APRS: does pre-emphasis make much difference

This morning I have been observing a weak APRS signal from a digipeater located on a prominent hill about 200km away, VK1RGI-1.

VK1RGI-1 is characterised by having a pre-emphasised transmission, but about 1.5KHz deviation, about 6dB low.

Nevertheless, VK1RGI-1 packets can be fairly reliably decoded at a strength that does not show any segments on the IC2200H S meter, less than -115dBm (by measurement). Continue reading APRS: does pre-emphasis make much difference

APRS: how popular are Kenwood trackers

APRS has evolved to be all things to all men.

It inventor, Bob Bruniga, generalised the original meaning Amateur Position Reporting System to Amateur Packet Reporting System as he extended APRS to include all manner of generalised broadcast and point to point messages.

Perhaps in doing that,it was recognition of the inability for many reasons of APRS to provide reliably good position reporting performance in practice.

Any thinking person understands that there is conflict between use for timely reporting of position and loading the radio channel up with generalised message traffic, or even extending the RF coverage of a position report by repeating it many times into adjacent precincts. But simple minds muttering “use it or lose it” ruin APRS for position reporting by sending repeating traffic without a demonstrated demand.

Attempts to emphasise position reporting performance tend to be opposed in my experience mostly by owners of Kenwood trackers (eg DM-710), and the weather propagators. Many Kenwood owners like to see position reports from a very wide area repeated into the local area so that they can be displayed on their radio. There are other features that generate traffic that appeal to Kenwood owners.

This article looks at how significant the Kenwood users are in terms of unique packets reaching APRS-IS, the basis of mapping for most users these days. Continue reading APRS: how popular are Kenwood trackers

Is the W5VJB J dipole novel

A reader of my article Are gamma matches as bad as all that asked whether the W5VJB dipole was a gamma match given the hint in the article that the traditional gamma tuning capacitor is not essential to a gamma matched antenna.
Screenshot - 10_11_2014 , 06_50_04
Above (Britain 2006) is the subject dipole which Britain describes as a partial folded element having a J shape which is grounded at the midpoint of the longest portion of the element. ((Duffy2010b) refers to it as a half folded half wave dipole.)
Continue reading Is the W5VJB J dipole novel

Are gamma matches as bad as all that

Seeing the comment recent online comment about a gamma match as I have noted from research online, NEC does not support modelling of the match component reminds one of the unreliability of online sources. This appeal to non-authority is fallacious, this writer writes as if it is fact that NEC does not support modelling a gamma match, and that is quite wrong.

NEC has limitations on geometry elements relative to each other and to wavelength, and those apply not just to the gamma match, but the entire model. It is the modeller’s challenge to stay within those limitations.

This article documents an NEC model of a real antenna and the model reconciles with the real antenna. Continue reading Are gamma matches as bad as all that

Where is the best place to measure feed point VSWR

The subject question is often asked, and the usual responses are mindless recitals of Rules of Thumb (RoT).

In the light of the discussion at Feed line length affect on VSWR and The half waves of coax rule for measuring VSWR accurately, lets consider the subject question and develop a rational answer. Continue reading Where is the best place to measure feed point VSWR