## NEC – vertical monopole radiation resistance study

A recent online discussion contained an analysis of the radiation efficiency of a vertical monopole over real ground.

The poster dismissed the values calculated by 4NEC2 and proposed his own formula $$RadiationEfficiency=\frac{35.6}{\mathbb{R}Z_f}$$ where 35.6 is the radiation resistance Rr of a quarter wave monopole over a perfectly conducting earth (PCE).

The reasoning seems to depend on Rr being independent of the ground type, but that is quite flawed.

## NEC insight

Let us look at an example of a quarter wave monopole with 120 shallow buried radials, average ground, at 3.8MHz.

Above is the model geometry. Continue reading NEC – vertical monopole radiation resistance study

## 4NEC2 – summary statistics discussion

I am a frequent user of 4NEC2 despite its many defects. It is a great work of software, in need of improvement that I suspect will never happen. Its author explained some time ago that it was developed in VB6 and with Windows upgrades, he no longer has a working VB6 development platform.

So, despite its defects, it is a very useful tool.

I will sometimes substitute _ for – in some 4NEC2 quantity labels for clarity in mathematical expressions. Continue reading 4NEC2 – summary statistics discussion

## FT240-43 matching transformer for an EFHW – NEC model at 3.6MHz

The article End Fed Half Wave matching transformer – 80-20m laid out a design for a EFHW transformer based on the readily available FT240-43.

This article builds an NEC model for an EFHW antenna at 3.6MHz incorporating a realistic model of the above transformer.

NEC provides for a NT card characterising a two port network using Y parameters.

## Y parameter model for the transformer

The Y parameter model is based on measured input impedance with port 2 open circuit, and short circuit, and the observed turns ratio.

Impedance was measured with the transformer at 3.6MHz using an AA-600.

Above, the calculated Y parameter model including a prototype NT card. This model captures the various loss components of the transformer, mainly magnetising loss, at 3.6MHz. Continue reading FT240-43 matching transformer for an EFHW – NEC model at 3.6MHz

## Small efficient matching transformer for an EFHW – NEC model at 3.6MHz

The article Small efficient matching transformer for an EFHW laid out a design for a small EFHW transformer.

This article builds an NEC model for an EFHW antenna at 3.6MHz incorporating a realistic model of the above transformer.

NEC provides for a NT card characterising a two port network using Y parameters.

## Y parameter model for the transformer

The Y parameter model is based on measured input impedance with port 2 open circuit, and short circuit, and the observed turns ratio.

Impedance was measured with the uncompensated transformer at 3.6MHz using an AA-600, the compensation in the reference article has little effect at 3.6MHz.

Above, the calculated Y parameter model including a prototype NT card. This model captures the various loss components of the transformer, mainly magnetising loss, at 3.6MHz. Continue reading Small efficient matching transformer for an EFHW – NEC model at 3.6MHz

## Exploration of feasibility of ground wave comms on 80m VK2OMD-VK1EA

To be useful, S/N needs to exceed 10dB.

Ham mythology has it that ground wave is good for 100km on 80m without much qualification.

## Ambient noise

Ambient noise is very important, it is one factor of the Signal / Noise ratio which sets the limit for communication.

Using ITU-R P.368-9 we find that expected median noise figure in a residential precinct at 3.6MHz is 57dB. Continue reading Exploration of feasibility of ground wave comms on 80m VK2OMD-VK1EA

## nanoVNA – VSWR in terms of 400Ω

I was asked whether the nanoVNA can display VSWR in terms of 400Ω or some other arbitrary impedance.

Some antenna analysers and VNAs support display of results in terms of some specified impedance other than 50Ω, sometimes only a limited fixed set.

The direct answer to the question is “probably no, not directly on the ‘original’ nanoVNA today”, there are many firmware forks and many PC clients for nanovna, and now many significantly hardware versions appearing. Things may change.

However, if you can save a set of impedance measurements, they can be converted to VSWR relative to some other impedance reference.

## An example

At Implementation of G5RV inverted V using high strength aluminium MIG wire – impedance measurements a set of impedance measurements made with a Rigexpert AA600 is presented.

A similar set of measurements could be made with a standalone nanoVNA and saved, but given that it is such a clumsy device and its USB socket has become so unreliable, I will not repeat the measurement but use the data used for the article above.

So we have a table of frequency and (R,X) measured looking into a ‘real’ 450Ω feed line, so we will calculate wrt 450Ω but we could use any reference.

Above is a plot or (R,X) looking into the feed line. Continue reading nanoVNA – VSWR in terms of 400Ω

## Annual inspection of antenna reveals balun box issue

During annual inspection of my HF antenna system, I removed the lid from the common mode choke.

The choke is housed in a (German) Hensel PS electrical terminal box. Hairline cracks were observed radiating out from one of the terminal screws, and adjacent to both screws in the lid. The lid is obviously not strong enough to be secured by two screws in the way done. Continue reading Annual inspection of antenna reveals balun box issue

## Review of W8TEE, AC8GY STL (Radcom Feb 2020)

(Purdum 2020) describes a small transmitting loop (STL) which is a little novel in that it uses an arrangement of four circular conductor loops, two in parallel in series with the other two in parallel.

The article goes on to claim some pretty extraordinary efficiency calculated from radiation resistance for a loop structure that is shown at a height of perhaps 2m above natural ground. Continue reading Review of W8TEE, AC8GY STL (Radcom Feb 2020)

## Stub matching loss can bite you

An article by K2PO in QST Feb 2020 entitled An SWR shifting T illustrates the pitfalls in naive design and implementation of transmission line matching systems. I say naive because the article does not address the matter of loss, yet QST publishes it as an example. Continue reading Stub matching loss can bite you

## A thinking exercise on Jacobi Maximum Power Transfer #4

The article A thinking exercise on Jacobi Maximum Power Transfer #3 discussed Kurokawa’s power reflection coefficient as in indicator of mismatch at a system node.

Above is a demonstration circuit in Simsmith, a linear source with Thevenin equivalent impedance of 50-j5Ω. The equivalent voltage is specified by useZo, which like much of Simsmith is counter intuitive (as you are not actually directly specifying generator impedance):

Vthev and Zthev are chosen so that ‘useZo’ will deliver 1 watt to a circuit impedance that equals the G.Zo. Zthev will be Zo*.