About the WSPRDaemon noise graphs - and repurposing for use in monitoring WWV(B) signal levels over time.

What is WSPRDaemon?

The WSPRDaemon program is largely a work of Rob, AI6VN and it exists for the purpose of facilitating the reception of WSPR signals off air, processing the data, and making that data available publicly - not only to wsprnet.org, but also via the wsprdaemon.org web site.  The WSPRDaemon script runs on a Linux computer - typically a Raspberry Pi - and typically takes data from a web-connected receiver (typically a KiwiSDR) - but it can also use the ubquitous RTL-SDR dongle or even the "raw" audio input from a receiver via a sound card.

For each of the (approximately) 2 minute receive "cycles" of WSPR transmissions (which are, worldwide, scheduled to start precisely at the beginning of each even-numbered minute) an audio (.wav) file is recorded for each receiver - and this audio is then processed using the "WSJT-X" program to decode the WSPR transmissions found within that audio stream.

Features not found in WSJT-X that are made available in the wsprdaemon script include the scheduling of receivers (e.g. switching a limited number receivers to most favorable bands) and the "merging" of several receivers - possibly using different antennas for diversity - that are listening to the same band so that only the best of the WSPR "spots" is reported, avoiding duplicate reports of a given transmission.

• More information about wsprdaemon may be found on the wsprdaemon Github page.
• For more about WSPR, see the WSPR Wikipedia page and the other pages to which it links - especially K1JT's page.

What it does:

While not precisely calibrated, having long-term, disparate records of WSPR signal reception from all over the world can provide useful information to amateur radio operators and researchers alike, providing a glimpse into the propagation of LF, HF and even VHF (and higher bands) which can help divine when propagation is occurring between two stations and when it occurred.  Having a "live" and past database of these events can help validate/tweak models of the Earth's geomagnetic field and its interaction with the sun on the ionosphere - and to satisfy the intellectual curiosity of anyone who wishes to study this by themselves.

Accumulation of "noise" data:

In addition to the accumulation of WSPR data, the wsprdaemon software is able to measure the apparent noise floor on the specific frequencies associated with WSPR transmissions on the amateur bands and with some hardware it is possible to calibrate this measurement in absolute terms of intercepted noise power.

If receivers taking these readings are found at disparate "RF quiet" sites around the world, this data can be informative of the natural, background noise which can be indicative of the state of the ionosphere and the Earth/Space environment:  In some cases, it is possible to observe the rising/setting of strong radio noise sources such as Sagittarius A and, occasionally, "noisy" planets in our solar system such as Jupiter not to mention the tremendous noise that can result when the Earth's magnetic field is being abused by our local star.

The software makes two separate noise measurements:

• The noise "floor" within the passband.  This reading (the "RMS" level, in Red on the wsprdaemon.org graphs) is the calculated noise floor level and its processing attempts to "remove" the effects of other signals within the detection passband.

• The signal power within the passband.  This reading (the "FFT" level, in Blue on the wsprdaemon.org graphs) is the total power within the passband.  Unlike the RMS reading, this is indicative of the cumulative power intercepted and will always be higher.

Even though the apparent detection bandwidth of these receivers is on the order of 400 Hz, all readings are scaled (by approximately 26 dB) such that the power readings reported are relative to a 1 Hertz detection bandwidth.

A typical graph may be seen below:

Figure 1:
A typical graph showing the last 24 hours of noise on the 40 meter amateur band a receiver at the Northern Utah WebSDR.

The displayed time, in UTC, shows the noise level rising at around local nighttime and then dropping off at night.  This graph is "skewed" somewhat by the overnight presence of strong thunderstorms in the Eastern United States, the intensity of which gradually tapered off overnight and into the day and by the fact that the antenna used is a log periodic beam has significant gain that is pointed in the direction of those storms.

Almost the "inverse" of this is the noise graph from another receiver at the Northern Utah WebSDR site:

Figure 2:
This graph depicts a 24 hour plot of the noise floor on the 20 meter band.

The noise floor can be seen to increase during the daylight hours, but drop to the floor during the night when propagation and ionospheric stimulation by the sun effectively ceased.  As expected - particularly during the period of low sun activity during which this is being written - the noise floor and signals decrease during local nighttime.

In theory, careful analysis of the noise data used to produce the above graphs can provide the opportunity to analyze HF propagation modes and the effects of Earth-Space environment.

Monitoring signals from consistent sources:

It occurred to me that the wsprdaemon script also afforded the opportunity for something else for which it was originally intended - the monitoring of consistent signal sources of known transmitter power and location.  From the Utah location, one source of signals - those from the NIST in the form of the WWV and WWVB  transmitters - was obvious.

A quick modification of the wsprdaemon script allowed the addition of additional frequencies:  While there would clearly be no WSPRnet reporting on these non-amateur channels, the noise measurements would still be posted as the graph below depicts:

Figure 3:
 A noise graph of the WWVB transmission at 60 kHz in Fort Collins, Colorado.

The red line shows the received signal level from WWVB - the same signal used to set many automatic clocks - on 60 kHz.  During daylight hours the signal level is pretty consistent at the "-100dBm" mark while during the night, signal levels vary a bit - particularly during sunrise/sunset where ionospheric perturbations are evident.

