Resonance

Resonance is the condition where the antenna displays a resistive load of predictable value at the frequency of interest. You get there by cutting a dipole to a half wavelength, breaking it at the center and attaching a feedline there. Then raising the whole mess in a flat top configuration a certain distance above electrical ground.

This is what is called a resonant antenna. Its resistive load is generally 75 ohms. Its height is at least one quarter wavelength above ground (or multiple thereof). Under those conditions its SWR is very close to 1:1. It will absorb all the power you feed it as long as the feedline is also 75 ohms and your radio has a 75 ohm output. Such an antenna will also provide the maximum in signal on receive.

A non-resonant antenna requires an antenna tuner. The antenna tuner adjusts the antenna system and brings the non-resonant antenna into a resonant condition. That resonant condition may or may not be 75 ohms resistive. Most commonly it will be higher than 75 ohms and that condition will be handled with open wire line, window line, or twinlead down to the antenna tuner. The SWR on the feedline may be much higher than 1:1 but that is okay because there is very little loss in open wire line. SWRs as high as 10:1 can be allowed without adverse effects as long as open wire line is used.

The antenna tuner will match the higher, resonant impedance to the radio. It will also ensure that the non-resonant antenna behaves as a resonant device in combination with the tuner to form a resonant antenna system.

Depending on the choice of antenna and tuner this is a nice way of providing coverage over the entire HF bands with one antenna system. You end up with a true multiband system where the system only works on one frequency at a time. The frequency to which it is tuned. Untuned multiband systems are tuned to multiple bands at the same time and may result in undesired radiation of harmonics. Such complications are avoided in tuned systems at the cost of requireing tuning.

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2 Comments on “Resonance”

  1. VK2FNET Says:

    Hi K5DKZ,

    Great article. Since it’s an article on resonance, i think it would benefit many people if you talked about how one calculates resonance (eg taking into account velocity-factors, etc).

    And a bit of clarification of resonance vs impedance.

    So many hams, especially the new ones (and i’ve been guilty of it before i knew any better) snip their dipoles to achieve that elusive 1:1 match and indirectly move their dipole off-resonance.

    Of course, as you mention, those using ATUs can deal with it, but i think it would make a really good article anyway, to help people understand resonance and impedance better.

  2. admin Says:

    I am not sure you can calculate resonance. For a dipole, resonance is a condition that is met when the antenna is a half wavelength long, fed at the center, and elevated one quarter wavelength above electrical ground ( or multiple quarter wavelengths ). The you get a 75 ohm resistive antenna load. That is it. There is no formula (other than the normal for cutting the wire to a half wavelength). Velocity factors do not enter into the mix.

    If you miss any of those special conditions you loose resonance. Getting it back by sniping the dipole is not necessarily bad as long as it helps you get power into the antenna. The length of the dipole is not the only thing that contributes to resonance. If the length is on and the height is off it will no longer be resonant. So a little snipping might be in order.

    I guess a more accurate measure of resonance is the point at which the antenna exhibits a resistive impedance and a 1:1 SWR. That could occur at any one of a number of impedances depending on conditions.


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