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Saturday, March 16, 2024

2024 Solar Eclipse DXing

DXing the mediumwaves promises to be an exciting event on April 8 during the 2024 total solar eclipse.

I've been mulling over the DX possibilities a lot lately and have come to some conclusions. I think it boils down to three promising DX scenarios.

Scenario 1. For those who live within or very near the path of totality, I believe best chances of DX would be first to listen to your southwest, along the path where totality is approaching. Darkness will already have happened in that direction, and a certain amount of residual de-ionization of the ionosphere will still remain. After the point of totality passes your location, I would swing my attention to the northeast.

Scenario 2. For those living within about 800 km (or about 500 miles) of the path of totality I believe best chance would be a perpendicular path across the totality path to a point roughly equidistant on the other side. This puts the signal reflection point right at the center of the totality path, or the deepest point of darkness.

Scenario 3. For those living more than about 800 km from the path of totality I believe best chance would be along a line from your receiving site to a perpendicular intersection to the totality path. This should define the greatest shaded path.

I think scenarios #1 and #2 have the best possibility for DX. Incidentally, across the U.S. the totality path varies from about 170 km to about 200 km wide, or 105-125 miles.

Important to keep in mind - skywave signal strength analysis is based almost entirely on the condition of the ionosphere at the reflection point, not at the receiving site. For single hop propagation, normally the reflection point is at the halfway point to the station along the great circle route.

That 800 km distance from the totality center I wouldn't hold as gospel. I'm throwing that figure out as a point where scenario #2 may start to transition to scenario #3.

Timing is of the essence for DXing. The shadow velocity exceeds 1000 mph, increasing from 1587 miles per hour at Eagle Pass, Texas to 3176 mph at Houlton, Maine. You may have only minutes to DX.

I'll be in Rochester, NY at the time of totality, and we are right at dead center. I'll be scenario #1. My plan is to listen to my southwest initially, where totality is approaching. I'll be listening particularly for WLW-800 in Cincinatti, OH, WHAS-840 in Lexington, KY, and others along or near that path.

Scenario #2 possibly holds the most promise. Calculate your distance to the path center line and look for stations on a direct line across the totality path and at an equal distance on the opposite side of the path from you. One such scenario might be WSB-750, Atlanta to a reception point in northwestern Illinois, central Iowa, or southern Wisconsin or southern Minnesota. Many possibilities on cross-paths exist here. I feel best results would be with a signal path that crosses the path of totality closest to 90 degrees.

A question was raised about the possibility of DX from Spokane, Washington, an extreme distance from the path of totality. That particular scenario would be scenario #3, more than 800 km to the path of totality. Maximum obscurity should be when northeast Texas (let's say the Dallas area) is experiencing full totality, as the great circle line to the totality path intersects at approximately 90 degrees to the line at that point. This would be at about 1848 UTC. I would listen for any signals along a great circle path between Spokane to anywhere from the Dallas area and northward.

Obviously, Spokane to Dallas is an extremely long one hop path, at about 2450 km. At that distance, the reflection point is near Denver, which will have a solar obscuration of 65.1 percent at maximum.

A Dallas area reception would be next to impossible I would think, but there are many more stations along that great circle path one could try for. Closer stations will obviously move the reflection point closer and start to reduce the solar obscurity. I did a scan along that path and there are some 340 stations within 200 km either side of the line of the great circle path between Spokane and Dallas.

Check out these links.

https://nationaleclipse.com/cities_partial.html

https://eclipse.gsfc.nasa.gov/SEpath/SEpath2001/SE2024Apr08Tpath.html

https://eclipse2024.org/eclipse_cities/statemap.html

Click image for full size.



Sunday, January 14, 2024

C.Crane Twin Coil Ferrite Signal Booster DISCONTINUED!

It's always been in my head to try an actual C.Crane Twin Coil Ferrite Signal Booster. I've used tuned Q-stick devices before with a lot of success. They have sharp nulls and lots of inductive gain. The Crane unit promised more of the same with its 8-inch twin coil ferrite rod.

https://ccrane.com/twin-coil-ferrite-am-antenna-signal-booster

Checking the Crane website the other day, I was shocked to find that they have discontinued it. Now, if I want to give it a try, I'll have to find one elsewhere - maybe eBay. I see there are a few of them out there yet. I'd advise you to pick up one of these fabulous goodies before they are gone forever.



