Lack of Short "Skip" on 75 Meters

[Steve - WB3HUZ]

The best explaination I've seen on why we often see close-in stations fade out at times on 75 meters over the past few winters.

From the ARRL Propagation Letter

A number of people have inquired about short vs long skip on 75/80 meters, wondering why short skip is often non-existent, but long skip is enhanced. Dennis Carlson, K9ZMI, of Arlington Heights, Illinois, provides an explanation: "As I understand it, the effect we are seeing is that the F-layer ionization is weak because of low sunspot activity. A weakly ionized layer has a low index of refraction (a term used in optics) which impacts the amount of bending of the HF radio wave impinging on it. Low index of refraction equals not much bending. So an HF radio wave leaving an antenna is typically headed toward outer space but is bent back (refracted) towards Earth when it reaches the ionized layers above Earth. The amount of bending depends on the index of refraction and, for a given index of refraction, the angle of impingement determines if the radio wave will return to Earth or not. HF radio signals impinging on the weakly ionized layer at a high angle (necessary for short skip communications between stations close to each other) are not refracted enough to be 'turned back' to Earth and they simply radiate into space. Signals impinging on the weakly ionized layer at a low angle are refracted enough to be 'turned back' to earth and they appear at a large distance from the signal's origin, which is long skip."

[Bacon, WA3WDR]

The lower angle of solar incidence caused by wintertime, the solar variations that track the rotation of the sun, and the low level of sunspot activity, can cause the vertical-incidence Maximum Usable Frequency (MUF) to drop below 4 MHz at night.  Terman (Radio Engineer's Handbook) said it got down below 2 MHz during some winters at solar minimums.

One thing I noticed while experimenting with circular polarization on 75 meters back around 1990 was that signals from stations in the 0 to 500 mile range would be observed to be arriving with one circular sense, and when 75 meters started 'going long', signals from closer stations would reverse sense and continue coming in, weaker, in the opposite circular sense, and then they would fade out almost completely.  The reverse-sense signals continued to be receivable for a while, I think about 15 minutes, after I lost the normal-sense signals.

Supposedly, signals reflecting off the ionosphere split into two types, 'ordinary' and 'extraordinary'.  It's supposed to be an effect of the earth's magnetic field and the plasma in the ionosphere.  A signal will go up to the ionized layers with some linear polarization, and it will refract back in two parts: one stronger part with one general circular sense, and one weaker part with the opposite circular sense.  The stronger one is called 'ordinary', and the weaker one is called 'extraordinary'.  The degree of circularity seems to vary, but the two refractions have opposite general circular 'sense'.   Evidently the 'ordinary' part fades out first when the band goes long, and the 'extraordinary' part continues a little longer, before fading out.  Put another way, the MUF of the ordinary path is lower than the MUF of the extraordinary path, and as the night progresses, the ordinary path will fade out before the extraordinary path does.

I saw that difference because I was experimenting with a circularly polarized antenna.  With a linear polarized antenna like a dipole, we usually just see the near-in station signals get weaker, and then fade out, while the distant stations continue to be received.

On a slightly different subject, ionospheric refraction appears to be a reflection from a higher layer because of the angles of transmission and arrival, but it is interesting that the time delay of the path is consistent with the apparent reflecting height, even though the refraction is taking place at a lower, sometimes much lower height.  The radio signals must be moving rather slowly in the plasma when we get near the MUF, and the speed seems to be related to the bend angle.  That makes sense, but for the angles and the delays to agree, and to point to a higher apparent reflecting layer that is not there, is surprising to me.

[Steve - WB3HUZ]

Looking at the ionosonde data recently, the MUF has gone down into the low 3 MHz range. Bye-bye short skip on 75 meters.

[K1JJ]

"Signals impinging on the weakly ionized layer at a low angle are refracted enough to be 'turned back' to earth and they appear at a large distance from the signal's origin, which is long skip."


Steve and I have been seeing this effect and the opposite on 40M lately while working Euro DX.

Steve's 40M delta loops at ~ 1/2 wavelength high have a take-off angle somewhere in the high 20's degree area - while my 40M Yagi stack is about 13 degrees. Recently we have had similar signal reports into the closer in European countires. This is cuz there has been some minor sunspot activity and the higher angles are becoming more useful as described. Sometimes my stack just skips over western Europe as shown by the low dipole being close to the stack in strength.  Though farther east into Russia the stack plays well again.

Chuck recently told me I wouldn't be happy with such a low angle later into the sunspot cycle on 40M as the optimum angles moved higher. I will need to add an "out-of-phase" config to get the stack to switch to a higher 30 degree angle when needed.  Or, a long boom wire Yagi at 55-65' will do the same thing.

