The unphysical free-space path loss

Propagation of wave energy to distance d, 2d, 3d
 showing how the size of the surface that the energy
spreads over increases in proportion to 
d², (2d)², and (3d)²

Many places it is stated that the lower the frequency, the smaller the free space loss. 

This is very counterintuitive, because if one solves the wave equation, the amplitude of the solution is independent of frequency. The physics of the problem is that intensity just falls with distance squared according to how the surface of a sphere increases with the radius as shown in the figure.

The Friis formula used in link budgets says something else. It gives received power, Pr, as a function of transmitter power, Pt, receiver and transmitter antenna gains, Gr and Gt; wavelength, lambda, and distance, d:

50 MHz Doppler shift from planes

This is the first time I have observed Doppler shift in the 6 meter band. It is not so hard to see on the waterfall display of FT8.

The two stations are LA9PJA (1415 Hz) and LB6D (2477 Hz). Both of them are located in the same square as me (JO59). The other weaker signals are British and French. One can see the direct signal which has a constant frequency, and then a time-varying frequency on top of that.

FT8 anomaly or long delayed echo?

My friend Alf, LA2NTA, has sent me these screenshots from when he has been operating FT8. The first image is when operating 10 meters and took place early in November.

Two of LA2NTA CQs being received by himself on 10 meter (in red)

It shows how his own CQ comes back to him at 10.54.00 and at 11.00.00 and is decoded in his own receiver. The echoes return at the time of transmission.

Magnetospherically ducted echoes in the San Francisco area

On 7. November 2015, several radio amateurs in northern California heard echoes in the 80 meter band. I was made aware of it by Jack, W6FB in Santa Clara, who recorded signals from K6YT some 25 miles away. According to W6FB, the echo effect was also heard north of Sonoma (several hundred miles north of him, reported by N6ZFO).

KM6I, Gordon, in Palo Alto also heard echoes of his own signals and recorded them. In his blog he analyzed the delay from the output of his transceiver and found 157 ms. He found that to be so close to the round-the-world time for signals of 138 ms, that he assumed that to be the cause.

I don't agree, so I took the location of W6FB at locator CM97ah (Santa Clara) as a starting point for computing delay. This is latitude 37.31 and longitude -121.96 and gives a geomagnetic latitude of about 42.5 degrees. Then I put it into my program for computing path length along geomagnetic field lines assuming a height of the reflecting ionosphere on the opposite side of 100 km. The result is shown in the figure and predicts a delay time of 126 ms. My estimate of uncertainty is +/-5 ms.

The delay value is slightly less than 138 ms and easy to confuse with a round-the-world path. The challenge with estimating delays like this from the signal is that amateur transceivers may have an unspecified delay between start of transmission and start of sidetone. Measuring on the audio output as done here, measures the sidetone, not the actual RF.

I discussed this source of error in my 2009 QST article "Magnetospheric ducting as an explanation for delayed 3.5 MHz signals." Therefore the measurement shown above may fit with 138 ms just as well as with 126 ms, it depends on the actual transceiver's delay.

Other properties of the echo, such as the amplitude of the echo which according to W6FB at times was louder than the direct signal, also point to the duct theory as the explanation.

Others have heard such echoes also:

• 165-168 ms, K4MOG, February 2006, 80 m. Listen to the signal here. This event is also the object of the analysis in my QST article referenced above.
  
• 210-220 ms, G3PLX, November 2006, 160 m. Listen to the signal here.
• 214-219 ms, W2PA, February 2008, 80 m.
  
• 237 ms, OZ4UN, January 2009, 80m, Listen to audio file where also OZ7BQ very close by heard the echoes.
• About 200 ms, G3ZRJ, January 2012, 80m. Also heard by GW3OQK, 100 km apart.

Other posts on the theme: Magnetospherically Ducted Echoes or Medium Delayed Echoes

Radio Ghosts Have Haunted the Airwaves for Nearly a Century

“The starship hypothesis is a very interesting one, and the one which seems to be the most popular one on the internet,” said Sverre Holm, a professor of signal processing at the University of Oslo. “Such theories always excite our imagination, but it builds on a very poor data set. Unfortunately I believe it says more about human imagination than anything else.”

Although scientists have yet to settle on a final explanation for these mysterious echoes, Holm believes this is has less to do with a lack of scientific knowledge than a lack of willpower.

“I think that with today’s satellites and sensors, the mystery of Long Delayed Echoes (LDEs) could probably be solved,” he said. “What’s holding us back is most likely the problem is not considered important enough—it doesn’t occur often enough and doesn’t affect important enough forms of communications.”

