## Backing out transmission line

Often we make measurements through a section of transmission line, and the measurements are wrt the reference plane, which for many analysers is the connector on the instrument.

Some analysers, or their associated software allow the effects of the transmission line to be backed out.

Above is a Smith chart view of measurement of a test antenna through some length of RG58. The antenna will have R<50Ω at minimum VSWR, so the angle of the complex reflection coefficient Γ will be close to 180° at the feed point. Antscope uses a different notation, but shows here the angle at the point of measurement to be -15.1°, so we need to increase it by 180–15.1=195.1°, which will take about half that electrical length of line, 97.6°. From TLLC, I calculate the length involved is 7.6m of RG58, which is an estimate that gives a starting point for backing out the cable. Continue reading Exploiting your antenna analyser #11

## External noise

External noise is the noise external to the receiver system.

(ITU-R 2015) gives some guidance on expected ambient noise.

Above is Figure 10 which gives guidance on the expected median ambient noise figure.

The table above gives calculates noise power in a 2kHz bandwidth receiver with lossless antenna system for the lower ham bands from the equations given in (ITU-R 2015). Note that the medians vary somewhat with location and time, see (ITU-R 2015).

From the table, business (city) man made noise (A) is 17dB higher than galactic (E), and rural (C) is 7dB higher than galactic. Unless there is some ionospheric absorption occurring, you are unlikely to observe noise below galactic level.

Noise can be expected to vary by hour of day, from day to day, and season to season. Importantly, noise may vary with direction, eg pointing through busy roads, office buildings, shopping centres etc, neighbouring buildings, powerlines etc.

Measurement of external noise is not too difficult, but rarely do hams understand quantitatively their own noise environment.

## Internal noise

Noise contributed by the various stages of a receiver system can be reduced to an equivalent input noise at the input of a noiseless receiver, and that is often expressed in the form of the receiver Noise Figure.

A typical receiving system for 6m would have a noise figure around 6dB (at the antenna connector). As will be seen, there is little need for better noise figure.

The noise floor of a receiving system with NF=6dB (4.5dB receiver and 1.5dB line loss) is -135dBm. In such a receiver, AGC action is typically delayed until the signal is around 25dB above the noise floor, around -110dBm in this case. There will be no S meter deflection in traditional receivers below AGC onset.

Above, the median expected noise falls below the typical AGC threshold (S meter threshold) for the example receiver (-110dBm), and apart from Curve A, the noise may not cause S meter deflection.

In a single isolated measurement at my location, I measured -123dBm which is in the range expected of a semi rural residential with underground power.

Receivers with an additional preamp may show significant S meter deflection, the preamp will increase gain without significantly improving S/N ratio, indeed they may degrade S/N ratio.

## Total noise power

The internal and external noise power add, but it is the power in watts or in equivalent temperature that can be added, not the dBm figure.

The chart above shows the effect of combining two additive power values. If they differ by more than 20dB, the sum is within 0.05dB of the higher power. For lesser ratios, the weaker power needs to be factored in, and the graph provides a simple means if you don’t want to crunch the numbers.

For example, if we took the median power in a quiet rural precinct from the table above to be -119.9dBm, and noise floor to be -135dBm, looking up -135–119.9=-15.1dB on the horizontal axis we see that the combined power is 0.15dB more than the higher power, so -119.1+0.15=-119.8dBm. This is unlikely to cause S meter deflection most of the time as the median is almost 10dB lower than AGC onset.

Working the numbers for residential precinct from the table above to be -114.6dBm, and noise floor to be -135dBm, looking up -135–114.6=-20.4dB on the horizontal axis we see that the combined power is 0.05dB more than the higher power, so -114.6+0.05=-119.8dBm., -114.6. This is likely to cause S meter deflection occasionally as the median is just 5dB lower than AGC onset.

## Low noise antennas

One of the recent market directions is so-called low noise antennas. The term is used to describe directional antennas with reduced side lobe response inspired perhaps by (Bertelsmeier 1987) who contrived a rather naive statistic based on the noise power captured by a Yagi in free space tilted up 30° from the Z=0 plane and excited by two arbitrary noise scenarios in the upper and lower hemispheres, the statistic labelled G/Ta, transformed by others to G/T in  ignorance of the true meaning of the industry term G/T.

Reduced side lobe response sounds a good idea, but what is the expected impact on total external noise power received?

For a Yagi in free space, if the distribution of external noise is uniform in three dimensional space:

• the same noise power will be received irrespective of the pointing of the antenna; and
• a lossless antenna captures the same amount of noise power irrespective of its gain.

But we are not in free space, are we.

For Yagi pointing horizontally over flat ground, if the distribution of external noise is evenly distributed over all azimuth headings:

• the same noise power will be received irrespective of the pointing of the antenna; and
• a lossless antenna captures the same amount of noise power irrespective of its gain.

The situation changes if the noise intensity is not uniform with azimuth bearing and elevation if there are more concentrated noise sources.

It should be apparent that reducing the average gain off the main lobe reduces power from noise sources to the side and rear, but if the pattern is not even, and it never is, then it is a matter of chance as to whether pattern nulls or peaks coincide with concentrated noise sources when pointing in a desired direction.

The complexity of this environment mitigates against a meaningful single metric for the noise capture of a Yagi.

One cannot argue against the logic that reduced sidelobe gain is an advantage in reducing off boresight noise, but it does imply increased main lobe gain (and possibly noise pickup) the question is really how much net advantage there is on 50MHz in your station with your noise environment on the paths you see as high priority.

