Lab 6

Introduction

For reference, here is the lab manual that was followed.

So we've built basically a whole radio, but what use is a radio with no antenna? The lab here will be exploring a very large and simple type of antenna, but our final project will explore smaller/better antennas. No promises on it actually being better.

The basics here are that an antenna is just an inductor, but as our radio has no filtering stage, we have to also tune this antenna by putting it into an LC circuit.

Antenna Construction

As we need to put this antenna in an LC circuit whose resonance will be the station that is received the strongest, our antenna becomes limited in design. The inductance must be in a range that our trimmer capcitor can combine with to be a tunable antenna. A good value for the capacitor in our kit comes out to around 14 wraps around a pizza box. Doing this wrapping can be a bit tedious as pizza boxes aren't the most willing subjects.

Antenna Performance

Measuring the inductance of the antenna is key to seeing if it will work. The lab lacks an LCR meter and so the oscilliscope was used. By connecting the LC to the RF port of the Oscope, a peak could be seen at 1230 MHz. This peak corresponds to the received station. By tuning the capacitor the amplitude of the station changed. Getting it to peak in the mid range of the cap means that the antenna has an appropriate inductance. Sadly we failed to take picutres of the oscope here, but there was a clear peak at the stations frequency that changed amplitude with the capacitor. Our antenna peaked around -75 dBm which is prety abysmal.

A working radio

As mentioned above, we only received about -75 dBm which is way too small of a signal to pass the biased diode in the original design. To get the radio working, we had to add a CC stage to the RF amp to buffer the ouput of the CS. This option was briefly mentioned in the RF amp manual but exact values were just a matter of experimentation. Correct values will be covered for the final project.

Conclusions and Reflections

Primary reflection from this lab is that I need to take more pictures. This seems to be a recurring problem

Making radios is hard and in the end, the simplicity of the biased diode really didn't pay off. The antenna works pretty well though considering its just wire and a pizza box. It's size is annoying but as I mentioned above there are approaches to fix that. The final project may need to switch back to the CFP to prevent this problem from happening in the future.

Final radio before project

The full radio built here incorporates:

An rf amp with three stages, CS-CC-CE. The cc was added as a buffer stage.
Biased diode AM detector. LM386 audio amp with gain increased to around 100.

Works reasonably well but has a huge amount of static on top of the signal due to poor selectivity of the antenna.