# 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.