Author: Owen

Fixes #1: magnetic stirrer with heating plate and digital display XB 85-2

Review: magnetic stirrer with heating plate and digital display XB 85-2 documented problems that prevented the device being very useful.

Attempts to tune the supplied PID controller above were frustrated by a lack of meaningful documentation supplied or found in searches on the ‘net, and the fact that the display is sometimes faked to appear that the temperature has stabilised. With any non-zero I term, it behaved badly and some observations suggest that it suffers from integral windup. It is truly a piece of Chinese junk and unusable.

Above is an independent logger capture of the temperature from switch on. There is a large overshoot, and then, no matter what the settings, it oscillates and the lowest amplitude obtained was 1°pp (above). The overshoot is almost as much as observed in manual warm up when power is cut at 40°. Continue reading Fixes #1: magnetic stirrer with heating plate and digital display XB 85-2

Last update: 23rd August, 2020, 7:29 PM

First full month of Amaysim broadband broadband Internet access

We have had wired broadband service delivered to these premises for almost ten years, supplied by three vendors: Telstra Bigpond, iiNet, and Amaysim.

Amaysim has supplied a 25Mb/s  broadband service for a month now and set an important baseline for service.

Above is a plot of the 90 percentile and 10 percentiles for each week so far. Continue reading First full month of Amaysim broadband broadband Internet access

Last update: 2nd October, 2017, 1:59 PM

A little programming adapter for 4 x 2mm pitch pads

I recently had need to attach four wires to a set of pads on a device for programming its microcontroller. The pads for these sort of things are often on difficult to solve pitch, this one is 2mm which is not too bad.

Above is the target and solution.

The target is the four vias right next to the LEDs on the daughter module. Continue reading A little programming adapter for 4 x 2mm pitch pads

Last update: 30th September, 2017, 7:03 PM

A low cost 50Ω termination for measurement purposes

This article shows just how easy it is to make an inexpensive low VSWR load for antenna analyser validation / measurements.

Above is an AA-600 sweep of the prototype from 10kHz to 100MHz. VSWR reads 1.02 in ‘All’ mode at 100MHz… better than the inherent accuracy of the instrument.

How expensive?

It is made from two 100Ω 1% 1206 SM resistors purchased on eBay for about $2/100, so about $0.04 for the resistors, and 40mm of bare copper wire (0.5mm phone / data wire in this case).

In use, it is held in contact with the coax socket (in this case an N type) with a pair of disposable plastic first aid tweezers (yep, you can buy them on ebay for about $0.20/pair).

While you are at it, make a good short circuit termination by scrunching up a bit of (clean) kitchen aluminium foil and press that against the coax socket conductors.

Try both of these on your antenna analyser and see how it stacks up.

Last update: 27th September, 2017, 4:46 PM

DL4YHF 50MHz counter on a Chinese TB-244746 PCB

DL4YHF published a frequency counter design based on the PIC16F628.

The design has been modified by many, copied by even more, and usually without attribution.

This article documents one of these copies (TB-244756 printed under the chip footprint), a $6 kit off eBay which comes with no documentation, though the screen mask is enough to correctly place components.

It turns out to be DL4YHF’s “DISPLAY_VARIANT 2”, the variation is that it uses a common cathode display.

Above the built kit with the 7550 voltage regulator replaced with a 78L05. Continue reading DL4YHF 50MHz counter on a Chinese TB-244746 PCB

Last update: 23rd September, 2017, 6:25 PM

“No short antenna is more efficient than a resonant-length antenna”

A recent online posting asserted that an antenna is optimal when itself resonant, and fed with a resonant feed line length so delivering a purely resistive load to a source, and further that implementors needed to be careful that a shorter dipole could be offset to some extent by a longer feed line but it would be inferior because:

no short antenna is more efficient than a resonant-length antenna

… but does that stand scrutiny?

An NEC experiment

Lets walk though an experiment using NEC-4.2 models of a dipole of 2mm copper wire at 10m height at 7.1MHz over average ground (σ=0.005, εr=13).

Key assumptions:

  1. source has a Thevenin equivalent source impedance of 50+j0Ω;
  2. feed line is lossless.

The results are sensitive to the model assumptions.

We will calculate the ratio of radiated power to the power delivered by the transmitter to a matched load, let us call it TransmitEfficiency for the purposes of this article. Continue reading “No short antenna is more efficient than a resonant-length antenna”

Last update: 22nd September, 2017, 6:21 AM

Small untuned loop with MMIC LNA on 80m

This article documents a small experiment with a quite small untuned loop, and LNA and receiver on 80m to assess its ability to copy signals on the band as well as the station transceiver on large antenna.

A significant factor at 80m is that ambient noise is quite high. Let’s consult ITU P.372-12 for guidance.

Curve E is the median city noise, at 3.6MHz Fa is about 62dB. At a more detailed level, P.372-12 gives the median noise figure for Rural precincts at 51.8dB and that figure is more appropriate to the test location (large block rural residential).

The test was carried in a few spots, at 50-100 m from the main station dipole. Continue reading Small untuned loop with MMIC LNA on 80m

Last update: 27th October, 2019, 3:58 PM

Review: magnetic stirrer with heating plate and digital display XB 85-2

I purchased a laboratory style stirrer / hotplate with PID temperature controller for some experiments.

Above, the 85-2 product from Chinese maker XB.

It certainly looks the part and for under $100 looked impressive value… but was it?

Is it safe?

First thing with ANY Chinese appliance is to test the electrical safety. Prior to a full PAT test, I plugged it in on an insulated work space and waved a non-contact voltage detector over the case. Beeeep! The case is hot. Earth continuity (earth pint to case) is zero, there is no connection. This came with an AU plug… so lets look inside at how they wired it up.

The green yellow earth wire is floating look, the end has been tinned so it was or was intended to be soldered to something. The end of one of the screws holding four rubber feet on is more silver coloured than the others, so it appears to have been tacked on to that. Three issues: the screw tension is cushioned by the rubber foot and pressure to case is low so it would not be a reliable low resistance connection; soldered ground connections can melt off in a fault and are unacceptable practice; and the even this had become disconnected and would not have passed a basic electrical test. Chinese Quality!

So with the cover off, it is apparent just a few months after this May 2017 dated build has been put together with rusty steel. Continue reading Review: magnetic stirrer with heating plate and digital display XB 85-2

Last update: 21st September, 2017, 10:20 AM

Failure to treat impedance as a complex quantity leads to…

A ham recently posted a graph on QRZ to educate his fellow hams on the behaviour of transmission lines under mismatch.

Above is one of his graphs (the red arrow is my annotation). It plots Impedance variation along a mismatched 75Ω transmission line. The curves look graceful, but are they science or just pretty artwork? Continue reading Failure to treat impedance as a complex quantity leads to…

Last update: 22nd September, 2017, 6:08 AM

Demonstration of the GR1606B for antenna Z measurement and calcs

This article demonstrates use of a GR1606B RF impedance bridge for measurement of the feed point impedance of a MHz loaded mobile whip. The antenna is roof mounted on a vehicle and measurements are made looking into 4m of RG58C/U, then transformed to feed point impedance using three tools:

Key metrics are:

  • F=7.05MHz
  • R=35Ω
  • ReactanceDial offset=200
  • ReactanceDial=442
  • X=(442-299)/7.05=34.33Ω
  • Z=35+j34.33Ω;
  • Feed line is 4m of Belden 8262 (RG58C/U).

Continue reading Demonstration of the GR1606B for antenna Z measurement and calcs

Last update: 13th September, 2017, 10:57 AM