Tuesday, November 25, 2008

Low-band DX'ing: The high-angle mode hypothesis - G0KYA contribution

In my previous post, I gave a few links mentionning the "high angle mode". I found extremely interesting G0KYA's paper "the twilight zone".
This is the introduction: Steve Nichols G0KYA, of the RSGB's Propagation studies Committee, believes that propagation around sunrise and sunset is not fully understood. Here he outlines the mechanisms behind grey line and other twilight propagation modes and a research project to help us understand them.
And a particulary interesting paragraph
Now imagine a radio wave hiting the ionosphere at about 75-85 degrees to the earth - a near vertical incidence wave (NVIS). Below the critical frequency, it would be returned. At some frequency close to fof2 it could be refracted through a large angle and could end up travelling almost parallel to the earth, giving a very long first skip distance. This is the condition for the Pedersen or critical ray, discovered in 1927, characterised as being high angle, long distance and close to and probably above the fof2 frequency. As there would be no intermediate ground hops the signal strength could be very high indeed.

Following this reading, I exchanged a few emails with Steve (one of them, reproduced here with
his authorization)
Hi Patrick, Thank you very much for your e-mail. I have had a quick look at your blog and it is very interesting. Your message was actually very timely as I have been looking at WSPR as a greyline beacon tool, but had put it to one side to concentrate on other things - I will pick it up again! Here are some thoughts and observations that might be useful.
1. VOACAP is not very good at predicting openings below about 5MHZ. It was never intended for this and is usually very pessimistic. W6ELProp is better.

2. You are quite right - it doesn't take into account sunrise enhancements I did some work on signals from VP6DX into the UK on 80m and using VOACAp looked at the number of modes (hops). It showed clearly that there are more modes around sunrise and the radiation angle goes higher. I can send you this if you want. I am therefore convinced that greyline has more to do with higher angle signals due to ionospheric tilting and multiple modes than the old "D layer not illuminated/F layer illuminated" model.
This also explains why you get good greyline openings at right angles to the terminator.

Thanks for thinking of me.
Steve G0KYA
RSGB Propagation Studies Committee
You will find other articles and powerpoint presentations on Steve page, definitely a "must read" for whoever is interested by those phenomenas (the above pictures are from Steve presentations).
Steve's VP6DX presentation is available as a PDF.
By the way, the "high angle mode" has a name: The Pedersen Ray... and nothing new, since it was discovered in 1927...

Saturday, November 22, 2008

Low-band DX'ing: The high-angle mode hypothesis - part 2

Following are the results of a quick test done this morning on K1JT 80m WSPR beacon. As for the previous post, I was using 2 antennas, my K9AY pointed NW and a magn loop, this time oriented NE, so roughly 90 degrees away from Joe's direction. The magn loop pattern at 90 degrees is particulary interesting because its response at low angle is minimum.
Following is the plot of the measured SNR's around sunrise.
Although the mag-loop is 90 degrees away from the TX-direction, it still takes the advantage just after sunrise (SR at 0645z). (you may click on the plots to enlarge them). The above plot is for the SNR's measured on K1JT WSPR-beacon on 2 antennas , a K9AY oriented NW and a mag-loop oriented NE. Despite the limited number of points, and the mag-loop orientation, the trend is clear: Just after sunrise the mag-loop provides slightly better SNR's than the K9AY, which is likely to confirm the "high-angle propagation mode hypothesis".
This plot shows the pattern of the magnetic-loop, for various elevation angles. The front to side ratio goes from 12dB at 5degrees elevation (red pattern) to 1dB at 60 degrees elevation (blue pattern). The orange pattern is for 20 degrees, while the purple is for 40 degrees.
This plot shows the compared patterns of the K9AY and the magn-loop turned 90 degrees away from the transmitter direction. In blue the magn loop pattern a 60 degrees, in light blue at 20 degrees. In red the K9AY pattern at 60 degrees, in purple at 20 degrees. The respective max gains being almost equal, the 2 patterns cross at an elevation angle of approximately 60degrees. Below 60 degrees, the K9AY should provide better SNR's while the Magn-loop takes the advantage at angles higher than 60 degrees.

