## Power standing wave null… solution

Power standing wave null… more left readers with “homework” to create the Pfwd and Prev traces.

Remember that Pfwd and Prev are interpretations in the context of some Zref of V and I at a point, and that $$P=P_{fwd}-P_{rev}=V_{fwd}I_{fwd}-V_{rev}I_{rev}$$ is valid ONLY if Zref is purely real.

So let’s plot Pfwd and Prev wrt:

• Nominal Ro (ie the real part of the nominal Zo of the RG58A/U at 10MHz);
• 50Ω; and
• 75Ω to demonstrate the effect of different contexts, ie Zref.

Above IndFwd50 and IndFwdRo are almost coincident (at 10MHz Nominal Ro is very close to 50Ω), as are IndRev50 and IndRevRo. IndFwd75 and IndRev75 are separated from the others. In all cases, the IndFwdxx-IndRevxx is equal to p.

Note that if Zref is close to the line Zo, the shape of Pfwd and Pref are essentially a logarithmic decay in the direction of wave travel with a small superimposed cyclic variation.

If Zref is quite different to line Zo, the exponential element still exists but with a much larger cyclic variation along the line.

Above is a model of load VSWR=10, and p, Pfwd and Prev wrt 50Ω, and there is still the exponential element and only a relatively small cycle variation.

So, from those we learn that if you were to insert a 50Ω directional wattmeter at various points along a nominal 50Ω line, even with high VSWR, there will be only a small cyclic variation with displacement  and the exponential decay will be more significant.

The cyan trace is the voltage along the line, and you may observe that the ratio of max to min near to load is almost 10, VSWR at the load is 10, but measuring the first peak brings a little line loss to bear. The solid magenta curve is 50 times the current (so that it is viewable under the right hand axis scaling).

So, under mismatch there may be a wide variation in voltage and current along the line, but that will not be so apparent on a directional wattmeter which responds to both current and voltage and is sensitive to their phase relationship.

## A point for pondering

Noting that in this example there is a small standing wave (VSWR=2), and that whilst Matched Line Loss (MLL per meter is uniform along the line, loss under standing waves is not uniform along the line. So the popular graphs that give you (uniform) line loss under standing waves based on VSWR and MLL are unsound, they are an approximation based on usually unstated assumptions.

The download below contains the original SimNEC model, and a revised one with the above calcs and traces added.

SWDisplacement.7z

## Power standing wave null… more

Power standing wave null? discussed the “Power Standing Wave” concept unfolding on social media.

Already a correspondent has asked if the graphs given in Power standing wave null? can be replicated in SimNEC.

They can. The original Mathcad graphs were wrt displacement from the source along the line to the load, and the sign of displacement is -ve (consistent with the Telegraphers Equation). So, that requires a bit of manipulation in SimNEC, and because SimNEC does not allow us to sample a TL element at an arbitrary displacement, the following model uses two TL elements of overall length 30m, and by adjusting the length of each we can move the observation point (T1 input).

The calculations of lengths and power are visible in the popups. Continue reading Power standing wave null… more

## Power standing wave null?

A social media posting in a very long thread with a lot of wooly thinking recently contained this explanation:

If you locate your power meter anywhere along the feedline other than at a POWER standing wave null, you will get a reading that is higher than the amount of power being delivered to the load.

A “Power Standing Wave”… hmmm, that is new to me.

The ensuing discussion may discuss this notion, probably in terms of lossless lines.

(Duffy 2008) develops several plots of interesting quantities with a load of 5+j50Ω on a length of RG58A/U using the  Telegraphers Equation.

Above is a plot from (Duffy 2008) Above shows P(x) vs displacement x, -ve x is on the source side of the load, at 10MHz with a load of 50+j50Ω where the modelled Zo is 50.4-j0.7Ω. Note that loss under mismatch is not uniform, the slope of P(x) varies with x. Continue reading Power standing wave null?

## An improvised intake block off adapter for the Husqvarna 372 XP X-TORQ etc

A really important test of two stroke engines is the crankcase vacuum and pressure test, as air leaks through crankshaft seals, intake boot, cylinder gasket, decompression valve, impulse tubes etc can cause them to run lean leading to premature failure of the engine.

