Monday, August 2, 2010

I’m confused about the concept of “SWR bandwidth.” Can you explain?

Q I’m confused about the concept of “SWR bandwidth.” Can you explain?

A “SWR bandwidth” is a term you’ll often encounter when you’re reading about antenna designs, or checking the specifications of commercial antennas. Basically, the SWR bandwidth is the frequency range after the antenna has been tuned at one frequency, over which the SWR is 2:1 or less. This is easier to explain visually, so take a glance at Figure 3. Let’s say that we have a 40-meter dipole antenna that is tuned to resonance at 7100 kHz. If our dipole has an SWR bandwidth of 200 kHz, we’d expect the SWR to rise to 2:1 at 7000 kHz and 7200 kHz.

Some types of antennas such as compact tuned loops have extremely narrow SWR bandwidths when tuned to resonance. Trap dipole and vertical antennas will have varying SWR bandwidths for each band, usually becoming narrower on the lower bands. Be wary of an antenna that claims a 2:1 SWR bandwidth covering all of a wide band, such as 80 meters. This band covers 3.5 to 4.0 MHz, a percentage bandwidth of more than 13%. While a wide SWR bandwidth may seem ideal, it’s often the hallmark of an inefficient design with high losses. After all, dummy loads have the “best” SWR bandwidths of all! Read all about broadband antennas in Chapter 9 of The ARRL Antenna Book.

From QST November 2000