IMHO, those plots clearly show that a high angle propagation mode appears at sunrise. It would seem, still for this particular path at this particular moment, that the take-off angle is at least 60 degrees, or higher. This explains why stations using low horizontal antennas, get "better than expected" results on a few DX-paths, although a low horizontal antenna
(below 1/4 wl) has a poor efficiency due to important ground losses. The little difference shown in the earlier published plot of DG0OPK signal (using a G5RV at 5m) vs mine (using a 1/2 wave vertical over a good soil), as received by VK7KRW on 40m long-path, may also have an explanation when looking at the "high angle mode" hypothesis.

For information I also looked at the SNR's measured on WA6MTZ (CA) 80m signal. On this particular 9300 kms path, there is no obvious benefit provided by the magn-loop. However as I have only a few spots, it might be too early to make definitive conclusions... so, more to follow !

Addendum: I experienced the same phenomena on YA/T61AA signal at sunset..
If you are interested by the topic, I found 4 online articles which mention the phenomena: By ZL1BPU, by G3CWI, by G0KYA and from antennex by ON5AU

Thursday, November 20, 2008

Low-band DX'ing: The high-angle mode hypothesis

Following the publication on this blog, of the VOACAP articles and the comments received from AB7E and OH6BG, my friend Seb F8CMF pointed me an old K3LR post on the "top-band reflector". Here is the post (reproduced with Tim's authorization).
Topband: Low RX Dipole
*To*: >
*Subject*: Topband: Low RX Dipole
*From*: k3lr at k3lr.com (Tim Duffy K3LR)
*Date*: Mon May 5 20:52:53 2003
Hi Gale!
It all depends on what other antennas you have. I have had up a 14 ft
high 1/2 wave dipole on 160 for many years. It has always impressed me
as a high angle antenna which limits its prime effectiveness to gray
line use. I also use it in tandem with other antennas and the MFJ
receive antenna noise canceling W8JI magic box (also an effective
antenna phasing unit). My low dipole's claimed fame was a sunrise QSO
with KH2. No other antenna (1100' beverage, vertical array, etc.) could
hear the KH2, but he was 569 on the low dipole.
Tim K3LR

Remembering that I received ZL1RS only on the magn-loop, this post interested me very much. Taking the opportunity of the "WSPR 80m Special Activity Day" I quickly setup a diversity receive test, using 2 receivers, one connected to the K9AY loop, oriented NW and one to the magn-loop (roughly E/W).

The compared patterns of the K9AY and the magnetic loop. Considering RX antennas, it should be made abstraction of the respective Gains and focus on the RDF. According to K7TJR tables, the RDF of the K9AY considered at 20degrees elevation is 7.2dB, vs 4dB for the magnetic loop, thus a 3.2dB advantage for the K9AY. Of course considering a higher angle, the difference will turn to the magn-loop advantage, as it exhibits a pattern more favorable to NVIS.

The plot shows the SNR's measured on 80m from 9 NA-stations during 2 nights on 2 RX-antennas. Over 500 spots, were used. By distance order VE1VDM (5205kms), W1BW, W1XP, K1JT, WD4KPD, W8LIW, KI4MTI, vE5MU, K4MF(7572 kms). In the middle of the night, the K9AY provides some 5dB average better SNR than the Magnetic-loop wich suggests an elevation angle lower than 20degrees.

The same as above, but focusing on the morning. The fact that the magn-loop takes the advantage near sunrise seems to be a confirmation that some "high angle" mode occurs at this particular moment and not only on paths following the grey-line. The respective antenna patterns suggest an vertical angle >45 degrees. A more "radical" NVIS antenna (like the "cizirf-special") has to be tried to confirm this hypothesis.
Last but not least: for the path F6IRF to W8LIW, VOACAP suggest a vertical angle of 8 degrees, with a peak at 15 degrees in the middle of the night. Once again the VOACAP-model is obviously far away from the above observations. As Dave AB7E mentionned "
It (voacap) is especially weak in predicting the effects of takeoff angles /// I'm convinced that much of gray line propagation is chordal hop, which is supported by the comments from K3LR and others (as well as my own observations) that high angle antennas often work better for such openings. When you draw a picture of the earth showing an F2 gradient at sunrise/sunset it is easy to see why that would be true."
What seems to show the above, is that "the high angle mode" does not only concern long DX-paths along the grey-line (such as F<>ZL) but also shorter distances DX paths.

Tuesday, November 18, 2008

The VOACAP mystic, an interesting comment...

