Month: August 2018

Loop in ground (LiG) – #2 – trial topology selection

* * * D R A F T * * * – a working document.

This article documents the selection of the trial loop in ground configuration as a development from the loop on ground antenna (KK5JY).

Baseline

The baseline is a minor variation of a design by KK5JY, a 15′ square loop 20mm above average ground, with 9:1 transformer and 50Ω load middle of one side.

Above is a plot of feed point impedance when the loop is driven. At 3.6MHz, the source impedance for a rx system is 43+j852Ω, and the mismatch loss to a 450Ω load is 11.0dB, a direct contribution to Antenna Factor (AF). Continue reading Loop in ground (LiG) – #2 – trial topology selection

Last update: 27th December, 2021, 6:58 PM

Loop in ground (LiG) – #1 Feasibility study for rx only on low HF

* * * D R A F T * * * – a working document.

This article documents a feasibility study of a smallish loop on or in ground as a rx only antenna for 160-40m, possibly with advantage in high noise environments.

Various ‘on ground’ antennas are discussed online etc, but there is a distinct lack of supporting scientific evidence though subjective anecdotal evidence abounds.

The approach used here is to determine the degradation of S/N resulting from a low gain antenna system in the context of expected ambient noise as per ITU P.372-13. The analysis leans to the conservative side. Continue reading Loop in ground (LiG) – #1 Feasibility study for rx only on low HF

Last update: 27th December, 2021, 6:57 PM

VSWR meter trap for the unwary

From time to time one sees discussion online about consistency of ‘measured’ VSWR at different power levels (on the same instrument).

A question often asked is:

I tune up at 10W and achieve VSWR=1.5, and when I increase power to 100W, the VSWR increases. Which should I believe?

The first thing to note is that good antenna systems SHOULD be linear, VSWR should be independent of power, it is if the system IS linear.

For the most part they are linear, even though many antenna systems contain elements such as ferrite cored inductors that may exhibit some small level of non-linearity in ‘normal’ operation.

Non-linearity caused by for instance saturation of magnetic materials, loss of permeability where the temperature of ferrite cores reaches Curie point, arcing of capacitors or other insulating materials is NOT normal linear operation of a GOOD antenna system. If high indicated VSWR at high power is caused by any of these effects, it is flagging a problem that requires attention.

That said, a significant non-linear element may be the VSWR meter itself.

A common, if not most common way to make these meters is to use a half wave detector to convert the direction coupler RF outputs into DC to drive an ordinary moving coil meter. These meters commonly assume that the detector DC output voltage is exactly proportional to the RF input voltage.

Lets look at the accuracy of that process.

Above is a plot of the detector output vs RF input voltage for a commercial 200W VSWR meter. The measurements cover input power from 10 to 100W.
Continue reading VSWR meter trap for the unwary

Last update: 7th August, 2018, 3:47 PM

Online calculator of ferrite material permeability interpolations

Many of my articles call for finding the complex permeability of a ferrite components from manufacturer’s data.

Let’s explore an example used in a recent article, Another small broadband RF transformer using medium µ ferrite core for receiving use – 50:450Ω.

The core used was a Fair-rite ferrite core of #43 material, and the magnetising impedance of a 5t winding needed to be found.

Above is a chart from Fair-rite’s catalog. Permeability is a complex quantity and is frequency dependent. One could scale from the graph, the values for µ’ and µ” at the frequency of interest. Continue reading Online calculator of ferrite material permeability interpolations

Last update: 6th August, 2018, 6:06 AM

Another small broadband RF transformer using medium µ ferrite core for receiving use – 50:450Ω

A simplified design for small broadband RF transformers using medium µ ferrite core for receiving use. The specific application is an impedance transformer for a nominally 450Ω antenna to a 50Ω receiver input. Intended frequency range is from 0.5 to 15MHz.

The characteristic of typical medium µ ferrite mixes, particularly NiZn, are well suited to this application.

This article continues with the design discussed at BN43-2402 balun example, but using a BN43-202 with 5t primary and 15t secondary for a nominal 1:9 50:450Ω transformer (though at high ratios, the transformation is only nominal).

Lets consider a couple of simple starting points for low end and high end rolloff. Continue reading Another small broadband RF transformer using medium µ ferrite core for receiving use – 50:450Ω

Last update: 10th August, 2018, 7:34 AM

Another small broadband RF transformer using medium µ ferrite core for receiving use – 50:3200Ω

A simplified design for small broadband RF transformers using medium µ ferrite core for receiving use. The specific application is an input transformer to a nominally 2kΩ receiver at around 9MHz (a panadapter).

The characteristic of typical medium µ ferrite mixes, particularly NiZn, are well suited to this application.

This article continues with the design discussed at BN43-2402 balun example, but using a 2t primary and 16t secondary for a nominal 1:64 50:3400Ω transformer (though at high ratios, the transformation is only nominal).

Lets consider a couple of simple starting points for low end and high end rolloff. Continue reading Another small broadband RF transformer using medium µ ferrite core for receiving use – 50:3200Ω

Last update: 10th August, 2018, 7:34 AM