The "Blue" line is largely influenced by the 17dB amplitude modulation of the WWVB carrier used to convey time and date information, but it is also prone to being "diluted" by peaks in the background noise as can be seen during the nighttime hours (from about 0400-1100 UTC) where propagated lightning static is evident.

From the same site, on a frequency that is orders of magnitudes higher we get this plot:

Figure 4:
 A noise graph of the signal and noise levels on the 10 MHz WWV/H frequency.

The graph of Figure 4 is a bit more cluttered as one might expect.  The Red "line" shows the wildly varying signal of the signal on 10 MHz - which could be from either WWV in Colorado OR WWVH in Hawaii.  As is the nature of HF, these signals can vary significantly - not only between day and night, but also from one moment to the next.  The blue line generally depicts the noise level at 10 MHz, but this may not be truly representative as it may be being affected by the ever-present modulation on the WWV/H carrier - primarily in the form of the 100 Hz time code modulation.

The utility of the graph in Figure 4 may be debatable as there is not one, single signal source, but it does provide a general perception of the signal levels that one might expect - and how the time of day affects them.

Final comments:

In addition to the general monitoring of the noise floor on the HF bands where WSPR monitoring is taking place, the wpsrdaemon script can also be used to monitor signals from known transmitters.  To be sure, this wasn't the intended use of this software and if such data is useful, it's likely that the utility and accuracy of such measurements could be improved.

* * * * * * * * * * * * * * * * *

Modifications to the wsprdeamon script for version 2.8a:

In the "wsprdaemon.sh" file, one need only add a few lines to the code to produce the "new" bands in the array "WSPR_BAND_LIST", as in:

"WWVB        58.5"
"WWV_2_5  2498.5"
"WWV_5      4998.5"
"WWV_10     9998.5"
"WWV_15    14998.5"
"WWV_20    19998.5"
"WWV_25    24998.5"
"CHU_3       3328.5"
"CHU_7       7848.5"
"CHU_14     14658.5"

Each new "band" is named by the first entry on the line (e.g. "WWVB") and the frequency of the carrier that one wishes to monitor is defined in kHz in the second entry.  Note that the frequency used here is 1.5 kHz lower than the actual carrier frequency to be monitored.

In the "wsprdeamon.conf" file where the receiver and its use is to be defined, these "bands" - defined in wsprdaemon.sh - are used in exactly the same way as any other band in the list.  For example, one might schedule the start a hypothetical receiver called "KIWI_1" on the WWVB signal at 0000 local time as follows:

declare WSPR_SCHEDULE=(
"00:00  KIWI_1,WWVB"
)

Comment:  At present, wsprdaemon will dutifully try to process the audio file for WSPR spots - but it will fail to do so.  It should be possible to modify the code to add an argument that will prevent this from happening to reduce processor loading.

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This page stolen from ka7oei.blogspot.com

[END]

Setting/calibrating signal and noise levels for signal/noise level monitoring using the "wsprdaemon" script

The WSPRDaemon script, a work largely of Rob, AI6VN, is designed to automate WSPRNet reporting of signals transmitted using the WSPR protocol - (The script may be found on GitHub, here).  While the WSJT-X program, itself, can report the signals that it hears in terms of callsign, Maidenhead grid locator and apparent signal-noise ratio, the WSPRDaemon script leverages the multi-receiver capability that can exist in various hardware configurations - including the KiwiSDR - which, with the appropriate platform, can receive as many as 15 signals on 15 different frequencies simultaneously.

Comments:

While this discussion is focused on methods and measurements exclusively using the KiwiSDR as that is the hardware with which I am most familiar, where similar capabilities exist, these same methods can be applied to other receive platforms.

Similarly, the methods discussed on this page need not be specific to the software mentioned and may be applied any time receiver signal level measurement is needed.

Care has been taken such that at the Northern Utah WebSDR, all of the WebSDR's receivers' S-meters are calibrated to within a dB or so in the center of their target band using the methods described below.

HF noise floor measurements:

In addition to facilitating the decoding and reporting of WSPR signals, the WSPRDaemon script can also divine the apparent noise floor within the receive bandwidth on some of these platforms.  In the case of the KiwiSDR - where an "S-Meter" reading is available via the remote interface - this reading can be used to derive absolute measurements and such data, taken from multiple locations over disparate geographical locations, can be used to glean information about the noise on the HF bands over time.

In practice, accurate, absolute signal level readings are quite difficult to obtain owing to the issue of meaningful calibration - and the more complicated the antenna system is, the more difficult it may be to get such measurements as one must consider things like:

• Cabling losses
• Splitter losses
• Filter losses
• Amplification
• Calibration of the receiver
• Gain of the antenna (e.g. dBi, dBd)

Fortunately, all but the last on the list may be accounted with a single measurement using a known-accurate signal source.

The "short" version:

The ultimate goal of this exercise is quite simple:

Input a signal of known amplitude into your antenna system - say, -60 dBm - and have the noise logger report that same signal level, +/- your known antenna gain.

The steps below are included in the event you may not be familiar with the methods/techniques involved.

