The Buzz

Couple weeks ago I ran a parallel RX test between SunSDR DX2 and Flex 6600M. With a disclaimer: not a pro lab test but rather an amateur shack test.

While the intention was to simply see how well the little SunSDR compares to big SDR Flex, I’ve overlooked one important detail: the two radios running two different versions of decoding software. In my mind, that was not hugely important but as a couple of subscribers pointed out, it made the results skewed towards the SunSDR. Which is fair enough.

Yesterday I decided to go back a step: to compare the FLEX-6600 with my old trustworthy IC-7700. After all, the old ICOM is still regarded as one of the best radios in its class. First introduced in 2007, this transceiver is revered for its receiver, especially the exceptional filtering.

Learning from the previous mistake: both radios were setup to decode WSPR signals with the original WSJT-X software.

A quick observation: the two radios are essentially completely different radios. The analogue old fashioned ICOM which does not have have a capability to process the audio signals internally vs. a modern SDR technology with integrated audio processing.

Every attempt possible was made to ensure that both receivers are as ‘identical’ as possible: the bandwidth, no pre-amplifier or attenuation, no enhanced processing, as well as ensuring that receiving frequency is set to receive all WSPR signals within 200 Hz bandwidth.

The first test run was on 10 MHz and the first decoding was satisfactory: identical number of decodes, signal strength to decibel. This was very encouraging indeed.

In the morning, well after sunrise, the band was set to 7MHz with the antenna directed to Europe. The radios were left on WSPR frequency to run, with decodes automatically being uploaded to WSPRnet database.

It is important to note the intermittent presence of a rather strong radar signal, which made the receivers work hard coping with the interference.

WSPR database, 7 MHz

IC7700

Total decodes 183, unique calls 31

FLEX-6600

Total decodes 161, unique calls 24

The next test was conducted on 21MHz. In the very start it was evident that without the pre-amplifier, FLEX-6600 was ‘hearing better’.

To level the play field, a 10dB preamp was engaged at ICOM 7700 and 8dB at FLEX.

After a couple of hours of uninterrupted decoding, the database was showing the following results:

WSPR database, 21MHz

IC7700

Total decodes 275 unique calls 42

FLEX-6600

Total decodes 348 unique calls 46

The question is this: how to interpret the results? And what exactly this parallel test means? One thing is certain: the more I compare and the more I test, I am discovering more factors outside the receivers themselves which have significant influence to the number of decoding. Here is an example: on 28Mhz there is a significant 50Hz buzz very close to WSPR frequency. This interference is not noticeable on IC7700 but it is very prominent on FLEX-6600 causing a major degradation in reception. What is causing it? A power supply, a computer the radio is connected to, the monitor – or the router? Or perhaps some other unsuspected device? On the other hand, on 21MHz Flex is clean as a whistle, while ICOMs spectrum exhibits faint but ever present 250 Hz lines.

Would it be even possible to create an absolutely levelled playing field before commencing a comparison test? Or should it be wise, before comparing two different radios, to do an actual test comparing two identical radios first, trying to figure out what roles the computers play themselves?

I feel a bit like a Renaissance man: kind to find more, explore, experiment while still in a dark universe of RF mysteries.

At this stage, one thing is certain: the hunt for the mysterious buzz is on. No A/B test until the imposters are eradicated.

The good news is that it’s not decibels that make contacts; it’s the ionosphere. The reason why we miss the juicy DX is not because of a decibel or two, or because of a decode or two – but because the radio is turned off.

I just love the sound of the IC-7700 receiver. It is soft, and I can listen to weak CW signals for hours, never getting tired. The Flex 6600M is going to be a main remote contesting radio, and I am saving hard for 4O3A amplifier and tuner. The SunSDR will then become a dedicated 6m radio. Nothing to complain!