Backscatter on 28 MHz - 2nd Nov 2023

For well over 12-months, I have had my radio tuned to the WSPR frequency of 28.1246 MHz on the 10m band and I feed the decoded signals up to the WSPRnet website.

While I can see the WSPR signals clearly on the waterfall display, I can also see the very slow morse QRSS signals as well just a few hundred Hz below.

The image above shows the QRSS signals I could see on the 2nd of November 2023 and the locations of the stations are shown in the map at the start of the post.

The screenshot above shows a good capture of AE0V in the USA at about a distance of 6000kms. Ned, AE0V reports using a solar powered transmitter with no battery storage running 100mW into a 1/4 lamba stainless whip about 8m above the ground.

The signals from the USA and Canada are easily explained as they are via F2 layer propagation. The signals I find unusual are the ones from the England which are in the region of 500 to 650kms.

The trace from the 0.2-watt signals of G0PKT and G0MBA are there nearly all the time when the band is open. It's not F2 propagation in the usual sense as it's too close and it's not Sporadic-E.

I believe that it's backscatter just like what the military use for their over the horizon radar systems (OTHR). 

In this case, the 28 MHz band is open with F2 layer propagation and the signals from G0PKT & G0MBA are being reflected back towards my location from some distant point.

As an example of how consistent these signals are, I have decoded the WSPR signal of G0PKT about 1,000 times in the last 3-weeks. And that's a signal that's supposed to be in my 'skip zone' where it's supposed to be hard to reach.

There's nothing new about this, it's just that in this modern age of weak signal modes and waterfall displays, we can now see these very weak signals more clearly. 

If you're using FT8 on the higher HF bands and you see lots of reports from stations that about 200-600kms away then F2 backscatter is probably the reason.

Backscatter on 40 MHz - 19th May 2021

Wednesday 19th May 2021: The day started with a comment by Joe, EI3IX on a Facebook post that he wasn't hearing the new EI1CAH 40 MHz beacon even though he is only 64kms from it. That set me off checking why that was the case and I ended going down a rabbit hole that saw me on the 8-metre band for most of the day!

Both the EI1CAH (40.016 MHz) and the EI1KNH (40.013 MHz) beacons are about 200kms from my location with plenty of hills and mountains in the way. I listened early on the morning of the 19th and I couldn't hear either beacon although I did get a meteor burst from EI1CAH so I knew that it was still operational.

A few hours later, I tried listening again and I found I could now hear EI1CAH all of the time! What had changed? As the signal was buried in the noise with a Signal to Noise ratio of -26dB, my initial thought was that it might be tropo? However, I couldn't rule out that it may have been to Sporadic-E backscatter either.

It was only later when I saw the signal improve to -6dB over the space of a few minutes then I knew it was Sporadic-E related. While there are no doubt slight tropo enhancements at 40 MHz, large changes like this are almost certainly due to Sp-E.

Later when I checked the EI1KNH beacon near Dublin, I could that as well. At that stage, I was 100% sure it was backscatter. Lloyd, EI7HBB reports hearing both beacons by backscatter as well.

Backscatter Sporadic-E: What is it? Refer to the diagram below...

The signal from the EI1CAH beacon (TX) is being propagated forward by an intense Sporadic-E region about 100-110kms above the Earth. The signal is then being reflected off something maybe 500kms or so away... maybe a range or mountains? The signal then returns by pretty much the same path and I hear it at my location (RX).

I've heard backscatter via Sporadic-E several times before on 10m and 6m. Back in the 'old days' when everyone was on either on SSB or CW, I remember beaming South to work stations in the UK on 50 MHz.

The signals seem to have a certain characteristic in that they are pretty constant but just at a very low level. This is what I was hearing... EI1CAH was a weak but constant signal for most of the time with the occasional jump.

The truth is that backscatter is probably there all of the time during Sporadic-E openings but we're not aware of it as the signals are so weak. When there is an intense Sporadic-E with the skip distances much shorter (e.g. <700kms), signals get a LOT stronger.

Imagine a signal being reflected off a distant mountain range which is 1500kms away which is a pretty normal skip distance for Sporadic-E at 40 MHz. Now imagine the range reduces to 500kms under intense Sporadic-E like there was on the 19th of May. Obeying the inverse square law, the transmitted signal at the mountain range would appear 9.5dB stronger. But crucially, this also applies to the reflected path resulting in an additional 9.5dB enhancement. This adds up to a 19dB improvement overall in signal which is a really big jump. 

Back in the 'old days' with SSB/CW, backscatter via Sporadic-E was something that was observed on an occasional basis. Now that we have digital modes like FT8 and PI4, we are 'hearing' signals that are much much weaker.

Assumptions about Propagation: It is for me a reminder that we should always question our assumptions about a propagation mode when we hear a weak signal. Is a weak signal from someone say 200-400kms away on 28/40/50 MHz really tropo, short skip Sporadic-E or via Sporadic-E backscatter?

When we look at those FT8 or WSPR maps at the end of the day, how were we hearing all of those stations in the 200km-500km skip zone? Meteor Scatter? Tropo? Back-Scatter? I think the real picture is a lot more complicated than we assume.

