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

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

(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)

AD9833 / PllLdr checkout

PllLdr is a generic microcontroller to load a PLL chip’s configuration registers using SPI. SPI is used by many PLL and DDS chips, data format and content varies from chip to chip.

This article documents checkout on an ADF9833 DDS chip.

The test was made on an inexpensive module purchased on eBay for about A$3 posted. This board has a 25MHz clock oscillator, but be aware they are sold with other frequencies as well, and is usable to about 40% of the clock frequency.

Above is the test board. It does not contain any form of output filtering, it really is bare bones. Continue reading AD9833 / PllLdr checkout

Rigexpert Antscope2 – v1.1.1

A new version of Antscope2 has been released.

Online posters are excited that it supports some versions of nanoVNA, and one thread attempts to answer the questions:

The SWR image shows that the SWR minimum is at the center phase angle as you would expect. My question is:

  1. what are the other points that look like resonance,

  2. and should I trim my antenna based on phase?

  3. If so which one?

They are interesting questions which hint the ham obsession with resonance as an optimisation tartget.

Properly interpreting VNA or analyser measurements starts with truly understanding the statistic being interpreted.

In this case, the statistics being discussed are Phase and VSWR, and their relationship.

What is the Phase being discussed?

Above is an Antscope2 phase plot for an archived antenna measurement. The measurements are of a 146MHz quarter wave mobile antenna looking into about 4m of RG58C/U cable. We will come back to this. Continue reading Rigexpert Antscope2 – v1.1.1

Rigexpert AA-600 N connector dimensions

A recent post by David Knight described dimensional issues with the N connector on his AA-600 and problems with the seller in having it resolved.

Warned of a potential quality issue, I measured my own AA-600.

Above, the test of the inner pin forward surface distance from the reference plane on the N jack on the AA-600. The acceptable range on this gauge for the female connector is the red area, and it is comfortably within the red range.

Above is a table of critical dimensions for ‘ordinary’ (ie not precision) N type connectors from Amphenol.

This dimension is important, as if the centre pin protrudes too much, it may damage the mating connector.

Pleased to say mine is ok, FP at 0.192″.

I used a purpose made gauge to check this, but it can be done with care with a digital caliper (or dial caliper or vernier caliper), that is what I did for decades before acquiring the dial gauge above.