200 and even this much requires good construction practices. The ratio between diameter and length to maximize Q is 2:1. Imagine running around with a spare tire-sized loading coil!

The Rg or ground loss resistance typically varies between 2 and 10 ohms, but can be much higher. It's mostly a factor of the size of the vehicle our antenna is mounted on, and how and where it is mounted. Remember our vehicle is not a ground plane for the lower frequencies, but rather a capacitor to ground. Rg can be minimized by proper grounding (bonding) all bolted-on parts including doors, hoods, trunks, tail and exhaust pipes, bumpers, etc. Mounting the antenna as high as possible on the vehicle also helps as this reduces the coupling to ground (we want the vehicle coupled to ground, not the antenna). Avoid using bumper mounts, magnet mounts, trunk lip mounts, and similar devices.

In just about every case, an HF mobile antenna will have an input impedance of less than 50 ohms, and typically between 12 and 35 ohms. Obviously if we wish to obtain a low VSWR, we need to match our 50 coax feed to the antenna. There are several ways to do this, but the easiest is to use a 4:1 unun.

Attached to this article is a schematic diagram of a 4:1 unun. Unun stands for Unbalanced to unbalanced, as opposed to Balun, which is Balanced to unbalanced. To the left is our 50 ohm input, and on the right a 12.5 ohm output. By tapping 1/4, 1/2, and 3/4 of the way along the top winding, 1.5:1, 2:1, and 3:1 ratios can be obtained. Or, 33 ohms, 25 ohms, and 16 ohms. In all but the extreme cases, one of these taps will closely match just about any mobile antenna. Using the chart in the ARRL Antenna Book, or by using an antenna bridge like MFJ's 259B, we can guesstimate or measure the input impedance of our antenna and select the best tap.

Construction is easy even for a novice builder, and the ARRL Handbook gives a lot of good tips on how to wind one. In short, the untapped winding should be insulated from the tapped winding with Teflon or high-temperature plastic sleeving or tape. A T200-6 is the best bet, but a T200-2 can be used. The core should be wrapped with glass tape if high power is contemplated. AWG 14 wire is adequate for 1,000 watts of power, and will allow an 11 to 12 turn bifilar winding. The transformer should be mounted close to the base of the antenna, and protected from the weather. Cores, glass tape, and Teflon sleeving may be ordered from Amidon, Palomar Engineering, and many other sources.

Now let's digress for a few moments and take another look at the inherent losses of a short, mobile antenna. Referring to our original formula above, Rt=Rr+Rc+Rg, let's read between the lines. If Rt is say 40 ohms, and affords us a 1.25:1 VSWR, what and where do you think the other losses are? Most generally they are in the coil, and to a lesser degree the ground losses. You might ask then, what does a lossy coil looks like? Well, one thing's for sure, it isn't spare-tire sized!

Many commercial mono band and multi band short mobile antennas will match 50 ohms quite well due to their resistive losses. This author recently measured a 6 foot long, helically wound, bumper mounted antenna with an MFJ 259B, and got 64 ohms at resonance on 40 meters! Its approximate efficiency is under 2%. That is to say, fewer than 2 of those 100 watts inputted were being radiated, and the guy bragged about how well it matched! Well guess what? I have an antenna with a 1:1 VSWR from DC to 1.2 GHz. It's called a dummy load! If your antenna matches as well as this one did, you need a better antenna.

One last bit of information... A VSWR bridge cannot measure phase angle, and therefore cannot be used to determine the actual resonate point of any antenna other than a purely resistive one, and that just doesn't happen very often, especially with mobile antennas. What it does measure is the (lowest) voltage, and because an antenna has reactance, the voltage and current are not in-phase. Without a good noise bridge or antenna analyzer, it is mostly guesswork. In short, don't rely on a VSWR bridge to adjust your mobile antenna.


Alan Applegate, KØBG

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