Analysis of a certain dipole animation – OCF implications

A correspondent having read Analysis of a certain dipole animation questioned the validity of the lossy transmission line model of the dipole, citing the case of an OCF half wave which has an approximately resistive feed point.

Since the OCF lacks the symmetry exploited in earlier study, we must consider each half of the OCF dipole and combine them. To assist, I have produced a similar plot of the transmission line but note the changed X axis.

The scenario is again a 2mm diameter copper wire, 3m above ground at 1MHz.

Zo can be approximated as 138*log(2h/r)=138*log(2×3/0.001)=521Ω.

Clip 199

Above is a plot of calculated V and I at displacements from the open end, and calculated phase of V/I. Continue reading Analysis of a certain dipole animation – OCF implications

Last update: 9th June, 2016, 7:08 PM

Exploiting your antenna analyser #19

Critically review your measurements

A recent post on an online forum provides a relevant example to discussion of this subject.

I have personally seen ratios similar to 3:1 or higher at the feed point become 1:1 at the rig over 100 or so feet of coax cable.

First point is that in good transmission line, it takes an infinite length to deliver the observations made above. Less might deliver almost VSWR=1 at the input end of the line.

Let us consider a practical scenario, 100′ of RG58A/U with a load of 150+j0Ω at 14MHz, the load end VSWR(50) is 3, the input impedance is 32.50-j22.86Ω and input VSWR(50) is 2.01. In this scenario, the line loss is 2.5dB which might be unacceptable for some applications. Continue reading Exploiting your antenna analyser #19

Last update: 21st May, 2016, 3:45 PM

Analysis of a certain dipole animation

Modern people look for videos and animations for their learning, and these are often not from reputable sources and raise more questions than they answer.

An example is an animation of a half wave dipole on the Internet, and being discussed on QRZ.

Dipole_antenna_standing_waves_animation_461x217x150ms

Above, the animated graphic.

Without trying to understand the problem, lets just extract two cases for further discussion, an analysis in the limits if you like. Continue reading Analysis of a certain dipole animation

Last update: 24th May, 2016, 9:34 AM

The old wheelbarrow will not die

We bought a cheap wheelbarrow 6 years ago, and it is like grandfather’s axe: 6 new handles and 3 new heads and its 100 years old.

This thing has had numerous tyre patches (some due to defects in the cheap Chines tubes), three new tubes, two new tyres, it is trying to rust out, and the concreters working here last Winter used it to carry fully loads of concrete… splitting the side of the poly tray. The split has been growing slowly with temperature cycling, I should have drilled some small holes to defuse the stress raiser at the ends of the split.

IMG_0539

Whilst I have replaced the barrow, we continue to use this one to exhaustion… and I succumbed and plastic welded the split in the tub. A similar bead on the inside dressed with a burr to remove edged that would catch on tools compete the job… we will se how long it lasts.

Last update: 21st May, 2016, 11:03 AM

Transformerless power supply for PAROT

This article documents design of a capacitive transformerless power supply for operating low voltage, low power logic from power mains. The intended application is PAROT (Duffy 2013), though it has potentially wider application.

(Microchip 2004) gives a method for design of a capacitive transformerless power supply for operating low voltage, low power logic from power mains. The equations seem simplistic for a circuit whose apparent simplicity belies the complexity of an optimal design that properly tolerates supply voltage and load variations. For that reason, a SPICE simulation was used to refine a design.

The immediate application is for the PAROT chip driving a 40A SSR.

Clip 197

Above is measured characteristic of a Fotek 40A SSR, it seems typical of several similar types on hand. It appears that much smaller SSRs in the 2A range require fairly similar current. Continue reading Transformerless power supply for PAROT

Last update: 11th June, 2016, 5:50 PM

Variac refurb

I purchased a Yokoyama 5A Variac quite some years ago which was unsafe as purchased (with a current Test’nTag tag) and boxed up for repair / restoration.