I received the following emails from Dave AB7E (reproduced here with his authorization). Think he is saying interesting things...Hi, Pat. Wow ... what a rigorous and interesting analysis. Very impressive.
My first reaction, though, is that an antipodal path (ZL-F) is possibly not the most valid for comparing VOACAP predictions to real life propagation. VOACAP assumes a single direct path (either SP or LP), whereas the actual path for an antipodal target could be almost any direction. VOACAP might render its calculation based upon a path over Asia while the actual signal might not take that route at all.
As a real life example, my Optibeam OB16-3 has a pretty narrow beam width on 20m (I can easily hear the difference in 20 degrees of beam heading), yet when working VQ9RD recently (close to antipodal for me), it was almost impossible to find a "best" beam heading within about a 70 degree spread. VOACAP, of course, is based upon heaps of experimental observations taken decades ago, with extrapolations based upon theoretical models of the ionosphere. It doesn't even come close to being a valid predictor of real-time conditions, a fact that the authors were very quick to point out but which hams have conveniently ignored.
It is especially weak in predicting the effects of takeoff angles, which can have a large impact. So overall I'm not surprised that your analysis would find large discrepancies between predicted and actual SNRs, but I suspect that the results would have been closer for a non-polar path of roughly 1/3 the circumference of the earth instead of 1/2.
73, Dave AB7E

pat_f6irf wrote:
Yes David agree... I think Jari that OH6BG summarized it very well in the part 4. The purpose of this article was to draw the attention of contesters and DX-men, on the fact that VOACAP is not suitable at all for low-band DX'ing... What the designers accept, but most hams ignore (including me b4 looking at it !); see http://www.voacap.com/lowband.html
Even the 10degrees vertical angle forecast may be wrong...

Hi again, Pat.
Yes, I think VOACAP is quite poor for low band use, especially since so much of it is gray line dependent. I'm convinced that much of gray line propagation is chordal hop, which is supported by the comments from K3LR and others (as well as my own observations) that high angle antennas often work better for such openings. When you draw a picture of the earth showing an F2 gradient at sunrise/sunset it is easy to see why that would be true. I don't think that VOACAP even considers chordal hop propagation mechanisms at all. I certainly hope you keep up the excellent work, and I think the diversity experiments should be very interesting as well. 73, Dave AB7E

Thanks Dave...
Your email reminded me that already ZL at long path (audio recording) was something for my "Cizirf-special" antenna, which was not designed to be a DX antenna.
Promised, I'll come back with the same type of analysis for shorter distance paths

By the way, I have updated the blog "best of" , an easy way to find articles...

Monday, November 17, 2008

The VOACAP mystic, a summary...

Why "the VOACAP mystic" ?
I would recommend to read this as a starter

Once done and for those who may have missed the early posts. Following is a summary of all the posts published on the a/m topic. It will allow to read them in the right order...

- VOACAP vs WSPR reports part 1 (study of a 30m transatlantic path)
- VOACAP vs WSPR reports part 2 (study of a 30m transatlantic path)
- VOACAP, the end of the myth ? part 1 ( study of a F<>ZL 40m path)
- VOACAP, the end of the myth ? part 2 ( continuation of the study of the F<>ZL 40m path)
- VOACAP, the end of the myth ? part 3 (study of a 40m long path F to VK7 + my temporary conclusions)
- VOACAP, the end of the myth ? part 4 (back to F<>ZL short/long path, OH6BG calculations and opinion)

May not be complete: I am still consolidating data on 80m F<>ZL path, which may lead to strange conclusions on the required take-off angle. Now that I have improved my knowledge of Voacap, I may also come back later on shorter distance paths. So, stay tuned !

Please note: So far these articles, exist only as "blog posts" (I like the spontaneous day to day blog form). Of course they may be made available for publication in a more conventionnal support, or public presentations if some interest is expressed. In any case, please contact me for any adaptation/translation...

