
<p>
<a href="hpr4526_image_1.png">
<img src="hpr4526_image_1_tn.png">
</a>
</p>
<p>
<br>
</p>
<p>
<a href="hpr4526_image_2.png">
<img src="hpr4526_image_2_tn.png">
</a>
</p>
<p>
<br>
</p>
<p>
<a href="hpr4526_image_3.png">
<img src="hpr4526_image_3_tn.png">
</a>
</p>
<p>
<br>
</p>
<p>
Goal: send a voice signal on VHF or UHF,  receive and record  with SDR via RTL dongle</p>
<p>
<br>
</p>
<p>
<u>
Core Problem and Resolution</u>
</p>
<p>
<br>
</p>
<p>
An issue may have been a <strong>
hardware I/Q Imbalance</strong>
&nbsp;in the RTL-SDR, which suppressed the desired signal, resulting in very low output volume that required excessive manual amplification.</p>
<p>
<br>
</p>
<ul>
<li>
<strong>
Initial Problem:</strong>
&nbsp;Very quiet, barely audible recorded audio, initially on UHF 433 MHz</li>
<li>
<strong>
Key Discovery:</strong>
&nbsp;An&nbsp;<strong>
I/Q imbalance</strong>
&nbsp;caused a repeating signal "image" in the spectrum,</li>
<li>
<strong>
Frequency Change:</strong>
&nbsp;Switched to the&nbsp;<strong>
VHF 145.500 Mhz</strong>
&nbsp;band, </li>
<li>
<strong>
Gain Override: </strong>
To overcome the defect and the failing AGC, gain stages were manually adjusted:</li>
<li>
<strong>
Squelch:</strong>
&nbsp;<strong>
Squelch (SQL) was set to OFF (0)</strong>
&nbsp;to prevent the software from muting the quiet, legitimate signal.</li>
<li>
<strong>
Adjusted the final Sound Output/Recording Gain</strong>
&nbsp;on the host OS to match the maximized digital output, achieving audible voice.</li>
</ul>
<p>
<br>
</p>
<p>
<u>
Summary of Key Radio and Software Settings</u>
</p>
<p>
<br>
</p>
<p>
<strong>
Transmitting Radio (Baofeng UV-5R):</strong>
</p>
<ul>
<li>
Operation: Simplex (direct radio-to-SDR).</li>
<li>
Mode: Narrowband FM (NFM).</li>
</ul>
<p>
<br>
</p>
<p>
<strong>
Receiving Software (SDR++):</strong>
</p>
<ul>
<li>
<strong>
AGC:</strong>
&nbsp;Disabled (Crucial override).</li>
<li>
<strong>
RF/Tuner Gain:</strong>
&nbsp;Adjust (Inject best signal power).</li>
<li>
<strong>
AF Gain:</strong>
&nbsp;Adjust (Produce loudest digital audio).</li>
<li>
<strong>
Squelch (SQL):</strong>
&nbsp;OFF (0) (Prevent muting of quiet signal).</li>
<li>
<strong>
Filter Bandwidth:</strong>
&nbsp;12.5 kHz(Correct setting for NFM).</li>
</ul>
<p>
<br>
</p>
<p>
The ultimate resolution was forcing input and output gain in SDR++  and volume adjustment in Audacity</p>


