# What is the bandwidth of an antenna ? and what happens when a narrow bandwidth is used?

I'm trying to understand the bandwidth of an antenna in order to choose the right one for my next project which involves a Nordic 2.4GHz radio transceiver (NRF24 series) that operates from 2400MHz to 2525MHz.

According to this

The bandwidth of an antenna is the range of frequencies over which it operates

I assumed this meant the higher frequency - lower frequency = the bandwidth, however, the datasheet of the 2.4GHz chip antenna used frequently with this chip, says it has a bandwidth of 180MHz, but the operating frequency range is 2400Mhz to 2488Mhz! if my assumption is correct, shouldn't it be 2580MHz (2400+180) ?

Also, if the higher frequency is 2488Mhz does this mean the antenna wouldn't radiate optimally if the radio is set to a channel over 2488Mhz or does it mean that it wouldn't radiate at all?

• Have you considered what happens when you stray from the center design frequency? You get higher SWR. The bandwidth is determined by measuring it and then documenting when the SWR goes above some arbitrary limit like 2:1. Narrow bandwidth means higher Q, and you get the maximum energy transfer. So straying away from that center frequency will very quickly result in higher SWRs. When receiving the signal strengths are too low to measure the SWR but it happens. Any antenna that can receive can also transmit, so you can characterize it with a small signal generator and an SWR bridge. – SDsolar May 12 '17 at 17:48

## 3 Answers

There are many different definitions of bandwidth and engineers are used to switching back and forth between them and comparing apples and oranges to get the correct answer that they both are fruit. For example, if one says that the bandwidth of a lowpass filter is 10 kHz, then usually it means that the output power of a signal at 10 kHz is attenuated by a factor of 2 (3 dB attenuation) compared to the output power at DC. It is not the case that signals above 10 kHz are blocked entirely; if that were the intent, then the filter would have been referred to as an ideal lowpass filter with a cutoff at 10 kHz. For commonly used low-pass filters, the output power decreases at the rate of n dB per octave as the frequency increases beyond the 3 dB point, where n depends on the filter order: sharper decreases require higher-order (and thus more expensive) filters.

Similarly, if your antenna is usable from 2400 MHz to 2588 MHz, then I would hesitate at using it for signaling at a carrier frequency of 2588 MHz since the upper sideband would be attenuated considerably compared to the lower sideband. You want to be sure that the entire signal bandwidth fits comfortably within the specified range of operation.

What happens when a narrow bandwidth is used?

Narrow bandwidth is never a problem with an antenna like this. However, the spec is a little misleading but the important factors are centre frequency and bandwidth.

Centre F is quoted at 2450MHz and the bandwidth is 90MHz either side of this i.e. 2360MHz to 2540MHz.

It sounds like your Nordic RF is within the limits of your antenna centre frequency and bandwidth.

• Hi, I'm really sorry I didn't comment earlier. Did you divide the bandwidth by two because the datasheet says it's 180MHz) ? I also have another one, same center frequency but 100MHz bandwidth, does that mean the highest frequency is 2500MHz ? why isn't it a problem if I use it ? – mux Jul 18 '13 at 18:47
• @mux it's not a problem provided the significant part of the bandwidth of the signal remains inside the bandwidth 3dB limits of your antenna. – Andy aka Jul 18 '13 at 19:08

The bandwidth of an antenna is the frequency range over which antenna give design performance ( Gain, efficiency, returnloss....). Bandwidth definition may vary as per requirement, in most of the cases, the designer takes 10 dB bandwidth ( frequency range in which antenna return loss is better than 10 dB). In some cases this requirement may be even tight like in the case of satellite remote sensing antennas this is 15 dB.

In wider bandwidth antenna, due to the wide range of frequency antenna can support multiple communication channels ( or devices) while narrow bandwidth antenna has limitation to work only for specific application.