VOACAP reliability, the end of myth ? part 4

Following the publication of the series of articles on VOACAP reliability, Jari, OH6BG (voacap.com) sent me the following email (reproduced here with his authorization). I have just underlined the key-elements.
Hi Pat,

Kari OH2BP wrote to me today pointing out your monitoring results vs. VOACAP predictions blog at
http://f6irf.blogspot.com/. A very interesting reading indeed, and a constant reminder for all of us that predicting HF propagation is indeed more of an art form than an exact science, hi!
I have made similar monitoring sessions with Faros on July 2007, and, on 14 MHz, VOACAP miserably failed to predict circuits from Finland to the United States in the middle of the night. I reasoned then that the twilight hours that prevail at that time of the year here in the North generate many hours of signal-enhancing "sunset" effect at this end, which VOACAP simply cannot see.
Below I gathered my personal observations, which can perhaps help you in your endeavour of tackling the issue in hand...
1. A quick run reveals that the use of the older IONCAP absorption model can, in your case, give up to 15 dB better SNR/SDBW values over the more conservative "Normal" absorption model, used in your calculations.
2. You must not only look at the predicted SNR values, please note also the predicted median Signal Power values, S DBW.
3. You must not only look at the predicted median values of Signal/Noise and Signal Power. Calculate also the predicted upper decile (10%) values that tells you the probability values for SNR and S DBW on 3 days (10%) in a month. The SDBW10 values specifically suggest an extremely poor, but probably distinguishable (if the real Noise Power is not too overwhelming), signal at the time periods that have been monitored, although the values themselves are extremely conservative, but to me, a right tendency can be seen. Formulas: SNR10 = SNR + SNR UP SDBW10 = SDBW + SIG UP
4. I have understood that extremely long circuits (such as this, far far beyond 10,000 km) can be troublesome for VOACAP and probably the predicted values are of a more conservative note. Therefore, I am pretty amazed at VOACAP being able to figure out the correct tendency anyhow.
5. In addition, I can only speculate that perhaps the greatest influence for the "better-than-predicted" monitoring values lies in the fact that the time periods of monitored best reception on 7 MHz seems to occur approximately at ionospheric sunset/sunrise on either side of, or along, the circuit. As far as I know, VOACAP is not able to take into account the signal-enhancing sunrise/sunset effect, which I can also confirm by my own monitoring on 14 MHz.
73 Jari OH6BG/OG6G

Attached to Jari's email was the run output table which I have plotted here vs "the observed average" over near to 3 weeks period (plot published in article part 2)This plot has been made using OH6BG run prediction table. It uses the old IONCAP absorbtion model, assumes Isotropic antennas at each end of the path, and a very quiet man-made noise level of -155dBW/Hz at 3Mhz. The purple curve is the "upper decile" (3 days per month) while the blue one is the SNR (as shown earlier equivalent to SNR50, so 15 days per month). The red curve is an averaged measured value over a 3 weeks period.

Following are the parameters that Jari used:

SHORT PATH: F6IRF to ZL2TLD, using IONCAP absorption model
CCIR Coefficients ~METHOD 30 VOACAP 08.1023I PAGE 1 Oct 2008 SSN = 7. Minimum Angle= 3.000 degrees f6irf zl2tld AZIMUTHS N. MI. KM 45.80 N 6.00 E - 40.96 S 175.59 E 61.06 306.11 10267.9 19014.6 XMTR 2-30 + 0.0 dBi[samples\SAMPLE.00 ] Az= 61.1 OFFaz=360.0 0.001kW RCVR 2-30 + 0.0 dBi[samples\SAMPLE.00 ] Az=306.1 OFFaz= 0.0 3 MHz NOISE = -155.0 dBW REQ. REL = 90% REQ. SNR = 4.0 dB
LONG PATH: F6IRF to ZL2TLD, using IONCAP absorption model
CCIR Coefficients ~METHOD 30 VOACAP 08.1023I PAGE 1
Oct 2008 SSN = 7. Minimum Angle= 3.000 degrees
f6irf zl2tld AZIMUTHS N. MI. KM
45.80 N 6.00 E - 40.96 S 175.59 E 241.06 126.11 11345.0 21009.3
XMTR 2-30 + 0.0 dBi[samples\SAMPLE.00 ] Az= 61.1 OFFaz=180.0 0.001kW
RCVR 2-30 + 0.0 dBi[samples\SAMPLE.00 ] Az=306.1 OFFaz=180.0
3 MHz NOISE = -155.0 dBW REQ. REL = 90% REQ. SNR = 4.0 dB

