To find the HF distance we use a range calculator and practical numbers obtained from RF receiver testing by the ARRL.
In our calculation example we use the Eton 750 radio receiver.
This is a high quality shortwave radio from the Eton Corporation. The receiver specifications are not provided in the data sheet. This is an important input to the distance calculator. We had to search a little deeper to find a review article by the ARRL where they tested the receiver sensitivity and other parameters.
The sensitivity at 7.49 MHz is 1.0 μV.
Use the μV to dBm calculator to calculate the equivalent -107 dBm amplitude. This is the minimum level required at the receiver. Note that we have not used the Minimum Detectable Signal of -130 dBm in this calculation, as our interpretation is that it represents the noise floor of the receiver.
Use the following numbers in the calculator above:
- Operating Frequency 7.5 MHz (40 meter band using calculator)
- Transmit power 10 Watt or +40 dBm (calculator)
- Receiver sensitivity -107 dBm
- Antenna Gain 0 dBi
- Cable and other losses 10 dB
The HF coverage distance is found to be 22,519 km or 14,074 miles.
Note that this is under ideal conditions.
Practically, using skywave propagation, HF can achieve a range of 1,900 miles.
A reddit user was able to achieve a range of 16,799 km with 30 watt of power.
How does this compare with the calculation? Ideally, with 30 Watt transmit power (+44.77 dBm) the range is 39,000 km.
The calculator uses the Free Space Path Loss (FSPL) equation:
FSPL = 20*Log10(d) + 20*Log10(f) + Lc + 20*Log10(4π/c) – GTx – GRx
HF Signal Strength Measurement
An easy way to measure the receive signal strength of an HF transmission is to use a spectrum analyzer like the Siglent below. While it might be tempting to use a lower cost Spectrum Analyzer like the TinySA, it doesn’t appear to have the required sensitivity in the HF band.