An example receive system:

As an example, the KiwiSDR-based portion of the system in use at the Northern Utah WebSDR (a browser-enabled remote HF receiver found at sdrutah.org) will be discussed and a simplified block diagram of the signal path may be seen below:

Figure 1:
Simplified KiwiSDR Signal Path used at the Northern Utah WebSDR showing several gain blocks and filters.
As noted, the directional coupler is connected in "reverse" as it is being used to inject rather than extract a signal from the signal path.
Click on the image for a larger version.

The components of the receive system depicted above in detail:

As with any antenna system, it begins with an antenna - but for the moment, we will ignore any intrinsic gain - or loss - that the antenna may have.

• Directional coupler:  At the Northern Utah WebSDR, the Mini-Circuit Labs ZFDC-20-3 is used at the main antenna input.  This device has minimal insertion loss (<0.25dB) on the "through" line and there is a separate port (labeled "CPL") that is coupled at 20dB from the through line.  This sort of device is typically used to extract a small sample present on the line while minimally affecting it and "in" and "out" ports are labeled for such purposes, but here it is being used in reverse to inject a signal into the receive signal path.

This device is convenient in that it has the advantage that it may be left in line all of the time with negligible effects at HF and it can put test signals into the receive signal path without affecting off-air performance.

This device is optional in that the same measurements may be obtained without it by connecting the signal source directly to the antenna port - the obvious disadvantage being that the signal path is interrupted - something that we try to avoid on a busy, multi-user system such as a WebSDR.

• Calibrated signal source:   It should be no surprise that having a known-accurate signal source is an absolute necessity when it comes to calibrating the signal level readings with available levels in the range of -60 to -30dBm being the most useful.  Having access to such devices (e.g. Communications Test Set or RF Service Monitor) is helpful - but other means of obtaining a usefully-accurate signal include:

• An "inexpensive" calibrated signal source like the Elecraft XG-3 - the >$300 price tag (when fully equipped) being the reason for the quotes.

• A "transfer" measurement from a signal source with a consistent level.  Some antenna analyzers - including the popular "NanoVNA" - can output a level that is fairly consistent.  Coupled with outboard attenuators (a switchable step attenuator and/or an assortment of fixed attenuators) one can use known-accurate test equipment (an RF power meter, signal level meter, spectrum analyzer) to obtain a table of signal power readings for each of the HF amateur bands.

• Because of the harmonic content of the NanoVNA, it is preferred that a selective means of measuring RF power like a spectrum analyzer be used rather than radiometric detector like a power meter - but the difference is unlikely to be significant.

• Important note:  The amplitude of the NanoVNA's output port is too high to be directly used as a calibration signal - especially if amplification is used in the signal path.  Expect to need attenuation in the range of 20 to 50 dB to provide a usable signal level:  It's recommended that one use a switchable step attenuator and/or a series of fixed attenuators to get reasonable signal levels.

• A transfer measurement from a piece of equipment that is likely to be "pretty close" - such as the KiwiSDR itself:  With the default "factory" S-meter calibration level of -13dB the KiwiSDR to which I have access have typically been within a couple of dB when comparisons were made.
• For calibrating, a signal source (e.g. NanoVNA) would be connected directly to the input of the receiver - via the attenuator(s) - and the S-meter levels noted for different settings on each of the amateur bands.

• AM BCB Filtering:  In many cases, strong AM broadcast band (mediumwave) signals will be present on the antenna port.  Even if these signals are not strong enough to overload the RF amplifier that follows, such signals can overload the receiving device downstream.  As mentioned later, there are advantages to strategically attenuating strong signals to minimize the total power impinging on the A/D converter of any direct-sampling receiver.
  

The AM broadcast-band filter used at the Northern Utah WebSDR is a rather complicated affair (and is described in this article:  Managing HF Signal Dyanmics and preventing overload... (link)).  Its main purpose is to reduce the amplitude of several very strong (50kW) local transmitters to prevent the "wasting of amplifier power and A/D bits" on very few signals.

• Amplification (post AM/BCB filtering):  In general, a 15 dB system noise figure is sufficient to allow the detection of a 10 meter signal above even the lowest expected level of background noise found at this frequency.  For this reason it is suggested that an RF amplifier be placed after the AM BCB filter (to minimize the probability of overload and intermodulation/mixing products).  Remember:  Once loss appears in a receive system due to loss/attenuation, no amount of amplification can restore the system noise figure beyond that point.

Such an amplifier should also be placed prior to any RF splitting as much as is practical as any loss - which can be from the feedline, in filters or in a splitter - will directly contribute to the the system noise figure.  In cases where very strong signals may be present on the antenna system (local AM BCB, strong Shortwave Broadcast signals - when the bands open) it is preferable to have an amplifier at this point that has only modest gain - in the area of 12-15 dB:  A high-gain amplifier (>20dB) may be more prone to overload in such conditions and the typical means of preventing this (addition of an attenuator in front of the amplifier) is not recommended as this will increase system noise figure.

