## Red Ironbark picnic table

I made a couple of picnic tables about 35 years ago. The design was broadly inspired by picnic tables deployed by the ACT administration at the time (local government), it used a galvanised water pipe frame for table and integral seats and hardwood tops.

I kept one of these tables, and the hardwood eventually degraded sufficiently to warrant replacement.

Durable timber has become very expensive, and the choice limited. Red Ironbark (a eucalypt endemic to the forests south east of here), GOS (green off saw) and DAR (dressed all round) was chosen, and stacked in the shed for a couple of years to dry down to 10% moisture content.

The timber was washed down, trimmed and edged, drilled and oiled (Organoil, a naturally drying oil mix), and fixed to the table with 304 stainless countersunk socket head M8 screws and nyloc nuts.

Above, the refurbished table. Total mass is 125kg, about half of that is in the hardwood and the rest in the steel frame.

The table will required replenishment of the oil finish every year, but should be a durable non-toxic lasting finish with that maintenance.

## Identifying ferrite materials #2

A correspondent wrote about Identifying ferrite materials asking how to use the method on an analyser that does not display X, but displays R and |Z|.

The method described calls for making a winding of the least number of turns for which measurement can reasonably be made, and finding the lowest frequency where R=X.

If your analyser does not display X (or |X|), you can exploit the relationship that |Z|=(R^2+X^2)^0.5. When R=X, |Z|=1.414R… so you would look for the lowest frequency where |Z|=1.414R.

Having found that, compare it with the table given or datasheet graphs to find candidate mixes.

## Reconciling my #52 choke design tool with G3TXQ’s measurements

A correspondent wrote with concern of the apparent difference between graphs produced by my #52 choke design tool with a graph published by G3TXQ of his measurement of 11t on a pair of stacked FT240-52 cores.

I published a note earlier about my concerns with a similar graph by G3TXQ compared to the Fairrite datasheet, and he reviewed the data, found the error and published a corrected graph.

The corrected graph above might at first glance appear different to my model’s graphs, and the first obvious difference is that G3TXQ uses a log Y scale (which is less common). The effect of the log scale is to compress the variation and give the illusion perhaps that in comparison with other plots, this balun has a broader response.

To compare the two, I have roughly digitised G3TXQ’s graph above and plotted the data over that from my own model (with linear Y scale). Continue reading Reconciling my #52 choke design tool with G3TXQ’s measurements

## Identifying ferrite materials

This Jan 2012 article has been copied from my VK1OD.net web site which is no longer online. It is for reference from other related articles. The article may contain links to articles on that site and which are no longer available.

One often wants to identify the type of material used in a ferrite core for use at radio frequencies. This article captures advice that the author has offered in online fora stretching back more than a decade, yet it seems uncommon knowledge.

The most common method is to make some measurements to determine the initial permeability µi, usually at audio frequencies, and to compare that to a table of µi for common core materials. This method might well indicate several mixes that have similar µi, but each may be quite different at higher frequencies.

The suitability for use at RF usually depends much more on complex permeability at radio frequencies than it does on µi at say 10kHz.

Above is a plot from the Fair-rite data book showing the complex permeability characteristic of #43 ferrite material. Continue reading Identifying ferrite materials

## Insertion Loss, Mismatch Loss, Transmission Loss

A correspondent asks about the effect of RCA connectors at HF on his proposed noise bridge. The question is very similar to that considered at Exploiting your antenna analyser #13 for UHF series connectors.

I have made a simple measurement of a BNC 50Ω termination (to check calibration) then inserted a BNC-RCA and RCA-BNC adapter.

Measurements of input impedance only for such an electrical short transmission line will not give useful data for determining TransmissionLoss which is the result of conversion of RF energy to heat. The measurements do give ReturnLoss and given that InsertionLoss=MismatchLoss+TransmissionLoss, they set a lower bound for InsertionLoss.

To jump to the chase, it also has a Smith chart plot up to 200MHz that suggests it might be well modelled by a TL segment of 30-35Ω.

Above is a plot of VSWR when Zref is adjusted for the flattest response from DC, and it can be seen that with Zref=33, response is quite flat to 200MHz. Continue reading Exploiting your antenna analyser #14

## Low Insertion VSWR HF Guanella 1:1 balun for instrumentation

The article describes a current balun intended for measurement use with low power instrumentation. It is lightweight and can be made with materials readily available in Australia (LF1260 cores are a little over \$1 each in packs of six). The target application is for a 600mm square small loop for field strength measurement below 15MHz.

## Design

The design is an implementation of (Duffy 2007) which used RG174 coax for the choke to give low Insertion VSWR.

