To do this, we split up tasks in lab. One of my partners worked on his chip based AM transmitter and a bunch of variable resistors. After getting it to work and realizing that it did not transmit at a high enough frequency, he perfected and tested my Hartley oscillator circuit. The other partner fine-tuned and tested his audio amp. I was tasked with making measurements of the radio, picking up a signal, and transmitting a signal from my cell phone using the lab oscillator.

    The measurements I made with of the radio included the frequency spectrum of the antenna node, the frequency spectrum of the mixer output (connected and disconnected from the bandpass filter), and the output signal of the oscillator. The antenna node frequency spectrum showed what seemed to be just noise. Though the spectrum seemed useless to measure at first glance, it proved to be useful to my understanding of the HET radio. We had left a variable capacitor in parallel with the antenna thinking that the resonant structure used for the one station radio was needed to pick up signals for the HET radio. I found this to be untrue after seeing that tuning that capacitor had little effect on the antenna node and no effect at all on the mixer output. So I took the capacitor out which made tuning the radio much less complicated.

    The frequency spectrum of the mixer output without the bandpass filter show a broad range of frequencies that shifted as I tuned the variable capacitor on the Hartley Oscillator. These were the products of the different frequencies picked up by the antenna being mixed with the oscillator frequency. When I connected the output to the rest of the radio with no bandpass filter, there was no sound. My guess is that all the different frequencies end up cancelling out making each of the signals too weak to be detected. 

    With the bandpass filter connected, there was a single frequency on the spectrum that never moved position, but disappeared when I cut the power to the radio. This was the intermediate frequency which is the difference between some broadcasted radio frequency and the frequency of the Hartley Oscillator. Tuning the capacitor on the oscillator did not change the output of the mixer because the filter knocks all the frequencies except the intermediate frequency. However, tuning the capacitor does change the output of the radio (a.k.a. the station) because as the oscillator frequency changes, the broadcasted AM signal needed to produce the IF frequency also changes.

    Although the frequency spectrum looked great, I was unable to get anything other than noise to come through the speaker for most of the lab. This slightly discouraged me, but I had seen it work before, so I just chalked it up to the building blocking out signals very well. So I moved on to transmitting a signal with the oscillator. To do this, we borrowed an old pizza antenna sitting around lab to for use as our transmitting antenna and hooked a cut auxiliary cord to alligator clips and plugged them in to the modulation input on the oscillator. Then we put the antennas in close proximity (inches apart). We first tried connecting the phone in directly as the modulation input. After a few minutes of changing both the transmitting frequency and the receiver’s oscillator frequency still had not gotten a sound. This was expected because the amplitudeof thephone output was much too small to have a significant effect on the RF signal. Next we put the audio amp (it had been tested earlier by plugging up the aux cord directly to the input and playing music from a phone) between the alligator clips and the lab oscillator's modulation input. This time we did eventually pick up the signal we had transmitted and even picked up a radio station, but the signal was very weak for both. By the end of the lab we had all of the pieces needed to make a working transmitter.