09 June, 2012

Medium delayed echoes

Medium Delayed Echoes or Magnetospherically Ducted Echoes (both MDE) are radio echoes delayed from 0.14-0.3 seconds. They represent a subset of Long Delayed Echoes (LDE) and is the only one of the five LDE effects discussed in "The Five Most Likely Explanations for Long Delayed Echoes" which is fairly well understood.

Here is a fascinating report of such echoes from G3ZRJ. His report is different from those of many others in two unique and very interesting ways.

Tony, G3ZRJ wrote to me and said that he had heard MDEs on 1 January 2012 operating portable from Clehonger, Herefordshire, UK (IO82) between 2118-2152 UTC (= local time) on 3524 kHz. He used 90 Watts into a low-hanging homebrew Carolina Windom supported at about 7 meters at the feed point. This antenna should have a lot of high angle radiation. He worked GW3OQK in Swansea which is about 100 km away. Both of them noticed the echo. G3ZRJ had also noticed a very similar effect on 4th January 2010 during the 80M CC CW Contest.

He said that the echoes were initially so strong that he had trouble monitoring his own keying on CW. The same was noticed by GW3OQK who commented that it was difficult keying while listening to the echoes.

There are two interesting observations here in addition to the rare observation of the echo phenomenon by itself:
  1. There was a relatively long distance between the two stations that both heard the MDE, about 100 km. I have only heard about stations that were 20 km apart that were able to hear this effect simultanteously previously so this should indicate a very wide duct.
  2. The other interesting observation is the effect of the echo on their own keying. It is known in psychoacoustics that certain delays will introduce stuttering. This was just recently used to make a speech jammer by K. Kurihara and K. Tsukada in "A System Utilizing Artificial Speech Disturbance with Delayed Auditory Feedback". Here they used a delay in the 0.2 second range. 
The description by both G3ZRJ and GW3OQK should indicate from the subjective experience of the delay that it was about 0.2 seconds.

This is also what is expected for magnetospheric ducting in this part of England/Wales. Previously I estimated the delay at the location of Peter, G3PLX in the north of England to be 211 ms (see figure above) and this was consistent with measurements he had of delays in the 210-220 ms range. G3PLX wrote a nice paper about this in Radcom in Oct 2007. I believe he there first coined the term Medium Delayed Echoes.

At a slightly more southern location relative to the magnetic North Pole a shorter delay is expected, so 0.2 seconds should be close. Further, the time of day and year and the frequency all match well with magnetospheric ducting.

You can listen to examples of such echoes elsewhere on this blog. Also my 2009 technical correspondence paper in QST entitled "Magnetospheric ducting as an explanation for delayed 3.5 MHz signals" will give more details about when and where these echoes occur.