Exploiting your antenna analyser #6

Shunt match

Now one of the methods that is often used to transform the impedance of an antenna to suit a 50Ω feed line is the shunt match.

Lets explore that with our test jig reconfigured.

Connect up the two line sections in cascade from the analyser, and terminate it with the two 50Ω loads on the tee piece. Don’t worry too much about what we have in terms of implementation, it provides a load to the analyser that presents a similar scenario to shunt matching a loaded short monopole.

So, measure the input impedance around 21MHz.

Screenshot - 10_12_2015 , 1_51_20 PM

Above is a scan with the Rigexpert AA-600 from around 21MHz. Ignore the |Z| line, it is irrelevant and confusing but I cannot switch it off, a shortcoming of the software.

What we are exploring is that as we change frequency, the parallel equivalent resistance changes at 21.275MHz above, it equals 50Ω. The full parallel equivalent is 50Ω//-j77.3. So, if we were to make a small inductor of 77.3Ω reactance (L=X/(2*pi*f)=580nH) and connect it in shunt, the resulting impedance will be 50+j0Ω. Continue reading Exploiting your antenna analyser #6

Exploiting your antenna analyser #5

Measure MLL using the Rin where X=0

Another method of estimating Matched Line Loss (MLL) from measurement is using the input resistance of a section that is an odd or even number of quarter waves in electrical length.

I say estimate because this method depend on an assumption of the value of Zo, and using purely real nominal Zo introduces some error.

The required length can be approximated by fining a frequency where X passes through zero. Again, this method is an approximation.

Simple formula

There is a simple formula published in many ham handbooks:

MLL≈8.686*Rin/Zo/length dB/unitlength

It is, a discussed at Measuring matched line loss, a crude approximation (and should be written with ≈ rather than =).

Better formula

A better formula is one I developed though it may not be novel:

MLL=-10log|(Rin-Zo)/(Rin+Zo)|/length dB/m

It is exact, but there is error introduced in using nominal Zo.

In practice

Low Z measurement

Lets measure Zin of our 4m o/c line section, and find the lowest frequency where X passes through zero, and note the value of Rin.

Screenshot - 10_12_2015 , 2_01_52 PM

Above is a wide sweep, the frequency we want to focus on is around 13MHz. Continue reading Exploiting your antenna analyser #5

Exploiting your antenna analyser #4

Measure MLL using the half ReturnLoss method

Again in the theme of measuring something known, let us determine the matched line loss (or normally quoted attenuation) of our cable at 3.5MHz.

To make the measurement, just connect the two line sections in cascade with a joiner, and one end on the instrument, other end open circuit, and measure ReturnLoss.

Most analyser manuals and lots of helpful articles in journals and handbooks will tell you that MLL=RL/(2*length) where RL is the ReturnLoss of an open circuit or short circuit line section (the only requirement is that the ρ=1 at the line end).

Screenshot - 10_12_2015 , 8_20_14 AM

Wow, that is so low, and using the traditional formula:

MLL=0.02/(2*4)=0.0025dB/m.

Of course we are measuring way low in the instrument’s capability and there is some considerable uncertainty… but when we consult a good transmission line loss calculator, we expect around 0.029dB/m… that is 12 times what we measured. Continue reading Exploiting your antenna analyser #4

Exploiting your antenna analyser #3

The sign of reactance

At Exploiting your antenna analyser #2, the matter of determining sign of reactance was mentioned.

If you have an analyser that does not measure the sign of reactance, this information should be important to you.

Screenshot - 9_12_2015 , 8_24_15 AM

Above is a Smith chart plot of  measurements from 15MHz to 25MHz.

One can see that the locus of Zin on the Smith chart forms an arc, and the points on the arc rotate clockwise about the arc centre with increasing frequency. Continue reading Exploiting your antenna analyser #3

Exploiting your antenna analyser #2

Reconciling the single stub tuner results

So having found that the length of the RG58 lines sections are both 1.98m (approximately 2m), let’s try to reconcile measurement and prediction of Zin at 9MHz.

The examples discussed in this series of articles are designed for the test jig as described at Exploiting your antenna analyser #1 with a pair of nominally 2m length RG58 patch leads, a pair of 50Ω loads and some tee pieces and adapters to connect it all up. If you duplicate it, you will experience the same learning opportunities (the examples are structured). If you presume to redesign the experiment, your outcome will probably be different.

