<?xml version="1.0" encoding="UTF-8"?>

<section xml:id="introForeknowledge" 
         xmlns="http://docbook.org/ns/docbook" 
         xmlns:xlink="http://www.w3.org/1999/xlink" 
         xmlns:xi="http://www.w3.org/2001/XInclude">

<info>
  <title>Basics: Repeater operations</title>
</info>

<para>
  This section briefly describes the common amateur radio FM-repeater operation. The majority of all
  licensed operators will be familiar with this topic. 
</para>

<para>
  If you are not yet licensed and interested into amateur radio, consider reading this section. 
  Otherwise, skip right to <xref linkend="introOrigin"/>
</para>

<para>
  The majority of connections between HAM operators are made in the so-called simplex mode. That 
  is, the two operators transmit and receive alternately on the same frequency<footnote><para>
    This is actually called <emphasis>semi-duplex</emphasis>, however the term 
    <quote>simplex</quote> stuck. The term simplex actually refers to the situation, where 
    there is only one transmitter and possibly many receivers (e.g., broadcast).</para></footnote> 
  and the connection is direct. This works very well on HF where world-wide direct connections can 
  be made.
</para>

<example>
  <title>Typical FM simplex operation</title>
  <mediaobject>
    <imageobject><imagedata fileref="fig/fm_simplex_a.png" width="7cm" align="center"/></imageobject>
  </mediaobject>
  <mediaobject>
    <imageobject><imagedata fileref="fig/fm_simplex_b.png" width="7cm" align="center"/></imageobject>
  </mediaobject>
</example>

<para>
  For this example, DM3MAT transmits directly to DL2XYZ on the frequency 144.500 MHz. The latter 
  then also answers directly on the same frequency.
</para>

<para>
  On higher frequencies, however, radio waves behave more like light and it gets increasingly
  difficult<footnote><para>Also on VHF and UHF, larger distances can be bridged using an elevated 
    location and larger antennas.</para></footnote> to bridge significant distances beyond
  the horizon. This fact limits the operating range of simple hand-held radios. To still cover 
  a large area, repeaters can be used.
</para>

<para>
  Repeaters are autonomous amateur radio stations that are usually located on a mountain,
  hill or high tower. This allows them to easily cover a large area. They receive signals from
  HAM operators and retransmit them at the same time. To do that, they cannot send and receive on 
  the same frequency (otherwise they would interfere with themselves). Therefore, repeaters operate
  in the so-called duplex mode. That is, the repeater receives on one frequency (the so-called 
  input frequency) and transmits the received signal on another frequency (the 
  output frequency) simultaneously.
</para>

<example xml:id="figIntroFMRepeater">
  <title>Simple FM repeater operation</title>
  <mediaobject>
    <imageobject><imagedata fileref="fig/fm_duplex_a.png" width="7cm" align="center"/></imageobject>
  </mediaobject>
  <mediaobject>
    <imageobject><imagedata fileref="fig/fm_duplex_b.png" width="7cm" align="center"/></imageobject>
  </mediaobject>
</example>

<para>
  For this example, DM3MAT sends on the input frequency 431.9625 MHz to the repeater DB0LDS. The 
  repeater transmits the received signal on its output frequency 439.5625 MHz. On that frequency,
  DL2XYZ receives the original call.
</para>

<para>
  The <xref linkend="figIntroFMRepeater"/> shows a common repeater operation on UHF. Here, the 
  operator DM3MAT transmits its call to DL2XYZ not directly but on the input frequency of the 
  repeater DL0LDS (431.9625 MHz). The repeater receives the call and transmits it simultaneously
  on its output frequency (439.5625 MHz). This signal is then received by DL2XYZ. Consequently, 
  the call has reached its destination, although DM3MAT and DL2XYZ may not be able to communicate 
  directly. The reply of DL2XYZ to DM3MAT follows the same path. Here DL2XYZ transmits on the
  repeater input frequency, and DM3MAT receives that call on the repeater output frequency. This 
  way the two operators can communicate with each other even if they are not able to reach 
  each other directly.
</para>


<section>
<info>
  <title>Echolink</title>
</info>
<para>
  However, there are situations, where two operators are far away and they cannot reach the same
  repeater. For these cases, it is possible to connect two repeaters via the 
  <link xlink:href="http://www.echolink.org/">EchoLink</link> network.
</para>

<example xml:id="figIntroEchoLink">
  <title>Repeater operation with Echolink</title>
  <mediaobject>
    <imageobject><imagedata fileref="fig/fm_echolink_a.png" width="10cm" align="center"/></imageobject>
  </mediaobject>
  <mediaobject>
    <imageobject><imagedata fileref="fig/fm_echolink_b.png" width="10cm" align="center"/></imageobject>
  </mediaobject>
  <mediaobject>
    <imageobject><imagedata fileref="fig/fm_echolink_c.png" width="10cm" align="center"/></imageobject>
  </mediaobject>
</example>
<para>
  DM3MAT connects repeaters DB0SP (near Berlin) and DB0LEI (near Leipzig) via EchoLink. Now, they 
  are able to communicate with each other.
</para>

<para>
  This network allows to link FM repeaters via <trademark class="trade">internet</trademark> or to 
  connect directly to a remote repeater via internet. Many FM repeaters are connected to the 
  EchoLink network, allowing for world-wide communication with simple hand-held radios. 
</para>

<para>
  Frequently, it is also possible to control a repeater over-the-air and to connect that repeater 
  to some other repeater via EchoLink. Usually, this can be done by sending the EchoLink number of 
  the destination repeater using DTMF to the local repeater. This is shown in 
  <xref linkend="figIntroEchoLink"/> above. There, DM3MAT sends the EchoLink number 662699 of the 
  repeater DB0LEI near Leipzig to his local repeater DB0SP near Berlin. Then the local repeater
  (DB0SP) will link with the destination repeater (DB0LEI) via the EchoLink network. For some 
  limited time, both repeaters will act like one logical repeater. That is, everything that is 
  received by one repeater will also be transmitted by the other. This way, the two operators 
  DM3MAT and DL2XYZ can communicate although they cannot reach a common repeater.
</para>

<note>
  <para>
    Once two repeaters are linked via EchoLink, they behave like a single repeater.
  </para>
</note>

<para>
  All over the world, there are FM repeater that are part of the EchoLink network. Therefore, it
  is possible to communicate world-wide at any time using simple hand-held radios that are as cheap as 
  40€ or even less.
</para>
</section>

</section>