Wednesday, August 20, 2008


Multiband Antennas
Multiband antennas are a good choice for many whether working HF or VHF/UHF. They allow you to cover several bands while using a relatively small amount of space and a single feedline. There are many, many different ways to create a multiband antenna, some are quite esoteric, some are ingenious in their simplicity, and some are downright odd. The latest QST describes a multiband antenna that is switched from band to band using air pressure to open and close relays which change the length of the antenna. (Thanks to Lee for reminding me of this article.) Today we are going to look at a particular technique for creating multiband antennas and that is using traps.

Multiband Antenna Basics
Let's go all the way back to antenna basics. One of the most important things you want in an antenna is for it to be resonant for the band where you want to operate. For a single band this is usually easy. You just figure out the length you need for your center frequency, remembering that the electrical length is different than the physical length, cut it to size, check the SWR and you are ready to go. The only concern might be if you have a wide band and a sharp Q. In this case you might get farther away from your center frequency than you want to be when you go to the extremes of the band. This would get you far away from resonance and would result in a high SWR. For this discussion we aren't going to concern ourselves with this kind of in-band resonance issue.

For a multiband antenna, the antenna must be capable of being resonant on more than one band; that is, it must have the capability to have multiple electrical lengths. In the case of the air-relay antenna, the relays change the length by switching open or closed. So it doesn't necessarily have to be resonant on more than one band at the same time, but it usually is.

What Are Traps?
Traps are electronic circuits designed to allow or stop the flow of AC current depending on the frequency. In a sense they are like the relays mentioned above but nothing is physically opened or closed, or changed in any way to change the length of the antenna. Instead, the trap is designed to be resonant at a certain frequency such that it has near infinite impedance. So at that frequency it looks like an open circuit and thus like the end of the antenna. Much like the simple antenna the trap is designed to be "close enough" to resonance over the part of the band where you will operate so that it will still be effective for whatever frequency in the band you choose. Things get more interesting when you send a signal at a frequency of a different band. To find out what happens there, let's go a little deeper into what makes up a trap.

Anatomy of a Trap
So we know that a trap resonates with infinite impedance at a given frequency. Just how does it do that? A typical trap is what is called a parallel LC circuit, also known as a tank circuit. Now that is a mouthful and it once again jumps ahead a bit of Lee's series. I won't go into this in any detail now. We can return to it after Lee has laid the foundation. I will just say that the L is inductance and the C is capacitance and to say that this is a parallel circuit element is to say that the inductor and the capacitor are connected in parallel. This allows current flowing through that part of the circuit to flow through either the inductor or the capacitor. Because of the nature of this circuit, when you are at the resonant frequency, the inductor and capacitor interact in a way to stop all current flow. This gives you the "end" of your antenna for the resonant frequency.

When you go off frequency, the LC circuit still modifies the signal, it doesn't just become a conductor the way closing a relay does. It turns out that as you lower the frequency, the LC circuit is inductive and increases the electrical length. If you raise the frequency, the LC circuit becomes capacitive and decreases the electrical length. Just how much it alters the frequency depends on the actual values you choose for the inductor and the capacitor. By choosing certain values you can decrease the physical length by quite a bit creating a more compact antenna. Nothing comes for free though and by making a physically smaller antenna, even though you have resonance, you will have a weaker field than if you had a full size dipole for that frequency.

Multiple Traps and Variations
Putting one trap on each side of a dipole will give you a two band antenna. You can add another trap and length of conductor to the end of that dipole to add another band. There are also other ways to use traps in combination with other techniques to increase the number of bands. I don't have any examples prepared but maybe some of our experienced hams can tell us of their own experiences using traps to make multiband antennas.

Coaxial Traps for Multiband Antennas, the True Equivalent Circuit

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