I recently became aware that there are companies selling Double Side Band radios for use on digital HF frequencies (FT8 specifically) on 40m, 30m, and 20m. In this post we’re going to take a look at what these rigs are, what they were originally designed for, and then you can decide whether these should be used for digital communications on HF.
Disclaimer: I don’t own a digital DSB rig. This post is intended to discuss the realities of such a design so that somebody looking to purchase a starter radio is completely informed of what they are getting into.
What is DSB?
Double Side Band describes an AM radio transmission which has had its carrier suppressed (aka filtered out). That means that only the sidebands are amplified, which in turn means that the transmission is 1/3 more efficient, because 1/3 of the useless signal isn’t transmitted. SSB (Single Side Band) goes a step further and filters out the redundant half of the transmission yet again, and so 2/3 of the original waveform is cut out, leaving a Single Side Band. This is what we’re accustomed to when communicating in most HF modes. Double Side Band foregoes the added filtering and still puts out everything but the carrier.
Why DSB digital?
Among the vendors who are selling kits for DSB Digital transceivers, at least one of them references Peter Parker’s (VK3YE, not Spiderman) Beach40 design as a design inspiration, But let’s look at what Peter actually says that the Beach40 was for. This is taken from his website and is for reference only. I hope you don’t mind, Peter!
It’s a project for the tinkerer… if you’re not quite ready to assemble an SSB transceiver, give this simpler DSB rig a shot.Peter Parker VK3YE on the Beach40 transceiver project
You’ll notice that it’s not intended for anything more than a project for a homebrewer– somebody who intends to build radios from scratch- to get on the air with without having the complexity of a SSB transceiver, and ostensibly to eventually graduate from. So, why are vendors putting out DSB digital transceivers that are solely intended for digital communications?
These digital DSB rigs are apparently intended to lower the cost and complexity of a transceiver by omitting the sideband filter and the alignment that goes along with it, and just transmitting both sidebands regardless of whether this is needed.
Is DSB good for Digital modes?
I’m not going to come out and tell you whether you should think DSB digital is bad or not. Instead lets look at some facts about DSB, and then you can determine this for yourself.
Extra Data Transmission
A SSB transmission is 3khz wide, but a DSB transmission is twice that at 6khz. When an audio signal is transmitted at say 1500hz, its opposite is transmitted at -1500hz. Thus you have two signals being sent but only one is needed.
Take for example 7074khz and 7078khz. These are the FT8 and JS8Call frequencies commonly in use. If you’re transmitting a signal on 7078khz, at 1500hz, then you’re outputting TWO signals: One at 7079.5khz, and one at 7076.5khz. Only the Upper Side Band can be decoded, because the Lower Side Band at 7076.5khz is technically “reversed” and cannot be decoded. So, if you were using one of these rigs for JS8Call, the FT8 crowd might not appreciate it very much. But there’s a catch.
You can’t use these rigs for JS8Call, because they are crystal controlled. They are not frequency agile. They can only be used for FT8. This is of course unless you go to the trouble of finding other crystals and figure out a way to switch them out.
How does power output look? They tout about 3W of DSB output on 40m, but that’s DSB output. That’s 1.5W per side band. And since only one sideband is useful, effective wattage is half that listed.
There is one other thing to consider: The receiver lacks filtering as well. That means that you’re effectively doubling the receiver bandwidth, which reduces sensitivity by at least 3db.
Counting the Cost
Lastly, there is cost. These digital DSB rigs are selling for about $40. That’s definitely cheap, but is it a value?
For $59 and a little bit of hackery you can have a BITX40 that is frequency agile and can be used on any digital mode or with voice, is Single Side Band and puts out 5-7W.
Now that the BITX40 is gone, the perceived value does go up a bit. It is an inexpensive entry into ham radio, and as an owner of QRP Guys pointed out below, it’s good for experimenting and learning with.
What do you think? To digital DSB rigs belong on the air? Let us know your thoughts in the comments below.