# What does antenna frequency range mean in practice?

I'm trying to wrap my head around software defined radios and antennas and noticed that lots of antennas are described in terms of a frequency range. For example, this one is described as operating from 300 to 1100 MHz. What confuses me is where these numbers come from / what about the antenna design (size? filters?) gives rise to these values.

If I were to hook such an antenna up to an SDR and attempt to transmit slightly outside of that range (e.g. 1200 MHz), what should I expect to happen? What about if I attempted to transmit massively outside of that range (e.g. 2 GHz)? Are there general principles for modeling these dynamics?

• Hello and welcome to ham.stackexchange.com! I understand that your question is about the technical aspect of what happens when transmitting at a frequency outside an antenna's frequency specification, but please do keep in mind that a license is required to transmit nearly everywhere outside the ISM bands. 1,200 MHz seems to be used for "radionavigation-satellite" and "aeronautical radionavigation" in the US. 2 GHz seems to be used for "space operation (Earth-to-space)" in the US. – rclocher3 Sep 16 '20 at 19:31

It means the range of frequencies in which the antenna is designed to operate. Operated outside that range, the antenna may not meet its specifications.

This particular antenna is a telescopic whip, a kind of monopole antenna. Such antennas are typically 1/4 wavelength long. It is advertised for 300 to 1100 MHz, and it's length can be adjusted from 24.5 cm to 9.5 cm. Not surprisingly, the bounds of these lengths correspond approximately to 1/4th the wavelength of the frequency bounds.

Ostensibly, the notion is that the length can be adjusted to be appropriate for the desired frequency. Adjusting the length changes the resonance of the antenna, and thus its feedpoint impedance. Your transmitter has a specification for the range of feedpoint impedance it wants to see: transmitting outside this range can mean reduced performance or sometimes even damage.

Receive applications are more forgiving: a mismatched antenna means at worst reduced performance. Some receivers degrade rapidly when the antenna is not matched, others are quite tolerant of a much wider range of antenna impedances. It depends on the design of the particular receiver.

A more general answer...

Antennas have a number of characteristics critical to their performance, including but not limited to gain, radiation pattern, impedance, and sometimes efficiency and polarization. These performance characteristics are all tied to frequency. When designing an antenna, these characteristics can be graphed vs. frequency and there will be peaks where each characteristic is optimal and areas where they are not.

The "frequency range" of the antenna is where all of the desired characteristics are within an acceptable range. The difference between the lowest and highest acceptable frequency is called "bandwidth". When describing an antenna's range, sometimes only one of these parameters is focused on, and which one depends on the intent of use for the antenna, but ideally multiple parameters are considered.

These graphs are not necessarily simple, and there may be multiple peaks where an antenna works, and this can be designed for to get a "multi band" antenna (for instance, one that works on both 2m and 70cm), although antennas typically work at harmonic frequencies anyway.

As an example, if an antenna is designed for the amateur radio 2m band, then it would be expected that it's impedance is close to 50 ohms from 144MHz to 148Mhz so that SWR will be below (hopefully) 1.5 for the entire range. Outside of the range, the SWR may climb rapidly and become unsuitable for transmission. Reception will be attenuated as well. Also, the antenna's gain and radiation pattern will be close to what it is advertised. If this is a directional antenna, this means that it remains directional within the range; outside of the range, it may become less directional or directional in unexpected directions. For an "omnidirectional" antenna, this usually means that its angle of elevation stays low enough for good gain. Outside of the range, the elevation angle typically rises until so much energy is going up that the gain in lower angles is very low. (Gain and directionality are tied together for antennas.)