• Limited attenuation high-pass filter:  Any receive system is capable of handling only a certain amount of total signal input power - and the direct-sampling receiver of the KiwiSDR is no exception.  The overload level of the KiwiSDR's 14 bit A/D converter is approximately -13dBm, so the total power arriving at its antenna port should be kept below that level and strategic, selective attenuation of local, strong carriers - often those of AM/mediumwave broadcast transmitters - should be considered - but there is another problem:

The sensitivity of a KiwiSDR is approximately -155dBm (in a 1 Hz bandwidth) over its frequency range which means that in a unity-gain HF antenna system that at 10 meters, the KiwiSDR will be "short" by at least 6-8dB from hearing the textbook "rural quiet" noise floor on that band, requiring 10-15 dB of overall amplification to place the 10 meter noise floor comfortably above that of the receiver's in that environment.  This amplifications has a cost:  With the addition of a 12 dB RF amplifier, the KiwiSDR will now overload at -25dBm.  It is not uncommon for the absolute power level of a single high-power shortwave broadcast station to exceed this level under good band conditions - and there will be many such signals on the bands - not including the wide-band energy from lightning static - that can combine in overall signal power and exceed this overload level.

What system noise figure is required for HF reception? This is a tricky question to answer because conditions on the HF band vary so much - most strongly on a per-band basis. Assuming quiet (e.g. "dead"), bands the lack of storm static. the absence of solar noise and the receiver being located in an area completely absent of man-made noise, noise figures equal to or lower should be attained for the receive system to have a "natural noise" limit: 1.8 MHz:  45dB NF 3.5 MHz:  37dB NF 7 MHz:  30dB NF 14 MHz:  24dB NF 21 MHz:  20dB NF 28 MHz:  15dB NF 50 MHz:  9dB NF 144 MHz:  2dB NF These numbers are approximate, but are a general guideline for system design.

Were the KiwiSDR being used over a very narrow range of frequencies - such as the case for most commercial direct-sampling receivers like the Icom IC-7300 and IC-7610 - this could be managed by gain adjustment in the signal path, but if the KiwiSDR is to be used over the entire HF spectrum, this cannot readily be done.  Instead, one can selectively insert attenuation where the signals+noise are likely to be the highest, namely below approximately 10 MHz and adjust calibration accordingly.

The use of this "limited attenuation" high-pass filter leaves signals above 12 MHz alone but offers significant attenuation (about 12 dB - around 2 "S" units) to signals below 8 MHz.  Because of the relatively high noise levels in the lower HF bands, additional attenuation - and the commensurate increase in system noise figure - can be tolerated without the loss of useful sensitivity.

Note:  This same issue of overload will occur with similar devices - such as the Red Pitaya - if operated as wideband HF receivers, without input band-pass filtering.  Simple math will indicate that even if the KiwiSDR had a 16 bit A/D converter - which would theoretically yield another 12 dB of dynamic range - one would still need to take these same precautions.  It is for this reason why all well-designed direct-sampling receive gear has narrowband filtering that precedes its A/D converter.

Unfortunately, the "limited attenuation" filter fits a niche requirement as is not a commercially-available device, but one may be built without too much difficulty:  Two versions are described here:

• A limited-attenuation high-pass filter for the KiwiSDR - link. This filter starts to attenuate below 12 MHz, offering about 12 dB below 8 MHz.

• Revisiting the limited-attenuation high-pass filter - link.  This version is similar to that above, but its attenuation pass range is limited  to the range of approximately 1700 kHz to 12 MHz, allowing the passage of signals in the AM broadcast band and below for the reasons discussed in the article.

• Because of the simplicity of these filters they exhibit a low return loss (high reflectance) at lower frequencies and  it is recommended that they be preceded with an amplifier to present the input feedline with a more consistent "50 ohm" impedance.

• Amplification (post high-pass filter):  Where high signal levels are likely to exist it is prudent to distribute the signal gain across several blocks rather than have a single high-gain amplifier near the "front" of the signal path.  An amplifier placed after the "limited attenuation" high-pass filter will see lower signal levels from the very strong lower HF-band signals and be less subject to overload.

• Splitter:  A 4-way splitter is depicted in figure 1.  A real-world 4-way splitter will have 6.5-7.5 dB loss between the input and any of its output ports which means that for the higher-frequency bands it is particularly important to have established the system gain and noise figure prior to the splitter.  Like any other block of gain or loss, the impact of such a device must be considered when evaluating the system overall.

Analyzing your receive system:

The procedure following this point assumes the availability of a known-accurate signal source:  See the heading "Calibrated Signal Source", above, for more information.

Again, we are characterizing the signal path beyond the antenna and ignoring any gain/loss of the antenna itself for the moment.

A procedure for obtaining the calibration of the receive signal path is as follows:

• Input a known signal level at the input of the receive signal path.  This signal would be input at the antenna port - or via the "coupling" port of the directional coupler.  This signal should be input at a frequency in the band of interest as calibration values should be obtained for every amateur band to be used.  For WSPR operation it is desirable that one generates test signals within the WSPR passband.

• Tune in the signal and note the signal level on the receiver.  A known signal level of at least 20dB (30dB is better) above the receiver noise floor and other signals in the receiver passband is suggested to minimize effects of the noise and signals - but the level should be no higher than necessary to avoid overload/compression of any amplifiers - or the receiver itself.

• If using a directional coupler as suggested, make sure that it is oriented correctly by comparing the signal levels when the generator output is fed directly into the receive signal path and again with signals fed via the coupler:  If its "in/out" is reversed the coupling level will be inaccurate. (Again, for a coupler used for extracting a signal from a coax, the connection will be reversed.)

