VSWR ripple

Having seen some recent discussion where the online experts opined that an example given of a VSWR plot that contained a fairly consistent ripple was quite normal, this article suggests there is an obvious possible explanation and that to treat it as quite normal may be to ignore the information presented.

Above is a partial simulation of a scenario using Rigexpert’s Antscope. It starts with an actual measurement of a Diamond X-50N around 146MHz with the actual feed line de-embedded. Then a 100m lossless feed line of VF=0.66 is simulated to produce the plot that contains a ripple apparently superimposed on an expected V shaped VSWR curve.

This is the type of ripple that the expert’s opine is quite normal. Continue reading VSWR ripple

nanoVNA-H – woolly thinking on MLL measurement

There is little doubt that the nanoVNA has made VNAs very popular in the ham community, possibly more so that any other device.

Eager owners are trying to apply them to solve lots of problems, often without sufficient knowledge or experience to properly inform the measurements.

An example that has a appeared a few times on online forums in the last weeks is measuring the matched line loss (MLL) of a section of RG6 coax… to inform a decision to discard it or keep it.

The common approach is to use a measurement of |s11| and to calculate Return Loss and infer the MLL.

DUT

For discussion, lets consider an example of 30′ of Belden 1694A RG6 solved in Simsmith. We should note that unlike most RG6 in the market today, this uses a solid copper centre conductor.

Short circuit termination

Some authors insist that the half return loss method is to be performed using a short circuit test section. Bird does this in their Bird 43 manual.

Above is a plot of calculated |s11| (-ReturnLoss) from 1 to 20MHz for the test section. The three plots are of |s11| wrt 50Ω, 75Ω and frequency dependent actual Zo (as calculated for the model). The cursor shows that the actual |s11| is -0.37474dB (ReturnLoss=0.37474dB). Using the half return loss method MLL=ReturnLoss/2=0.37474=0.187dB/m. Continue reading nanoVNA-H – woolly thinking on MLL measurement

RG6 cladding thickness report

N0TZU recently report his perception that a length of Logico COX3520 RG6 Quad cable he purchased exhibited higher than expected Matched Line Loss (MLL) at 10MHz.

Most RG6 type cable sold these days at low cost uses a copper clad steel centre conductor, and much of it has insufficient copper cladding thickness for copper like performance at HF.

Above is a pic N0TZU gave of the centre conductor cross section. It is possible to measure the cladding thickness from the pic knowing that the overall diameter is 1.024mm. The copper thickness measured 13.7µm, lets round it to 14µm. Continue reading RG6 cladding thickness report

MFJ ATU hand effects on capacitor knobs

The problem

Users of some ATUs may have noticed particular sensitivity to hands on the capacitor adjustment knobs. It is a common problem with cheap implementations of the T match as the capacitor rotor is usually at high RF voltage and if that shaft is extended to the adjustment knob, under certain circumstances tuning becomes very sensitive to hands on the knobs.

In some of these implementations, if the users hand touches the metal grub screw in the knob, or the metal panel bushing behind the knob they may get a significant RF burn.

The cause

Let’s use the MFJ-949E as a discussion example. It is a T match, and the metal capacitor shafts in the knobs and panel bushings carry RF voltages.

So why is this only sometimes a problem?

The RF voltage across the coil, and impressed on the capacitor shafts can be extremely high when using loads with small resistance and large negative reactance, more so on the lower bands. Continue reading MFJ ATU hand effects on capacitor knobs

WIA 4:1 current balun – further measurements

4-101a

I mentioned in my article WIA 4:1 current balun that the use of a single toroidal core in the above graphic compromises the balun. This article presents some simple measurements and analysis that question whether the balun works as so many users think.

The popularity of the balun derives from the work of VK2DQ and is often known as the VK2DQ 4:1 current balun (though probably not his invention).

Analysis at the limits

Often, analysis of a network as frequency approaches zero or infinity can simplify the analysis whilst allowing a reasonable test of the sanity of the design.

Above is a conventional transformer schematic of the WIA 4:1 current balun on a perfectly symmetric (balanced) load. At frequencies where the electrical length of each winding is very short, we can assume negligible phase delay along or between windings, simplifying analysis greatly. Continue reading WIA 4:1 current balun – further measurements

A common mode choke for a VDSL pair – LF1260 core

This article describes a common mode choke intended to reduce RF interference with a VDSL service.

The MDF is located where the underground cable enters the building. From here it rises vertically and travels some 25m across the ceiling to the VDSL modem. Continue reading A common mode choke for a VDSL pair – LF1260 core

nanoVNA-H – measuring an inductor – is it a no-brainer?

Let’s explore measurement of a test inductor with the nanovna.

Above is the test inductor, enamelled wire on an acrylic tube.

An online expert’s advice make this task look like a no-brainer:

For a 100 nH inductor you are probably using an air wound coil so you won’t see that much change in inductance with frequency. However, inductors made with toroids will because the permeability of the core goes down with frequency.

So, this is an air cored inductor, permeability is approximately that of free space, a constant 4πe-7 independent of frequency. Nevertheless we will see that apparent inductance can change with frequency. Continue reading nanoVNA-H – measuring an inductor – is it a no-brainer?

nanoVNA-H – continuing USB-C repair

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.

Anyway… it finally arrived after many months. A pack of 10 sockets cost $2.30 incl shipping, so it gives one a fair idea of how cheap the low grade connector that was used would have come.

Above, the replacement USB-C socket soldered in to the board without removing the display. The SB1 pad lifted of the board during removal of the old socket, no connection is made to it, so no harm done. Continue reading nanoVNA-H – continuing USB-C repair

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