In a later email Jari wrote:
Although I am by no means a specialist of low-band propagation, my little experience tells me that enhanced signal levels and openings on lower bands can be observed 1 to 2 hours before sunset and similarly 1 to 2 after (if not even longer) hours after sunrise. And both the stations need not be on the terminator zone, "greyline"... This particular case with ZL2 could fit into this framework.
When it comes to using VOACAP for predicting propagation on lower bands, I firmly believe that predictions on 80M, not to talk about 160M, are simply doomed to fail for any longer path... George Lane writes about this at http://www.voacap.com/lowband.html as follows, "... George Haydon said there was very little data below 4 MHz but there was some for short paths that did go down to 2 MHz. So they modeled a fit to those cases. Risky, but it has proven to give good results for Near Vertical Incidence Skywave (NVIS) situations. ..."
So, trying to use VOACAP to predict low-band DX propagation is asking for trouble
. Maybe you can get better results just by simply looking at sunrise/sunset maps! hi.

Many thanks Jari for your contribution... I think that the message is clear !

Friday, November 14, 2008

back to "another low band vertical"

13 monthes ago, I published a complete description of my "low band" vertical. This antenna continues to surprise me every day by the reports I am getting worldwide. Just as an example, I recently had several bilateral WSPR-exchanges with ZL1RS on 80m long path using 5W. This is what says Larry W8LIW (OH) " As for the spots of your station, sometimes I think your station and mine are connected by a wire :o) You are usually the first station I hear! Your vertical is amazing".
I recently improved the radials-systems: to the 30 burried radials, I added 40 "surface mounted" temporary ones, for a total of 70 (50x10m and 20x 20m = 900m of wire!). Unfortunately, my terrain allows me the 20m-ones, in one direction, so I have now reached the maximum... Next improvement will be to add a few top-loading wires to improve the performances on 160m.
The temporary "surface-mounted" additional radials. My attentive readers, may recognize them ;-)
Among my attentive readers, is my friend Dodo, F6EPY. He sent me the following pictures, of his portable vertical, made from a 18m spiderbeam mast (thanks Dodo).

Last, but not least, during the a/m exchanges with ZL1RS I was using this RX-antenna. I am now planning to put 2 of them in "end fire" configuration (using my MFJ phasing box)... More to follow...

Monday, November 10, 2008

160m: A good example of an asymmetrical path

Often on the top-band, it is not easy to contact a DX station that you hear quite well. Before, accusing the DX station to be deaf, have a look at this example. I think that it is quite a good example of an asymmetrical transatlantic path.This plot shows the 160m reported levels by K1JT and mylsef using K1JT's WSPR mode. You can see that K1JT decoded me several times around his sunset, while I could not decode him. On the other hand, near my sunrise (06:28z), I could copy him better than he could. Btw, you may note that the K9AY provides a 4dB better SNR than the vertical (I was using 2 receivers), which is conform to its RDF-factor vs the vertical one (see W8JI receiving pages, k7tjr tables or ON4UN's bible). Note that we were both using 20W, which explains why the plotted levels @1W go below the WSPR decoding threshold (-30dB in 2500Hz BW)

The explanation is provided by the following plot of the noise at my location (VOACAP). You can see a 12dB variation of the noise floor, with a minimum in the middle of the day and a maximum in the middle of the night. At Joe's sunset my noise floor is very high, while it is still low at his location. At my sunrise, the noise floor is lower at my location and maximum at Joe's location. CQFD !
As pointed out by W8JI in his receiving pages, the "man made noise" is not only local noise, but also distant "man made noise" propagated through the ionosphere. In fact, based on the above plot, it seems that even at my location, the local noise is not dominant on 160m (except in the middle of the day, or in case of rain).

This is the 2MHz noise plot for my location (45.8N, 06E). A "man made" noise level" of -155/dBW/Hz @3Mhz is assumed (CCIR quiet). Of course for a higher "man-made noise" level the variation is less important (especially if the dominant source of noise is local).

VOACAP reliability, the end of myth ? part 3

fig1: This plot shows the signal received by VK7KRW in Hobart, Tasmania, on 40m long path. I have also plotted DG0OPK signal (using a G5RV at 5m). In fact this plot alerted me on a problem on my vertical, as I was expecting more difference (the simulation shows -7dBi for the G5RV at 10deg -assuming it is broadside to the signal-, and about 0dBi for my vertical), Checking my antenna I discovered that one set of radials was disconnected. For this test, I was also using an auto-tuner, which is quite lossy for HighZ/50ohms impedance transformation (clearly visible on the antenna bandwidth).
Fig 2: This plot shows the VOACAP "RPWRG" output for the path F6IRF>VK7KRW. As explained in the text, all the parameters have been set to the most optimistic values... remains 40dB offset between the real world and VOACAP !