• In the case of the 20 dB coupler suggested above, the signal levels reaching the receiver will be 20 dB lower due to the coupling loss than that if the signal generator coupled directly and this should be remembered when taking measurements.

• If the levels of the test signal are low (less than 20dB S/N) and cannot be increased, narrowing of the receive passband can minimize the effects of noise - just observe the S-meter while making such adjustments to verify that the test signal remains within the passband.

• Because we are measuring the amplitude of a coherent signal within a finite bandwidth rather than trying to measure the noise power in a specific bandwidth, we must make sure our test signal is strong enough that the power of the background noise is a small percentage of the total.

• While it is necessary to compensate for detection bandwidth when measuring the noise within that bandwidth, one does not do so for coherent signals as all of the power of that signal is (theoretically) all on a single frequency.  It is for this reason that the measurement obtained of the coherent signal should match the actual power of that signal.

• Knowing the signal level being input to the signal path, observe the S-meter reading on the KiwiSDR on a per-band basis.  It is not important that the S-meter read correctly - except for the benefit of users connected to the KiwiSDR and looking at the meter - as the levels reported by wsprdaemon will be adjusted in its calibration file.

• If you are running wsprdaemon and are producing a signal within the WSPR passband, the signal will show up on the graphs after several minutes as a constant level - and this level can be used to verify calibration.

• The level displayed on the graph will be that of the KiwiSDR plus the amount of attenuation/amplification specified in the wsprdaemon.conf file for that band.  Use either raw data or the interactive Grafana data to read the signal levels precisely.

• At the Northern Utah WebSDR, the KiwiSDR S-meter calibration setting has been adjusted so that for bands above the roll-off of the high-pass filter (e.g. 20-10 meters) the S-meter read the same as the input signal level.  At lower HF bands, the S-meter reads low by the amount of attenuation of the filter.  Since there is currently only one S-meter calibration point that applies to all frequencies, there is nothing to be done to correct this.

Any adjustment of the KiwiSDR's S-meter calibration should be done before completing the steps below.  If it is changed, the final per-band calibration values will need to be adjusted accordingly.

• Record, for each band, the amount of RF you are applying at the antenna port (taking into account the coupler, if used) and the S-meter reading on the KiwiSDR.

• The difference between these two readings will be the amount of gain - or loss - in your signal path.  Again, we are not concerned about the absolute gain of the antenna itself at this point.

• If the reading on the KiwiSDR is above that of the absolute input level, signal gain is indicated at that frequency.

• If the reading on the KiwiSDR is below that of the absolute input level, signal loss is indicated at that frequency.

• We have now characterized the gain/loss of the signal path.  If our antenna had unity (0 dB) gain we would use the numbers that we'd just obtained as an offset in the wsprdaemon.conf file.  For example:

• If -50dBm were present at the input of the signal path and the KiwiSDR read -38dBm on 10 meters, we would know that we had 12 dB gain at that frequency.  To compensate we would put -12 db in the entry for 10 meters in the wsprdaemon.conf file.

• If -50dBm were present at the input of the signal path and the KiwiSDR read -52 dBm on 80 meters we would know that we had 2dB attenuation at that frequency.  To compensate we would put 2 dB in the entry for 80 meters in the wsprdaemon.conf file.

• Again, to verify our calculations we can place a carrier of known signal level within the WSPR passband for the band being tested.  If the measurements have been done correctly the levels on the graph should match those of the level of the RF at the input of the signal path.

Compensating for coax cable losses:

The next step would be to include the losses of any coaxial cable in the system before the point in the signal path where we injected our test signal.  This can be directly measured with a wattmeter and dummy load being used to measure the power lost over its length or - presuming a fairly low VSWR - can be estimated using the manufacturer's loss values on a per-band basis.

These loss values would be added to the per-band numbers in the wsprdaemon.conf file.

Compensating for antenna gain:

With simple equipment, calibrating the signal path for an absolute level is pretty easy - but the gain of the receive antenna at a given frequency is more difficult to determine as very few people have access to a known-accurate antenna range - so we will have to make an educated guess.  Because many HF antennas have definite patterns of of lobes and nulls one will have to decide if the peak gain of the pattern will be used, or some sort of averaged gain value.

Passive antennas:

For simple wire antennas, the gain can be estimated with reasonable (+/- a few dB) accuracy.  For example, a half-wave dipole above typical ground will likely be in the area of 1-2 dBi gain peak while the gain of a commercially made vertical or Yagi antenna might be available from the manufacturer's data sheet.

For multi-wavelength wire antennas such as long wires or rhombics, antenna simulation programs are suggested and may be the only reasonable means of estimating gain - but again, one will have to make a decision on what gain value to apply in light of the likely existence of lobes and nulls.

Active HF antennas:

If the feed from a conventional antenna (beam, dipole) includes an amplifier, the steps mentioned above may be used to estimate its gain and one need only add the loss of the feedline and gain of an amplifier to obtain the desired offset for the wsprdaemon.conf file.

Electrically-short active antennas such as the PA0RDT mini-whip pose a particular problem.  They are essentially a capacitive plate (or wire) that couples RF from the "aether" - but the amount of coupling (which correlates with the apparent gain of the antenna) can vary widely, the determining factors including:

• Height above "ground".  In general, the higher above the ground - earth or metal - the more signal such an antenna will intercept.

