Signal Strengths of VHF Sporadic E Propagation

[VK2ZRH]

An explanation of signal strengths observed on VHF via sporadic E propagation has never been detailed in the scientific or technical literature, let alone amateur radio publications. Until now.

SPORADIC E VHF PROPAGATION – 4x1 TO 5x9++ – HOW?

Roger Harrison VK2ZRH

Sporadic E propagation is a capricious thing. Exciting, but capricious. Anyone who has spent a summer season chasing Es DX cannot help but acknowledge that.

The band ‘opens’ , but you never know how long it will last. You hear other local stations working DX you can’t hear. You hear “rock crushing” signals from stations running a 10 watt rig and then scratchy signals from a station you know runs “the full bottle”. Capricious. Exciting. Fun.

I’ve pondered for decades about sporadic E VHF propagation and spent many hours of rewarding research asking that question uttered at five minute intervals by every 4 year-old – “but, why”? Over recent years I’ve managed to discover a lot of answers, which have featured in papers I’ve presented at Gippstech Conferences (http://www.vk3bez.com) every so often.

In February 2011, I published here on the Forum site “A New Model of Sporadic E VHF Propagation”, which explained, for the first time (ever), how MUFs beyond the ‘classical MUF’ are supported – by petit chordal hop propagation [1].

From recent research, I can now derive how the signal strengths of Es VHF propagation are achieved – from the weak and scratchy to the 5x9++ rock crushers.

I wish to acknowledge the help I had from Scott VK4CZ and Joe VK7JG (one of the ‘keepers’ of the VK7RAE beacon site), both of whom provided salient technical details and observations, Andrew Martin VK3OE, who provided useful comments, and Adam VK4CP, for the reliable provision and maintenance of the VK Logger.

The Quick Summary

Sporadic E VHF propagation is accompanied by two kinds of ionospheric loss – (a) absorption, and (b) polarisation loss. Absorption loss, although small, is dependent on the solar zenith angle and the sunspot number, amounting to a few dB. Polarisation loss may be as much as 6 dB.

Es VHF propagation is also accompanied by propagation gain – from raypath focusing, which can boost signal strengths by typically 6-24 dB, and as much as 35-40 dB.

Signal strength losses can also arise from destructive raypath mechanisms, which can reduce signal strengths by as much as 20-35 dB.

PART 1

Case Study for a Reference Es Path

Calculating path loss parameters to get an idea of signal strengths is nothing new for VHF tropospheric scatter, satellite communications and moonbounce, for example. It’s all set out in “the good books”. The path and communications system is divided into its component parts: transmitter, feedline, antenna, propagation path characteristics, antenna, feedline, receiver. Put numbers to the parameters and you can calculate an expected signal level at the receiver. It should be no surprise when you get what you expect (or close to it) when communication is attempted under “real” conditions. It’s exciting when that happens !

Likewise for HF ionospheric propagation via the E and F layers. HF propagation prediction programs for PCs have been offering signal strength estimates along with propagation forecasts for at least 25 years. But the algorithms for signal strengths on HF don’t work for VHF sporadic E. I found that out when I developed and marketed HF propagation prediction software 20-odd years ago.

Nevertheless, a sporadic E VHF propagation path can be broken down into components and an idea gained of where the gains and losses lie to enable calculation of expected signal strengths. If it’s that simple, why hasn’t anyone done it before ? Good question. Answer is, it’s not quite that simple.

To fix a few realistic parameters and eliminate as much “guess work” as possible, I settled on a known Es VHF propagation path that is frequently reported on the VK Logger, this being reception of the VK7RAE beacon in south-east Queensland (VK4). In my February 2011 posting on this Forum [1], I used a logger spot of VK7RAE by Scott VK4CZ as a case study in how sporadic E parameters from the Canberra ionosonde relate to the path’s MUF. Posting spots on the Logger is an extraordinarily useful practice.

The VK7RAE-VK4CZ path mid-point (the point of reflection from the Es cloud) is within the ‘view’ of the Canberra ionosonde (a circle with a radius equal to the Es height [1]). Figure 1 shows the salient path parameters.

Figure 1. The Reference Path – VK7RAE-VK4CZ – reception reported at 2304 UT 2/01/2009, RST 549; h’Es was 92 km.