Practical Application: It's all very well wondering about the propagation mode but most people will want to know what is the practical application? Imagine say you are a serious 50 MHz station and you want to work a new country which is only 400kms away and there are mountains in the way. You should never assume that just pointing your beam at a wanted station will always result in a stronger signal. If the signal is weak but constant then it may be via backscatter and you'll have to look for the direction which gives the strongest signals.

Addendum

For Reference: This map from DXMaps shows the suggested maximum usable frequency (MUF) at around 12:00 UTC on the 19th of  May.

It clearly shows one area over the south of England. Note however that this map is generated by users reports and areas of Sporadic-E out over the ocean are not shown. There could be an intense area to the west of Ireland and it would go unreported.

This was the Jet Stream at the time...

Changes in the direction of the Jet Stream are associated with Sporadic-E openings.

Data: Just for my own benefit, I've kept a list of the PI4 decodes below.

Free PDF articles from the UK Microwave Group

This might be of interest to those who operate on the microwave bands. Backscatter is a compendium of the best technical articles that have been in Scatterpoint, the UK Microwave Group newsletter over the period 1999 to 2006.

The original publication in 2008 was an A5 size book of 445 pages and is an excellent reference with many articles on all aspects of microwaves.

It is now freely available as pdf chapters via the link below:- http://www.microwavers.org/?backscatter.htm

28 MHz...Wed 3rd Oct 2012

Another day with some good conditions on the 10 metre band. I left the radio on the WSPR frequency for most of the day........which is great as I don't have to be there! ;o)

I had a quick listen at various times as well and it was different from the previous days. No Australians heard this time but DU1MGA did make it through with one spot on WSPR...

2012-10-03 10:48 DU1MGA  28.126068  -28  1  PK04lc 5   EI7GL   IO51tu 11213 331

On SSB, I heard VU3RAZ in India and A4 and A6 stations in the Middle East. Also, lots of Russian and Ukraine stations at one F2 hop distance.

As the map shows above, there was some Sporadic-E as well with some of the closer European staions in Germany and Spain making it through. Later, it opened to the USA again, the first time really since last Sunday and the aurora on Sunday night. The one of interest here was N6OIL making it through from California...


 2012-10-03 18:08 N6OIL 28.126067 -28 0 DM14fl 0.2 EI7GL IO51tu 8212 37 
 2012-10-03 17:50 N6OIL 28.126068 -22 0 DM14fl 0.2 EI7GL IO51tu 8212 37

Another point of interest was GW7PEO made it through again, presumably on back scatter?...



 2012-10-03 13:52 DB0ZDF 28.126080 -24 -1 JN49cx 5 EI7GL IO51tu 1178 287 
 2012-10-03 13:44  GW7PEO 28.126137   -21  -1   IO83gh 5 EI7GL IO51tu 370    246 
 2012-10-03 13:44 EA8FF 28.126107 -12 0 IL18pc 2 EI7GL IO51tu 2730 12 
 2012-10-03 13:44 DG2NBN 28.126031 -24 0 JN59nr 5 EI7GL IO51tu 1387 287 
 2012-10-03 13:40 CX2ABP 28.126066 -24 1 GF15wc 10 EI7GL IO51tu 10705 27 
 2012-10-03 13:38 EA8FF 28.126107 0 0 IL18pc 2 EI7GL IO51tu 2730 12 
 2012-10-03 13:38 DF6MK 28.126047 -24 0 JN68ik 5 EI7GL IO51tu 1544 292 
 2012-10-03 13:38 KK4XO 28.126004 -13 -1 EL96uf 5 EI7GL IO51tu 6527 43


Again, EA8FF was being heard at the same time with a reasonable signal indicating good conditions to the south. In reality, the back scatter could be from anywhere....south? south-west? south-east? Today, the spot from GW7TEO was at 13:44 UTC. The two spots from yesterday were at 14:30 and 14:52 UTC.

Overall.....some notes...
1) Over the last few days that I have been listening on the 10 metre band, the Solar Flux index has been around the 120-130 mark.
2) Lot's of multi-hop F2 conditions but very variable. Open to different areas on different days.
3) The most consistent signals as expected are the single hop F2 ones....East Mediterranean, Russia.
4) The most consistent beacons are those around the 3,250 to 3,750 km mark. e.g. 5B4CY/B in Cyrus, SV5TEN/B in Rhodes.
5) When conditions on 28 Mhz got very good, the skip distance shortened and the beacons around 2,400 kms were heard.
6) There is a fishing buoy on the WSPR frequency! It ID's itself as 'DJ' but seems a bit low in frequency so is not a issue.
7) Using WSPR to measure band conditions is somewhat limited by the number of people using it. For example, there is a lack of WSPR stations in the mid states of the USA. All you can use the WSPR map for is to gauge where the band is open to, not where it is not open to. The band can be wide open to southern Russia and there can be no WSPR spots.

Backscatter.........For anyone not familiar with the principles of back scatter, it works as follows....

A signal from a transmitter 'bounces' off the ionosphere and lands in a target area. A small percentage of that signal may be reflected back towards the TX due to mountains, waves on the oceans, etc. Received signals tend to be very weak.