Expected ambient noise

One of the casualties of the cessation of VK1OD.net was an article on expected ambient noise.

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The original work was based on ITU-R P.372-8 which has been updated to -10, -12 and now -13, but the updates do not alter the basis for the original article.

Since the work was a reference cited on my FSM pages, it has been updated and copied to Expected ambient noise level. The graphics and tables in the article and the PDF file all refer to ITU-R P.372-8 but remain correct wrt ITU-R P.372-13 (2016).

Reconciling my #52 choke design tool with G3TXQ’s measurements

A correspondent wrote with concern of the apparent difference between graphs produced by my #52 choke design tool with a graph published by G3TXQ of his measurement of 11t on a pair of stacked FT240-52 cores.

I published a note earlier about my concerns with a similar graph by G3TXQ compared to the Fairrite datasheet, and he reviewed the data, found the error and published a corrected graph.


The corrected graph above might at first glance appear different to my model’s graphs, and the first obvious difference is that G3TXQ uses a log Y scale (which is less common). The effect of the log scale is to compress the variation and give the illusion perhaps that in comparison with other plots, this balun has a broader response.

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To compare the two, I have roughly digitised G3TXQ’s graph above and plotted the data over that from my own model (with linear Y scale). Continue reading Reconciling my #52 choke design tool with G3TXQ’s measurements

Low noise Yagis and 50MHz noise

After reading my post Some thoughts on noise on 6m (50MHz)…, a correspondent asked about the content in the context of comments by G0KSC.

Under the altruistic heading “Low Noise Yagis Explained”, G0KSC takes a competitor to task over the accuracy of statements regarding the importance or not of G/T at 50MHz.

G0KSC says:


Have you ever heard the saying ‘A little knowledge is a dangerous thing’?…

What is G/T? Continue reading Low noise Yagis and 50MHz noise

A low cost home made USB CI-V interface with open collector and solid Windows drivers

This article describes an inexpensive USB adapter for Icom’s CI-V interface.

There are four common options for USB-serial adapters:

  • Prolific;
  • FTDI;
  • WCH; and
  • Silabs.

This article describes an adapter based on an inexpensive FTDI adapter (~$5 on eBay).


You will need the module, a Schottky signal diode (eg 1N5711), wire and a 3.5mm TRS plug or two. I have connected two plugs, one wired for TS (CT-17) and one for RS (OPC-478x). Continue reading A low cost home made USB CI-V interface with open collector and solid Windows drivers

Some thoughts on noise on 6m (50MHz)…

External noise

External noise is the noise external to the receiver system.

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

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

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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.

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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.

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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.


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

Duffy, O. May 2013. Noise and receivers presentation. https://owenduffy.net/files/NoiseAndReceivers.pdf.

  • 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.

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.


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

Attempting to reconcile W5DXP & G3TXQ’s comparison of K and 52 mix ferrites #2

This is a follow up to Attempting to reconcile W5DXP & G3TXQ’s comparison of K and 52 mix ferrites.

Steve saw the above article and revisited the FT240-52 measurements which he apparently did, and found them wanting: Continue reading Attempting to reconcile W5DXP & G3TXQ’s comparison of K and 52 mix ferrites #2

Attempting to reconcile W5DXP & G3TXQ’s comparison of K and 52 mix ferrites

Steve (G3TXQ) posted a graph comparing Cecil’s (W5DXP) measurements of two turns on FT240-52 and FT240-K.

It is interesting to reconcile the #52 curves with Fairrite’s datasheets. A simple reconciliation is to compare results at the frequency where µ’ and µ” curves cross over. Continue reading Attempting to reconcile W5DXP & G3TXQ’s comparison of K and 52 mix ferrites

Ferrite K mix

Among forum experts, there are ready recommendations for the ideal ferrite material (or mix) for a balun, often without knowing any detail of the application.

The ‘magic’ mixes include K. Perhaps they are devotees of Sevick.

Over some years I have searched for manufacturer’s data on K mix, and found only two references:

  • Amidon who give a very brief table summarising characteristics, inadequate for RF inductor design; and
  • Ferronics who give characteristic curves, albeit in less common format.

Problem is that Ferronics µi is 125 against Amidon’s 290… so their K materials are different.

One has hoped that an interested competent person might have made measurements of some samples from Amidon to give full characteristic curves, it isn’t that hard. Continue reading Ferrite K mix

What does an Antenna Tuning Unit (ATU) do?

An Antenna Tuning Unit (ATU) performs a simple but important function in many transmitting systems.

Almost all things called an ATU are simply impedance transformers, and almost always, narrow band impedance transformers (meaning that when adjusted, they achieved the desired transformation over only a narrow frequency range).

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ATUs come in a range of configurations, each designed for a specific set of characteristics. Above is the heart of a Palstar AT2K T Tuner, just three real passive components that are fully explained by conventional linear circuit theory. Continue reading What does an Antenna Tuning Unit (ATU) do?