These are excerpts from an interview in an article entitled "Radio Ghosts Have Haunted the Airwaves for Nearly a Century" on Motherboard Vice written by Daniel Oberhaus. It builds on a web page that I created some years ago after having spent days studying the archives from the 20's of professor Carl Størmer at the National Library in Oslo.

Nice, watery CW signals from Alaska

 Great circle path between Anchorage and Oslo

Here's what a transpolar signal sounds like (click here to listen) and looks like (below). This one has traveled from Anchorage, Alaska to Oslo, Norway.

The characteristic sound of a CW signal that has passed over a moderate geomagnetic disturbance (Kp = 2) isn't too hard to recognize with a little practice. There is also some static on this short recording as we had a local thunderstorm coming in with the first summer days here.

The great circle distance between the two cities (actually their two airports) is 6446 km as illustrated by GPSVisualizer. Anchorage and Oslo are about at the same latitude, 61° N and 60° N, respectively. As they are at longitudes 149° W and 10° E, there is a difference of 159 ° - almost 180 ° - and this means that the signal almost passes directly over the North Pole, although a bit hard to see on the projection used in the map.

Long Delayed Echo on VOA Chinese Service

Thierry, F4EOB from Paris is still hearing strange echoes on the VOA Chinese service broadcasts. There isn't really any good explanation for this phenomenon.

Now during winter he is hearing it both on 13650 kHz from 9 to 12 UTC and on 21590 kHz from 9 to 11 UTC. The 21590 kHz transmission has been heard by him for a long time and I mentioned it here last year also. As then the echo is about 2 seconds. Thierry also made a youtube video of it with a recording.

The transmitter locations are in Asia. The 19 m band site is on the Mariana Island (Tinian) and the 13 m band transmitter is in Tinang in the Philipines.

In my blog last year I discussed possible explanations such as multiple transmitters or multiple round-the-world travel. But since the delay is so consistent and has had the same delay for such a long time, the probability that it is man-made is rather large.

Thierry tells me that this LDE can easily be heard with the WebSDR at the University of Twente in the Netherlands also. I would be curious to hear from people outside Europe who could compare the Dutch WebSDR with their own local reception and see if the same echoes are heard everywhere.

The most interesting contact in the CQ WW Contest

Sometimes looking up remote stations on QRZ.com or other sites gives a glimpse of the person behind the callsign. I did this for the Chinese station BY5CD which I contacted on 40 m during the CQ Worldwide contest this weekend.

It turned out to be a club station called "YinZhou Middle School Amateur Radio Club Station" which is located just south of Shanghai.

It is interesting to consider the age of the operators as one can see from the picture. More pictures and some information can be found on their QRZ.com website. With this many young people entering ham radio in China, maybe we will see more stations there in the future. The number of stations is unreasonably low compared to the enormous population of China.

And the most interesting signal was that of GM5X on 21 MHz at 1207 UTC on 25. November. It had a distinct echo which seems to indicate that the signal travelled both on the direct path of about 800 km and the long path of about 39200 km.

The image shows "GM5X GM5X Test". The long path signal seems to fade in and out as there is much less of it in the last part, the word "test", than in the second "GM5X".

F/LA3ZA on Long Delayed Echoes

Laurent, F6GOX, who is one of the primary forces behind the ARP, Radio-Club de Paris, has written a nice little French-language presentation of me since I am a former member of the club. It also includes my interest in Long Delayed Echos (LDE).

Here he also talks about the presentation I gave for the Paris club of radio amateurs on 18 March 2009 on this subject during my year-long stay in Paris. That was a very nice evening which I remember with pleasure.

I also talked then about the interest that the French general Gustave Ferrié (1868 - 1932) took in this phenomenon (The link is in French, but read about Ferrié here in the English Wikipedia). There were French studies of LDEs in Indochina, Senegal, and Mauretania, French territories at the time, which he played a major role in.


Merci Laurent!

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.

More long delayed echoes

4.5 metre high UK acoustic mirror used as early warning system
during WW1. Not so relevant for Long Delayed Echos, but something
which definitely reflects well. Wikipedia Commons, Paul Glazzard

The mystery of Long delayed echoes is still without a scientific solution. With that I mean an explanation in terms of an effect which is testable, i.e. where one can predict the value of the delay and under what conditions it occurs.

It is likely that there isn't just one explanation, but several. I found it helpful some years ago to list all 15 possible hypotheses and then narrow them down to The Five Most Likely Explanations for Long Delayed Echoes. This list is now being referenced on the RSGB site for Long-delayed echoes (LDE) as well as in the Wikipedia article on the subject.

