nanoVNA – measuring cable velocity factor

With the popularity of the nanoVNA, one of the applications that is coming up regularly in online discussion is the use to measure velocity factor of cable and / or tuning of phasing sections in antenna feeds.

‘Tuning’ electrical lengths of transmission line sections

Online experts offer a range of advice including:

  1. use the datasheet velocity factor;
  2. measure velocity factor with your nanoVNA then cut the cable;
  3. measure the ‘tuned’ length observing input impedance of the section with the nanoVNA; and
  4. measure the ‘tuned’ length using the nanoVNA TDR facility.

All of these have advantages and pitfalls in some ways, some are better suited to some applications, others may be quite unsuitable.

Let’s make the point that these sections are often not highly critical in length, especially considering that in actual use, the loads are not perfect. One application where they are quite critical is the tuned interconnections in a typical repeater duplexer where the best response depends on quite exact tuning of lengths. Continue reading nanoVNA – measuring cable velocity factor

PllLdr application – ATTiny44 & AD9833 in Codan 6801

The Codan 6801 is an older SSB transceiver using a single crystal per simplex SSB channel, for up to 10 channels. The channel switch selects the crystal and also a band pass filter for that channel.

The cost of crystals to populate the 6801 runs towards $1000. A recent project implemented a functional replacement for the crystals using PllLdr and an inexpensive DDS module.The cost of crystals to populate the 6801 runs towards $1000. A recent project implemented a functional replacement for the crystals using PllLdr and an inexpensive DDS module suitable for use in the ham bands.

Above, the modified radio with 8 channels on ham bands (this radio is missing the last two channel filters, so it is only equipped internally for 8 channels). Continue reading PllLdr application – ATTiny44 & AD9833 in Codan 6801

An experimental propagation beacon on 144MHz

An experimental beacon on 144MHz has been deployed for evaluation.

Details:

  • frequency: 144.245MHz, 144.244MHz USB dial freq, 144.245MHZ dial frequency in CW mode on modern transceivers (accuracy should be withing 200Hz);
  • power: 20W EIRP towards Melbourne (225°) from Bowral, NSW, horizontally polarised, antenna is 6m AGL;
  • modulation: ~5 minute cycle uses A1 Morse modulation (OOK) QRSS1 (1s dits) callsign (VK2OMD) followed by key down for the rest of the cycle.

The oscillator on the keyer can have accuracy as bad as 1000ppm, and a power interruption would cause it to restart at a random time, so the modulation pattern is not syncronised to the wall clock.

The narrow band modulation means it can be decoded in 1Hz receiver bandwidth, allowing decoding with packages such as SpectrumLab some 20dB or more lower than by ear.

Above is a screenshot from SpectrumLab, albeit a relatively strong signal where S/N in 2kHz is about 0dB… but as can be seen from the plots, there is around 30dB of margin left. A settings file for SpectrumLab is linked below. Continue reading An experimental propagation beacon on 144MHz

Arduino Nano – FT232RL test pin floats

The Arduino Nano leaves the FT232RL TEST pin floating which may give rise to initialisation and communications problems.Grounding the test pin by bridging pins 25 and 26 with a small solder bridge seems to overcome the problem.

 

NanoMod

Above, a fixed chip.

Small untuned loop for receiving – it’s not rocket science

I have written several articles on untuned loops for receiving, as have others. A diversity of opinions abounds over several aspects, but opinions don’t often translate to sound theory.

This article analyses a simple untuned / unmatched loop in the context of a linear receive system.

An example simple loop for discussion

Let’s consider a simple single turn untuned loop with an ideal broadband transformer. The example loop is 3.14m perimeter and 10mm diameter conductor situated in free space. The loop has perimeter 0.0744λ at 7.1MHz, less than λ/10 up to 9MHz, so we can regard that loop current is uniform in magnitude and phase. This simplifies analysis greatly.

Above is a schematic diagram of the example loop. The transformer initially is a 1:1 ideal transformer, it serves to isolate the loop from a coaxial feedline, allowing fairly good loop symmetry and reduction of common mode feed line current contribution to pickup. This works, and subject to symmetry and a good transformer design, it will work well over the stated frequency range, though its gain at some frequencies might not be sufficient to overcome receiver internal noise. Continue reading Small untuned loop for receiving – it’s not rocket science

YouLoop-2T and the self resonance bogey at MF/lowHF

Small untuned loop for receiving – simple model with transformer gave a simple model for analysing a loop and and Towards understanding the YouLoop-2T at MF/lowHF  applied that to the YouLoop-2T.

Above is the Airspy Youloup-2T. Try to put the two turns thing out of your mind, it is misleading, panders to some common misunderstanding, and so does not help understanding.

It would seem that many are quite confused by information from Airspy. The following quote from an online forum captures the confusion. Continue reading YouLoop-2T and the self resonance bogey at MF/lowHF

Oyster conversion – #2 – 24W

Conversion of oyster luminaire to LED discussed a first conversion effort. This article describes a conversion of a oyster that used a T8 32W flourescent tube.

First step was to strip the magnetic ballast, power factor correction capacitor (if fitted), clips for the tube, labels that are misleading, and to check / provide the needed protective earth connection.

The LED plate used is that reviewed at LED plate analysis – 24W round plate with driver but with a new driver that delivers 260mA (though rated at 300mA).

Above is the 24W LED plate. The plate has 48 0.5W 5730 LEDs in a 24×2 configuration. If we allow that the 24W rating is total input power, driver loss is typically around 2W so the LEDs themselves will draw 22W. We expect that the voltage at 22W will be around 80-85V, and will require ~275mA current. The original LED driver supplied (180mA) is not capable of that current and was discarded. A nominally 300mA LED driver was procured for about $10 for five, and they fall short, delivering 260mA but that is good enough for this implementation. Continue reading Oyster conversion – #2 – 24W

LED plate analysis – 24W round plate with driver

This article reviews a 24W LED plate and driver commonly sold on eBay and Aliexpress for around $12.

Above is the 24W LED plate from the sellers pics. The plate has 48 0.5W 5730 LEDs in a 24×2 configuration. If we allow that the 24W rating is total input power, driver loss is typically around 2W so the LEDs themselves will draw 22W. We expect that the voltage at 22W will be around 80-85V, and will require ~275mA current. It is shown here with a 220mA LED driver or 80% of what is required for 22W. Continue reading LED plate analysis – 24W round plate with driver

(How) does this balun work? (Tonna / F9FT balun for Yagis)

This article proposes an explanation of how the balun used on some Tonna Yagis works.

It appears Tonna no longer manufactures these antennas, I do not know if this design is novel. I do not recall seeing them used by other manufacturers, they may be protected by patent.

Above is a pic of the balun structure on a 2m antenna.

Above, the manual shows that the black sleeve on the balun sleeve would be slid up over the coax connector, making a neat finish. There are slightly different versions for 70cm antennas. Continue reading (How) does this balun work? (Tonna / F9FT balun for Yagis)