• Type of "ground".  The ground above which the antenna is mounted could be earth (dirt. soil) which is lossy or it could be a metal roof (much preferred!) to which the feedline's ground is bonded.

• Size of the "antenna".  The traditional PA0RDT antenna uses a PC board plate for coupling - but there are many variations on what size of coupling conductor - wire or plate - is used, all of which can affect the apparent gain.

• Gain response of the amplifier with frequency.  In most - if not all - implementations of an E-field whip are subject to gain drop-off with higher frequency and this is not easy to measure directly and determining a suitable number is almost entirely guesswork.

• Unfortunately, such drop-off conspires with the high RF levels at low-medium HF frequencies.  With a broadband, direct-sampling receiver like the KiwiSDR, overload will surely occur if enough gain is added to the system to attain the natural noise floor at the higher HF bands (e.g. 10 meters) unless strong compensation (e.g. something like the high-pass filter described above) is used.

However one might obtain the antenna gain, this value would be applied to the wsprdaemon.conf file:  Positive gain would mean a reduction of this number by the dB value while negative gain would mean a commensurate increase in this value.

What to expect:

In theory, the noise floor one sees should generally reflect that predicted in the ITU-R data, reproduced in figure 2, below:

Figure 2:  "Typical" noise floor for various radio environments.  Because the above chart is based on a 500 Hz bandwidth, one would subtract 27dB from its vertical axis to scale to an equivalent 1 Hz noise bandwidth when measuring noise power.
Click on the image for a larger version.

To be sure, the values depicted in Figure 2 are very generalized - and one would be lucky to approach the values of "quiet rural".  In general, these value depicted above would be most representative when the respective frequency band is "dead" (e.g. daytime for the lower bands, nighttime for the higher bands) with a "quiet sun".  While usable only as a general guideline, one should be suspect of their calibration settings if the "quiet band" noise floor values they measure on their system is markedly lower than those of the chart above.

Cross-checking noise level measurements with the receiver:

It is possible to check the noise level measurement provided by wsprdaemon using the receiver in question, but to do this two conditions must be satisfied:

• You must know the receive bandwidth.  For most SDRs this is pretty easy:  The "bandwidth" setting - which is typically at the -3dB or -6dB points - is "fairly close".

• The passband must contain ONLY noise and no other signals.  On the WSPR frequencies this may not be possible, but measuring on a "nearby" frequency (up to a few 10s of kHz away, provided that such measurements aren't affected by narrow band-pass filters that might be present).
• Wider (SSB) bandwidths are fine, but it may be necessary to user narrow bandwidths to avoid signals within the passband.

 The method of making this measurement is approximately thus:

• Note the average signal level reading (in dBm).
• Since we are looking for what amounts to a rolling average over time, the readings of occasional noise bursts must be considered which means that a "fast" AGC should be used and one should not be looking at any "peak" readings.
• Since we are likely doing a "sanity check" simply staring at the signal meter and guesstimating the average.
• It is best to do this during "quiet" conditions on a given band - that is, when it is dead:  Trying it on 80 meters during the night where there may be a lot of summer static is not recommended.

•  Note the receive bandwidth.
• On an SDR, this is likely to be the "bandwidth" figure which is probably taken from either the -3dB or -6dB points - which will be "close enough.

• Calculate the bandwidth noise power ratio.
• Because wsprdaemon's noise power measurements are scaled for 1 Hz detection bandwidth, the reading obtained on any receiver with a wider noise bandwidth than that will be much higher and the reading (in dB) must be offset by that amount.
• As an example:  If you are using a 500 Hz receiver bandwidth (e.g. a CW filter) you need to account for a 500:1 power ratio which is:
• 10 * log10(500) = 26.99 = 27 dB - that is, one would subtract 27 dB from the average level that you observed.
• In other words, to scale to the 1 Hz wsprdaemon bandwidth:
• Your signal level - (10 * log10(RX bandwidth in Hz))

Conclusion:

While it is unlikely that the typical amateur will be able to be absolutely confident of the accuracy of their absolute HF noise floor measurements, the steps above will greatly limit the uncertainty of the system's measurement to the the properties of the antenna itself.

[End]

 This page stolen from ka7oei.blogspot.com

Analysis of a repeater's antenna pattern

Back in 1997 the antennas on the Utah Amateur Radio Club's 146.760 repeater were relocated and replaced - this, because the original, guyed tower on which the antennas were located was being replaced by a free-standing 120' tower.

Because the (separate) transmit and receive antennas were, at that time, over 20 years old (but still in perfect condition owing to radome placed over them when they were originally installed) we decided to start anew with the 2 meter antennas, putting the new antennas at the locations prescribed by the owner:  The receive antenna on top at the 120 foot level and the transmit antenna at the 60 foot level.  Upon installing the new antennas and running the new Heliax (tm) in a cable tray with almost nothing else in it (yet) we noted that we were the first to attach anything to the (also) brand-new ground system.  (We also noted that some hardware for part of the ground system had been installed incorrectly - which we fixed!)