A sporadic E cloud or patch is a thin layer – hundreds of metres to a few km thick – of metallic ions with an accompanying electron density that is very much greater than the E region in which it is embedded. Indeed, sporadic E is a “stranger” within the E region at 90-120 km. Because the Es is thin, it is a very efficient reflector in refracting radio waves back to earth. It is said to be a specular reflector – that is, “mirror like”. It is the electrons that do the work of reflecting radio waves.

So, in Figure 1 we have a 6m (50.057 MHz) beacon transmitter at VK7RAE in Devonport, Tasmania, the 6m station of VK4CZ on the north side of Brisbane, a sporadic E cloud at the midpoint between them and the raypath, VK7RAE-Es-VK4CZ. The terrestrial distance between VK7RAE and VK4CZ is 1648.7 km – as reported automatically in the VK Logger spot as the Logger “knows” the latitude and longitude of the two stations.

Path Distance Loss

Leaving aside the two stations’ parameters for the moment, we can calculate the signal loss arising from dispersion of the radio wave (the radio wave spreading its power over greater and greater area the further it goes), generally known as the “Free Space Path Loss”. The formula for this, widely published in “the good books”, is:

Free Space Path Loss (dB) = 32.45 + 20 log(D) + 20 log (F) – 1.0

where D is in km, and F is in MHz.

Now wait a minute ! The distance travelled by the radio wave is NOT the same as the terrestrial distance. The trigonometry of calculating the raypath distance is a bit beyond high school maths, but fortunately, someone’s done it for us before [2]

Here’s what Sir Edward Appleton prepared for us earlier:

In this instance, the raypath distance turns out to be 1674 km.

Substituting the beacon frequency and the raypath distance into Equation 1.0, the free space path loss works out to be 130.9 dB.

VK7RAE Beacon Parameters

Transmitter output: 20 watts.
Antenna: crossed dipoles (“turnstile”); ie.10 watts per dipole.
Feedline: 9 metres of RG213.

The beacon is housed in a small corrugated iron shed having a sloping roof. The antenna is mounted on a pole about 1.3 metres above the shed’s ridge line. For the purpose of estimating a radiation elevation pattern for the beacon antenna, without doing a complex computer simulation, I decided to assume that the roof ‘ground plane’ averaged about 2.7 metres below the crossed dipoles, or about half a wavelength. The elevation radiation pattern probably looks something like that in Figure 2, derived from an EZNEC simulation of a dipole a half wave above ground.

Figure 2. Likely elevation radiation pattern of the VK7RAE 6m beacon antenna.

From Figure 1, the raypath elevation angle is 2.6 degrees, so the VK7RAE antenna radiation at that angle is approximately -9 dbi. Don’t despair

The feedline loss is about 0.5 dB and there may be, conservatively, another 0.5 dB loss in connectors and the phasing harness. Total loss from transmitter to either dipole – 1 dB.

On to Part 2.

[1] Harrison, R. VK2ZRH 2011, "A New Model of VHF Sporadic E Propagation", viewtopic.php?f=43&t=9832

[2] Appleton, E.V. and W.J.G Beynon, 1940 “The application of ionospheric data to radio-communication problems: Part 1”, Proceedings of the Royal Physical Society, Vol. 52. (Available online).

[VK2ZRH]

SPORADIC E PROPAGATION – 4x1 TO 5x9++ – HOW? – PART 2

VK4CZ Station Parameters

Antenna: 9-element 6M2WLC Yagi; 14.0 dBd free-space gain [3], 25m above ground (see: http://vk4cz.blogspot.com/).
Feedline: 40 metres of LDF5-50 from shack to tower; 25m LMR400 to the Yagi.
Receiver: Yaesu FT-920.

Scott’s antenna elevation radiation pattern likely looks like the EZNEC simulation in Figure 3.

Figure 3. Likely elevation radiation pattern of VK4CZ’s 6m antenna.

Note that, while the peak gain is about 19 dBi at 8 degrees, at the raypath elevation angle of 2.6 degrees, the antenna’s gain is about 13 dBi. As a line from “The Blues Brothers” movie has it – it’s got good pickup

The feedline run to the rig might be long, but Scott’s choice of cables nets a low total loss – just 0.33 dB for the LDF5-50 and 0.74 dB for the LMR400. Add an estimated 0.3 dB for connector losses (conservative) and the total loss from antenna to receiver comes to 1.4 dB.

