Month: July 2023

Thoughts on the ARRL EFHW antenna kit transformer

Several readers have asked my thoughts on the ARRL EFHW kit.

I have not built and measured the thing, but have done the first step in a feasibility study.

The transformer design is not novel, it is widely copied and this may be one of the copies. The design is usually published without any meaningful performance data or measurements.

The article Select a ferrite core material and sufficient primary turns for a low InsertionVSWR 50Ω broadband RF transformer – comparison of measured and predicted laid out a method for approximating the core loss of a EFHW where the load is adjusted to that input VSWR50=1, ie input Z=50+j0Ω.

That method will be applied here for a good initial estimate of core loss.

I will present calcs for 80m and 40m since there are lots of articles and videos encouraging people to extend the antenna to 80m (with and without a loading coil).

It is quite practical to build an EFHW transformer with less than 0.5dB (11%) core loss.

Amidon FT240-43 toroid with 2t primary

The first point to note is that Amidon’s 43 product of recent years is sourced from National Magnetics Group, and is their H material. It is not a good equivalent to Fair-rite’s 43 mix.

Above from Amidon’s #43 datasheet, identical to NMG’s H material data (apart from the page header).

Let’s make a first estimate of core loss at 3.5MHz.

We can estimate the complex permeability which is needed for the next calculation. Continue reading Thoughts on the ARRL EFHW antenna kit transformer

Last update: 3rd September, 2024, 12:28 AM

What passes as an optimised small transmitting loop?

Whilst researching another article, I came across a Youtube presentation on  the subject of “Optimum Magnetic Loop Antenna.” It described the authors design of the “10-40m “epicenter” 42″ square 1.5″ dia loop” as an example optimised loop.

This article attempts to analyse the presented data to estimate the performance of the loop, specifically radiation efficiency at the lowest operating frequency.

Loop parameters:

  • shape: square
  • conductor diameter : 38mm
  • perimeter: 4.47m

The presentation gives a measured VSWR curve at maximum capacitance. Minimum VSWR is just below the 40m band, so we might expect that performance on 40m is very similar.

I did not see description of the measurement environment (height above ground, soil type, proximity to structures etc) in the presentation, they are all relevant if the experiment was to be replicated.

Above, the VSWR curve at maximum capacitance. Unfortunately it is not matched for low VSWR50 so we must make assumptions. Lets assume the impedance at resonance is lower than 50Ω, we can estimate the half power bandwidth from the VSWR=10 bandwidth of 480kHz, see below. Continue reading What passes as an optimised small transmitting loop?

Last update: 22nd July, 2023, 12:24 PM

Review of “The Truth About Magnetic Loop Antennas – MYTH BUSTING!”

A reader referred me to a video of a presentation to a radio club, the subject being “The Truth About Magnetic Loop Antennas – MYTH BUSTING!”

The presentation includes prediction and measurement of a small transmitting loop. This article tries to reconcile the claimed radiation efficiency between prediction and measurement.

Radiation Efficiency

The presentation liberally uses the term “efficiency”, let us take that to mean Radiation Efficiency:

the ratio of the total power radiated by an antenna to the net power accepted by the antenna from the connected transmitter.

Note that Radiation Sphere requires that radiated power must be measured / determined / summed in the far field.

Radiation fields decay inversely proportional to distance, other fields immediately around an antenna decay more quickly and are insignificant for the purpose of radio communications at great distances. Hence, Radiation is the usual objective of radio communications antennas.

The calculation tool used in the presentation defines:

Efficiency (%): The percentage of input energy that is actually radiated and not lost as heat.

The prototype loop antenna

The prototype loop antenna is described:

A table is given which appears to be predicted performance based on https://miguelvaca.github.io/vk3cpu/magloop.html . Continue reading Review of “The Truth About Magnetic Loop Antennas – MYTH BUSTING!”

Last update: 19th July, 2023, 8:48 AM

200h maintenance – Toro MX 4250

My Toro MX 4250 residential zero turn mower, has 200h of service over 8 years… which is not very much.

Above is a recent capture of battery voltage (red) and battery current (blue) using a Hantek 1008C that was under evaluation. (The image colours were inverted for the article.)

Valve lash measurement

Starter current measurement hinted that valve lash was incorrect in at least one cylinder

The exhaust valves should be 0.15-0.2mm lash, and were measured within that range for one cylinder and between 0.2 and 0.25mm (Cylinder #1). This probably accounts for the difference in cranking current at compression, the compression relief is degraded by excessive valve lash.

The inlet and exhaust valves were adjusted as needed.

Above is a capture of starter current after the valve lash adjustment. Note that the peaks in each pair are approximately equal.

Spark plugs

Spark plugs were removed and inspected. The gap was good, the electrodes had sharp corners, Nevertheless, new plugs were reinstalled (per the service manual).

Leak down test

A leak down test was conducted using an OTC 5609. Leak down was less than 1% @ 400kPa in both cylinders, excellent.

Oil change

Oil and oil filter changed.

Fuel filter

Fuel filter changed.

Air filter

The prefilter was washed and re-oiled. The paper cartridge was pretty clean, but was blown out with compressed air.

Control dampers

The right hand control damper is not effective, and sufficiently bad to impact driving.

Replacements are unavailable at this time.

Last update: 2nd July, 2023, 8:38 AM