FSM Sensitivity Improvement Experiment 1

This week-end I finally attempted to increase the sensitivity of my Field Strength Meter (FSM). I originally built it with a 50uA analogue panel meter from Jaycar, but have reported previously that I found it rather insensitive.

I was thinking this was simply due to the 50uA meter having a high resistance of 3,000 ohm. So for an experiment I grabbed myself a 1mA meter from Jaycar which only has a resistance of 200 ohm.

Well, it appears that I forgot that ultimately they're calibrated dials and as a result I found that the change of dial made no different to the sensitivity at all. To try and understand this, I resorted to the maths and all became very clear.

50uA Analogue Panel Meter
Resistance: 3000 ohm
Minimum reading: 1uA

Thereby, the minimum signal strength that will be detected (in uV) can be calculated:
E = IR
= 1e-6 x 3000
= 1e-6 x 3e3
= 3e-3
= 3mV or 3000uV

1mA Analogue Panel Meter
Resistance: 200 ohm
Minimum reading: 0.02mA

Thereby again, we calculate the minim signal strength:
E = IR
= 20e-6 x 200
= 20e-6 x 2e2
= 40e-4
= 4e-3
= 4mV or 4000uV

For comparison, I hooked up the FSM to my digital multi-meter (DMM) instead (as suggested by the original article) for improved sensitivity. I placed the meter on the 200mV range and had a few tests.

And as suggested, there was a huge increase in sensitivity! With the analogue meters, once I was more than about 0.5m away from the unit nothing was detected. But with the DMM in use, I could stand at one end of the house and still make a reading when transmitting with only 1W.

As comparison, the minimum reading I can accomplish with the DMM in 200mV range (seeing it supports 1 decimal place) is 0.1mV or 100uV. This is a significant difference, so let's summarise.

Summary

The minimum signal strength that the following meters can display:
50uA Panel Meter: 3000uV
1mA Panel Meter: 4000uV
DMM: 100uV

Changing between the two panel meters almost had no effect – although the 1mA meter is slightly less sensitive than the 50uA meter (opposite to what was expected). On the other hand, the DMM in theory is around 30x more sensitive than the 50uA meter.

Going forward

I'm actually now tempted to find a uV Panel Meter and see how that goes. My understanding is that in commercial radio gear it is much more common for signal strength to be measured by a uV meter rather than an S meter, so this seems like a good idea. However, I know Jaycar does not sell them so I've no idea how hard they'll be to source.

After that, I'll again look at an RF amplifier.
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