In this blog post there are examples of multisecond echoes on 21 and 27 MHz.

Unusual HF Propagation Phenomena

Earthrise from Apollo 8 (NASA)

My blog has just been updated by migrating posts from my old web site containing a collection of audio samples from unusual HF propagation phenomena. They range from round-the-world propagation (delay 138 ms) to ducted transmission in the magnetosphere (delay ~140-300 ms) ending with moonbounced 7 MHz signals (delay 2.39 sec).

The posts are:

• Magnetospherically ducted echo
• Three paths from Japan to Norway along the grayline
• Propagation via aurora reflection
• Polar flutter
• Moonbounced echoes on 6792.5 and 7407.5 kHz

Multiple 28 MHz propagation paths and excellent conditions

Final 'F' in UA3MIF. Notice echoes of final dot
marked by arrows. Press image to enlarge.

There were excellent conditions during this weekend's Ukrainian contest. I was active on 10 m and noticed several reverberant signals coming from Russia and Kazakhstan around 11-12 UTC on 6. Nov.

Here is an example from UA3MIF (28 MHz, 11.30 UTC 6. Nov 2011, press link to hear audio) as received on my vertical end-fed dipole (omnidirectional) for 10 m and the K3.  See the picture for the final 'F' in his call sign and what seems to be multiple echoes. The final dot starts at 3.247 sec and multiples seem to occur at 3.322 and 3.373 seconds, i.e. after 75 and 126 ms.

Although 126 ms is almost 138 ms, the round-the-world travel time, I could need some help to interpret these delays based on the locations of the transmitter relative to me near Oslo, Norway. It was easier for a similar reverberant signal from JA3YBK some years ago.

Aurora, 50 MHz and the Elecraft P3

Tonight had nice aurora conditions on 6 meters due to a geomagnetic storm. The Elecraft K3/P3 combo did an excellent job in showing how wide the bandwidth of these CW signals has become after having been reflected off the aurora.

The first image shows the green cursor on SM7FJE's signal, the bandwidth is almost 1 kHz.

Magnetospherically ducted echo

Magnetic field lines in the
magnetosphere (Ill. NASA)

Signals in the 1.8 - 4 MHz range may pass the ionosphere and be ducted in the magnetosphere out to a distance of several earth radii over to the opposite hemisphere where they will be reflected on top of the ionosphere.

The round-trip time varies with the latitude of the transmitter, or to be more accurate with the position relative to the magnetic Equator. Typical delay times are 140 - 300 ms. At my location near Oslo, Norway, the expected delay is about 308 ms, but unfortunately I have yet to hear such an echo.

Three paths from Japan to Norway along the grayline

The Japanese station JA3YBK was received in Oslo (press here for audio), 30. November 2003, 08:20 UTC on 21.004 MHz during the CQ WorldWide contest. The sound is reverberant due to the multiple copies that are received. This can be seen as extra signals, and it is in particularly easy to see after the first short dot (‘e’ in ‘test’).

Propagation via aurora reflection

Northern Lights in Tromsø, Norway 

Here is a recording of the Swedish station SM6CNN in contact with me on 29. October 2003, 22:42 UTC on 28.024 MHz. The direct distance is about 275 km.

Note the noise-like signal which is typical for signals that have been reflected off the aurora on the higher HF and on VHF frequencies. There was a strong geomagnetic disturbance that day with the K-index at Dombås, Norway reaching a value of 9 which explains the occurrence of aurora reflection.

Polar flutter

Arctic areas

Here is a recording of JW/DJ3KR on Spitzbergen from 24. August 2007, at about 21:45 UTC. This was only a few minutes after I had contact with him on 7.002 MHz.

Notice the fluttery character of his signal. On this day it wasn’t too bad and it wasn’t too hard to copy his signal, but on other occasions, signals that travel over the polar zone may be impossible to copy correctly. The K-index in Tromsø, which lies between Oslo (60 deg. N) and Spitzbergen (78 deg. N), at the time was 0.

[Image source: regjeringen.no]

Moonbounced echoes on 6792.5 and 7407.5 kHz

Recently, a facility with big enough antenna (300 by 365 m) and high enough power (3.6 MW) was used to set a new record for how low in frequency one can go and still get echoes from the moon.

This was also done during the HAARP moonbounce experiment which encouraged radio amateurs to listen on 19 and 20 January 2008. At my location in Oslo, I only heard the direct signal from Alaska, but many in the US heard good moon echoes.