The FT-920 is an interesting rig. Reviewed in QST for October 1997, the ARRL lab measurements reveal that a 50 microvolt signal at the antenna showed S9 on the S-meter with the internal preamp on. That’s -73 dBm.

Hang in there, all these pluses and minuses are summarised in a table a bit later.

You’re probably asking why I’m bothering with all these fractional dB figures – this ain’t moonbounce or tropo scatter. Well, it’s necessary to establish a base line from which ionospheric processes that contribute to observed signal strengths can be assessed – both high (5x9+) and low (4x1) signals.

Ionospheric Parameters

Here comes the interesting bit.

The HF band operators among you have some understanding (and experience) of ionospheric absorption, particularly on the bands below 14 MHz. Daytime ionospheric absorption is why DXers on 40-80-160 metres wait for nighttime to chase DX. Some of you will know that ionospheric absorption is inversely proportional to the square of the operating frequency [2, 5]. At 50 MHz, you’d expect the ionospheric absorption to negligible, not worth considering. I asked myself the question, “just how much is it?” The signal raypath scenario being considered here is illusrated in Figure 4.

Figure 4. The signal raypath traverses the D region and the lower part of the E region to reach the sporadic E layer. This is where ionospheric absorption is a consideration.

Kenneth Davies’ 1965 book [4], “Ionospheric Radio Propagation”, details a very useful method for determining the value of “non-deviative absorption”, which occurs mainly in the D region where there is little or no deflection (deviation) of the raypath. Deviative absorption occurs around the level of the reflection point and is only a consideration in the low HF bands where the raypath travels relatively significant distances in the reflecting (refracting) layer(s).

Ionospheric absorption is controlled by the sunspot number, R, and the angle of the Sun with respect to the time of day at the propagation reflection point – called the “solar zenith angle”. So, first we have to calculate the absorption index, for which Davies [4] provides this equation:

For the VK7RAE spot by VK4CZ at 2304 UT on 2 January 2009 (0904 local solar time – LST – on 3 January), the sunspot number from the Solar Influences Data Center [6] was zero (no surprise). So the first term of equation 1.2 becomes, simply, 1. The solar zenith angle for the LST on 3 January 2009 can be conveniently calculated using the online tool at: http://solardat.uoregon.edu/SolarPositi ... lator.html. For the latitude and longitude of the path mid-point, the solar zenith angle was 40.9 degrees.

Substituting in equation 1.2, I = 0.76.

Part 3 gets closer to the goal.

[1] Harrison, R. VK2ZRH 2011, "A New Model of VHF Sporadic E Propagation", viewtopic.php?f=43&t=9832

[2] Appleton, E.V. and W.J.G Beynon, 1940 “The application of ionospheric data to radio-communication problems: Part 1”, Proceedings of the Royal Physical Society, Vol. 52. (Available online).

[3] http://www.m2inc.com/index.php?ax=amateur&pg=94

[4] Davies, Kenneth, 1965 Ionospheric Radio Propagation, US Dept. of Commerce, National Bureau of Standards, Monograph 80. Online at: http://digicoll.manoa.hawaii.edu/techre ... /NBS80.pdf

[5] Davies, Kenneth, 1990 Ionospheric Radio, Peter Perigrinus Press, ISBN 0-86341-186X, Chapter 7.

[6] Solar Influences Data Center (SIDC) at http://www.sidc.be/products/meu/index.php

[VK2ZRH]

SPORADIC E PROPAGATION – 4x1 TO 5x9++ – HOW? – PART 3

To derive the amount of absorption per hop, Davies [4] wraps up all the complex calculations into a really neat nomogram – a handy graphical means of relating all the variables – operating frequency, raypath elevation angle and the absorption index – to obtain the value of absorption wanted. In Figure 5, I have redrawn this nomogram, extending the frequency scale to 52 MHz and reducing other scales for the sake of clarity.

Figure 5. Nomogram to obtain the ionospheric absorption per hop.

