## NEC – vertical monopole ground wave study

The article NEC – vertical monopole radiation resistance study discussed ‘radiation’ in the strict sense, this article takes a look at ground wave propagation from the same antenna.

## NEC insight

Let us look at an example of a quarter wave monopole with 120 shallow buried radials, soil σ=0.005  εr=13, average ground, at 3.8MHz.

Above is the model geometry. Continue reading NEC – vertical monopole ground wave study

## 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.

This article set about explaining interpretation of the summary statistics shown on 4NEC2’s main form.

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

## LDF4-50A shield prep with simple hand tools

Commscope makes a range of prep tools that have good productivity but are quite expensive for the hobbyist.

There are different forms of connectors for LDF4-50A, this article discusses a modern type that uses a collet to clamp the cable to the connector body.

There are many ways to prepare the shield end. This article describes one using a fine tooth pull saw which makes for good results for a novice.

Above, an Excel thin kerf razor saw #55001 which has a K5 handle and 30490 46tpi pull saw (~\$20 on eBay).

Above, a 3.6mm (0.14″) zip tie is pulled quite firmly into the valley of the corrugated shield to serve as a saw guide. Note the partial cut. The width of the zip tie is critical, and this width is common. The objective is to trim the shield just a little towards the end from the middle of the crest. (Normally you might have the jacket trimmed further back to accommodate the o ring and back of the connector, but it can be trimmed when the cut is complete.) Continue reading LDF4-50A shield prep with simple hand tools

## nanoVNA-H – continuing USB-C woes #2

I have reported issue with the USB-C plug / socket arrangement on the nanoVNA-H.

It is very sensitive to any jiggling of the cable or connector, causing a reset of the nanoVNA which almost always means lost work. The supplied cable was a partial cause, but sadly the jack on the PCB is also faulty.

This has progressively gotten worse to the point the nanoVNA-H is unusable. I have had a replacement socket on order for months from China where public health problems are causing chaos, it has only just shipped so could be some months yet.

I do realise that this is replacing cheap Chinese junk with cheap Chinese junk.

Replacement of the USB-C socket will be difficult, fortunately it is the 12/16 pin version rather than the full 24 pins… but I do wonder at the wisdom of using a USB-C over the proven micro USB connector.

Above, the old socket has been removed from the board. One pad came off with the socket, but it is the unused SB1 pad. Of greater concern is whether the slight movement of the some other pads might cause conductor cracking. I do see signs that a couple of pins might not have tinned the full pad area, a hint of low quality board fabrication and a possible contribution to intermittent connection.

An alternate recovery is to cut the end off a USB cable and permanently wire it directly into the board.

So for now, the nanoVNA-H awaits parts again.

## RF transmission lines – quite old art

From time to time I see online discussion about determining matched line loss (MLL) from Return Loss.

Something like 150 years ago a self educated genius thought about the problem of ‘smearing’ of telegraph signals in submarine cables and developed a model for the behavior of transmission lines that remains a very good model today. (The guy’s day job was as a telegraphist.)

He needed to develop new mathematics and new concepts like reactance, and he did this in the face of recognised engineers of the day dismissing the ideas.

The smearing problem was different propagation speed of the fundamental and harmonic components of the telegraphy waveform, he referred to it as distortion and in terms of his RLGC transmission line model he defined the requirements for a “Distortionless Line”. Zo for a Distortionless line is a purely real number, and a Lossless Line is a special case of a Distortionless Line. Continue reading RF transmission lines – quite old art

## 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