Signalink USB PTT disable

Many years ago I bought a Tigertronics Signalink USB, principally for measurements using FSM and NFM.

A nuisance with the device is the lack of a simple means of disabling PTT. This is particularly important if the system is being used for noise figure measurement with an expensive calibrated noise source which will not withstand transmitter output.

This note describes the fitting of a front panel PTT disable switch. Continue reading Signalink USB PTT disable

A simple preamp for dynamic capsules to suit radios using electret microphones

A lot of radios are made today to use an electret microphone which has a higher output than common dynamic capsules and may require a preamp to comfortably obtain sufficient audio.

This article describes a microphone + preamp intended as a drop in replacement for an electret capsule.

An electret capsule contains an integral FET preamplifier, and the common two terminal version of the capsule depends on an external load resistor (commonly around 4.7kΩ and source of supply current (commonly from 6-12V via the load resistor.

Screenshot - 10_09_2014 , 11_47_44Above is the preamp schematic from an LTSpice model. A common emitter amplifier operating in this circuit has re of about 20Ω, and so voltage gain from collector to base is very high at 3000/20=150 (assuming total collector load of 3kΩ). Continue reading A simple preamp for dynamic capsules to suit radios using electret microphones

Roger Beep fitted to MFJ-1263 microphone / rig switch

Some years ago I purchased a used MFJ-1263 microphone / rig switch with a view to using it as an ongoing test bed for my Roger Beep module.


The Roger Beep module was assembled with 6 way 90° single row header pins for easy connection and disconnection.

RB-MFJ1263-01The module was built, tested and adjusted for audio output level and encapsulated in clear heat shrink. The cable was made from parts from ready made 3 way RC servo jumpers with the wires removed and inserted into more suited back shells. Continue reading Roger Beep fitted to MFJ-1263 microphone / rig switch

End fed wires – new hams love ’em

Hams impoverished for space to erect a ‘full size’ antenna for HF pursue a range of options and end fed wires figure highly.

New hams who heard the maxim that ‘any antenna is better than no antenna at all’ will try to use bed frames, window frames, balcony railings, anything metallic to get on air. This cry for mediocrity is to prevail over a quest for understanding, ham radio as it has evolved.

The ‘net abounds with conflicting advice:

  • (Yates 2010) recommends a T50-2 powdered iron core;
  •  (VK2AVR 2014) states do not use a powdered iron toroid.. they won’t work.

They cannot both be correct. Continue reading End fed wires – new hams love ’em

TNC-X – channel access behaviour

I have been investigating ‘lost APRS packets’, a result of many root causes.

One is a perception that when I drive in the local area, I often do not hear my own digi ‘echo’ yet I find that it did receive a packet and submit it to APRS-IS. This problem is much more apparent using a TNC-X that with any other KISS TNC in its place. Continue reading TNC-X – channel access behaviour