Wednesday, January 10, 2024

U.S. Broadcast Station Counts 1922-2022

100 years of AM broadcast in the U.S.

Shown below on the graph are the counts of licensed stations in the FCC's AM broadcast service for the years 1922-2022. You will see a steady rise in counts from 1922 to 1990.

I remember listening as a kid in Philadelphia in the very early 1960s. The band was literally alive with stations every night from as far away as St. Louis (KMOX), San Antonio (WOAI), New Orleans (WWL), Minneapolis (WCCO), and many others. Even the Mexican border blasters with the likes of Wolfman Jack were a normal catch every night.

We are in slow decline over the past thirty years and gaining more downward speed in the last ten. When will it end? What decade was the heyday of AM broadcasting? You decide.



Friday, November 24, 2023

Mediumwave Skywave Prediction #6 - Wrapping Things Up

Let's close up by defining a few of the terms used in the formulas from the last post, and finish by talking a bit about diurnal and seasonal effects.

WHAT IS POLARIZATION COUPLING LOSS?

Polarization coupling loss, sometimes depicted as Lp, is the fraction of incident power lost on entry into the ionosphere. Further polarization coupling loss occurs when the wave which emerges from the ionosphere induces a voltage in the receiving antenna. Polarization coupling loss depends to some extent on frequency and angle of incidence at the ionosphere. Polarization coupling losses are low in higher latitudes because the Earth's magnetic field is almost vertical. At the magnetic equator, however, the Earth's field is horizontal and polarization coupling losses on east-west paths are large.

Polarization coupling loss at MF is an important factor in skywave propagation. It arises because the Earth's natural gyromagnetic frequency lies within the frequency band being considered. The gyromagnetic frequency of the Earth's ionosphere varies between 800 kHz in the equatorial regions and 1600 kHz near the magnetic poles. When a linearly polarized mediumwave frequency radio wave enters the ionosphere, it gets split into two waves known as ordinary and extraordinary. At the gyromagnetic frequency the extraordinary wave is so greatly attenuated that it makes a negligible contribution to the received signal. As a consequence, the extraordinary wave can be disregarded within the mediumwave band. The propagation is therefore by the ordinary wave.

To explain further, conventional antennas at mediumwave radiate vertically-polarized waves. At MF, the wave which is accepted by the ionosphere and which will propagate back to Earth usually differs in polarization somewhat - hence the ionosphere may not be excited efficiently by the incident wave. We have decreased coupling efficiency, or polarization coupling loss. The wave which subsequently emerges from the ionosphere is in general elliptically-polarized and in-turn may not excite the receiving antenna efficiently because antennas near the ground are most sensitive to vertical polarization, resulting in additional loss.

WHAT IS SEA GAIN?

For long distance paths (1000 to 6000 km or greater), when the path is over the sea and at least one end of the link is located on or near the sea coast, the phenomenon of sea gain can add from 3 to 10 dB to the predicted field strength. 

Gains peak at the usual single, double, and triple hop distances of 2000 km (8 dB), 4000 km (10 dB), and 6000 km (10 dB). Only about 3 dB is gained at the 1000 km distance. A dip in gain (to about 5 dB) occurs at about the 2500 and 5000 km distances.

A knowledge of the land-sea boundary information is necessary to assess the sea gain phenomena. Generally, in the skywave calculation, the sea gain correction is normally set to 0 dB without this knowledge. To take any advantage of sea gain, one of the terminals (transmitter or receiver) must be within about 10 km from the sea coast. Even at 10 km inland, the penalty is about -4 dB. At 4 km, about -2 dB. At 3 km, only about a -1 dB penalty.

SOLAR CYCLE LOSSES

Solar Cycle 25 is well on its way now, having started its general upward trend in sunspot count by late 2020. The daily sunspot count for August 30, 2023, for example, was 104.

Do sunspots effect nighttime skywave propagation at the medium waves? Yes they do, at a small but noticeable level. Here are the details.