The outer scale on the lower curve is the “effective frequency” – the sum of the operating frequency (f) and the gyrofrequency (fH) – the natural ‘spin rate’ of electrons in the ionosphere. At the VK7RAE-VK4CZ path midpoint, fH is 1.6 MHz [7]. Thus, the effective frequency is 50.057 + 1.6 MHz = 51.657 MHz. The nomogram scale compresses rapidly after 26 MHz, you will notice. The inner scale is for the raypath elevation angle. The upper curve shows values for the absorption index (I) on the inner scale and the dB/hop absorption on the outer scale.

Determining the absorption is straightforward. A straight line is run between the elevation angle and the absorption index scales. A second straight line is then run from the f + fH scale so that it intersects the point where the first line cuts the central horizontal line. The dB/hop can then be read from the upper curve’s outer scale.

For a sunspot number of zero, as it was in January 2009, the absorption per hop was 1.9 dB.

As you can see, for the sake of interest, I also calculated the absorption index, I, for sunspot numbers of 100 and 200. The absorption per hop increases, as you’d expect. For a 1-hop path such as this, the absorption is no big deal in the general scheme of things, but it becomes significant for multi-hop paths – particularly for “extreme” multi-hop paths of four or five hops crossing the mid-latitudes.

The remaining transmission path parameter is polarisation loss. On entering the ionosphere, radio waves split into ordinary and extraordinary waves because of the ionization and the presence of the Earth’s magnetic field. The two wave components are polarized differently, with the wave power divided between them. On exiting the ionosphere, the radio wave will be circularly polarized and a linearly-polarized antenna will only respond to the corresponding wave polarization [4]. At worst, the polarization loss will be 6 dB.

Finally, the Signal Strength – Calculated and Observed

Table 1 summarises all the gains and losses assessed or calculated above to provide a ‘baseline’ characterization of the VK7RAE-VK4CZ 1-hop VHF sporadic E propagation path on 6m.

Table 1. The grand summary. NB: S9 on the FT-920 S-meter is at -73 dBm, which is 50 microvolts.

Without revealing my calculated result for signal strength. I put the following question to Scott VK4CZ:

When 6m opens to VK7, at what general signal strength would you hear VK7RAE?

Scott replied as follows:

In respect to normal signal levels, these are highly variable, as I can often hear the beacon from the band opening (and hence very low signal levels) to band close (same again) with anything from moderate to very strong signals in between. But typically, for [VK7]RAE I wouldn't expect signals much beyond S5 for a normal opening, just [that] when the intensity of the Es builds . . . it would be higher.

Scott also added that he receives it at S9 less often than at lower signal strengths; he takes his spot reports come from the FT-920’s S-meter.

Satisfyingly, calculation and observation agree remarkably closely.

Part 4 to come.

[4] Davies, Kenneth, 1965 Ionospheric Radio Propagation, US Dept. of Commerce, National Bureau of Standards, Monograph 80. Online at: http://digicoll.manoa.hawaii.edu/techre ... /NBS80.pdf

[7] http://www.ips.gov.au/World_Data_Centre/2/1/28

[VK2ZRH]

SPORADIC E PROPAGATION – 4x1 TO 5x9++ – HOW? – PART 4

How can it be Stronger, or Weaker ?

The next question is, what mechanism or mechanisms makes the signal much less or much greater than S5 on this path?

The signal level can be raised by ionospheric raypath focusing, a topic discussed by Davies in his 1965 book [4]. The concept is illustrated, for ‘plane’ Es, in Figure 6.

Figure 6. Raypath focusing acts to raise the signal strength.

From the transmitter antenna, raypaths diverge. When refracted by the Es layer, the variation in electron density of the layer will refract the rays by differing degrees so that suitable raypaths converge on the ground at a given skip distance, creating a focal point “footprint” on the ground. The process is not ‘perfect’ and there’s a “fringe” of weaker signal strength around the focal point footprint. That’s why a nearby station to you can hear a DX station at 5x9 while you can only hear it at 4x2 – or not at all !