It is needed for a current project, so time to fix it!

variac01

Above is the terminal block of the Variac. The defects include exposed input active and neutral terminals, exposed single insulated conductors, and the earth terminal has no spring washer or like and the screw also secures the resilent plastic P clip so it does not provide a reliable low resistance independent connection to the frame. There is no sign that there was ever a cover for this terminal block. It is noted that the terminal markings have been somewhat defaced.
Continue reading Variac refurb

Last update: 3rd June, 2016, 5:27 PM

An Emergency Stop switch for the mill

In a non-thinking moment, I had an accident with the mill because the head had not been clamped fully. I found myself fumbling for the power switch and the incident reinforced the need to fit an emergency stop button. I had procured parts for this a long time ago, it was time to put them to use!

ES01

Above is an inexpensive emergency stop button from eBay, about $6 including the box. This switch had NC and NO contact sets, for this application only the NC set is used.  A gland is used in the bottom of the box to let a 3 core 1mm^2 flex into the box. Continue reading An Emergency Stop switch for the mill

Last update: 13th May, 2016, 5:33 PM

Chinese AD8307 power measurement module #3

At Chinese AD8307 power measurement module #2 I concluded that the modified AD8307 was useful on HF, and through to 54MHz depending on accuracy requirements.

This article looks at combining the AD8307 module with a display option based on an Arduino Nano.

RFPM201

Above is a demonstration of the display prototype. The module in the foreground is an Arduino Nano (~$6), and behind it a 16×2 LCD with I2C module (combined, ~$4). The Android tablet is connected to the Nano using a OTG cable (~$1) and is logging the measurements (optional) and powering the equipment. The red and black clips are connected to a power supply to simulate the voltage from the AD8307. The same configuration should work with any Arduino phone or tablet if it supports OTG. Continue reading Chinese AD8307 power measurement module #3

Last update: 21st October, 2016, 8:15 AM

Chinese AD8307 power measurement module #2

At Chinese AD8307 power measurement module #1 I documented the first phase of checkout of a low cost AD8307 module.

There are two requirements for accurate power measurement:

  1. input impedance of the power meter must be very close to 50+j0Ω (say input VSWR<1.2); and then
  2. gain from the SMA terminals to the AD8307 input terminals must be independent of frequency.

Though the module was clearly junk in terms of criteria 1 as supplied, it was possible to modify it to present a low VSWR 50Ω input impedance, and that was documented in the last chapter.

This article carries on with criteria 2 above, the amplitude response.

Application requirements

The application requires an adjustment of the AD8307 calibration to 20mV/dB with -90dBm intercept, meaning it will produce 1800mV at 0dBm input and have a slope of 20mV/dB.

Though the original circuit shows the necessary components R3 & R4, and R5 & R6, they are not fitted and must be fitted to the board. I have used 50kΩ 20t trimpots for R3 and R6, and 33kΩ for R5 and 47kΩ for R4.

The technique used to calibrate slope and offset is that described at (Duffy 2014).

Slope and offset calibration, and log conformance / scale linearity at 10MHz

Clip 194

Above is a sweep from -65 to -6dBm after calibration of slope and offset. The linear fit to the blue curve shows slope is 20mV/dB and intercept 1.8015 for 0dBm means the offset is -1.8015/0.02=-90.08dBm. Log conformance is 0.2dB.

Clip 195Above is the fuller plot from -65 to 15dBm, and it can be seen the linearity degrades above -5dBm, but the error is small for this class of measurement chip.

Frequency response

The module was swept from 1 to 500MHz and response at 0dBm captured.

Clip 196

 

Above is the response from 1 to 50MHz. Response is down by 1.5dB at 55MHz, but within 0.5dB to 30MHz so quite suited to the intended application, a HF common mode current meter.

Conclusions

  • The module as supplied was cheap Chinese junk, it had 35dB slope from 10MHz to 1MHz, input VSWR  from 1.6 to extreme over the range 1-500MHz.
  • Reworking the input circuit delivered very good input VSWR to 240MHz.
  • Amplitude response with the reworked input circuit is within 0.5dB from well below 1MHz to 30MHz.
  • Flat response at VHF – UHF would require an equalised input circuit and appropriate PCB layout.

References

Last update: 12th May, 2016, 8:23 PM