QRPp WSPRing - With a Strange Anomaly.

Over the past few weeks, I've been busy gathering 5W resistors for homebrewing a step attenuator.

Before I lose the graphic (courtesy of N2KI)  on which I based the design, here is the circuit, complete with resistor values needed:

5W resistors aren't that easy to come by in the UK, but Ebay sellers in China and Hong Kong provide them with admirable quality and speed for ludicrously good prices.

Having not yet received quite all the resistors needed to complete the project, I decided to throw together a fixed value attenuator, consisting of two pi-mounted resistor circuits yielding a total of about (mindful the resistors are tolerance rated only at +/- 5%) -10dB.

In practice, after using the admittedly very limited power meters at my disposal, I found the attenuation provided by this unit to be roughly 11 - 12 dB, which is perfectly acceptable.  This allows 2 Watts at the rig to be reduced, allowing for losses, to about 100 - 150mW.  Without a traceable, calibrated power meter, I can't be more accurate than that.

Here, for those who want to build their own, is the simple circuit, which, unlike the larger, switched unit, needs no special care in preparation, save for good soldering.  As you can see from the associated SARK 110 analyser plot, the impedance and SWR both come in at very good values.

The resistors, incidentally, are 200 Ohms (four needed, bonded to the case at one end), and 33 Ohms (two needed.)

A simple, fixed value attenuator of roughly -10dB.

Analyser shows good characteristics at 14MHz.

So, having connected-up and fired 2W, attenuated to about 100mW through the unit from 18:00UT on July 21 2016 overnight, I was pleased to find this tiny power level was getting across the Atlantic quite comfortably.

The overnight run into August 01 2016 produced an average reception SNR at K9AN, always my reference station in the US, of -19dB.  This value didn't change by very much for the whole run, except for one outlier, which showed a huge, 24dB enhancement in SNR, or a factor of 250 times above the average reception strength!   Here's the plot of the evening's proceedings, where you can see the enhancement to +5dB SNR at 23:12UT:

This anomaly is really interesting.  I've noticed similar enhancements in the past with WSPR and other digital modes, when signals are, broadly speaking, taking a path that yields a glancing blow with the auroral oval where local magnetic midnight is roughly half way between the transmitter and receiver.  It's interesting to note that the enhancement occurs shortly after connection with K9AN is re-established, after about an hour where I wasn't being heard.

The auroral situation on 2016 July 31d 23h12m UT (NOAA)

The grey line situation at the same time.  Courtesy DX Atlas, with permission.

 
The auroral involvement might be supported by looking at GM4SFW, transmitting 5 Watts from Scotland, where the auroral oval has a greater and more frequent effect.  Remarkably, his signal, more variable with time due to the aurora, shows an even greater enhancement an hour after mine (2016 August 01d 00h16mUT), of a whopping 29dB - or a factor of 795 times!

I'm not terribly clever at explaining radio propagation, and not many texts I've read fill me with confidence that anyone else does, either.  That said, I'm sure there will be someone out there with insight, whether theory or proven principles.