Monday, June 4, 2018

Field Strength Meter Monday: Let's Calibrate Baby!

If you are catching up on the HVDN Smart Field Strength Meter project or coming across this for the first time, here are past articles in the "Field Strength Meter Monday Series"
Frequency Calibration Comments

With the brief US Memorial Day weekend break over, the first post of June in the series will focus on ensuring the RF power sensor is calibrated for somewhat accurate measurements useful to the amateur radio community. Key design frequencies are:

The frequencies chosen between the 1 MHz and 25 MHz were chosen because they are in the sub bands for digital text transmission modes such as PSK31 and FT-8.  Both modes have great signal to noise ratios and permit very long distance communication with low transmit power. Ensuring optimal antennas are used, especially for portable or temporary operation, is one use case for the field strength meter to get the most out of these most modern modes for those interested in HF communications.

Frequencies chosen between 28 MHz and 925 MHz were chosen with a little less formality. The 28.120 MHz  and 50.290 MHz were chosen because of similar use cases as those in the HF band.  146.000 MHz and  223.000 MHz were chosen as good approximate "middle" frequencies in the 2m and 1.25m bands. The three frequencies in the 70cm band were chosen since its the widest and most used amateur band. Lastly, 925 MHz was chosen for representation of the 33cm band which is not very well used, but can help determine the top end of the field strength meter range.

Antenna Calibration Comments

With such a wide design range, it would be hard to use a 1/4 wave antenna for each frequency for calibration, even though that would be optimal. Only 3 antenna lengths will be used to calibrate the field strength meter.
  • 6in/15.3cm = Chosen for small size and near 1/4 wave length for 70cm UHF testing
  • 19.2in/48.5cm = Chosen for mid length size and near 1/4 wave length for 2m VHF testing
  • 39.2in/1m = Chosen as a standard metric length to be used for general HF and VHF testing
The same exact material diameter will be used for all three antenna lengths

Unit Measurement Calibration Comments

Using the tables found in the "RSSI, dB, Oh My!" article is where we will get our calibration targets of 50 uV for the lower 8 frequencies chosen and 5.0 uV for the upper 8 frequencies chosen.

The upper frequencies will be easier to calibrate based on easier to set up test sites compared to the lower frequencies which require larger reference antennas, so calibrations will be more general for HF compared to more accurate for the higher frequencies.

Calibration Goals

The general theory in ensuring an S9 signal level can be generated at specific frequencies from a chosen distance from the transmitting antenna is how we will arrive at our calibration figures. The transmit power from the signal source will also be uniform at 37 dBm or the equivalent of 5 watts for all measurements to gauge the distance needed to generate the S9 signal level.

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