While the receive antenna - being the tallest thing on the tower - worked quite well we could tell that something was amiss with the transmit antenna.  From the time that it had been installed we got reports that the signals to the north were noticeably weaker than they had been on the old tower/antenna and anecdotally, they seemed to get worse as the tower was finally built-up and more antennas, dishes and cables were gradually installed over the years.

Table 1
Serial-port S-Meter readings versus signal input (as read via the serial port) on 2 meters for my FT-817 as shipped from the factory.

Reading (HEX)	SSB/CW/AM signal strength (dbm)	FM signal strength (dbm)
0	<-108	<-114.5
1	>-108.3	>-113.8
2	>-107.3	>-113.0
3	>-106.7	>-111.6
4	>-106.0	>-110.2
5	>-105.1	>-108.8
6	>-104.2	>-106.4
7	>-103.0	>-104.7
8	>-100.4	>-102.8
9	>-84	>-101.0
A	>-74.5	>-99.5
B	>-70.1	>-97.8
C	>-58.9	>-96.8
D	>-50.8	>-95.8
E	>-40.8	>-94.6
F	>-30.1	>-93.5

Not wanting to rush into these things, it wasn't until 2001 that we decided to make some scientific measurements.  One option was to drag along a signal level meter or spectrum analyzer and, every so-often, stop and make signal level measurements.  Since this method was likely to be very tedious and, in some areas may not even be very practical, I decided that there had to be a better way!

The FT-817 as a test instrument:

Not too long before this I'd bought a Yaesu FT-817 and noticed that it had the capability of reading the S-Meter via the serial port, but it had a rather useless signal strength span when it came to making meaningful measurements of real-world repeaters.

As can be seen from TABLE 1 the readings aren't entirely useful.  While each step is approximately 1 dB (more or less) the useful range goes from -114.5 to about -93.5 dBm - this entire range being generally weaker than what one might see from a local repeater.  At the same time I also made measurements of the S-meter reading when in SSB/CW/AM mode to see if that would be useful and while it covered far more range, the steps were uselessly small at the weak signal end (e.g. <1dB) but uselessly large at the high-signal end! (This indicates another, well-known problem with the FT-817's AGC, but that's another story...)

At about this same time I'd become interested in another aspect of the FT-817:  It's "soft" calibration settings.  I believed that these settings, in a special "calibration" menu, were too numerous and tedious to have someone on an assembly line adjust so I figured that there MUST be a way in which a radio was semi-automatically calibrated at the factory - and I was right!

What I found were some "undocumented" commands via the serial port - some of which obviously read from and wrote to the EEPROM - and I quickly wrote a program that would allow me to determine what memory locations were used for what:  The program would download the current EEPROM content, I would change a setting, and then the program would tell me what had changed after downloading it again.  I'd documented my findings on a web page and in the years that followed, all sorts of things followed-on from this information (e.g. "FT-817 Commander", the "SoftJump" program, various remote meters for FT-817 signal strength, ALC, SWR and transmit power - just to name a few).
 

Table 2
Serial-port S-Meter readings versus signal input using FM mode (as read via the serial port) after the described recalibration of the FM-S1 and FM-FS parameters. 

Reading (HEX)	FM Signal Strength (dbm)		Reading (HEX)	FM Signal Strength (dbm)
0	<-110.7		8	-94.2
1	-108.9		9	-91.5
2	-106.2		A	-89.2
3	-104.2		B	-87.2
4	-102.3		C	-85.2
5	-100.6		D	-82.1
6	-98.9		E	-78.1
7	-96.7		F	>-75.7

In  this early stage there were two "Soft Calibrate" (and now, EEPROM) settings that most interested me:  The ones that corresponded with S-Meter calibration, namely #9 - "FM-S1" and #10 - "FM-FS" which, I correctly surmised, related to the settings for the S1 and Full-Scale readings.  Through experimentation by using a calibrated signal generator and observing the readings on the serial port I determined that the original settings badly shortchanged the dynamic range of the FM S-meter and simply by readjusting these two settings could provide a wider and more useful FM S-Meter range as TABLE 2 demonstrates.

Now the meter was useful over a range of more than 30 dB and it still had reasonable resolution - between 2-3dB per step, but I still had a problem:  The usable range - from about -108 dBm to about -80dBm was still too low for the expected signal strength of typical, local repeaters which could vary from about -50 to -80 dBm at the receiver's input terminal.

Fortunately, I knew of another setting or two within the radio that proved to be useful - Calibration menu # 5 "VHFRXG".  This setting adjusted the bias of a PIN diode in the FT-817's IF and I found that it could usefully add at least 30 dB of attenuation, extending the S-meter to signals stronger that -50dBm!

What was more, I found that this setting - because it was done in the IF - was the same for every band (using the corresponding calibration points for HF, 6 meters and UHF) and it turned out to be consistent (within a 2-3dB) over a very wide temperature range (e.g. "Hot Car" to "Deep Freeze").  I found three more values for the "xxxRXG" parameter that adjusted the gain by approximately 10 dB (and precisely measured that amount of attenuation) and was ready to go!
 

Table 3
Sample values of the VHFRXG parameter (soft calibration menu item #5) versus the signal input level.  The bottom row shows the average difference between the "unattenuated" reading (VHFRXG = 99) versus the reading obtained with differing amounts of "attenuation".
Note:  The above values are for my FT-817.  Every '817 will be different, requiring individual calibration to assure accuracy.