Concerning medium wave, sunspots and the increasing solar flux are relevant to skywave field strength and are accounted for in most modern (nighttime) skywave prediction methods. In general, mediumwave skywave field strength is slightly better during low or zero sunspot periods, at the bottom of the solar cycle. The calculation of the additional path loss in dB is dependent on location.

Greater consideration is given to paths within North America and Europe (nearer to the north geomagnetic pole), and Australia (nearer to the south geomagnetic pole). The North American loss factor is 4 times that of Europe and Australia, and rises for all as we get to the higher latitudes. Longer paths, those between North America and Europe are usually interpolated.

The ITU skywave prediction method is one such method which incorporates these added loss factors due to sunspots and solar flux. Figures below have been extracted from that prediction method.

Below are increased single hop skywave loss factors in dB as the sunspot count goes up.

Paths within North America:

Sunspot count = 0 a net added loss of zero
Sunspot count = 7 an additional loss of 0.28 dB
Sunspot count = 25 an additional loss of 1 dB
Sunspot count = 50 an additional loss of 2 dB
Sunspot count = 100 an additional loss of 4 dB

Paths within Europe:

Sunspot count = 0 a net added loss of zero
Sunspot count = 7 an additional loss of 0.07 dB
Sunspot count = 25 an additional loss of 0.25 dB
Sunspot count = 50 an additional loss of 0.5 dB
Sunspot count = 100 an additional loss of 1 dB

Paths between North America and Europe:

Sunspot count = 0 a net added loss of zero
Sunspot count = 7 an additional loss of 0.175 dB
Sunspot count = 25 an additional loss of 0.625 dB
Sunspot count = 50 an additional loss of 1.25 dB
Sunspot count = 100 an additional loss of 2.5 dB

Admittedly, these extra losses are small but important enough that they are factored in for skywave calculations. Be aware that 3 or 4 dB can make a difference logging a station or not. A single S-unit is 6 dB.

DIURNAL EFFECTS

The final determination which really completes our skywave field strength calculation must include three more tweaks:

1. Diurnal hourly losses/gains
2. Sunrise and sunset enhancements
3. Seasonally-driven losses/gains

The D-layer of the ionosphere is characterized as having a strong dependence on frequency, but this is present only during the daytime. The E-layer is the dominant contributor to LF and MF propagation at night and is only mildly dependent on frequency, so the effects of frequency of this layer can be neglected for most practical purposes.

Although daytime ionospheric propagation is relatively unimportant, it cannot be entirely disregarded at the upper end of the band, since ionospheric attenuation decreases with the square of the frequency. Nor can it be entirely disregarded at the lower end of the band, where partial reflection from the lower edge of the D region may occur, especially in winter at temperate latitudes.

The critical frequency of the normal E layer is about 1500 kHz at sunset, but it then falls rapidly as a result of electron-ion recombination and will assume a value of about 500 kHz late at night. Skywaves may be reflected from the E layer, or they may penetrate the E layer and be reflected from the F layer, depending on the frequency, path length, and time of night. Simultaneous reflection by both layers is also possible in some circumstances. 

Upper MW band diurnal (or daily) morning enhancement can show effect as late as 3 hours after sunrise. The start of the pre-sunset afternoon enhancement is delayed a little to about 2 hours before sunset, gradually building to sunset. The lower part of the band shows little of this effect, morning or night.

The diurnal enhancement described in the last paragraph is not to be confused with the short sunrise and sunset enhancements on extreme DX due to what is called "greyline effect", the signal traveling along, or partly along, the sunrise/sunset terminator.

Skywave propagation does indeed exist during the daytime hours, and its strength varies greatly, seasonally.

Daytime, noon-hour skywave is generally pegged at approximately 30 dB lower than the nighttime field-strength prediction, and this will vary considerably seasonally. An ionospheric transition period occurs immediately surrounding sunset and lasts till approximately four hours after sunset, and another occurs during the period from 2 hours before sunrise until sunrise where the field strength goes through this 30 dB change with a very steep slope. The shapes of the curves are not symmetrical for the transition from day-to-night and night-to-day.

WRAP UP

In this series I have attempted to present to you first a little history skywave propagation analysis, who developed the formulas and how they are geographically dependent, and the formulas themselves. I hope it has brought some perspective to the process and you have enjoyed it.