Amateur Radio – Page 15

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

Last update: 4th March, 2020, 7:47 PM

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

This article is a desk study of the likelihood of ground wave communications over the path VK2OMD-VK1EA, about 150km.

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

Last update: 31st March, 2020, 7:46 AM

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

Last update: 29th February, 2020, 2:43 AM

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.

Last update: 27th February, 2020, 6:17 AM

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

Last update: 27th February, 2020, 5:52 AM

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

Last update: 26th February, 2020, 6:38 AM

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

Last update: 16th May, 2020, 7:00 AM

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)

Last update: 30th November, 2020, 6:03 PM

Reinforcement of nanoVNA-H connectors – performance discussion

At Strength of reinforcement of nanoVNA-H connectors I showed a method I used to reinforce the SMA connectors to reduce the flexing of the PCB when the SMA connectors were torqued to specification for reliable measurement.

This has been commented on by online experts stating that Hugen, the designer of this board, posted notes about his efforts to keep the grounds for tx and rx port circuits isolated to some extent.

Opinion by some is that the modification I performed above which electrically bonds the two connectors through a brass bar of about 60mm length is likely to significantly degrade performance. Continue reading Reinforcement of nanoVNA-H connectors – performance discussion

Last update: 20th February, 2020, 5:43 PM

Windowed ladder line – single core CCS 21% IACS

There is some evidence that the common 1.024mm (#18) single core CCS windowed ladder line advertised as 30% IACS conductivity supplied recently may be closer to 21%. This is based solely on comparison of measured DC resistance with specification, but that is a strong hint that the copper cladding is less than specification.

Theoretical prediction

This article presents a theoretical prediction based o A model of current distribution in copper clad steel conductors at RF of the matched line loss (MLL) at 1.8MHz.

The assumption is a 1.024mm steel cored conductor with 30.7µm copper cladding.

Above is a plot of the predicted current magnitude and phase distribution in the conductor. Continue reading Windowed ladder line – single core CCS 21% IACS

Last update: 18th February, 2020, 6:24 AM