Are the WSPR reported levels accurate? I checked the reported levels with a "path simulator" (AE4JY's PathSim, a great tool !), using the same soundcard that I used for the above ploted files . Taking into account that the path simulator uses 3kHz BW for the noise and WSPR 2.5 kHz thus 0.8dB for the BW relation, I found the WSPR reported levels to be within 1dB from the Path simulator displayed SNR's. I also used several "Path algorithms" (ie CCIR good, moderate and poor) with no impact on the WSPR reported levels; so the WSPR reported levels should be considered accurate (let say +/-2dB depending on receiver and soundcard used). By the way, the only simulator path algorithms for which WSPR does not provide any decoding are the "CCIR flutter" and "High latitude disturbed"- this explains why it is so difficult in Western EU, to get a report from US-west coast (as the path goes through the aurora oval). Under the same "perturbed" conditions, JT65A works perfectly.

Where the VOACAP error may come from ?

- I think that the first source of error, especialy on low bands, is the RX antenna gain. If you reverse the TX and RX antenna you will see that the calculation provides moreless identical results in terms of SNR (+/-3db due to the noise variation along the day at the receiver location). The hams familiar with low bands DX'ing know that a receiving antenna may have an absolute negative gain, but a better receiving performance. In other words the gain of a receiving antenna is generaly meaningless in terms of SNR.The typical examples are the beverage or the K9AY loop, which have negative gain (or very negative gain for the K9AY), but provide better receiving performances than an omni antenna. A suggestion might be to use W8JI's RDF factor, rather than the absolute gain for the RX antenna
. The problem is to know if the source of noise is in a different or in the same direction as the useful signal... not simple! In any case, based on W8JI table, and unless a long beverage is used, this error can't be more than 3 or 4dB compared to the isotropic antenna. Here none of the receiving stations were using a RX-antenna, therefore using the isotropic as RX-antenna can't be that wrong ! ."A contrario" using a TX-antenna on the RX side may produce exageratly optimistic results... as explained by W8JI, on HF bands, a higher antenna gain does not mean a better SNR, as the software assumes!
fig3: NEC2 simulation of VK7KRW antenna. Using an Isotropic antenna in the VOACAP model is also exageratly optimistic !

- Another potential source of error might be the 3Mhz noise level at the receiver location. Measured here on a base-loaded 18m monopole tuned at the frequency, the minimum noise level I can get (depending on the time, and the meteorogical conditions) is around -80dBm in 3000 Hz bandwidth (S7.5 according to my S-meter calibration table). This corresponds to -115dBm in 1 Hz bandwidth, thus -145dBW (the recommended average level, which is used for example in HAMCAP). I know by experience (having transported my transceiver in several other contest locations), that my location is noisy (I have 380kV power line passing some 200m from the antenna). It means that in most of the cases, for the radioamateur service, it might be more representative to use a lower noise level than the default one (-145dBW/Hz). The minimum that can be applied with some effect on the output is -160dBW (although the contextual help says: range 100-200), which represents a maximum of 15dB gained on the signal to noise ratio, not taking into account that there are also interferences (ie pactor BBS's, SSB, ...) on the bands that we are using, especialy on he 40m band... so the 15dB here should be considered as an absolute maximum on an ideal QRM-free band...Obviously not the case of the 40m band ! On the other hand, the bilateral data with ZL2TLD, shows that the reports offset, between his station and mine is just 4dB at long path and 10dB at short path (here, the band is far more noisy in the evening than it is in the morning), therefore applying -150dBW/Hz to 155dBW noise level at ZL2TLD and VK7KRW location looks appropriate as they both reported having a quiet site...
My location can't be considered as "radiolectricaly quiet" !