The balun is intended for low power measurements and will withstand dissipation of a few watts.

The LF1260 cores are made from a medium µ ferrite and have an ID of 7.8mm.

Above, the cores will accommodate four round conductors of diameter 3.2mm, so they will comfortable accommodate the four passes of RG174 (2.5mm dia). (For the mathematically minded, the minimum enclosing circle diameter for four equal circles is 1+√2 times the diameter of the smaller circles.)

## Implementation

Above, the balun cores  are housed in a small Jiffy box with a BNC-F flange mount connector at one end and a pair of M4 screw terminals at the other. Small brass tabs were made as non-rotating terminal tags and the M4 brass screws soldered to them. The cores are attached to each other with a piece of double-sided foam tape to prevent them shattering, and two pieces of the same used to secure the cores to the box. A packing between the cores and lid helps to hold them in place.

As discussed at Baluns – show me the numbers, implementation details without quantitative models or measurement are of little value. Continue reading Low Insertion VSWR HF Guanella 1:1 balun for instrumentation

## A little rigging this week (20160204)

A little rigging job this week, taking down some towers at another place…

I thought there were no spectators, but the inevitable video has emerged!

And some stills…

Abusers of Nally towers know what that is on the hook!

## APRS duplicate removal – trial #2

The undetected long-delayed duplicate posits that are a feature of APRS VHF are a significant corruption of mapping.

Program code has been written to collect posit records direct from APRS-IS and where the record contains a HMS timestamp (as mine do), to apply a timestamp consistent with that timestamp, and then to not write records with duplicate timestamps to the database.

The above map has two journeys, one a loop to the north of home to Mittagong, and another SW to Marulan and back. The Marulan trip extended to 40km from the prominent digi VK2RHR-1, and coverage in the area became poor, evidenced by the poorer APRS mapping in that region.

The map can be browsed interactively at http://owenduffy.net/map/20160204.htm. The red trace is APRS VHF derived using the processing detailed below.

The cyan trace is from stand alone GPS logger above attached to the case of the TinyTrak 3+. The transmitter is 65W output to a quarter wave in the middle of the wagon roof. Continue reading APRS duplicate removal – trial #2

## Lithium battery – 1S protection boards

Some of my projects use a single Lithium cell for power, and the ready availability of low-cost battery protection boards offers opportunity for better projects.

Above, a 1S board rated at 4A and which sells for about A\$1 posted in lots of 5.

Specifications:

Description:
New and good quality
Use BM112 protection chip + AO8810 MOS tube
The protection board is used to protect the battery overcharge, over discharge, so can’t use as a charger,when you want to charge the battery you need to use the dedicated charger,because the protective board has a time to response to the short circuit, can’t to connect too large instant impact current, such as drills and so on

The main performance parameters:
1. PCB Size: 39 * 4 * 2mm
2. Overcharge protection voltage: 4.2750 ± 25MV
3. Over-discharge protection voltage: 2.88 ± 75MV
4. The overcurrent protection: 4-8A
5. Continue working current: > 4A

Note: Only for the equipment which instant start-up current less than 4A,those starting current instant is great, such as high-current motors, drills, etc., are not suitable for use.

Above are protected battery assemblies based on the board and a 1200mAh LiPo (sells for about A\$4) and Panasonic 18650 Li-ion 3400mAh (sells for about A\$12). The connectors used are 3A rated JST RCY connectors as used in RC battery applications and readily available with tails for way less than A\$1 each set.

The 18650 cell has tags spot welded to the battery contact points, the LiPo has contact tags as supplied.

To use these, the power source needs to supply about 4.5V so as to ensure charging when necessary. The power source needs to be current limited unless you choose to depend on the protection boar’s limit.

## Insertion Loss, Mismatch Loss, Transmission Loss

A correspondent having read Exploiting your antenna analyser #12 asks whether the measurement provides evidence of loss of the connectors, and referred me to (Arther nd) where he reports some measurements of UHF series adapters and conclusions.

## Duffy

Let’s deal with interpretations of my own measurements first.

Measurements of input impedance only for such an electrical short transmission line will not give useful data for determining TransmissionLoss which is the result of conversion of RF energy to heat. The measurements do give ReturnLoss and given that InsertionLoss=MismatchLoss+TransmissionLoss, they set a lower bound for InsertionLoss.

Above is a plot of ρ and ReturnLoss for the DUT. ReturnLoss curiously is plotted ‘upside down’ as ReturnLoss increases downwards… a quirk of AIM software, but remember that ReturnLoss in dB is +ve.