No-name video baluns common mode impedance

I purchased a bunch of no-name, no-spec, video baluns on eBay for use with radio antennas at MF and low HF. A pair of these are for transporting 75 ohm coax video signals over twisted pair LAN cable.

Above, the two styles of baluns purchased. Given the label, nondescript as it is, there is quite a possibility that the internals are the same. Continue reading No-name video baluns common mode impedance

Matching a centre loaded 80m vertical – a shunt match tutorial

This article describes a method of measurement and adjustment using an antenna analyser or VNA to quickly set up a shunt match, a narrow band match (ie for one band, or even only part of the band).

The article uses Rigexpert’s Antscope as the measurement / analysis application, the techniques will work with other good application software.

To demonstrate the technique for matching such an antenna, let’s use NEC-4.2 to create 80m feed point impedance data for a 12m high vertical with 8 buried radials (100mm) and centre loading coil resonating the antenna in the 80m band for simulation of measurement data.

An s1p file was exported from 4NEC2 for import into Antscope, to simulate measurement of an example real antenna.

Analysing the ‘measured’ data

Step 1

Above is the VSWR curve displayed in Antscope. Note that the actual response is dependent on soil types, antenna length and loading etc, but this is a good example for discussion. It is not real bad, another example might be better or worse. Continue reading Matching a centre loaded 80m vertical – a shunt match tutorial

Another small efficient matching transformer for an EFHW – 2643251002 – #2 – prototype bench measurement

The article Another small efficient matching transformer for an EFHW – 2643251002 – #1 – design workup lays out the first steps in a design. This article documents bench measurements of a prototype transformer.

Above is the prototype transformer wound with 14t of 0.71mm ECW tapped at 2t. The mm rule gives some scale. The turns are close wound, touching on the inner diameter of the core. Continue reading Another small efficient matching transformer for an EFHW – 2643251002 – #2 – prototype bench measurement

Another small efficient matching transformer for an EFHW – 2643251002 – #1 – design workup

The article Another small efficient matching transformer for an EFHW – 2643251002 described a correspondent’s , Luis, CT2FZI, implementation of the transformer.

I have also had lengthy discussions with Faraaz, VK4JJ, who is experimenting with a similar transformer.

This article describes my own design workup and measurements using a Fair-rite suppression core, 2643251002. The cores are not readily available locally, so I bought a bunch from Digi-key.

I really resist  the tendency in ham radio to design around unobtainium, it is often quite misguided and always inconvenient. In this case, the motivation for these cores that use quite ordinary #43 material is the geometry of the core, they have ΣA/l=0.002995, a quite high and rivalling the better of binocular cores. High  ΣA/l helps to minimise the number of turns which assists broadband performance. See Choosing a toroidal magnetic core – ID and OD for more discussion.

Design criteria

  • EFHW;
  • InsertionVSWR<2 3-22+MHz;
  • nominal 49:1 transformation;
  • compensated;
  • autotransformer; and
  • 50W average power handing.

Some key points often overlooked in published designs of EFHW transformers:

  • Insufficient turns drives high core loss; and
  • leakage inductance is the enemy of broadband performance, so the design tries to minimise leakage inductance.

Note that high number of turns drives high leakage inductance, so the design is to a large extent, a compromise between acceptable core loss and bandwidth.

Initial design estimate

From models, I expect that a turns ratio of 2:14 (ie 14t tapped at 2t) is likely to deliver the design criteria (with suitable compensation capacitor).

Above is a perhaps ambitious initial objective using a simple model of the transformer, dotted line is Loss and solid line is InsertionVSWR. Continue reading Another small efficient matching transformer for an EFHW – 2643251002 – #1 – design workup

Holzforma / Farmertec G372XT chainsaw – initial evaluation

I purchased a Holzforma G372XT chainsaw, it is a Chinese clone of the now discontinued Husqvarna 372XP X-TORQ. It is a relatively new technology carburetted engine without introducing electronic auto tune, a 20 year old design.

There are plenty of online discussions about the 372XP X-TORQ overheating, hot seizures, and pics of blued big ends, scored pistons and bores etc, so whilst the design achieves quite high power output for 71cc, one is warned to pay attention to mixture, fuel quality, and lubrication. This might be an engine that should not be used with other than full synthetic two stroke oil. Continue reading Holzforma / Farmertec G372XT chainsaw – initial evaluation

Low power Guanella 1:1 balun with low Insertion VSWR using a Fair-rite 2843009902 binocular – measurement of Zcm

The article Low power Guanella 1:1 balun with low Insertion VSWR using a Fair-rite 2843009902 binocular – design workup describes a current balun with low Insertion VSWR for operation at modest power levels. The design was based on a low cost Fair-rite 2843009902 binocular core (BN43-7051).

This article documents measurement of the complex common mode impedance Zcm, and calibration of the predictive model.

Zcm is a most useful quantity, it can be used in NEC models of an antenna system. Continue reading Low power Guanella 1:1 balun with low Insertion VSWR using a Fair-rite 2843009902 binocular – measurement of Zcm

RTKLIB using stunnel for TLS connection to NTRIP host

RTKLIB does not currently natively support TLS, hopefully it will in time.

This article details configuration of RTKLIB rtknavi to work on a Win10 workstation using stunnel to connect to a NTRIP server that is only available using TLS.

The version of rtklib used is v2.4.3b33, and stunnel v5.60.

Continue reading RTKLIB using stunnel for TLS connection to NTRIP host

nanoVNA – measure common mode choke – it is not all that hard!

It seems that lots of hams find measuring the impedance of a common mode choke a challenge… perhaps a result of online expert’s guidance?

The example for explanation is a common and inexpensive 5943003801 (FT240-43) ferrite core.

Expectation

It helps to understand what we expect to measure.

See A method for estimating the impedance of a ferrite cored toroidal inductor at RF for an explanation.

Note that the model used is not suitable for cores of material and dimensions such that they exhibit dimensional resonance at the frequencies of interest.

Be aware that the tolerances of ferrite cores are quite wide, and characteristics are temperature sensitive, so we must not expect precision results.

Above is a plot of the uncalibrated model of the expected inductor characteristic, it shows the type of response that is to be measured. The inductor is 11t wound on a Fair-rite 5943003801 (FT240-43) core in Reisert cross over style using 0.5mm insulated copper wire. Continue reading nanoVNA – measure common mode choke – it is not all that hard!

Chinese batteries – replacements for Makita 1220 battery pack

Buying Chinese batteries is a minefield. Experience has been that they rarely meet specified capacity, and less that 10% of specified capacity is not that unusual.

So, with that background I searched eBay for replacement batteries for a Makita drill / driver. The current pair of batteries are Chinese source, purchased about 5 years ago, label rating is 2.0Ah, and measured capacity now is 1.3-1.4Ah… which is not too bad, they were 1.7-1.8Ah when new (yes, below spec).

There were very many sellers selling a lot of two packs rated at 3.6Ah for around $34 including delivery. That is very cheap, too good to believe? Let’s rely on eBay Buyer Protection to put them to the test.

The two batteries arrived quickly and were put through charge / discharge cycles to condition them.

Above are the C/5 discharge curves for both batteries on the third cycle, capacity is 1.3Ah, just 36% of the label rating. Continue reading Chinese batteries – replacements for Makita 1220 battery pack