Whilst constructing my first quarter wave vertical antenna, a radio friend said…
“Don’t worry about the feed impedance or SWR at the moment; just find the resonance for now. Raising the antenna feed point & sloping radials towards the ground enables easy adjustment of feed impedance, reduces ground losses (often disguised by a ‘good’ SWR) and significantly lowers radiation angle enabling improved DX.”
I realised that I didn’t fully understand how SWR and the resonant frequency could be different, so I embarked on a journey of discovery. These are notes and videos that helped me understand the differences between SWR and resonant frequency, and why they may be different.
From the MFJ-259B Analyzer Manual – Suggesting SWR may not equal resonant frequency

Additional Information

This article explains that if an Antenna is does not have an impedance of 50 Ohms, then the resonant frequency and lowest SWR may be different
Now let’s look at the classic center-fed half-wave dipole in free space. At the resonant frequency, the antenna has an impedance of 73 Ω, purely resistive. The SWR can be calculated by taking the ratio of the impedance to 50 ohms, giving SWR = 73/50 = 1.5. (By the way, for impedances less than 50 ohms, the SWR is calculated using SWR = 50/R.) Dipole antennas generally work well, so the antenna efficiency will be high and depends on the actual construction of the antenna. Note that the SWR is not equal to 1 at resonance, it is a bit higher. However, an SWR of 1.5 does represent a good match and is normally considered just fine.
Now let’s take a look at an antenna that is nowhere near 50 Ω at resonance, the half-wave folded dipole antenna, described in this article. This antenna has an impedance of about 280 Ω at the resonant frequency. If we connect this antenna to a 50 Ω transmitter, the SWR is 280/50 = 5.6. So here is an example of a resonant antenna that has a high SWR. At the resonant frequency, this antenna will radiate efficiently but will present a difficult impedance to a 50 Ω transmitter. The impedance match is poor and we will struggle to deliver power from the transmitter into the antenna. While we might choose to accept this high SWR, a more practical approach is to add a matching network to produce a 50 Ω impedance.
Many of the antennas we use are designed to be close to 50 Ω (SWR = 1) when they are resonant. For this case, the SWR is a good indicator that the antenna is resonant, which is why most hams associate low SWR with resonance. Low SWR does not tell us anything about how well the antenna is working (antenna efficiency). A dummy load has excellent SWR but fails to radiate. Some antennas are like that, too.
The Importance – or Not – of Antenna Resonance
A resonant antenna does not necessarily have a low SWR. Antennas with a feed point impedance of 5 + j0 or 500 + j0 are resonant, by this definition, but either will have an SWR of 10 : 1 when fed with 50 ohm cable. And either may still be a useful antenna. Note, however, that one cannot have a low SWR when the load impedance has significant reactance. To have a low SWR on a cable with a characteristic impedance of 50 + j0, R must be close to 50 ohms and X must be close to 0 ohms. So, if there is no other matching in the system, having a low SWR means the antenna must be close to resonance, but the reverse case is not true when all the feedline has the same impedance as the SWR meter. Things get more complex when we use feedlines with different impedances.
Other Comments from Websites & Forums
Antenna is Z=100+j0. Transmitter and line are Z=50+j0. Antenna IS RESONANT even though you will see “reflected wave” (SWR is 1:2)
What’s the difference between “minimum SWR” and “resonance”?
https://ham.stackexchange.com/questions/2482/whats-the-difference-between-minimum-swr-and-resonance
Should I trim my dipole so it’s at the resonant frequency or the lowest SWR?
https://www.reddit.com/r/amateurradio/comments/1669ceu/should_i_trim_my_dipole_so_its_at_the_resonant
Good SWR & Antenna Resonance
https://www.hamradioschool.com/post/good-swr-antenna-resonance
YouTube Videos
Resonant Frequency Often Not Lowest SWR
https://www.youtube.com/watch?v=lrOWq-2CYaw
Impedance and Ham Radio Antennas Explained
https://www.youtube.com/watch?v=orS0Qodaats
Resonance or Lowest SWR
https://www.youtube.com/watch?v=JrwMh7FYn5U
Ham Radio Antenna Basics
https://www.youtube.com/watch?v=QtIqJ60KhoY
Impedance and Ham Radio Antennas Explained
https://www.youtube.com/watch?v=orS0Qodaats
Is Your Antenna Really Efficient? SWR isn’t the only thing
https://www.youtube.com/watch?v=miTt_DtxKAY
Demystifying Smith Charts for Ham Radio Beginners
https://www.youtube.com/watch?v=7sWOtdaGQgs
Fun with SWR
https://www.youtube.com/watch?v=BpIQH6PgpeA
The Truth about SWR: Debunking the Myths and Misunderstandings
https://www.youtube.com/watch?v=L1_NLEpsW90
NanoVNA Example that uses SWR, Reactance & Smith Chart