## References

• Bertelsmeier, R. 1987. Equivalent noise temperatures of 4-Yagi-arrays for 432MHz. DUBUS.

• ITU-R. 2000. Recommendation ITU-R S.733-2 (2000) Determination of the G/T ratio for earth stations operating in the fixed-satellite service .
• ITU-R. Jul 2015. Recommendation ITU-R P.372-12 (7/2015) Radio noise.

## Canberra trip – APRS / GPS logger

I had occasion to make a trip to Canberra for a few days this week.

It presented as a good opportunity to explore APRS by VHF radio and the stand alone GPS logger above attached to the case of the TinyTrak 3+. The transmitter is 65W output to a quarter wave in the middle of the wagon roof.

Over the three days, the extract from findu was 6240 APRS records (the vehicle was in a covered car park by night), and 389693 NMEA records (~20MB) logged to the logger, from which 6150 track records were extracted using GPSBABEL for track simplification. Both track sources were converted to kml files.

Above is a screen shot comparing the track around Majura park for GPS logger in red and APRS in cyan. There are several instances of false double backs on the APRS track (a problem inherent to APRS and quite common), and very coarse track despite being less than 8km from the prominent digi on Black Mountain, and very good view of the digi on Mt Ginini.
Continue reading Canberra trip – APRS / GPS logger

## Panasonic NCR18650B Lithium Ion cells on eBay

Further to 18650 Lithium Ion cells on eBay I purchased a pair of Panasonic NCR18650B cells, nominal 3400mAh, from an Australian supplier for about A\$22 posted.

Above is a pic of a cell.

Above is a zoomed in view of the same pic with increased contrast. The feint QC code printed on the underlying steel container is visible. It is usually visible through the jacket on genuine Panasonic cells.

It is always hard to know whether the product is genuine, the Chinese are better at copying the looks than the internals.

The cell was charged, then discharged at 1C on a battery analyser.

Above is the first three discharge cycles, the cell achieved just under 3000mAh to 2.8V, about 93% of datasheet rated capacity of 3200mAH, 85% of the advertised nominal 3400mAh capacity.

The actual discharge curve is fairly similar to the 1C curve from the datasheet.

These cells look more promising than the GTL red 5300mAh cells previously evaluated.

## ANS T1.523-2001, Telecom Glossary 2000 and Return Loss

A correspondent has suggested to me that my practice of giving Return Loss as a positive dB value is wrong, citing US FS-1037C.

US FS-1037C has been superseded by ANS T1.523-2001, Telecom Glossary 2000, and the wording in the latter is identical to the former, so let’s discuss the more current document. Continue reading ANS T1.523-2001, Telecom Glossary 2000 and Return Loss

## Update for FSM software (v1.11.0)

FSM has been updated to v1.11.0.

The update adds an export to Gnuplot file of the wave file to allow visual examination of the recording on which the measurements is based.

This replicates the utility of the existing Dplot export, but with the freely available Gnuplot package.

Above is an example of the receiver noise recording, and whilst it might not seem very interesting, it is interesting that it is of the character of white noise.

Of more interest are cases where there is a distant cyclic pattern at twice the AC power frequency which hints insulator failures on each half cycle. Some other types of periodic modulation are helpful in identifying possible sources of emissions.

## References

• Duffy, O. 2005. Field Strength Meter software (NFM). https://owenduffy.net/software/fsm/index.htm (accessed 11/01/2016).

## On negative VSWR

(Terman 1955) gives a meaning for the term SWR (or VSWR).

The character of the voltage (or current) distribution on a transmission line can be conveniently described in terms of the ratio of the maximum amplitude to minimum amplitude possessed by the distribution. This quantity is termed the standing wave ratio (often abbreviated SWR)…

Standing-wave ratio=S=Emax/Emin

Note that the use of capital E implies the magnitude of voltage, so Emax/Emin must always be a positive number.

## Lossless line example

Let’s look at an example of a 5Ω load on a line with Zo=50+j0Ω at 0.1MHz.

The standing wave is observable, the expression VSWR=Emax/Emin seems straight forward enough. The voltage along the line could be sampled and VSWR determined, seems all very practical. Continue reading On negative VSWR

## Can the magnitude of the complex reflection coefficient (ρ) be greater than 1

Let’s examine a number of transmission line loss calculators on the following scenario:

• line type Belden 8259 (RG58A);
• length 1m; and
• frequency 0.1MHz,

## 18650 Lithium Ion cells on eBay

I have been intrigued by the huge number of sellers of very low cost 18650 Li-ion cells on eBay.

Could they be any good?

As a reality check, Panasonic cells around 3000mAh sell through traditional channels here in Australia for around A\$20 per cell, there are Australian eBay sellers selling cells advertised as Panasonic for around A\$22 per pair posted.

Above, the GTL red LS18650 5300mAh Li-ion cell purchased in a lot of five for \$1.30 each (inc post from China).  The rated capacity is more than 50% higher than the maximum from brand name products. Continue reading 18650 Lithium Ion cells on eBay

## Quiet solar radio flux interpolations calculator

Further to the posting Quiet solar radio flux interpolations calculator

With a change in hosting and the changes to adapt to supported and unsupported features, one of the positives is that the ‘short term’ fetching of data on which Quiet solar radio flux interpolations calculator is based is now happening two hourly at 5 minutes past the even hours. Continue reading Quiet solar radio flux interpolations calculator