So, take confidence that 160-40M will become more local friendy as the spots come back and the lower antennas will start to perform very well for DX again.  The low angle antennas are king during the extreme minima area, but like everything, that will change and they will be booted out on their asses, just like our leaders in the next election, caw mawn.

T

[Bacon, WA3WDR]

Interesting that this happens as the sunspot activity picks up.  I thought that the virtual height of the reflecting layer is higher when you get close to the MUF for the path, and the higher MUF resulting from the increases solar activity would make the optimum take-off angle lower for a given distance contact on a given band as the solar activity increased.  But you are seeing the opposite.

Do you think it has to do with a different number of hops that are possible because of higher angle reflections that can now happen because of the higher solar activity?  I think I need to do some iono-geometry.

[WA1GFZ]

Tom,
That is exactly why I think the feed lines need to come to the ground so you can configure the system for the conditions without having to go up the tower.

[Todd, KA1KAQ]

Short skip has been great on 40m lately. More like daytime conditions of 4-5 years ago and before. Had a great chat with HUZman yesterday along with Beefus, ICQ, K3JRR and Dave WB4IUY. We carried on for a couple hours and things were still going strong when I checked out. Came back a few hours later and was still hearing a number of strong signals well into mid-late afternoon.

Lack of local propagation really knocked the snot out of AM operations on 75/80 over the last few years. Hopefully things are starting to level out and improve.

[Steve - WB3HUZ]

I don't think it's as much that lower angle propagation gets worse, as higher angle propagation gets better.

Interesting that this happens as the sunspot activity picks up.  I thought that the virtual height of the reflecting layer is higher when you get close to the MUF for the path, and the higher MUF resulting from the increases solar activity would make the optimum take-off angle lower for a given distance contact on a given band as the solar activity increased.  But you are seeing the opposite.

Do you think it has to do with a different number of hops that are possible because of higher angle reflections that can now happen because of the higher solar activity?  I think I need to do some iono-geometry.

[K1JJ]

Yes, the best definition is "optimum angle" for a given path  -   and it's always changing in both the short term and long term.


Bacon:  Logically what you say seems correct and I used to think the same thing. I'm no prop expert, but after talking wid the gurus about what the solar cycle effects on the lower bands (160-40M - based on their experiences in the past) it appears the higher angle antennas will begin to gain on the lower angle antennas again with the increase in sunspot and general magnetic activity. I understand the low Yagis on 10-20M have their place at solar peaks too.

I noticed the effects on 75M too as we moved from the mid cycle into the minima. My low 75M dipole was many times equal to the high Yagis to the point of thinking there was something wrong. But throughout the night the angles wud shift around and the high Yagi (low angles) would prevail at times.  Then came the solar minima and the differences were sometimes 20-25db! It was amazing. The low dipole was deaf into Euroupe on a relative basis cuz of the lossy path for higher angles.

The reasons are well described in readings and many times debated - but the actual effects are repeatable throughout the years.

The bottom line is, as always, it's best to have both a higher and lower angle antenna for a given band to "match" up the angles we wish to use. It's much like choosing a certain iron number to give the correct stroke and elevation for a golf shot.  Forward gain with a Yagi might be only 4 to 5db, whereas choosing the proper takeoff angle may mean a 15-20 db advantage or more.

T

[Steve - WB3HUZ]

It really showed later on last evening (maybe an hour after sunset) on 40 meters. I was surprised the number of times we received similar or very close signal reports. Your antenna has quite a bit more gain than mine, but as you said, it's at a much lower angle. To Western Europe though, that advantage seemed to disappear it times.

Check out this video from the Balearic Islands - Mallorca. It looks like your signal is about one more LED segment (not sure how many dB it represents), but the audio and SNR from our two station is virtually identical. I would have never expected this.


http://www.youtube.com/watch?v=-7fgf_64pW8

[K1JJ]

It's cool to hear and see the recording.

It looks like each LED bar is about 4db or so.  Gawd, I wud hate to live my life looking at those bars vs: a standard analog meter...

Yes, the EA's are very close to us. In fact, I was thinking there's times like this when you probably have the QTH advantage of being FARTHER away to let the skip work out....    It's almost like when we can hear the W8's well, but the other W8's can't hear each other, etc.

Well, I guess soon we'll have to look back at those solar minima low angle days with fondness. But lest we forget, here's one of these "special" nights when the lower angles were optimum for a long haul trip to TA4AU  in Turkey - exactly at the solar minima last Sept, '09...

http://www.youtube.com/watch?v=TvrMV6q5Lhs

T

[Steve - WB3HUZ]

Ah yes, the good old days.... 

[KM1H]

Hey Steve, does that mean we should start trying for 80M moonbounce?

Carl

[Steve - WB3HUZ]

Now would be the time. In a few years it will be impossible.