Author Archives: Thomas

Southgate ARC: What Is a Balun and How to Make One Cheaply

(Source: Southgate ARC)

Ham Radio – What Is a Balun and How to Make One Cheaply

A Balun is a transformer which allows coaxial cable, which is an unbalanced lead to be connected to a balanced load, for example a resonant aerial.

The 1:1 balun is often called a choke balun, it works by eliminating Radio Frequency currents flowing on the outside of your coaxial cable, this is important because it prevents the pattern of the dipole becoming distorted and prevents the braid of the coaxial cable radiating.

Connecting coax to your resonant dipole which can be configured in a V shape or in a sloping fashion, however serious power loss and feeder radiation can occur without the use of a balun transformer. No one wants to induce Radio Frequency currents into Television receivers or Broadcast equipment as it may result in disputes with neighbours and could possibly lead to your station having to close down. Try to keep the centre of your aerial away from buildings as it may induce Radio currents into the mains wiring.

Building a choke is not difficult all you need is some plastic pipe available from many Do It Yourself outlets and warehouses. I use a length of about 1 foot of plastic pipe and 21 feet of coaxial cable, the length of the pipe and diameter is chosen because it works well if using R-G-5-8 coax the ends of the pipe can be sealed after placing a SO-239 connector for your coaxial lead. I use two bolts and solder tags at the other end of the pipe to connect the ends of the dipole to the choke.

When wrapping your coax around the pipe don’t use too much force as it may damage the inner braid and space the turns away from each other by a millimetre or two. R-G-2-1-3 coax around 21 feet used with 5 inch pipe will handle 400 watts pf power. Wire ties can be used to hold the turns together along the length of the pipe. Using these measurements your choke will cover all of the ham radio bands from 1.8 Megahertz through to 28 Megahertz and will keep the radiation pattern.

Using a dummy load connected to the choke and transmitting 100 watts from my transmitter indicated an S.W.R. readings of around 1.5 to 1 at 3.5 Megahertz when testing 28 Megahertz the S.W.R. reading came down to 1.1 to 1 which is an excellent match. Using the choke as it should be at the feed point of a dipole cut for 40 metres give an S.W.R. reading of 1.2 to 1. The highest reading was 1.5 to 1 when using 18 Megahertz but the rest of the High frequency bands gave me very acceptable matching.

John Allsopp G4YDM

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Cloud-SDR: a remote access system for a number of popular SDRs

Many thanks to SWLing Post reader, Sylvain (F4GKR), who writes:

I am Sylvain, from Cloud-SDR, a new company selling software to get remote access to your existing SDR receivers through the Internet.


  • currently supports : SDRPlay, Perseus, BladeRF and AirSpy. LimeSDR to come when received and tested.
  • client software is free (can be used as standard SDR receiving software with locally connected devices),
  • server software starts at 110 Euros, VAT may be added depending on customer location,
    easy to use

From the Cloud-SDR website:

The Cloud-SDR Concept

You need a way to remotely access your existing SDR receiver? To stream your IQ samples to another remote computer over the network?

Cloud-SDR is a commercial software solution to enable real time RF data sharing or processing through the cloud.

Cloud-SDR can collect real-time IQ complex samples from an SDR hardware device connected on one machine, stream the samples to a second machine for demodulation or analysis, then send the resulting stream to third machine for storage.

In standalone mode, Cloud-SDR can execute signal processing tasks described with embedded JavaScript DSP engine.

Because network bandwidth is limited compared to SDR receiving bandwidth, the core concept of Cloud-SDR is to move the processing along the cloud to where it is required or possible : the DSP chain is divided in sub-tasks that are spread between computers interconnected through Internet.

Very cool, Sylvain!

I like the fact your system supports popular affordable SDRs like the SDRplay RSP & AirSpy and that you can stream your IQ samples.

Click here to read more on the Cloud-SDR website.

Build a Raspberry Pi-powered SDR

Many thanks to SWLing Post contributor, Jeremy Clark (VE3PKC), who writes:

I have developed an SDR receiver that may be of interest to your readers. It can be used directly with the Raspberry Pi2B/3B. It comes in several versions DIP/SMD.

Check out this MP4 movie:

I have two eBooks which are basically design manuals for the DIP and SMD version of my SDR receiver. Each eBook has an Internet linked parts list, so that the reader can get their own parts to keep costs down. The DIP eBook is $7.90 CAD and the SMD book is $14.90 CAD.

Click here to view Jeremy’s website.

Thank you, Jeremy–this looks like a fascinating construction project!

Readers: just to be clear, Jeremy is selling eBooks detailing construction–not kits. He did mention if you’re interested in obtaining the PCB, he can sell this as well.

eBay Find: Sangean ATS-818 shortwave portable

While browsing eBay a few moments ago, I noticed this Sangean ATS-818 which has a BuyItNow price of $30.00 + $15.00 shipping. If this would have been the ATS-803A/DX-440, I would have snatched it up in an instant if only for nostalgic reasons. I know many people that cut their teeth on the ATS-818 or the RadioShack equivalent. The price seems fair, the unit is supposedly functional and the seller has stellar ratings.

If you’ve been looking for an ARTS-818, this might be a good time to pull the trigger.

Click here to view on eBay.

The company and history behind the Boeing 707 HF antenna coupler

(Source: Boeing)

Many thanks to David Giba who shares the following via the Gary J. Cohen’s Shortwave Listeners Global Facebook page:

The Boeing 707 is my favorite commercial jetliner. If one looks at the tail they will see a probe pointing forward off the top of the tail. That is the HF shortwave antenna.

(Source: VIP Club)

It was matched by an antenna tuner/transmatch much like we have in our HF radios. This is the story of the Univac Coupler. Very interesting.

Click here to read on the VIP Club website.