- VOACAP does not take into account the K or A index, but just the smoothed monthly SSN. Personnaly, I think that using the SSNe might be more representative for day to day conditions on high bands, as the SSNe which is obtained from the Ionosonders data, takes into account those variations of the K and A index. Anyway VOACAP does not accept negative SSN values and the designers insist that only the smoothed monthly SSN should be used. IMHO, everything depends on the considered path. For transequatorial pathes on 40m band such as the ones considered here, the SSN has almost no influence on the prediction (you can verify by yourself that, on this 40m path, any SSN value provides moreless the same output result). In fact, it seems that the SSN is mainly used to determine the MUF, which on low bands can't be considered as a predominant factor...
- Another critical parameter, is the "required reliability" percentage. As shown in the earlier published plots, using a too low percentage has for effect to predict earlier band opening and later band closure, than the observed one. Anything >50% seems to match quite accurately the path opening. As also shown in those plots, the "SNR" output provides absolutely identical results than the SNRxx when set to 50%. For the fig plot I used 10%... also exageratly optimistic, for a path that proved working every day over the month, with an excptionnal stability !
- The multipath tolerance, also has an impact. For conventionnal analog ham-modes, I think that this parameter should be ignored (setting multipath tolerance to 0 as for effect to disable the impact of this parameter). Of course for digital mode, it becomes more critical, but on the pathes considered here, the probabilility of multipath effect is very low (might be different on higher band, where both short and long path might be open simultaneously - ie EU to JA during EU mornings).

- The minimum angle might also be a critical factor. I have a quite bad take-off to the W-SW sector (long path) with some 8 degrees blocking due to a hill. However even 8degrees does not impact the prediction result, as the predicted radiation angle is some 10 to 12 degrees for the considered path. This is of course different on higher band, where the vertical radiation angle might be as low as 3 degrees, on a certain number of DX pathes.

- Remains the F-PROB parameters... must say that this is above my competency... I can only use the defaults. However those parameters (at least on 2 of them) seems to be linked to the E-layer effect (E and E's), which on this particular path at this time of the day, on this frequency, are unlikely to provide any improvement of the SNR, but more likely a degradation due to higher absorbtion.

The fig2 output was produced using exageratly optimistic parameters...still remains a large 40dB offset on the F>VK7 long path. How can it be explained ?

Fig4 the grey line position at the path peak. None of us are on the grey line...

- Everybody has heard about the grey-line improvement effect... If you look at the path at the observed signal peak, you'll see that none of us is on the grey line at the observed signal level peak...
- Another Hypothesis, is some "exotic" propagation mode that has not been taken into account in the model: for example and as suggested by several authors, some ducting within the ionosphere (something similar to the troposheric VHF/UHF modes, but in the F2 layer), or some other mode (ie interaction between E and F layer, or direct F2-F2 (or F2-F1 mode), avoiding the signal to be returned to earth in the middle of the path), thus reducing the path attenuation. The problem, is that "exotic" does not apply to a propagation mode which occurs every day with a remarquable stability !
- The last one is that VOACAP has never been calibrated for those "long DX" pathes, as the main objective was broadcasting at distances not exceeding 10,000 kms. As shown in my first study, I personnaly still have doubts about the proper calibration of VOACAP, even at shorter distances, but this may be the topic of another study...
Anyway, and as a temporary conclusion, I think that this humble contribution shows that VOACAP predicted levels cannot be considered reliable for long DX pathes. Remains that VOACAP predicts quite accurately the best time for a given path and most probably the proper required min radiation angle, those 2 parameters being in fact the most importants for a DX-man or a contester... Anybody who is familiar with the 40m band knows that it is quite easy to work VK/ZL from EU on 40m at long path in the EU mornings, even using Low Power and a modest antenna. I personnaly had many CW QSO's with VK/ZL just using a R7 vertical and 100W. VOACAP tells you that it is not possible, unless you use "broadcast type" antennas and power... Of course VOACAP is completly wrong on such pathes, but it is the vocation and the beauty of ham radio to investigate unexplored terrains ! Just remember that the radioamateurs were the first one to explore the HF bands. At that time, all specialists were considering those frequency ranges as completly useless, so even if VOACAP tells you that a path is not possible, just try it, VOACAP can be wrong ! Patrick Destrem - F6IRF October 2008
Fig 5 Using what look "reasonnable parameters" for the path and stations setup, the offset can be as large as 73dB...

Acknowledgements: I would like to thanks, all the radioamateurs who have contributed to this article, consciently or not; between others: AE4JY, DG0OPK, G4ILO, K1JT, VE3NEA, VK7KRW, W1BW, ZL2TLD, ...