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

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.


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.


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

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 (1W 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

QRP antennas

We see more and more reference to “QRP antennas” online these days, and it begs the question, what makes an antenna more or less suited to QRP.

To a novice, the obvious possibilities for a low power antenna system are that they are:

  1. highly efficient to offset the lower power; and/or
  2. unable to withstand higher power.

Continue reading QRP antennas

Does VSWR damage HF ham transmitters

This Jan 2011 article has been copied from my web site which is no longer online. It is for reference in further articles discussing the popular reflections explanations. The article may contain links to articles on that site and which are no longer available.

The statement is often made to the effect that:

VSWR will damage a HF ham transmitter, and the mechanism is that the ‘reflected power’ in a standing wave will be absorbed by the Power Amplifier (PA), increasing heat dissipation and damaging the PA.

There are two problems with this statement: Continue reading Does VSWR damage HF ham transmitters

Matching a quarter wave monopole with two variable caps

Two recent correspondents have discussed matching a quarter wave monopole with two variable caps.

Two capacitor shunt/series match

The matching scheme involves a shunt variable cap at the end of the coax feed line, and a series variable cap to the monopole base. The radials are of course connected to the feed line shield.

This type of matching scheme requires that the monopole feed point has sufficient +ve reactance, ie the monopole is longer than resonant. Lets assume the R component of feed point Z is 35Ω.

This scheme incorporates the simple shunt match, and the value of the shunt capacitor can be found knowing the R value to be matched to 50Ω.

Screenshot - 08_07_2015 , 06_43_07

Above is a Smith chart of a model of the match at 14MHz. The monopole has been lengthened to have 100Ω reactance along with 35Ω resistance. In this case a series cap of 148pF and shunt cap of 150pF are required. Continue reading Matching a quarter wave monopole with two variable caps

Baofeng GT-3TP MkIII review

This article is a review of the Baofeng GT-3TP MkIII, a hand held 2m/70cm FM transceiver.

The radio is supplied with a bunch of useful accessories (even if the power supply lacks the necessary Australian approval) and a brief and inadequate manual.


Above, the Baofeng GT-3TP purchased for about A$85 delivered overnight from Sydney. Continue reading Baofeng GT-3TP MkIII review

Feeding at a current maximum, and three other options

Feeding at a current maximum visited the common practice of designing to feed a multi band dipole with open wire feed at or very near to a current maximum.

I explained that feeding at the current maximum may provide sub-optimal performance on the popular T-match ATU as its losses tend to be worst with low R loads, aggravated by the use of 4:1 baluns for even lower R.

On the other hand, feeding at a voltage maximum might exceed the ATU’s voltage capacity, or perhaps be outside of the matching range of the ATU.

Well if neither of these is optimal in all cases, what about half way between. It has been done, the odd eighths wave feed line on an 80m half wave is another of the recipes you will hear.

Lets explore the options of a half wave dipole at 3.6MHz with four different feed line lengths (Wireman 551). Continue reading Feeding at a current maximum, and three other options