[ Home ] [ Up ] [ Reporting Forms ]

Measurements Using Amateur Station Receiver S Meters

Not all amateurs have access to spectrum analyzers or other sophisticated test equipment. In that case, they may have to rely on S-meter readings. S meters are not known for their accuracy, but as a general indication of interference levels in terms that many radio operators will understand, S meter readings will provide useful information. When presented, S-meter data should also include station information, such at the receiver used, its mode, bandwidth, preamplifer or attenuator settings, etc. Also, if possible, S-meter readings should be provided in fractional S units. While the resolution of S-meter readings is not as good as fractional S-meter readings might imply, those fractional readings may be useful in other tests, some of which can scientifically assess the relative amount of noise, expressed accurately in dB. 

Baseline Measurements: 

If BPL is coming to your area, it will be helpful to know just what the present noise levels are in your community. It would also be helpful to record the typical signal strength of standard time and frequency stations (such as WWV and CHU), as well as specific shortwave broadcast stations, for future reference. To do either of these tasks, you can solicit the help of hams in the area. Also, mobile stations can go to the area where BPL is to be deployed and obtain a baseline of the noise levels there per band. Later, the BPL noise levels can be compared to these baselines.

Most hams will do this using their receiver S meters. This can be useful, if the readings are made in fractional S units and all information about the receiver and station is logged. Later, when BPL comes to the area, the test can be repeated and the difference in S meter reading will provide some quantified data on the degradation of that station. It may also be possible to calibrate that S meter against a calibrated signal generator, improving the accuracy of the S-meter readings. If you do, it is important to do a separate calibration for each band, as some receivers’ S meters vary a lot from band to band. They also can vary a bit over time, but that really can’t be helped, so just follow the usual “warm up for 30 minutes” practice to ensure that your receiver is at thermal equilibrium.

If you don’t calibrate your receiver S meter against a signal generator, the approximate relationship between S meter readings and RSLs is shown as Appendix A to this document.

Appendix B shows a suggested reporting form. 

BPL Signal Measurements Using S Meters  

If the baseline measurements have been made, they can later be compared to the BPL noise levels after the BPL system is deployed. That way, as a minimum, an amateur can say, for example, that the noise level before BPL was installed was S2.2 (some noise is present), but it is now S9+10 dB – an extremely strong level of noise. Estimating at 6 dB per S unit, this is a degradation of approximately 63 dB. If the S meter were calibrated against a signal generator, an even more accurate estimate could be made.

Those S meter levels can also be used to obtain RSLs, accurately if the S meter were calibrated against a signal generator, or approximately by using the levels in Appendix B. If desired, the RSLs can be converted to field strength using the formula described earlier in this document.

Measurments Made With a Step Attenuator  

A good step attenuator with 1 dB, or smaller, steps can be used to make relative measurements. There are two ways to do this.

The first uses the receiver S meter. Let me explain this with an example. If you took a baseline reading with your receiver and obtained a reading of S2.5, then found S8 noise later with exactly the same receiver settings, you could estimate that you had about 33 dB of degradation. If you inserted a step attenuator in the coax feeding your receiver, you could then adjust the attenuator until the S8 noise read S2.5 on your S meter. The attenuator setting would be the amount of degradation that the S8 noise was giving to your station. 

You can also make measurements using a step attenuator, your receiver and an audio voltmeter. This generally must be done over a relatively short period of time, because you can’t touch the receiver volume control during this test. This can be used to measure a signal that is turning off then on, or to compare the levels outside a BPL area with the levels inside the BPL area, perhaps by using a mobile receiver. Here are the steps: 

o     First, tune your receiver to the desired frequency, adjusted for normal operation and with the audio set to a usable level.

o     Crank the attenuator to maximum level and disconnect the antenna.

o     Readjust the receiver volume control to a usable audio level.

o     Connect the AC voltmeter to the receiver output. This should be an RMS-reading meter (such as most Fluke instruments), but an average reading meter will still provide usable, if less accurate, data.

o     Obtain and log the measured voltage as Voltage#1. If the signal being measured is noisy, this will usually be easier with an old analog meter with a pointer, as you can watch the pointer bob back and forth a bit and estimate the center point by eye.

o     Do not change the receiver volume control or other setting in any way for the remainder of this test.

o     Reconnect the antenna.

o     Now, adjust the step attenuator so the level of the actual band noise is just above the hiss level in the receiver. This will typically be about a 10-dB change in the audio voltage, or about 3 times the Voltage#1. Log this level as Voltage#2.

o     Now, without changing the receiver volume control or other settings in any way, wait for the noise you want to measure to appear, or drive the mobile station into the BPL area.

o     When you encounter BPL or another signal you want to measure, adjust the attenuator setting until the signal being measured gives the same AC voltage as Voltage#2.

o     The attenuator setting is the amount of degradation that was caused by the BPL signal.

Appendix A: S Meter to Received Signal Levels Table 

 This shows the approximate correlation between receiver signal-strength-meter S units and received signal level. The actual levels vary from receiver to receiver. The signal strengths reported by amateurs on the 3.5 MHz band will typically represent the levels in a 2500 to 3000 Hz receiver bandwidth. The S meter in most receivers becomes less linear toward the low end of the scale. S9 is 50 uV across 50 ohms, and each S unit is nominally 6 dB.

Appendix B: Baseline Measurements Reporting Form is available here

[1] The slant range distance would be calculated using the formula for the hypotenuse of a right-angle triangle. For example, if a power line were 10 meters in height and the measurement were made a 1 meter height, 10 meters horizontally from the power line, the slant-range distance would be sqrt(10^2 + 9 ^2) = 13.45 meters.

[2] Most amateur antennas are located at greater height than the power lines. NTIA has also performed all of its skywave calculations based on the premise that the BPL emitters are radiating no more than Part-15 levels skyward. Aeronautical, Inc. and Boeing have both expressed grave concerns about BPL interference to aeronautical communications, so it is important that the emissions above the power line be accurately determined.