Monday, May 1, 2017

VOACAP Online Point-to-Point (P2P) User Manual (August 1, 2017 Edition)


UPDATE August 1, 2017: The UI of this service has been revamped considerably.

The VOACAP Online Point-to-Point (or P2P for short) HF propagation prediction service uses VOACAP (Voice of America Coverage Analysis Program) as its calculation engine, requiring that SVG (Scalable Vector Graphics) is supported in your web browser (for the prediction wheel). For example, the latest versions of Microsoft Internet Explorer, Mozilla Firefox, and Google Chrome are known to work. The earlier versions may not be supported. If you encounter problems with the page, please try first to upgrade your browser to the latest version available. If you think you have found a bug or if you wish to help translate the user interface into your language, please contact me at


The VOACAP P2P web interface is divided into three sections and their sub-sections as follows:

  1. A Google Map for setting the transmitter (TX) and receiver (RX) site coordinates. The easiest way to set the coordinates is to drag the red (TX) and blue (RX) markers to appropriate locations on the map. Under the map, the distance from TX to RX is given in kilometers and miles, and the bearing in degrees from True North. If you need to zoom in or zoom out the map for better details, just scroll the mouse wheel up and down over the map. To quickly swap the TX and RX points, just double-click one of the markers.
  2. A Prediction Charts section which offers 15 different charts, including, among others, individual prediction charts for all the amateur radio bands from 10 meters (28 MHz) to 80 meters (3.5 MHz). The charts show e.g. the probability (or, the REL parameter in the VOACAP language) for a communication contact (i.e. a QSO) between the TX and RX sites. The "REL Short-Path" chart is the default. Other key prediction charts include SDBW (Signal Power) and MUFday. The calculations are available for the path both via Short-Path and Long-Path. The 24-hour prediction wheel -- which displays exactly the same prediction data as the new "REL Short-Path" chart -- is now available via a link under the "Receiver Site" section. See image above.
  3. The input values for the prediction can be set in the area below the Google Map and the prediction wheel. There are five sections here:
  1. Google Map
  2. Date
  3. Propagation Parameters
  4. Today’s Sunrise/Sunset Times
  5. Transmitter Site (TX), and
  6. Receiver Site (RX).

The input parameter sections under the Google Map and the predictions chart section.

1. Google Map -- for smooth entry of coordinates

Originally, a smooth and an easy coordinate entry for the Transmitter (TX) and Receiver (RX) sites was one of the single most important design features at VOACAP Online. The stand-alone PC version of VOACAP does not offer this, and, in fact, not many others do, either. Choosing Google Maps for this purpose lowered the threshold of using VOACAP considerably.

On the initial Google Map, there are two markers -- red and blue -- placed on the equator line in Africa. The red marker signifies the transmitter's location (TX) and the blue marker is the receiver location (RX). Perhaps typically, the transmitter is your QTH and the blue one is the DX station, or any way you like.

a) Two great-circle paths: short-path and long-path

There is always a red line connecting the red and blue markers, showing, by default, the great-circle path (short-path) between the two locations. A long-path great-circle line can be shown when you choose "Long-path" from the pop-up menu by the "Specials" label in the Transmitter Site section. The small red circle along the red great-circle line indicates the geographical midpoint between the Transmitter and Receiver.

b) Distance and bearing

One of the features under the Google Map is the on-the-fly calculation of the distance between TX and RX, and the bearing from TX to RX in degrees, calculated from True North. The details can be found under the map (see image below).

Distance (in kilometers and miles) and bearing
(in degrees from True North, not your compass north).

And if you would like to swap the TX and RX locations, there are two ways to do it. The easiest way is to double-click on either of the markers. The bearing value will be re-calculated at once. You can also click on the "Swap TX-RX" button by the "Specials" label. Whenever the locations have been swapped (or moved), the propagation prediction is re-calculated and shown on the prediction wheel immediately.

2. The Date

The whole concept of setting the date in VOACAP Online has been changed from the original setup after I implemented the grayline terminator functionality over the Google Map, and this has been quite a while ago. Earlier, I was showing the grayline terminator but it was always fixed to the current time and day -- the user was not able to set it to a specific time and day in order to see how the grayline terminator looked like at a particular point of time. I felt that a more flexible grayline zone map could be used as a way of trying to determine signal enhancements on the low bands, and therefore a new way of setting the time and day was needed.

This was the reason I chose to use a pop-up calendar for this purpose. Also, any month the user would select from the calendar for the grayline zone would also be used as input for all propagation predictions. The pop-up calendar is located just below the Google Map, and looks like this:

Always select a day number in the pop-up calendar, and press the Set button.
Press the Reset button to return to the current time and date.

To set a date, click on the calendar icon on the right of the date field. It will prompt a calendar where the user can browse the months (and years) backward and forward, by pressing the arrow icons. You select a month by picking any day number in that particular month. Please note that you must select a day (although the day selected is not used for any propagation calculations)!

The month selected will be used for propagation prediction calculations, and the day selected (and the time set by the user) will be used for drawing the grayline zone terminator over the Google Map. Please note that the selected day will not be used for propagation prediction calculations as VOACAP will not calculate any daily predictions.

When you have selected a month and a day, and have set the time correctly for your purposes, then press the Set button. This will finally use all the parameters set. To return to the current month, day and time, press the Reset button.

3. The Propagation Parameters

The section "Propagation Params" include a number of user-adjustable parameters that will affect propagation predictions.

a) Es, or setting the ionospheric sporadic E layer (Es) on and off. This may (or may not) prove useful during summer months when Es propagation conditions are quite common. The default setting is OFF (No). Please note that the use of the Es layer is otherwise discouraged as the sporadic-E model was not fully tested during the development of VOACAP. Nevertheless, the effects of the sporadic-E layer are not totally excluded in VOACAP calculations although the layer would not be set.

b) Model, or selecting the propagation model. Three choices are available here: Auto, Ducted, and Ray-hop.

  • The default "Auto" or automatic model refers to Method 30 in the VOACAP speak. It's a propagation model that chooses automatically either the ray-hop model or the ducted (forward-scatter) model to predict the signal power. There is also a smoothing function for ranges of 7,000 km or greater.
  • The (forced) "Ducted" model refers to Method 21 in the VOACAP speak. Typically, this model is used for paths of 10,000 km or more. The Ducted model forces VOACAP to simulate the ducted or forward-scatter mechanisms that can prevail usually at distances having three or more hops. This model may produce unrealistic results at shorter distances where the ray-hops should occur.
  • The (forced) "Ray-hop" model refers to Method 22 in the VOACAP speak, typically used for all circuits less than 10,000 km. It's a model that contains multiple ionospheric reflections, and includes all of the ionospheric and earth bounce losses. This model may produce extremely pessimistic predictions at the distances beyond the third ionospheric hop where ducted/forward scatter mechanisms can occur.

c) SSN, or user-settable smoothed sunspot number. Here you can set a specific SSN (i.e. sunspot number) to be used for calculations. Note that VOACAP Online knows the current predicted smoothed sunspot numbers so it may be advisable not to set any value to the SSN field unless you wish to conduct your own propagation experiments. After you have entered a value in the SSN field, press the TAB key (instead of the ENTER key) to validate the number entered, and run the new prediction for the prediction wheel.

At this point, I would like to take a few moments to discuss the pros and cons of this feature. By default, VOACAP Online does internally know the current SSN to be used for all the months of the years available. You can ask how can that be as the sunspot number varies day by day? The simple answer is that VOACAP does not operate on daily SSN figures but smoothed monthly SSN figures which are being predicted for many years ahead and which are re-adjusted at regular intervals.

The predicted SSN figures are based on the Lincoln-McNish smoothing function, and they have been maintained by the National Geophysical Data Center (NGDC) / National Oceanic and Atmospheric Administration (NOAA). These are the sunspot numbers used in the database reduction for the worldwide ionospheric maps used in IONCAP and now VOACAP. This is why only these figures should be used with VOACAP. Read George Lane's discussion on the sunspot numbers for VOACAP use.

However, please note that, at the end of 2016, the NGDC/NOAA discontinued providing this invaluable predicted data, instead directing users to the SIDC website.

Jim Watson, one of the key figures behind VOACAP Online and a maintainer of his own Proppy website, has conducted a survey, comparing the effects of three SSN data sets: Original NOAA values, SIDC values, and adjusted SIDC values. The results indicate that VOACAP performs best with the original NOAA SSN values. Now, going forward, we will need to use 'adjusted' SIDC SSN values as a substitute. Read Jim's blog post or the full version (as PDF).

In addition, there have been months in the past where the conditions have been well above the average for a couple of months, and a re-adjustment of SSNs would have been appropriate. Now this power has been given to the user. Just remember that, strictly theoretically speaking, entering a daily SSN value in the SSN field does not generally give you better (or more precise) predictions as VOACAP is not suited to real-time predictions at all. Read more about the theoretical background of VOACAP in my Quick Guide.

d) Min. TOA, or setting the minimum takeoff or arrival angle for antennas at the steps of 1 degree, starting from 0.1 degrees (the default), up to 5 degrees. My default value has always been 0.1 degrees, due to some practical reasons. However, in the VOACAP literature, a value of 3 degrees is commonly recommended, as it can be a common lowest angle for arriving skywave signals due to the roughness of the terrain. Also, 3 degrees may be a good choice if your antennas are not located in a flat, unobstructed area. And if you are using isotropic antennas, you should avoid huge amounts of antenna gain at angles below 3 degrees. You are encouraged to experiment between 0.1 and 3 degrees to see differences in predictions, using different antennas.

4. Sunrise and Sunset Times

The section under Propagation Parameters labeled as "Today's Sunrise/Sunset Times (UTC)" offers the Sun's rise and set times at various regions of the ionosphere, calculated at both the transmitter and the receiver coordinates. All times are UTC.

These calculations were originally inspired by Steve's (G0KYA) more-than-15-year-old article about grayline propagation. In short, the best predictions for grayline propagation or trans-terminator enhancement on low bands can probably be achieved by a close examination of grayline maps. Some also swear by W6ELProp. To make this section even more informative, a new Point-to-Point prediction option - All-year grayline - has been developed. See below for more information. Also, you may be interested in VOACAP Greyline.

The abbreviation GND (for Ground) refers to sunrise and sunset at the sea level. The letter "D" refers to sunrise and sunset at the bottom of the ionospheric D region. Similarly, the letter "F" refers to sunrise and sunset in the ionospheric F region.

In the summer, if you place the TX or RX marker close to the Arctic Circle, you will see that "--:--" will appear in the D and F region fields. This simply means that sunrise and sunset times cannot be calculated for those regions e.g. because the sun does not set/rise during the summer at high latitudes. Alternatively, in the winter, the sun may not rise/set.

Please note that all the calculated values can be clicked on (as they are in fact buttons), and the Google Maps above will show how the terminator line will run across the map at the time clicked on. This can be very useful in determining the best times for low-band signal enhancements.

5. The Transmitter Site

a) Selecting QTH

In the Transmitter Site (TX) section you can, besides dragging the red marker to the appropriate location on the map, choose the location from a list of DXCC countries. The QTH pop-up menu features 483 locations around the world, including all DXCC entities. When you choose a location from this list, its name and the coordinates (latitude and longitude) will automatically be entered in their corresponding fields below. Much care has been taken to find the exact coordinates of even the smallest of the islands! If you happen to find a location with wrong coordinates, drop me a note!

When you drag the markers over the map, you are advised to use the Name field for entering a label for the TX site (otherwise, the Maidenhead grid locator will be used). And you can also enter the Maidenhead grid locator in the Name field, and press the "Loc calc" button: the corresponding coordinates will then automatically be calculated from the grid locator and entered in the Latitude and Longitude fields. The latitude and longitude values can also be entered manually. When you do that, please press the TAB key to validate them and run the prediction.

b) Selecting antenna and power

At the moment, only one antenna can be chosen for all amateur bands. If you need more freedom in selecting different antennas for different bands, you can try VOACAP Propagation Planner. In the VOACAP P2P service, the default antenna is a quarter-wave vertical over a very good ground. All TX and RX antennas are artificial in the sense that they are omnidirectional, which allows the user to see all possible openings to all parts of the world. In dipole-type of antennas, the height of the antenna is related to the elevation angle and the number of elements to the gain. When you choose an antenna, you should think about the elevation angles and gain, rather than the physical structure of the antenna.

In the TX power, you can select powers from 1 watt to 1500 watts at the steps given. 1500 W is the default setting. Some line loss is assumed so that the actual power used for the calculation is 80% of the power chosen. In the TX mode, you can choose from CW, SSB and AM. CW is the default setting.

c) Short-path and long-path, and specials

You can decide whether you want the predictions to be calculated via Short-Path or Long-Path. Short-path means the shortest distance between the TX and RX, and this so-called great-circle path is visualized with a red line on the Google Map. If you set this to Long-path, you will go from TX to RX in the opposite way: the longest great-circle path. On the red great-circle line, the geographical midpoint of the path is shown as a small red circle.

Last but not the least, there are three buttons:

  • Swap TX-RX,
  • Set Home, and
  • Unset Home.

If you click on the Swap TX-RX button, the TX and RX locations will be swapped: the current TX location becomes the RX location, and the RX location becomes the TX location. You can accomplish the same effect by double-clicking the red (TX) or blue (RX) marker on the map. In this way, you will see that the predictions for circuits are not always 100% reciprocal. In VOACAP calculations, this is mostly due to the different level of noise power in the RX site.

By clicking on the Set Home button, the TX Name, Latitude and Longitude information is stored in a cookie, as well as the RX Name, RX Latitude and Longitude, along with the TX and RX antenna selections. And when you press the Unset Home button, the cookie will be destroyed. Remember to allow your browser to set the cookie on this page if you want to allow this feature to work.

6. The Receiver Site

In the Receiver Site (RX) section, the input options are similar to those of the Transmitter Site. The RX location can be selected from the pre-defined DXCC list, or coordinates can be entered manually in the Latitude and Longitude fields. If you enter the values manually, please then remember to press the TAB key.

The Name field is used to give a label for this site, or alternatively you can enter a Maidenhead grid locator in this field and press the "Loc calc" button, and the latitude and longitude values will be calculated automatically.

Also the receiving antenna selection is exactly the same as for the Transmitter Site. Also, there is an option of choosing the noise level at the RX site. This will affect the QSO probabilities: when there is a lot of man-made noise, the probabilities are lower; when the noise level is minimal (e.g. “Quiet” (default) or “Remote”), then the probabilities are better.

Five types of P2P predictions available

So, after setting all the input parameters properly, you can always see the immediate results on the prediction charts section on the right of the Google Map. For most users, the first of the charts -- REL Short-Path -- may be enough for an overall understanding of the predicted propagation conditions.

The new Prediction Charts section, replacing the previous 24-hour circular prediction wheel, shown below. The circular wheel is now on a page of its own.

Why change to new charts?

The reason is simple: the new charts contribute to a better assessment of HF propagation. You can paint the big picture with a single VOACAP output parameter but, for a more accurate picture, you will need (at least) three: REL (used by the earlier 24-hour circular chart which is still available via a separate link: but also SDBW (signal power) and MUFday. These are now offered to the ham community, in addition to band-by-band predictions, which even visualize the signal power distribution (upper decile, median and lower decile).

Each chart offers multi-colored lines for the various parameters. And thanks to the JavaScript framework (plotly.js) used for plotting these graphs, all visible legend parameters/frequencies can all be toggled on and off by clicking on the legend values on the bottom of the graphs, helping the user focus on relevant parameters/frequencies only. Also there is a versatile toolbox on the top-right corner of each graph that allows the user to save the graph as PNG, zoom in/out, compare results data on all frequencies on mouse hover, pan the chart, and more.

But let's now make a recap of what the three parameters -- REL, SDBW and MUFday -- mean to you.

The REL or Circuit Reliability. The REL is related to VOACAP's output parameters of SNR (Signal-to-Noise Ratio) and REQ.SNR (Required Signal-to-Noise Ratio), and is defined as a circuit reliability factor. It tells us the percentage of days in the month when the SNR value (which is not shown in the charts as a separate parameter) will equal to or exceed the REQ.SNR. The REQ.SNR is an internal value set by me, related to the transmitting mode selected. For CW, the REQ.SNR is set to 24 (dB-Hz), and for SSB, it's 38 dB-Hz.

SDBW or Signal Power. The SDBW indicates the dBW (the strength of a signal expressed in decibels relative to one watt) value (the green line in the chart) that can be maintained on 50% of the days (ie. on 15 days) in the month. In a similar fashion, the SDBW90 indicates the dBW or signal strength value that can be maintained on 90% of the days (ie. on 27 days) in the month. And finally, the SDBW10 is the dbW value that can be maintained on 10% of the days (ie. on 3 days) in the month. However, it does not tell us which days are good or which days are bad. The SDBW10 and SDBW90 values are the top and bottom boundaries (respectively) of the light-gray area that is now always visible in all band-by-band prediction charts. The signal power distribution is calculated for Short-Path circuits only. The SDBW values are all translated to corresponding S-Meter readings in the charts.

The MUFday will tell us what percentage of the days in a month at that hour will be below the predicted MUF (Median Maximum Usable Frequency) for the most reliable mode (MRM). The MRM is the mode with the highest reliability of meeting the Required Signal-to-Noise Ratio, or REQ.SNR (see above).

These three output parameters are being calculated via Short-Path and Long-Path.

At the same time, I changed the default setting for the Transmit & Receive Antenna and Transmit Power. Now the TX & RX antenna is a quarter-wave vertical antenna over a good ground, and the TX power is set to 1.5 kW.

So, how to use the new charts? I regularly follow this sequence:

  1. Check the bands of the best REL values for the path in question.
  2. Then check the SDBW values for the best bands.
  3. When you have your candidate bands selected, then go to the charts for those bands.
  4. In band-by-band charts, pay attention to MUFday values, and the signal power (SDBW) distribution (the light gray area). If the distribution is extremely wide, there is a chance that VOACAP unfortunately does not have a good idea of what's going on.
  5. Be sure to check the Long-Path predictions, too! On long-haul paths, Long-Path may bring nice surprises.

The 24-hour prediction wheel shows the Circuit Reliability (or VOACAP's REL parameter). The prediction is always re-calculated as soon as any of the input parameters changes. The colors indicate the probability of a contact (QSO) over the days of a month at the given hour, using the TX mode set.

Then again, three more advanced prediction options are available, and these predictions can be calculated by clicking any of the three buttons below the Prediction Charts section:

  1. All-year grayline
  2. All-year prediction, and
  3. 1-month prediction
  4. BxB (band-by-band predictions charts)

1. All-year grayline, or all-year solar info to support determining grayline propagation

The All-year grayline calculates a wealth of solar-related information for the Transmitter and Receiver, and for the geographical midpoint of the circuit, covering the entire year from January to December.

There are seven different times which will be calculated for each site (TX or RX): three related to sunrise, three related to sunset, and one related to solar midnight. The eight time parameter (MIDPT MNITE) is the solar midnight at the geographical midpoint of the circuit in question.

Sunrise-related times:

  • DAWN = a point in time when the sun is 6 degrees below the horizon before sunrise
  • RISE = the sunrise time at the horizon
  • POST = a point in time when the sun is 3 degrees above the horizon after sunrise

Sunset-related times:

  • PRE  = a point in time when the sun is 3 degrees above the horizon before sunset
  • SET  = the sunset time at the horizon
  • DUSK = a point in time when the sun is 6 degrees below the horizon after sunset

Solar midnight

MNITE (and MIDPT MNITE) = This is the time opposite to solar noon when the sun is closest to the nadir (the direction pointing directly below a particular location), and the night is equidistant from dusk and dawn. The solar midnight rarely coincides with midnight on a clock. Solar midnight is dependent on longitude and time of the year rather than on a time zone. [Wikipedia:]

POST and PRE times 

The POST and PRE times are based on an educated choice; there is no pre-meditated scientific theory behind "the 3 degrees above the horizon". We know from experience that the low-band propagation starts to deteriorate at some point after sunrise, and that the propagation starts to get enhanced before the actual sunset, and "3 degrees" was my personal choice for this purpose.

There can be cases where no time is calculated but "--:--" is shown instead. This means that the sun does not reach the degree position set for the calculation.

Predicting probable grayline propagation enhancements

Not going to any deeper theoretical discussions here, there are basically three periods of time when distinct propagation enhancements have been reported on low bands.

These are as follows:

1. Both the Transmitter (TX) and Receiver (RX) are situated in the terminator zone. In my calculations, the terminator zone has been defined as the zone limited by DAWN and POST (post-sunrise) in the morning as well as PRE (pre-sunset) and DUSK in the evening. If there is an overlap between the Transmitter's and Receiver's morning/evening terminator zones, the times will be colored as follows:

a) TX DAWN-POST and RX DAWN-POST zones overlapping:


b) TX DAWN-POST and RX PRE-DUSK zones overlapping:


c) TX PRE-DUSK and RX DAWN-POST zones overlapping:


d) TX PRE-DUSK and RX PRE-DUSK zones overlapping:


2. The Transmitter (or Receiver) is in the terminator zone and the Receiver (or Transmitter) is in darkness. Whenever this condition is met, the times in the DAWN-RISE-POST and PRE-SET-DUSK columns will be colored in green. This is one of the most common cases for signal enhancement on the low bands.

3. The Transmitter (or Receiver) is in the terminator zone and the Receiver (or Transmitter) is in darkness AND the midpoint of the path is in the solar midnight. This is a special case of Number 2 in this list but a very important one. Currently, the solar midnight period at the midpoint (labelled as MIDPT MNITE in the tables) is defined as a time period of plus minus 7 minutes from the time calculated and shown in table. The MIDPT MNITE time is the exact calculated solar midnight time but, in my calculations, I consider that the midpoint midnight period is plus minus 15 minutes from that time.

There can be three colors for the time: black (default), red and blue. When the color is black, the midpoint midnight time does not overlap with the DAWN-RISE-POST or PRE-SET-DUSK times of neither the Transmitter not the Reveicer.

The colors of red and blue will be assigned as follows:

a) Midpoint midnight period and TX Terminator zone (morning or evening) overlapping: RED
b) Midpoint midnight period and RX Terminator zone (morning or evening) overlapping: BLUE

As a special mention, if the solar midnight period, a period of plus minus 7 minutes from the MNITE time at TX/RX, overlaps with the Midpoint Midnight period, the times in the MNITE column will be colored as follows:

a) TX midnight period (MNITE) and Midpoint Midnight period overlapping: RED
b) RX midnight period (MNITE) and Midpoint Midnight period overlapping: BLUE

Visualizing the grayline related information on a Google Map

[30 June 2017] The grayline related times can be investigated on a Google Map by clicking any of the days of the year.

Times of signal enhancements are marked with colors. Click on any date in the DATE column to view the calculated times on a Google Map for better intelligibility.
[Click on the image to enlarge
As soon as a date in the DATE column is clicked on, a Google Map will appear with the times of enhancements as buttons on the top of the map. Click on any of the buttons to see how the day/night terminator is running at that given time on that given day.

The times placed on the top of a Google Map as clickable buttons. Explanations of the buttons in red. The explanations are available as tooltips over the buttons on the page.
[Click on the image to enlarge]

2. All-year prediction, or point-to-point predictions for all months of the year at one go

The All-year Prediction calculates the point-to-point predictions for the circuit (from TX to RX), covering the entire year from January to December. The colors in the table indicate the probability of making a contact between the TX and RX, using the TX mode selected (CW, SSB or AM). All user-settable input parameters will be observed, except the Sunspot Number (SSN).

The All-Year Prediction Tables are extremely informative as they nicely combine the Circuit Reliability (REL), Signal Power (S DBW) and MUFday data as well as solar data for TX and RX in one compact, interactive format.

Click this line to read more about the importance of the MUFday on Above-the-MUF frequencies!

An all-year prediction for the circuit, showcasing the months of January 2017 and February 2017.

In the table, the elements of the top row are as follows (from left):

  1. Label for TX and RX,
  2. Short-path (SP) or Long-path (LP),
  3. the distance (kilometers & miles) of the circuit,
  4. the bearing (in degrees from True North) from TX to RX, and
  5. the Sunspot Number (SSN) used for calculations.

Below each prediction table, the sunrise and sunset times for TX and RX locations have been calculated and visually presented in the table cells below the UTC time row. The gray color denotes night-time and white day-time. The exact sunrise (SR) and sunset (SS) times (in UTC) will come up as you hover the mouse over the TX and RX label texts on the left column. Here, in this example, the sunrise (SR) at TX is at 0605 UTC and the sunset (SS) at 1813 UTC. The similar calculations are available for the RX site, too. The day used in the calculations is always the 15th day of the given month.

The sunrise and the sunset times for the Transmitter (TX) site.

All charts are interactive: if you hover your mouse over table cells, you will see a pop-up text, indicating the (VOACAP's REL) probability in percentages, and the (VOACAP's S DBW) signal power values in dBW (it’s always a negative number!). For instance in the example below, the signal power value of “-127” equals to a median signal strength of S5 whereas “-93” would correspond to S9 on the S meter. Read more about translating the signal power values (S DBW) into S-meter values here: .

The pop-up window over the table cell shows the VOACAP's REL output value in percentages and the signal power as a (negative) S DBW value

All colors, except grey, indicate QSO-making probabilities. In the prediction table per se, white means 0%, blueish 10%, greenish 30-40%, yellowish 50-60%, yellow-orangeish 70-80% and orange-reddish 90%, and pure red 100%. The color of grey does not indicate any probability value. Instead, it shows that, although VOACAP does not predict any probability for that specific hour, some signal power has been predicted which may translate into workable conditions. So, in a sense, grey indicates a heads-up note -- "a grey area" where QSOs may (or may not) be possible. Typically, these grey areas can mostly be found in low-band predictions (40 to 80 meters).

Propagation predictions use a color scheme from white (worst) to red (best).

All predictions charts start at 01 hours UTC. You may ask, "Why not start at 00 UTC?". Well, it's a matter of taste. All VOACAP predictions span 60 minutes but not necessarily the way you may think. A prediction for 01 UTC does not span from 01:00 to 02:00 but, in fact, from 00:30 to 01:30 UTC! So, I decided, being inspired by the original makers of VOACAP, to start at 01 UTC and end at 24 UTC. Following the same logic, 24 UTC means a time frame of 23:30 to 00:30 UTC.

3. 1-month prediction, or the original VOACAP P2P graphs

The button “1-month prediction” is the original button used at the VOACAP Online P2P site, but now it’s with a new name, and pressing it will calculate the detailed propagation prediction graphs for the entire frequency range from 2 MHz to 30 MHz, showing the REL (Circuit reliability) and S DBW (Signal Power) graphs for the circuit.

The Circuit Reliability graph, one of the two graphs calculated for the "1-month prediction".

4. BxB, or all the new Prediction Charts displayed on one page

If you wish to see all the new prediction charts displayed on a single page, click the button labelled "BxB" (short for Band-by-Band).


  1. Hi. How can i do for insert "The 24-hour prediction wheel" in my web page???

    1. Hi, you don't as it's not available. Other charts are available for this purpose, see:

  2. Hi Jari -

    I am getting more used to the new charts and like them a lot, thank you for the work!

    On a "nice to have" feature request, it sure would be nice to allow users to "save" (in a similar fashion to the "Set Home" button) the bands which we normally use. For example *most* of the time I only want to see the 80m and 40m charts as I am an early morning operator and don't operate on 60m. I would sure like a shortcut to avoid clicking the band selections unless I am considering an unusual case.

    However, it's not a show stopper and the more I know about it, the better I like the new presentation!

    73, Larry KJ1RE