To perform a vacuum and pressure test of the Husqvarna 372 XP X-TORQ chainsaw engine, the intake needs to be blocked off, and the exhaust blocked off, and the test can be performed using the impulse hose.

The intake boot in the Husqvarna 372 XP X-TORQ has a rubber tongue that projects into slots in the carburettor throat, part of the strato engine.

Above, Husqvarna 578-09-56-01 intake block off adapter for the Husqvarna 372 XP X-TORQ. The open end of the ‘tube’ mates with the rubber intake boot, and the recess accommodates the tongue mentioned. Continue reading An improvised intake block off adapter for the Husqvarna 372 XP X-TORQ etc

## NEC model of 600mm a side square loop for field strength measurement

This article Reconciliation of transmitter power, EIRP, received signal strength, antenna factor, ground wave propagation etc @ 576kHz used a 600mm a side square loop which was originally designed for field strength measurements on the 40m in an effort to understand and document BPL (PLC) emissions.

As part of validation of the antenna, a free space NEC model with external excitation was developed. This article publishes a graphic summary of the antenna characteristic. The model antenna is loaded with 50+j0Ω and includes 10m of RG58A/U which was used for the BPL related measurements with FSM. Continue reading NEC model of 600mm a side square loop for field strength measurement

## Crystal substitute using si5351 – part 3

Continuing the Crystal substitute using si5351 series…

Above is an example pair of inexpensive modules, less than \$10 for the pair (incl shipping). Both boards are powered from 5V, the left hand module is a ATTiny85 dev board, it has a small 3.3V regulator on board. The dev board uses a DIP chip, so it can easily be programmed in a device programmer and then inserted in the socket. Continue reading Crystal substitute using si5351 – part 3

## Traditional directional wattmeter uncertainty due to coupler Directivity

NanoVNA – how accurate does the LOAD need to be – part 1? discussed the importance of the calibration parts to accuracy of the NanoVNA.

Let’s digress for a moment at look at a directional wattmeter, a traditional way of measuring ReturnLoss and VSWR. This article examines the effect of coupler Directivity alone on uncertainty. There are other contributions to uncertainty, they are outside the scope of this article.

## Directivity

Let’s review the meaning of directional coupler Directivity.

Above is a diagram from Mini-circuits. Continue reading Traditional directional wattmeter uncertainty due to coupler Directivity

## New e-delay feature in DisLord NanoVNA-H4 firmware v1.2.30

Recent articles discussed the use of e-delay to approximately compensate for cables connecting the DUT to the reference planes.

NanoVNA has had provision for an e-delay compensation for some time, it is a single value that is used to correct the s11 and s21 measurements.

It is very commonly the case that the optimal e-delay values for s11 and s21 compensation are different, so one needed to save a .s2p file for each of the two values and then merge the s11 measurements from its file with the s21 measurements from its file. The requires some work and risk of error.

I recently suggested to DisLord that provision be made in his NanoVNA firmware for specification of separate e-delay values for s11 and s21. He took the suggestion up and with days delivered a beta version with that facility.

In this article, unless stated otherwise, reference to |s11| and ReturnLoss are to those quantities expressed in dB. Note that |s11|=‑40dB is less than |s11|=‑20dB. ReturnLoss and |s11| are related, ReturnLoss=‑|s11|.

Loss terms used are as defined at Measurement of various loss quantities with a VNA. Continue reading New e-delay feature in DisLord NanoVNA-H4 firmware v1.2.30

## NanoVNA – how accurate does the LOAD need to be – part 1?

A reader of EFHW transformer measurement – how accurate does the load need to be? asked whether the discussion applies more generally, in particular to the loads used for calibration and measurement with a VNA.

In this article, unless stated otherwise, reference to |s11| and ReturnLoss are to those quantities expressed in dB. Note that |s11|=-40dB is less than |s11|=-20dB. ReturnLoss and |s11| are related, ReturnLoss=‑|s11|.

## Measurement 1

As a basis for discussion, let me offer an example measurement.

Above is a scan of a certain DUT after SOLT calibration of the NanoVNA. Continue reading NanoVNA – how accurate does the LOAD need to be – part 1?