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Analysis of a series of NEC-4 models of a low loss small transmitting loop at 7MHz at varying height

This article documents a series of NEC-4 models at 7MHz inspired by Paul Casper’s (K4HKX) small transmitting loop using 3″ conductor described on his web page at http://qrz.com/db/K4HKX .

The basic loop dimensions derive from 3″ (76.2mm) OD copper tube, with octagon side lengths of 27″ (685.8mm).

This series explores the effect of antenna height. (Note the models have not been calibrated to Paul’s scenario, they are stand alone models of a somewhat similar scenario for the purpose of studying the effect of height.) Continue reading Analysis of a series of NEC-4 models of a low loss small transmitting loop at 7MHz at varying height

Last update: 23rd July, 2015, 7:47 AM

Do gamma matches necessarily degrade bandwidth

Gamma match is not a broadband impedance transformer, so it must be classed as a narrowband match.

But, does that mean that ham folk-lore that it necessarily degrades antenna VSWR bandwidth is soundly based? Continue reading Do gamma matches necessarily degrade bandwidth

Last update: 12th December, 2014, 7:30 AM

The Army Loop (Patterson match)

The ARRL and other publications refer to the Army Loop or Patterson match.

Patterson described his antenna system at (Patterson 1967). Hams seem to call any configuration that uses only capacitors in the matching circuit a Patterson or Army loop, though they are incorrect.

The ARRL Antenna Book 21 has a nonsense circuit that cannot work.

Another ARRL example, one that does work

Above is a diagram from a much earlier ARRL and as far as I can ascertain, this is McCoy’s version the so-called ARMY Loop. (McCoy 1968) gives the middle capacitor as 500pF variable which would reduce the matching range. Continue reading The Army Loop (Patterson match)

Last update: 11th November, 2015, 4:56 PM

An example of Eb/N0 design with the Field strength / receive power converter

I have been asked whether the Field strength / receive power converter can be used to solve a Eb/N0 (E_b/N₀) design problem.

Eb/N0 is a method often used for specifying the relationship of signal and noise that will give adequate bit error rate in a data demodulator.

Whilst the calculator was not specifically designed for that purpose, and you cannot directly enter the desired Eb/N0, with the help of a hand calculator for simple calculations, a solution can be found. Continue reading An example of Eb/N0 design with the Field strength / receive power converter

Last update: 9th December, 2014, 8:50 AM

Exploitation of NEC’s GM card

NEC-2 and NEC-4 support a GM card to translate and rotate a structure in three dimensions.

This article gives a simple but practical example of exploitation of this often overlooked facility. Continue reading Exploitation of NEC’s GM card

Last update: 5th December, 2014, 2:44 PM

Packet loss by digi collision

It has been apparent to me that the two local digipeaters often collide.

One uses UIDIGI and therefore is conformant with the p-persistence channel access algorithm.

The other is a Argent Data T3-135 which does its own thing.

To discover how many packets are detected with bad CRC, I installed 6PACK on a TNC. 6PACK reports receive CRC failures which are logged by the Linux kernel. Data was collected for seven hours through the middle of the day to ensure that there was traffic, but that channel utilisation was low. Continue reading Packet loss by digi collision

Last update: 1st December, 2014, 4:16 PM

Another Argent Data T3 incompatibility

I have written on incompatibility of Argent Data TNCs with other devices.

In pursuing apparent packet loss, I have run up a Paccomm Tiny-2 MK-2 TNC with 6PACK firmware, and my aprx server using Linux AX25 kernel support and 6PACK driver.

It has become apparent that although the system reliably decodes packets from a multi-packet burst from VK2AMW-1, it only ever decodes the first packet of a multi-packet burst from VK2RHR-1. Frame check errors are logged. Continue reading Another Argent Data T3 incompatibility

Last update: 1st December, 2014, 4:17 PM

The KISS TNC – too simple, too stupid?

An important element of early AX.25 networks was the Terminal Node Controller (TNC). Essentially, a TNC was a packet assembler / disassembler (PAD) pretty much equivalent to the PAD of X.25 networks but adapted to AX.25, and commonly, an embedded modem.

The TNC-2 was a hardware configuration which became a de-facto standard, and various firmware packages became available each with their own advantages and disadvantages. and a range of protocols for the host connection.

One of the inventions was the KISS protocol (KISS for keep it simple stupid) from Mike Chepponis (K3MC) & Phil Karn (KA9Q), and an implementation for TNC-2. Continue reading The KISS TNC – too simple, too stupid?

Last update: 30th November, 2014, 6:12 PM

Chameleon CHA M-LOOP

Chameleon has announced a small transmitting loop, the Chameleon CHA M-LOOP. Continue reading Chameleon CHA M-LOOP

Last update: 17th April, 2015, 7:07 AM

RPi headless aprx server using soundmodem

I have posted several articles on headless APRS servers based on RPi. This article describes one based on RPi, AX25 soundmodem using a $2 USB sound card, and aprx v2.09.

Above is the server hardware. It uses the Sailer sound modem and kernel AX.25 support. This is currently configured as a RX only iGate for 30m, hence no tx audio path and no PTT (though on HF, PTT can be done simply using transceiver VOX). The small black USB module is a Belkin WLAN adapter. Continue reading RPi headless aprx server using soundmodem

Last update: 16th April, 2015, 3:46 AM