CARC

Welcome to Crawley Amateur Radio Club

Latest News ▾
You are here: Home / Archives for News

CARC Newsletter January 2025

By

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:

  • “Review of 2024” – Editorial by Mike G0KAD, CARC Chair
  • Christmas 2024 Dinner
  • “Think amateur radio, think friendship” – World Thinking day, 15th February 2025
  • Is lowest SWR always the same as the resonant frequency? (Peter M0LMG)
  • CARC WSPR Completion
  • How “Auntie” broadcast English to west Africa and south America
Post Views: 0

Is lowest SWR always the same as the resonant frequency?

By

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

Post Views: 0

CARC WSPR Competition 2025

By

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 

Post Views: 0

Online Amateur Radio Community (OARC) GB3MH Net

By

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.

Post Views: 0

CARC Newsletter November 2024

By

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:

  • Measuring coax length with a NanoVNA (I used this some time back, and it works very well – PJ)
  • JOTA 2024
  • From the CARC Newsletter 20 years ago…
  • Rotten Signals: How to cure them
  • Marconi Instruments RF Amplifier TF2167
  • Experiences of reporting QRM to Ofcom
  • CARC programme updates
Post Views: 0

JOTA: Jamboree on the Air 2024

By

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.

Scouts enjoying building the RSGB ‘Snail’ Morse Key

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.

Scouts passing short messages with PMR radios

Out in the forest, the Scouts had a game of hide and seek, using radio location beacons and directional antennas.

Scouts enjoying Hide & Seek

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.

Local scouts enjoying some DX

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

Post Views: 0

CARC Newsletter September 2024

By

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:

  • “Getting on the Air” – Editorial by Mike G0KAD, CARC Chair
  • GB7MH DMR Network update
  • “Jamboree on the Air” (JOTA) – Saturday 19th October
  • Almost 259, written by Ted (G7OBF)
  • Rotator Refurbishment, written by David (M0WID)
Almost 259, written by Ted (G70BF)
Post Views: 0

CARC Newsletter July 2024

By

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:

  • GB3MH update
  • CARC 2024/25 Club Project(s) – An Outline – G4PEO
  • RSGB Discovery scheme relaunched in new format
  • Events Calendar

Post Views: 0

HF Operating for Beginners

By

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:

Post Views: 0

FT8 Superfox Mode

By

The following is taken from the excellent website of the Jarvis Island DXpedition 2024.

This great new mode will enable even more FT8 QSOs and enables the chasers (us) to be sure that we are listening to the real DXpedition.

Read on and enjoy.

1. What is SuperFox mode in WSJT-X?

SuperFox mode allows an expedition to make digital QSOs at rates higher than possible with previous versions of WSJT-X.  In previous versions the Fox would transmit up to 5 streams, with each stream being 50 Hz wide.

The SuperFox transmits a full power constant-envelope waveform 1512 Hz wide.  The SuperFox may send reports or RR73s to as many as nine SuperHounds simultaneously.  Importantly there is no reduction in received signal strength to do so.  A SuperHound will experience the same received signal strength regardless of the number of Hounds being contacted by the SuperFox.

In addition, the SuperFox transmits a digital signature of authenticity.  This signature allows the SuperHound to verify the legitimacy of the expedition, thereby reducing or eliminating piracy.  This is covered in more detail soon.

2. Do I need special software?

Yes, both the SuperFox and SuperHound need to run WSJT-X version 2.7.0-RC5 or newer.  (expect the release around July 1).

3. Do I need to set my receiver or transmitter to a certain bandwidth for SuperFox decoding?

No, as the SuperFox transmits a signal 1512 Hz wide, using the same settings you use now with WSJT-X will work fine.

4. What software settings do I need to be a SuperFox or SuperHound?

For the SuperFox: In the Settings—Advanced tab of WSJT-X, select the Fox radio button, and check the SuperFox mode box.  You also need to obtain a Key and fill in the Key field.  (More detail on the Key field is below). 

For the SuperHound: In the Settings—Advanced tab of WSJT-X, select the Hound radio button, and check the SuperFox mode box.  No Key setting is needed.

In both cases a red SuperFox or SuperHound marker will be displayed in the lower center of the WSJT-X screen when the Settings window is closed.

5. Where will the SuperFox transmit?

Using a 1512 Hz bandwidth, the SuperFox will start at 750 Hz and go up to 2262 Hz.

6. Where should the SuperHound transmit?

The previous limitation of Hounds sending above 1000 Hz has been removed.  A SuperHound may transmit anywhere from 200 Hz and up.  Unlike the old-style Fox and Hound operation, the SuperHound’s frequency is not shifted down after being called.

7. Do I need to change the rig Split operation settings for SuperFox or SuperHound modes?

No, you can use the same Split settings as before.  For “Fox” you may also use None, as no VFO shifting will be done.

8. Are there special SuperFox frequencies on each band?

This is up to each DXpedition to define. Expeditions should post their desired frequencies on websites or live via a DX spotting network. SuperFox stations should never transmit on standard FT8 frequencies.

9. How will I know the SuperFox is not a pirate?

The SuperFox will transmit a digital signature which will be verified by the SuperHound when received. A displayed message will state the SuperFox’s callsign has been verified.  If no such message appears, the transmission is not verified.

 

10. Do I need special equipment to work a SuperFox?

No, your hardware will remain identical to your current usage. As stated in question #2, only your WSJT-X software needs to be upgraded.

11. How do I know the QSO is complete?

An RR73 message will be received as in previous versions of WSJT-X. If you did not receive RR73 after repeatedly sending your R+report, you can wait a few minutes and call again.

12. How do I know that the mode is SuperFox mode vs regular FT8?

A normal FT8 signal is only 50 Hz wide; the SuperFox signal is 1512 Hz wide. The sequence of tones is markedly different and it is easy to tell that SuperFox is not a normal FT8 signal.

13. Do I keep calling on every odd cycle until the SuperFox comes back to me?

This is difficult to answer. A Hound or SuperHound has no way to know where you are in the Queue or if you are in the Queue at all. Generally, it is suggested to continue calling until a reply is received.

14. Is old-style Fox/Hound Mode still available?

Yes, there are now three major FT8 sub-modes in WSJT-X: Normal FT8, Fox/Hound, and SuperFox/SuperHound.

Post Views: 0