Before you read on, take a measurement of Zin at 20MHz and write down the impedance value. Do whatever you do to determine the sign of the reactance. If your instrument displays the sign properly, use it, otherwise use the method in your user manual or whatever wisdom you trust.

Done that? If so, read on… Continue reading Exploiting your antenna analyser #2

Exploiting your antenna analyser #1

I often see posts in online fora by people struggling to make sense of measurements made with their antenna analyser.

This article is about exploitation of a modern antenna analyser beyond its capability as a self excited VSWR meter. The latter is fine, and it is often not only all you need, but the best tool in optimising some antenna systems… but if you want to exploit the other capabilities of the instrument, read on.

Great benefit can be obtained by measuring some known loads, and reconciling the measurement with the known.

A quite simple set of equipment can be used to create a scenario rich with opportunity to prove your understanding of the basics of complex impedance, transmission lines, and measurement. Lets explore a simple example.

AnTestJig01

Above is a test jig. It is two equal lengths of identical coax connected to a tee piece on the analyser. The end of one piece of coax has a tee piece with two nominal 50+j0Ω loads as used on Ethernet 10base2 networks. The analyser is a Rigexpert AA-600. Continue reading Exploiting your antenna analyser #1

Expression of VSWR as a simple decimal real number

Voltage Standing Wave Ratio is the ratio of the voltage maximum (antinode) to the adjacent voltage minimum (node) on a transmission line. (This assumes a fully developed standing wave, that the maximum and minimum are not forced by end of line). Continue reading Expression of VSWR as a simple decimal real number

VSWR=1 and X≠0

Another of those threads has broken out on eHam illustrating that lots of hams do not understand the complex nature of impedance and cannot see the consequences of the formula to calculate VSWR from load impedance and transmission line characteristic impedance.

Most methods of measuring VSWR are indirect, and they are based on an assumed Zo which is purely real (ie Xo=0Ω), and we speak loosely of that as the VSWR even though the standing wave that might exist on a practical transmission line is a little different as a consequence of that assumption being a little bit in error. Continue reading VSWR=1 and X≠0

AUT – MobileOne M40-1 40m helical

This article describes an Aerial Under Test (AUT) that features in some of my experiments and write ups and is subject of some current experiments. It is a MobileOne M40-1 helically loaded vertical for 40m installed in the car roof. It is in the style of the popular US antenna, the Hamstick, but this is a little longer and the results are not directly applicable.

I hasten to add that this configuration is not suited to travelling, it is just a rather ideal mounting of a helically loaded whip without the questions that arise from the effects of roof racks, bumper mounts etc.

M40-1

The M40-1 is fitted in the centre of the station wagon roof, the roof is 1.5m above ground and the antenna is 1.5m long including a 200mm unloaded tip (tip of the antenna is highlighted with a pink dot). (The setting is not the test site.) Continue reading AUT – MobileOne M40-1 40m helical

RC PWM – ON/OFF switch

This article describes a remote ON/OFF switch which uses an RC receiver and adapter chip to convert the RC PWM signal into an ON/OFF output. (Suitable RC transmitters are on hand.)

The immediate application is for remote ON/OFF PTT or KEY of a transmitter for field strength testing at various locations.

Background

Remote control hobbies have long used a multi channel digital proportional protocol for control of planes etc. The simplest multi channel receiver has an independent PWM output for each servo.

The PWM signal is a 1000-2000µs pulse with a repetition rate from about 50Hz up to 500Hz or so, the duration of the pulse conveys the information.

Implementation

The converter chip is a ATTiny25 MCU with firmware that monitors the PWM stream and provides ON/OFF and OFF/ON output pins. For the immediate application, the ON/OFF (or non inverted) output drives a 2N7000 FET with ‘open collector’ output suited to the PTT and KEY lines of most modern transceivers.

The firmware ignores PWM signals with duration outside the range 900µs to 2100µs, and switches ON at 1600µs, and OFF at 1400µs to provide some hysteresis. If PWM input is lost for 125ms, the output will fail safe OFF.

rcsw01Above is the schematic. The 2N7000 is good for 60V, can handle up to 100mA without a heat sink, and had a body diode to absorb transients if the load is a relay. Continue reading RC PWM – ON/OFF switch