Inductance of a loop of CAT5 pair

An online expert recently reported:

I tried to make an antenna loop for longwave with cat 5 and after it did no good I realized the twisted wires canceled each other out.

Or did they really cancel?

Parallel connection

I constructed a loop of one Cat 5 pair and measured its inductance when both wires are bonded at the ends.

The conductors are 0.5mm diameter and spaced 0.9mm. To estimate the inductance we use the geometric mean radius (GMR) as the equivalent radius of the pair. GMR=(0.5*0.9)^0.5=0.67, diameter=1.34mm. So let’s calculate the inductance of a single turn circular loop of 0.8m perimeter and round conductor of 1.34mm diameter.

The estimate above is 850nH.

Above is the measurement, the screen is not readable, but it is 852nH, very close to the estimated 850nH. Continue reading Inductance of a loop of CAT5 pair

Calibrating the Elecraft N-GEN

The Elecraft N-GEN is a low cost noise source which is quite suited to many applications, more so if the Excess Noise Ratio (ENR) is known.

ENR is a commonly used property to describe the noise power density of a source, it is calculated as ENR=10*log(Tne-T0)/T0 dB where Tne is the effective noise temperature and T0 is 290K.

This article describes a calibration procedure. Note that the calibration is specific to the device and cannot be applied to another N-GEN.

Above is a screenshot of the Spectrum Analyser scan. A text file of the frequency,power pairs is saved for input to a spreadsheet to calculate ENR vs frequency. Continue reading Calibrating the Elecraft N-GEN

Effective noise bandwidth – IC-7300 SSB Rx Filter2 (2400Hz)

For a lot of experiments, knowledge of the Effective Noise Bandwidth (ENB) of a receiver is necessary. The ENB is the bandwidth of an ideal rectangular filter with the same gain as some reference frequency, 1kHz is usually specified for SSB telephony receiver sensitivity measurement.

Though filters are often specified in terms of bandwidth at x dB down, that metric is of relatively little value, the x is often 6dB but not always, the filters depart significantly from ideal or even common response.

In brief, a white noise source is connected to the receiver input, Filter2 (nominal 2400Hz bandwidth soft response) selected and set to standard PBT, and the audio output captured on a PC based audio spectrum analyser, Spectrogram 16 in this case.

Spectrogram is set to integrate over 30s to average the variations due to the noise excitation. The resulting graph and text spectrum log are saved.

The method is explained in detail at Measure IF Bandwidth.

Above is the spectrum plots, as receivers go this is relatively flat, lacking the usual tapering off above 1kHz (a technique to cheat on sensitivity specs).
Continue reading Effective noise bandwidth – IC-7300 SSB Rx Filter2 (2400Hz)

GPS antenna plot

I bought an inexpensive GPS antenna on eBay, the requirement was for one that operated from 3-5V to suit both of my GPSDOs. The antenna is mounted in an electrical junction box on conduit above the roof for reliable coverage.

The question was whether the active antenna with 5m of RG174 coax and SMA male connector at A$6 posted was any good.

To map its behaviour, it was attached to a Trimble Thunderbolt GPSDO and Lady Heather used to plot rx signal level over about 30h.

Above is the plot. The pattern is not quite symmetric as there are obstructions in play, in fact some of the dips in performance are explained by specific trees and the roof profile. There is a gap to the south at low elevations, GPS satellites don’t fly there (MEO inclined 55° from the equator) .

Overall, it reaches similar strength at the zenith as other antennas tried.

Overall evaluation, it seems to work ok though the coax is a bit rough.

Inductance of sensorless brushless DC motors

A reader of A Demagnetisation Risk Index for a sensorless brushless DC drive asked whether the inductance of a sensorless brushless DC motor could be measured with one of the inexpensive LC meters available on eBay.

Motor inductance line-line typically ranges from several µH up towards 100µH. Importantly, the fundamental frequency of flux change in the laminated iron core under normal operation is typically less than 2kHz.

Validation of the LC200A

To verify the instrument, a test inductor was made with 3t on a FT-240-43 ferrite core.

Above is an estimate of the expected inductance of the test inductor, 9.65µH. Keep in mind that the tolerance of ferrite is quite wide, 20% variation is not unusual. The test inductor measured 9.1µH at 10kHz on a classic RLC meter.

Above, the LC200A measuring an inductor comprising 3t on a FT240-43 ferrite core, measurement frequency was 670kHz. The measured inductance is 8.98µH, 7% lower than the estimate but well within tolerance of the ferrite core, and less than 2% below the value measure with a classic RLC meter. Continue reading Inductance of sensorless brushless DC motors

Standardised processing of instrument screen captures

I have several instruments and software packages that can create screen captures, and the capture files commonly need some mix of image processing including for example:

  • cropping;
  • scaling;
  • brightness, contrast, gamma adjustment;
  • transparency change;
  • format conversion; and
  • file copy / archive / cleanup.

Above is an example. Though WordPress presents a small image inline, if you click on it, there is a 640×480 image that was created from a QVGA (320×240) screen capture file scaled and gamma adjusted.
Continue reading Standardised processing of instrument screen captures

Review of Hantek DSO8102E hand held oscilloscope

This article is a brief review of some issues that were found with initial testing of a Hantek DSO8102E two channel 100MHz hand held oscilloscope.

The DSO8102E is a member of the DSO8000 series (DSO8060, DSO8070E, DSO8100E, DSO8150E, DSO8200E), and shares most specifications across the series.

The specifications are very impressive, and price at just under $1000 for a Chinese brand seemed reasonable (hand held oscilloscopes are expensive compared to bench oscilloscopes).

The test scenario was a practical application, observation of the data traffic to/from a DHT22 temperature and humidity sensor in the project ESP8266 IoT DHT22 temperature and humidity – evolution 2. Continue reading Review of Hantek DSO8102E hand held oscilloscope

Cheap and nasty 50Ω SMA terminations

Chinese sellers offer low cost 50Ω SMA terminations mostly without specs, but some sellers specify VSWR<1.2 to 3GHZ.

Above is the internals of one, it is a 51Ω 5% metal film resistor.

They often fail a DC test and tapping them gives erratic resistance readings up to hundreds of ohms, and of course they can be unreliable at RF.

They rely upon the resistor pigtail to make a spring contact with the inside of the barrel, and give that the pigtail is soft copper with little spring the contact is not very reliable. Continue reading Cheap and nasty 50Ω SMA terminations

A check load for antenna analysers with UHF series socket

Hams embrace the UHF series connectors like no one else, including for its use on test equipment where its performance is lacking.

This is the likely reason why it is so hard to find low VSWR 50Ω terminations with UHF series plug. It is rare to find something with VSWR quoted in specifications, and nigh on impossible to find one at a reasonably low price.

On the other hand, SMA terminations start at about $2 each (posted), and it is not too hard to find ones specified with VSWR<1.2 to several GHz.

Above is a low cost, low quality solution. It is a SMA termination selected from a bunch using a high accuracy DMM (selected, R is 49.86Ω) and a SMA(F)-UHF(M) adapter, total cost $7 (posted) (but you might be advised to buy 5 loads to select the best one). Despite the specification, they are probably only good to 100MHz, and can be unreliable. Continue reading A check load for antenna analysers with UHF series socket