Test equipment – Page 16

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

Last update: 27th June, 2017, 11:49 AM

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.

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

Last update: 26th December, 2015, 8:45 AM

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

Last update: 7th December, 2015, 11:13 AM

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

Last update: 7th December, 2015, 9:12 AM

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.

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

Last update: 4th December, 2015, 4:06 PM

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.

Above 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

Last update: 3rd December, 2015, 10:51 AM

Proposed 60m amateur band power limit

The WIA announced on 19/11/2015:

The World Radiocommunication Conference (WRC-15) in Geneva has agreed on a secondary allocation of 5351.5-5366.5 kHz for the Amateur Service, with regional power limits of 15 watts to 25 Watts measured in effective isotropic radiated power (EIRP).

This prospect has lots of hams excited, and some wringing their hands over the power limit expressed in the release.

Compliance with this power limit (15W EIRP in this region) may well challenge most modern hams (though EIRP power limits are not new to the Amateur LCD) and perhaps if that is to be the limit, a set of simple compliance guidelines be endorsed by ACMA to assist compliance. Continue reading Proposed 60m amateur band power limit

Last update: 30th August, 2017, 5:33 PM

Trial of prototype stand alone GPS logger

An upcoming project calls for a stand alone GPS logger.

The requirement is for a GPS stream that allows correction using RTKLIB, but this trial is of a lesser GPS as proof of concept.

Above, the equipment consists here of a Ublox NEO-6M based GPS module (~A$15 incl on eBay) at left, an Openlogger (~A$15 incl post on eBay) at right, and a 12V-5V converter (~A$7 from Hobbyking) at bottom. The latter is a 5A converter, way overkill, but it was on hand. The GPS module has a 3V regulator on board for the NEO-6M chip.
Continue reading Trial of prototype stand alone GPS logger

Last update: 28th November, 2015, 12:51 PM

Return Loss sweep using IC7410, RL bridge, and RFPM1

This article demonstrates an automated Return Loss scan of an antenna using:

IC-7410 transceiver with CIV;
40dB power attenuator;
Return Loss Bridge (RLB);
RFPM1 with USB data logger (A prototype data logger for RFPM1); and
a PC orchestrating the test.

Automation facilitates:

measurement of a large number of data points;
improved accuracy by reducing the risk of recording errors; and
reducing the tedium of a measurement task.

Continue reading Return Loss sweep using IC7410, RL bridge, and RFPM1

Last update: 26th December, 2015, 9:03 AM

A cheap and cheerful data logger

I had need of a portable serial data logger for proof of concept of a supplementary data logger for an APRS tracker.

The requirement is to capture RS232-TTL data at 4800bps, 8N1 to a data file for later extraction. The logger needs to restart automatically and append new records to the existing file.

A spare Raspberry Pi2 was applied to the job as a headless data logger.

Above is the RPi2 with an inexpensive FTDI USB/RS232-TTL adapter. Only the ground and RD wires attach to the modified TinyTrak. Continue reading A cheap and cheerful data logger

Last update: 31st October, 2015, 8:05 AM