Reading (HEX)	VHFRXG 99	VHFRXG 57	VHFRXG 49	VHFRXG 43
0	<-110.7	<-98.5	<-88.1	<-78.7
1	-108.9	-96.8	-86.7	-77.7
9	-91.5	-79.5	-69.6	-60.4
D	-82.1	-70.5	-60.2	-50.9
E	-78.1	-66.3	-56.5	-46.9
F	>-75.7	>-63.8	>-53.6	>-44.5
Average  Difference  (db)	-	12.0	22.0	31.1

Putting it all together:

I could now get down to the business of writing a program that would take all of this data and make sense out of it.

What I had now were lots of bits of information that I could use to analyze the problem related to the repeater's transmit coverage:

• Using the FT-817, I could now read the signal level arriving at its antenna terminal.
• Knowing the type of antenna and amount of coax, I could make an estimate of antenna gain and other losses to correct the signal level reading.
• The GPS location of the repeater was known from previous on-site measurements.
• The repeater's transmit antenna gain and losses (coax, cavity, etc.) were known.
• Using a portable GPS receiver connected to the computer, I knew MY location via the NMEA strings emitted by a GPS receiver and fed to the computer.  The laptop that I was using had only one serial port so I used a relay controlled by the handshake line two switch between the FT-817 and the GPS receiver every 30 seconds or so to record the location.
• Knowing my location with respect to that of the repeater, I could calculate the distance between my antenna and the repeater's antenna as well as the bearings to/from the two antennas.
• Using fairly simple formulas, I could calculate the free-space path loss between my current location and the repeater antenna.
• Knowing the transmit antenna gain and loss parameters, my own receiver's antenna gain and loss parameters and the amount of expected path loss, I could could calculate how much signal I should (theoretically!) expect from the repeater.
• Since I was able to directly measure my received signal strength, I could calculate the "Excess Path Loss" - that is, the difference between the predicted signal level and the actual signal level.  This value could vary from being negative, indicating a higher signal level than expected, to positive, indicating greater path loss than expected.  Both a "real-time" and a "sliding average" reading were made available, the latter smoothing out short-term variations in signal level due to Fresnel effects, uncertainty in measurements and the effects of nearby obstructions such as buildings and vehicles.
• Since it was a computer, this was done automatically and the results saved to a text file for later analysis.  This included time stamps and all of the raw data as well as the "cooked" data such as excess path loss, bearing to/from the site, etc.
• The program also allowed brief text notes to be inserted in the file permitting one to take notes about local obstacles that might skew readings, etc.

What this meant was that while I drove a path that circumnavigated the 146.760 repeater, my passenger could look at the computer's screen which was providing a real-time display of the calculated parameters.  The biggest advantage was that we could be zooming down the highway, taking readings very frequently.  With the real-time display we could also take a different route if we suspected that some local obstructions excessively skewed the readings.

So, during April 2001 - after testing the program on a few other local repeaters and finding that the readings agreed within a few dB of theoretical - Gordon, K7HFV and myself took a day-long drive, circumnavigating the 146.76 repeater.  While much of this was via paved roads there was a significant segment consisting of high-clearance four-wheel drive dirt and gravel roads that took more time to traverse than the rest of the trip put together!

Having made the trip "behind" Lake Mountain to the west we were coming close to closing the circle when, while driving along the highway, Gordon started reading out "additional path loss" numbers like "-10... -15... -25... -35... -25... -15... -10..."  While in full, line-of-sight view of the transmit antenna we had passed through a 30+ dB deep null in the transmit pattern while traveling a fairly short distance!  Not sure of what we just saw, I did a legal U-turn and re-traced the path going the other way - and then back again, each time seeing the same numbers go by on the display on three occasions!

Figure 1: 
The measured antenna pattern (the shaded circle near the center) and the calculated coverage of the 146.760 repeater based on this pattern and actual terrain data.
Click on the image for a larger version.

We now had our answer as to how severe the null was - and the results may be seen in Figure 1.  After analyzing the logged data I was able to determine the approximate antenna pattern and input this data into the "RadioMobile" program by VE2DBE.  As expected, it showed a rather deep null almost exactly straight north, encompassing a significant portion of the Salt Lake valley and communities to the north.

What to do about the null?

Even though we've known about this problem for some time now, the big question is "What to do about it?"  On this site, the receive antenna is just that:  A receive-only antenna, and we cannot transmit from that location - which, being on the top of the tower, is free of this null.  At the level of the transmit antenna we have the problem of there being very limited options as to where and how we may mount our antenna to avoid the mechanical obstacles.  We have some ideas in mind, but we are still considering the options!

A slightly more in-depth version of this article may be found here (link).

For more information about the FT-817's inner workings, visit the KA7OEI FT-817 pages (link)


Update:

In the fall of 2014 a "fill" antenna was added to (hopefully) minimize the null caused by the "tower clutter".  While anecdotal evidence indicates that this has improved coverage in the "null zone", at the time of this update (3/15) we have yet to re-do at least part of the circumnavigation to quantify the effect of this change.

[End]

This page stolen from ka7oei.blogspot.com