Resources

Antenna Loading

There are a number of ways to make the small antenna "look electrically longer". Most of the available methods involve some form of loading.

The slow wave structures: zigzag, helix, fractal, slots, notches, meandering and others; although they can be applied to HF antennas, are more suitable for cell phones and other small low power UHF devices.

Two of the most common and practical loading methods utilized by HF radio operators are capacitive and inductive loading. Although these two loading methods are applicable to most antenna designs, for simplicity sake, a vertical antenna will be used as an example.

Loading will either reduce the inherent capacitive reactance of an ESA to a manageable level, or cancel it out completely. In either case, the object is to get the antenna to be resonant, where there is zero reactance and only pure resistance.

How you accomplish this resonant state will determine your small antenna's efficiency and useable bandwidth.

Using "Capacitive" Top Hats

Capacitive top loading can be used to increase the apparent electrical length of an antenna (up to 2 times).

When you increase the apparent electrical height of a short antenna, you get several advantages:
- The radiation resistance increases; increasing efficiency.
- The degree ampere-area expands: increasing signal radiation.
- The Q value decreases: resulting in an increase of bandwidth.
Since the radiation resistance increases with the square of the added length of an antenna, an optimal top hat can increase the radiation resistance by a factor of 4 (2x added apparent length).

Although the apparent electrical height has increased to match 1/4 wavelength (it is now resonant), the radiation resitance is still well below that of a full 1/4 wavelength antenna.
- Example: 1/4 WL unloaded vertical (Radr = 36 ohms, (Rg = 12ohms).... Radeff = 36/36+12 = 74%
- Example: 1/8 WL vertical base coil (Radr = 2 ohms, (Rg + Rc) = 15 ohms) Radeff = 2/2+15 = 12%
- Example: 1/8 WL vertical top loaded (Radr = 8 ohms, (Rg = 12 ohms)........ Radeff = 8/8+12 = 40%
A top hat, by raising the effective height of the antenna also changes the current distribution in a favorable manner, resulting in an increase of current flowing over a longer part of the radiator.

What Are the Dilemmas Involving Capacitive Hats?

In order to bring a shortened antenna to resonance using only a capacitive hat, the total top hat length would need to be about 2 times the missing height. This is not likely to be practical with a very short low band antenna!

The larger the top hat becomes, the more you will have wasteful high elevation horizontally polarized radiation and the less you will have useful low elevation vertical polarized signal.

Antenna currents above the top hat taper off very quickly, so any antenna length beyond this point does very little radiating.

Ideally then, you would like the capacitive hat to be at the very the top of the antenna. Stability alone will often make this impractical

When you top load, you can shorten the antenna, BUT a shorter antenna means less linear ampere-feet for radiating signal.

In the example below, the choice of the smaller top hat allowed for more antenna radiating length and stability.

Using A Loading Coil

A short antenna can be tuned to resonance with an inductor (coil) to cancel out the inherent capacitive reactance of an ESA.

Like the capacitive hat, the inductor can also be used to favorably alter the current distribution, resulting in more radiated power per length.

The radiation resistance can be increased by moving the coil up the antenna. Moving from base loading to center position can increase the radiation resistance considerably, depending on the frequency involved.

What Are the Dilemmas Involving Loading Coils?

A loading coil will reduce the antenna length required for resonance, BUT the shorter you try to make the antenna with inductance, the more coil resistive losses will diminish the efficiency.

You want to move the coil up the antenna to increase the radiation resistance, BUT after a certain point, approximately 50% up, the increasing inductance required cancels out the benefit due to increased coil resisitive losses.

Resistive losses can be reduced by using a high Q coil, BUT high Q coils tend to be larger and heavy and as we move the coil farther up the antenna, we really need the coil to be small and light.

Coils can "tune out" the negative capacitive reactance of a short antenna, BUT a loading coil may also have a high resistance that, in combination with ground and other resistive losses, may equal 50 ohms. This will result in a 1:1 SWR reading (assuming that the reactance has been cancelled out to zero). In this case, you would not have a good antenna, but a very inefficient antenna - probably an efficiency in the single digits.

A dummy load has an SWR of 1:1 because it is 50 ohms, but no one (except for a Russian) would mistake it for an efficient antenna.

Making the Small Antenna Look Big