My recent article Near-field field strength measurements using the RFPM1 described a technique using VK3AQZ’s RF Power Meter which is based on the AD8307 log detector.
There are many ways to measure low level RF power or voltage, and this article describes methods that I have used using a simple diode detector attached to the HF loop, and measuring the DC output voltage using a small digital panel meter with 9V battery for a self contained measurement system with little risk of significant common mode current. Such a system can be hauled to some height and read remotely with a telescope.
Note that diode characteristics vary from diode to diode, and they are temperature sensitive.
Examples
HP-423A
The HP-423A is a moderately expensive crystal detector good to microwave frequencies.
Above is a calibration curve created from measurements of a particular detector. It is good for measurement purposes from 20dBm (7.7V) down to about -15dBm (40mV).
The non-linear transfer characteristic means that some conversion must take place, the meter is not direct reading.
1N34A
The clip on RF ammeter described at (Duffy 2011) used a simple half wave detector using a 1N34A Germanium diode with a 22kΩ DC load.
Above is the transfer characteristic at the low end. It is good for measurement purposes from 30dBm down to about -15dBm (40mV).
The non-linear transfer characteristic means that some conversion must take place, the meter is not direct reading.
Above, the detector response from 0.2V to 1V input. Linearity is very good, though the Y intercept is non-zero. However, it makes for easy conversion from measured DC voltage to RF input voltage (Vrf=(Vdc+0.13)/1.09).
Lets look at at example at the low end of that linear range. If we measured 100mV DC, Vrf=(0.1+0.13)/1.09=0.211Vrms which is -0.5dBm (using Convert Vpk-Vrms-dBV-dBu-mW-dBm).
Above, the result of spot frequency calculation using Calculate small loop Antenna Factor. In this instance, the field strength is 141dBµV/m.
This system is not nearly as sensitive as the VK3AQZ RF power meter (RFPM1) which can measure power to about 45dB lower than the simple diode detector… but the diode detector is usable in capable hands… though you may want an antenna with lower Antenna Factor, and higher transmitter power to make measurements at distance.
Other diode types and configurations
Other diode types, most commonly Schottky diodes, and other configurations (eg voltage doubler) are commonly recommended by hams. My choice of a half wave Germanium detector was an informed choice, informed by examining diode characteristics (eg plots at (Duffy 2011) and experience. The Germanium diode is very well suited to this application, and they are readily available on eBay usually for a few dollars a hundred if you look around.
Conclusions
- It is possible to make reasonably accurate measurement of near-field strength using relatively simple equipment.
- A simple half wave detector with a calibration curve can be used to measure RF power in the loop load, and converted to field strength using the loop’s Antenna Factor.
References / links
- Duffy, O. Sep 2006. Small square un-tuned loop for field strength measurement – NEC models. VK1OD.net (offline).
- ———. Jun 2007. Small un-tuned single turn square loop for field strength measurements. VK1OD.net (offline).
- ———. O. May 2011. Measuring common mode current. vk1od.net (offline).
- VK3AQZ RF power meter (RFPM1)
- Calculate small loop Antenna Factor
- Convert Antenna Factor and Gain
- Field strength / receive power converter
- VK3AQZ RF Power Meter kit (RFPM1).