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The Complaint That Wasn’t

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Jun 092011

The Society recently received a complaint from a new member that another LPFM Station was swamping his signal. A visit by a Society member revealed a different story. The Society member reported back:

  • The problem is in your antenna/coax
  • Impedance was 24 ohms. should be 50
  • SWR 2.1:1. should be 1.5: max. (But  better if closer to 1.1:1)
  • Z=10. Z should be as close to 0 as possible but certainly below 3 or  4
  • The coax RG58 is 6 years + old and looks a bit suspect.
  • the connections to the JPole appear to be perished and probably letting water into the cable..
  • I would remake the RF choke with 4 turns at 100 mm dia rather than the 2 turns as at present.

It has been reported that the station is now back on air. Another great result from the Society.

 Posted by on 09 Jun 2011 at 08:03

Samson Audio Meteor Mic

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Apr 032011

Future of Audio Recording?Looking for a new way to record content for your radio station?. Well look no further. Samson Audio have released the Meteor Mic based loosly on the look and feel of the Shure 55SH. Connect the Meteor Mic  to your iDevice running MultiTrackDAW from Harmonicdog and you have a full digital multitrack with portable microphone.


Pira 75 Analyzer

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Nov 282010

One of the most challenging elements of FM broadcasting is accurately setting up modulation levels. Schedule 1 h of the GURL specifies

(h) Unwanted emissions shall not exceed the limits as follows;

Unwanted Emission            Frequency Range                    Spectrum Analyser
Limits – in the                         from carrier                              Resolution  Bandwidth
resolution bandwidth

-25 dBW                                   128 to 240 kHz                          Not to exceed 10 kHz*
-35 dBW                                   240 to 600 kHz                         Not to exceed 100 kHz*
-56 dBW                                   > 600 kHz                                    100 kHz

* The spectrum analyser shall be set for 5 minute maximum hold.

But what does all this mean? I can guarantee most LPFM broadcasters just plug everything in and adjust the volume to sound similar to another station. The danger of this approach is that many stations use specialised and expensive processing. High volumes are obtained by reducing the dynamic range. Trying to match the volume of a commercial radio station will inevitably lead you to breaching the GURL. With a PIRA75 modualtion monitor you can set your modualtion levels correctly. It will also assist with the ability to deal with interference complaints. This will be available to members.

Nov 262010

As an added service to members “The Society” has available to borrow a MFJ-269 antenna analyser. This analyzer allows you to verify that the impedance of your antenna is correct thus ensuring all your power is actually radiated. A request form will soon be available in the members section of this site

Covers 1.8 to 170 MHz and 415 to 470 MHz — an MFJ-269 Exclusive! It includes all the features of the MFJ-259B, the worlds most popular SWR Analyzer plus much more! Large easy-to-read two line LCD display and side-by-side meters clearly display your information Built-in Ni-Cad/Ni-MH charger circuit, battery saver, low battery warning, smooth reduction drive tuning. Use as signal source for testing/alignment. Built-in CoaxCalculator — calculates coax line length in feet given coax length in electrical degrees and vice versa for any frequency and any velocity factor (great for building matching sections and phasing lines) — an MFJ-269 Exclusive! New 12-bit A/D converter gives much better accuracy and resolution than common 8-bit A/D converters — an MFJ-269 exclusive. Super easy to use! Just set the bandswitch and tune the dial — just like you transceiver.
1.8 to 170MHz range features
MFJ-269 gives you a complete picture of you antenna. Read antenna SWR and Complex Impedance 1.8 – 170 MHz. Built in frequency counter. Read Complex Impedance as series equivalent resistance and reactance (Rs+jXs) or as magnitude (Z) and phase (degrees). Also read parallel equivalent resistance and reactance (Rp+jXp) — and MFJ-269 exclusive! You can determine velocity factor, coax loss in dB, length of coax and distance to short or open in feet.
You can read SWR, return loss and reflection coefficient at at any frequency simultaneously at a single glance. Also read match efficiency. Also read inductance in UH and capacitance in pF at RF frequencies. You can measure SWR and loss of coax with any characteristic impedance from 10 to over 600 Ohms, including 50, 51, 52, 53, 73, 75, 93, 95, 300, 450 Ohms — an MFJ-269 exclusive! For more information see MFJ-259B. New 415 to 470 MHz Range features. Just plug in your UHF antenna coax, set the frequency and read SWR, return loss and reflection coefficient simultaneously. You can read coax cable loss in dB and match efficiency.
You can test and tune stubs and coax lines. You can manually determine velocity factor and impedances of transmission lines. You can adjust and test RF matching networks and RF amplifiers without applying power. Has easy-to-read LCD logarithmic SWR bargraph and SWR meter for quick tuning.

Take it anywhere
Fully portable take it anywhere — remote sites, up towers, on DX-peditions. Built in “N” connector Free N-to-SO-239 adaptor. Use 10AA Ni-MH or Ni-Cad or alkaline batteries (not included.) or 110VAC with MFJ-1312D, $14.95


Coverage map service now available to members

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Nov 132010

LPFM Society members can now request coverage maps here. If you are not a member you can join here. All coverage maps will be posted here unless a full member requests privacy.

Here is a YouTube video to show how to create and export a KMZ