Mobile Antenna Notes

[WOLF, EARNEST G]

Another of interest to those mobile folks amongst us!

Mobile Antenna Notes
Alan Applegate (K0BG) on January 20, 2003=20
View comments about this article!=20

The majority of HF mobile antennas are electrically short. For example, =
an 80 M mobile antenna 8 feet long is electrically 11=B0 in length as =
opposed to 90=B0 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:=20



Rt=3DRr+Rc+Rg=20



Where Rt=3Dtotal 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.=20

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=3DRr+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.=20


Alan Applegate, K=D8BG



Thanks,



Glenn

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