This is all very well for ‘plane’ Es, which is essentially ‘flat’ a cloud parallel to the Earth’s surface. As I explained in A New Model of VHF Sporadic E Propagations, Es clouds can also be rippled, or crinkled. This manifests itself on ionograms by ‘spreading’ the Es reflection trace. Hence, it’s known as “spread Es”. This condition also leads to raypath focusing, as illustrated in Figure 7.

Figure 7. Raypath focusing with rippled Es.

Here, raypath 1 approaches the ripple at A at a grazing angle and is gradually refracted such that the raypath emerges horizontally and then encounters the ripple at B, where it is likewise refracted back towards the ground.

Raypath 2, is at a slightly higher angle to raypath 1 and penetrates the Es ripple at A to a greater distance within the layer, where it is refracted at an angle greater than for raypath 1 such that it emerges horizontally below raypath 1. Raypath 2 then meets the Es ripple at B where it is refracted towards the ground at an angle similar to the refraction angle at A.

Under suitable circumstance, the two raypaths converge on the ground, with effects similar to that described for the plane Es case.

From the baseline path parameters detailed in Table 1, the amount of signal gain derived from these raypath focusing mechanisms can be calculated.

When Scott VK4CZ hears VK7RAE at S9 on his FT-920 S-meter, the raypath focusing gain is the difference between -73 dBm and -97.2 dBm, or 24.2 dB !

On those occasions when the VK7RAE beacon is heard at S9+, the ionospheric gain might exceed 35-40 dB !

For the circumstances detailed here, Es layers at heights greater than 92 km will raise the raypath elevation angle, improving antenna responses at both VK7RAE and VK4CZ and thus improving the signal strength. For an Es layer at 110 km, for example, the raypath elevation angle is about 4 degrees. At that angle, the VK7RAE antenna response would improve to -4.6 dBi, raising the signal by 4.4 dB. The incoming raypath at VK4CZ’s antenna would be at 16 dBi, raising the signal by 3 dB. Thus, the total improvement would be 7.4 dB and the signal strength -89.8 dBm, or just over S6. An S-point is an S-point !

Signal strengths weaker than S5 can result from a number of things:
(a) ionospheric defocusing (which causes the raypaths to diverge) [4, 5];
(b) multi-path wave interference (raypath signals out of phase) [4, 5]; and
(c) reception in the fringe of a focal footprint (as described above).

These mechanisms can plunge the signal strength to S1 (-121 dBm, 0.2 microvolts), as seen in VK4CZ’s VK Logger reports.

The amount of signal loss under these ionospheric circumstances is the difference between -97.2 dBm and -121 dBm, or 23.8 dB.

If the propagation is holding up for other stations, but the signal strength falls to the noise floor at VK4CZ, the loss can increase by a further 10-12 dB (depending on ambient noise levels at VK4CZ).

Conclusion

For the first time, the parameters of a sporadic E transmission path have been defined and quantified, including the gains and losses arising from common ionospheric mechanisms – non-deviative absorption loss, polarization loss, raypath convergence (focusing gain), raypath divergence (defocusing loss) and multi-path wave interference.

The procedure detailed here can be applied to any other sporadic E VHF propagation path – 1-hop or multi-hop – provided the relevant parameters are known or reasonable representations of them can be determined.

73, Roger Harrison VK2ZRH

[1] Harrison, R. VK2ZRH 2011, "A New Model of VHF Sporadic E Propagation", viewtopic.php?f=43&t=9832

[2] Appleton, E.V. and W.J.G Beynon, 1940 “The application of ionospheric data to radio-communication problems: Part 1”, Proceedings of the Royal Physical Society, Vol. 52. (Available online).

[3] http://www.m2inc.com/index.php?ax=amateur&pg=94

[4] Davies, Kenneth, 1965 Ionospheric Radio Propagation, US Dept. of Commerce, National Bureau of Standards, Monograph 80. Online at: http://digicoll.manoa.hawaii.edu/techre ... /NBS80.pdf

[5] Davies, Kenneth, 1990 Ionospheric Radio, Peter Perigrinus Press, ISBN 0-86341-186X, Chapter 7.

[6] Solar Influences Data Center (SIDC) at http://www.sidc.be/products/meu/index.php

[7] http://www.ips.gov.au/World_Data_Centre/2/1/28

[VK3ALB]

Excellent work Roger. Thanks for keeping us abreast of new developments in your research.