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Restricted Space Antennas

by Walt Fair, Jr., W5ALT

Shortened Dipoles

Since we're interested in antennas for restricted space, a short dipole is an obvious option. For purposes of this and following compromise antenna discussions, the examples will be focused toward 80 meter operation at 3.6 MHz, since that is a major challenge for restricted space environments.

What is it? A shortened dipole is simply a dipole antenna that has been shortened. Since it is shorter than its resonant length, it will not be resonant and will exhibit both resistance and reactance at the feed point. Shortened antennas tend to have a capacitive reactance and therefore need an inductance to cancel the capacitance and bring the antenna back to resonance. Normally the resistive impedance also drops as the antenna is shortened, so additional impedance transformation will be needed to effectively match the antenna.

A shortened dipole will act similar to a full sized resonant dipole in many ways. The effect of ground will still be important, current will be maximum in the center and very close to zero at the ends with maximum voltage at the ends and minimum in the center. If the antenna is center fed, it will still be balanced, with equal voltage and current distributions on both legs.

From the principle of conservation of energy, we know that if we can feed energy into an antenna, it will radiate. We also know that with a suitable matching network, we can feed power into nearly anything, including a shortened dipole. So what's the trade-off? Let's look at a shortened dipole mounted 20 feet above an average ground. The antenna will be made using #14 wire and fed in the center.

Length and Impedance. Notice that in the above figures the impedance is shown with 2 components: resistive and reactive. The left hand figure shows that at a length of about 133 ft the antenna is a full sized dipole and resonant, with about 46 ohms resistive impedance and no reactance. As the antenna is shortened, the resistive impedance drops fairly rapidly and the capacitive reactance rises. Notice that the reactive impedance scale is in thousands of ohms, while the resistance scale is in ohms. The right hand figure is a close-up view of the shorter lengths. Remember that a 40 meter dipole was about 66 ft long, so we are looking at what happens when the dipole is shortened to half of its normal size or less. Conversely, it can be viewed as what happens when we try to use a dipole for a higher band at 80 meters.

Note that at around 40 ft, the resistive impedance is 4 ohms and below about 20 feet, drops to less than 1 ohm. Meanwhile the reactive impedance at 40 ft is about 2000 ohms and at lengths less than 20 ft rises above 4000 ohms rapidly. That's not a problem in itself, since we can always use a coil to cancel the reactance and then match the resistive part with a matching network.

The problem is that our coil and matching network will be made of real components and will have resistance in it. Usually the coil contains the most wire and will normally have the most resistance and the Q of the coil is the ratio of its reactive to resistive impedance. So a fairly good coil might have a Q of about 600. That means for 2000 ohms reactance, the coil will offer around 2000/600 = 3.3 ohms resistance. That means that the coil resistance is getting very close to the radiation resistance and the efficiency is approaching 50%.

But it gets worse. At less than 20 ft antenna lengths, the reactive impedance rises much faster. For 4000 ohms reactance the resistance in the coil would be 4000/600 = 6.7 ohms, while the radiation resistance is 1 ohm or less. Thus most of the power will be used up in the coil resistance and not radiated.

That's is the problem with short antennas in general and with dipoles in particular, in this case. The lower radiation resistance and the high impedance causes significant losses in any sort of matching network. Failure to consider those losses will lead to an antenna that loads, but doesn't radiate.

For this reason, shortened dipoles can work quite well as long as the shortening isn't too severe. If they get really short, extreme care will be needed in the matching network to make sure that the resistive losses are low. Remember, if you can feed power into an antenna, what isn't lost to resistive heat will radiate, so the resistive losses relative to the radiation resistance must be minimized.