The club newsletter (put together by Richard G3ZIY) for March 2025 is now available to download here. The newsletter will now open directly in your web browser without having to use Adobe PDF Reader.
Features in this issue include:
The club newsletter (put together by Richard G3ZIY) for January 2025 is now available to download here. The newsletter will now open directly in your web browser without having to use Adobe PDF Reader.
Features in this issue include:
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
The event will run for the calendar month of March 2025 as defined by https://www.wsprnet.org/drupal/node. Adjudication will use the March 2025 file from the download section of their website.
The band will be 40m.
Any antenna may be used including separate antennas for transmit and receive if that is the preference of the entrant.
Maximum power is 5W.
There will be three classes:
Transmit
Receive
Two-way
A two-way QSO is defined as an entrant being heard by the other station on the same calendar day as the entrant heard them.
Entrants will be automatically included in all three classes.
Points are awarded as follows: 1 point for each station heard/hearing the entrant. 1 point for each large maidenhead square (e.g. IO91, JO00 etc) heard/hearing the entrant. The score will be total stations multiplied by total squares.
Two way will be the total individual stations heard and also hearing the entrant with the day totalled over the month.
An entrant may make multiple entries so long as a distinct callsign is used for each entry.
There is no need to submit a log, all necessary data will be gleaned from the WSPRnet archive, but entrants must advise G3YSX (stewart@g3ysx.org.uk) of their intention to enter together with the callsign(s) that are using.
Members of local clubs are welcome to join in.
Stewart/G3YSX
The Online Amateur Radio Community (OARC) will be running series of nets in 2025 on repeaters which include both Analog and Echolink access. This allows a wider participation as it removed the need to join via RF (although this is certainly welcome).
The first of these repeater-based nets will be held on GB3MH, operating from Turners Hill, West Sussex, on 3rd January at 1830 hrs.
You can show your support to GB3MH with a donation via Paypal.
The club newsletter (put together by Richard G3ZIY) for November 2024 is now available to download here. The newsletter will now open directly in your web browser without having to use Adobe PDF Reader.
Features in this issue include:
On 19th October, Scouts from 1st Langley Green, 5th Crawley and 7th Crawley took part in the Jamboree On The Air at an event run by Crawley Amateur Radio Club (CARC) in their hut in Tilgate Forest. As the name suggests it’s a chance for Scouts from around the world to “meet” each other over the airwaves. CARC provided a range of activities, starting with a brief explanation about How it All Works, including how radio waves going in a straight line can reach round the other side of the world. HF radio is the answer, which bounces off layers in the atmosphere after being transmitted from this huge antenna.
The Scouts had the opportunity to take part in a number of activities. Here’s one group making “Snail” Morse keys, so they could learn and practice sending messages in Morse code. It’s the shape of the key that gives it the name.
The Scouts also had a go using PMR446 hand held transceivers. Or “walkie talkies” as they’re known as by everyone outside of radio world. Using the phonetic alphabet and correct RT procedure they passed short messages.
Out in the forest, the Scouts had a game of hide and seek, using radio location beacons and directional antennas.
The main objective of the day was to contact other Scout troops using amateur radio. Atmospheric conditions weren’t brilliant and the airwaves were a bit crowded due to a couple of competitions being held. However as well as talking to Scouts in nearby Merstham, contact was made with two stations in the Netherlands and one on Scotland plus places in between.
Huge thanks to CARC for all their hard work. The event, as well as being great fun helped the Scouts work towards the World challenge badge and the Communicator activity badge.
Words and images used with the permission of the 1st Langley Green, 5th Crawley and 7th Crawley Scouts
The club newsletter (put together by Richard G3ZIY) for September 2024 is now available to download here. The newsletter will now open directly in your web browser without having to use Adobe PDF Reader.
Features in this issue include:
The club newsletter for July 2024 is now available to download here. The newsletter will now open directly in your web browser without having to use Adobe PDF Reader.
Features in this issue include:
For everyone who might be interested in the mysteries of working HF, Norfolk ARC have produced a series of four videos, each about an hour and a half long introducing new amateurs to the dark art. They can be found on the NARC YouTube channel. This is the link to the first in the series: