et-voacap

The ability to plan short-range, mid-range, and long-range contacts on HF is central to effective emergency communication. The Voice of America Coverage Analysis Program (VOACAP) HF prediction software aids in the planning of HF contacts between two geographic points.

The challenge is that this software is very difficult to use by the average radio operator requiring many configuration settings. In keeping with the K.I.S.S. philosophy of EmComm Tools, et-voacap is designed to reduce the number of settings required and opts for meaningful defaults to give a “good enough prediction” for the average off-grid operator.

Power users are encouraged to use voacapl as et-voacap makes many assumptions about the RF path between two points. For example, the antenna used is not configurable in the current version of et-voacap.

Command Line Parameters

Open a terminal.
Run et-voacap to get the list of command line parameters supported.

$ et-voacap 
Usage: et-voacap.sh [OPTIONS]

Station options (choose one per TX and RX):
  --tx-call CALLSIGN        Transmitting station callsign
  --tx-grid GRID            Transmitting station Maidenhead grid
  --tx-latlon LAT,LON       Transmitting station coordinates in decimal degrees
  --rx-call CALLSIGN        Receiving station callsign
  --rx-grid GRID            Receiving station Maidenhead grid
  --rx-latlon LAT,LON       Receiving station coordinates in decimal degrees

If no --tx option is specified, your position will be pulled from a supported
ETC GPS unit or fallback to the grid square configured by 'et-user'.

Other options (required):
  -p POWER                  Output power [5|10|20|50|100|500|1500]
  -m MODE                   Mode [am|ardop|cw|js8|ssb]

Running a Simple Prediction

Running a simple prediction involves a minimum of three command line options:

The remote location, specified by one of the --rx-* options.
The output power of the transmitting stations with the -p option.
The operating mode being used with the -m option.

The --tx-* parameter is optional and specifies the transmitting station. This is typically you and your location. If a --tx-* parameter is not specified, then EmComm Tools attempts to use your location if a support GPS unit is detected, otherwise it uses the maidenhead grid square defined by et-user.

For example, let’s assume that you want see what the chances of making a contact with Jason KM4ACK using JS8Call at 20 watts.

et-voacap --rx-call km4ack -m js8 -p 20

EmComm Tools R5 has a small, offline, geocoded database of all the U.S. amateur radio operators as of 11/12/25 that it uses for doing a reverse geographic latitude and longitude lookup using the on-file zip code with the FCC.

If the command is successful, it will generate a report containing the circuit details (transmitting and receiving stations), their distance, the bearing from the transmitting station to the receiving station, and a reliability report of making the contact over a 24-hour period (expressed in UTC) on each of the amateur radio bands.

VOACAP Prediction via Short-Path. Nov 2025: SSN 123. Power = 0.016kW, AM
TX (33.90N, 112.04W) to RX (35.85N, 86.39W): 2343 km, 1456 mi, 77 deg

  | 01|02|03|04|05|06|07|08|09|10|11|12|13|14|15|16|17|18|19|20|21|22|23|24|
10| d                                      d  B  A  A  A  A  A  A  A  A  B |10
12| C  f                                   C  A  A  A  A  A  A  A  A  A  A |12
15| A  C  f                             f  A  A  A  A  A  A  A  A  A  A  A |15
17| A  A  C  f           f  f           C  A  A  A  B  B  A  A  A  A  A  A |17
20| A  A  A  B  C  d  C  B  C  d     e  A  A  A  A  C  C  C  C  A  A  A  A |20
30| A  A  A  A  A  A  A  A  A  A  B  A  A  A  C  C  e  f  f  e  d  B  A  A |30
40| A  A  A  A  A  A  A  A  A  A  A  A  A  C  d  f              f  d  C  B |40
60| B  B  A  A  A  A  A  A  A  A  A  A  B  d  *                    *  d  C |60
80| C  C  B  B  B  B  B  B  B  B  B  B  C  *                          *  d |80
  | 01|02|03|04|05|06|07|08|09|10|11|12|13|14|15|16|17|18|19|20|21|22|23|24|

A = 90 - 100%   d = 25 - 49%  * = REL 0%, but Signal Power over Noise
B = 75 -  89%   e = 10 - 24%
C = 50 -  74%   f =  1 -  9%

In general, contacts are typically possible, on average, for those bands (left column) under the hours displaying an A (90% to 100%).

A big thanks to Jari OH6BG for developing the scripts that formats the above table.

Advanced RX Options

If an amateur radio callsign is not available in the current offline database, the user can set the receiving station using one of two options:

latitude/longitude: --rx-latlon LAT,LON
Maidenhead grid: -rx-grid GRID

LAT,LON must be expressed in decimal degrees and contain no spaces (33.8851,-111.9349).

GRID must be a 4-digit (DM43) or 6-digit (DM43AV) valid maidenhead grid square. The case does not matter.

Example: Lat/Lon to KT7RUN

et-voacap --rx-latlon 33.8851,-111.9349 -m js8 -p 20

Example: Maidenhead Grid to KT7RUN

et-voacap --rx-grid DM43av -m js8 -p 20

Advanced TX Options

The transmitting station, typically you, is based on your current GPS position (if available) or the grid specified by et-user. To override this behavior, use one of the three --tx-* parameters.

callsign: --tx-call CALLSIGN
latitude/longitude: --tx-latlon LAT,LON
Maidenhead grid: --tx-grid GRID

Example: KT7RUN to KM4ACK

et-voacap --tx-call kt7run --rx-call km4ack -m js8 -p 20

Less Nerd Prediction

If you want less nerd prediction, check out the next section as it covers a new graphical HF prediction application called et-predict-app.

[Next Up: et-predict-app]