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<!DOCTYPE html>
<html lang="en-GB">
<head>
<meta charset="utf-8" />
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<title>RX and TX offsets — OscarWatch Help</title>
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<a href="index.html" class="brand-home" title="Home"><img src="oscarwatch-icon.png" alt="" class="brand-icon" width="56" height="56" /></a>
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<h1>RX and TX offsets</h1>
<p class="tagline">When to nudge listen and transmit frequencies</p>
</div>
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<main>
<p class="lead">
Offsets are small shifts added to the <strong>nominal</strong> downlink and uplink frequencies shown in the frequency panel.
They are saved <strong>per satellite</strong> so your favourite AO-91 spot stays remembered next pass.
</p>
<h2>What you see in the panel</h2>
<ul>
<li><strong>Downlink / Uplink</strong> — centre frequencies from the satellite database (plus Doppler when tracking).</li>
<li><strong>RX offset</strong> — added to the receive frequency sent to the radio.</li>
<li><strong>TX offset</strong> — added to the transmit frequency.</li>
</ul>
<p>
Doppler correction is separate — OscarWatch moves the “centre” as the satellite passes.
Offsets are <em>your</em> choice of where to sit inside the passband.
</p>
<h2>When to use RX offset</h2>
<table>
<thead>
<tr><th>Situation</th><th>What to do</th></tr>
</thead>
<tbody>
<tr>
<td><strong>Linear SSB/CW</strong> — signal not on the nominal frequency</td>
<td>Turn the <strong>receive</strong> dial until you hear the station, or set RX offset in the panel. OscarWatch can <a href="radio-doppler-tuning.html#passband-knob">learn from your dial</a> on many rigs.</td>
</tr>
<tr>
<td><strong>FM downlink</strong> — repeater sounds off-centre</td>
<td>Small RX offset (a few kHz) if the transponder passband is wide and you want a clearer spot.</td>
</tr>
<tr>
<td><strong>ISS SSTV / beacon</strong></td>
<td>Only RX matters — adjust RX offset until the picture or beacon is clean.</td>
</tr>
<tr>
<td><strong>Testing</strong></td>
<td>Leave offsets at 0 until you are on the air; then tweak once you hear yourself or a reference.</td>
</tr>
</tbody>
</table>
<h2>When to use TX offset</h2>
<table>
<thead>
<tr><th>Situation</th><th>What to do</th></tr>
</thead>
<tbody>
<tr>
<td><strong>Linear transponder</strong> — you hear yourself off-frequency</td>
<td>Adjust TX offset so your downlink echo lines up with others (often a few kHz on SSB).</td>
</tr>
<tr>
<td><strong>FM uplink</strong></td>
<td>Usually leave TX offset at 0 if the database uplink is correct; use offset only if the channel plan for that bird needs a fixed shift.</td>
</tr>
<tr>
<td><strong>Split across two radios</strong></td>
<td>Each leg still uses the same logical uplink — TX offset applies to the uplink radio’s frequency.</td>
</tr>
</tbody>
</table>
<h2>Offsets vs spinning the dial</h2>
<p>
On linear passes, many operators <strong>hunt by ear</strong> on receive first.
OscarWatch remembers that shift as an offset so Doppler can continue without losing your spot.
Typing offsets in the panel does the same thing if you already know your favourite kHz correction.
</p>
<p>
On FM with CAT locking the dial, use the <strong>offset fields</strong> (or Pause CAT) instead of expecting to spin the knob during tracking.
</p>
<h2>Resetting</h2>
<ul>
<li>Clear RX/TX offset fields to <strong>0</strong> to return to database centres (Doppler still applies).</li>
<li>Offsets persist in your settings for that satellite — handy for regular birds, but reset when you change transponder mode.</li>
</ul>
<div class="tip">
<strong>More detail on the panel:</strong> modes (Voice/CW/FM), Doppler scope (Full / RX / TX), and CTCSS are covered in <a href="frequencies.html">Frequencies</a>.
</div>
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