Jacob M. Ramey EGRE 307: Integrated Circuits 5/6/2015

Final Lab – Individual Report

Jacob M. RameyEGRE 307 – Integrated Circuits

Electrical EngineeringVirginia Commonwealth University

Abstract and Theory- This lab report details the design of the Antenna and Impedance matching network and how important the

antenna design was to the overall function of a communications system

The design of an Antenna for the purpose of sending a sound wave (music) over the air and receiving it on the other end to be played. There are two main elements of the communications system - each containing several important parts that make up the Transmitter and the Receiver. Inside each of these are filters, amplifiers, modulators, buffers, and impedance matching networks. Even if all these parts are working flawlessly without a good antenna design the signal will have too much attenuation to propagate notable distances. Antenna's are essentially just stubs that are (normally) comparable to the electrical length of the frequency which they are designed to emit the most power at. For this labs design specification, the frequency of transmission is from 600k-620kHz. This corresponds to a wavelength given by

c=fλ

λ= 3∗108

[600 k :620 k ]=[483.87 :500](m)

The wavelength of the signal is very important because it determines the length that the antenna needs to be.

Though an electrically short antenna may be used and can be very effective, a very well made impedance matching network is necessary and it also needs to work over a wide band. This is a limitation when working with L-network matching because these are normally made to work around a single frequency and not a range of frequency. Because of this, there is distortion in the signal because different frequencies will attenuate at different rates. The antenna design most familiar for my group and I is the Quarter Wave Transformer (QWT). The radiation spectrum gives by a QRT is given by Figure 1

Figure 1: Grounding plane increases directivity of the antenna.

As the name implies, it requires an electrical length equal to a quarter of the wavelength, in our case this corresponds to 410.1 ft of wire. This is enormous for an antenna that needs to be used inside at small distances. The solution to this problem was using a concept called a 'loading coil'. The idea of a loading coil came from the early telecommunications field when engineers needed to send signals a larger distance.

The wire is wrapped around a core instead of The only problem was that the antenna emitted

Jacob M. Ramey EGRE 307: Integrated Circuits 5/6/2015

going straight up into the air - this allows us to make an antenna that requires a length of 410ft in a physical height of the solenoid of around 3ft. The wrapping of the wires enable us to keep the size of the antenna down and also contribute to the inductive effect of the wire being wrapped around the solenoid core. This is a good thing for us because naturally the wire of such a length will have a capacitance that negatively impacts the performance of the antenna by lessening the power it would receive. Such lost power would be absorbed by the antenna and sent back towards to generator. The last thing we want is to lose power that would otherwise be used to send out signal at a further distance with less distance at the final output. Amplification of a weak signal works but also it amplifies the noise of the signal as well. The inductive effect of the wrapping of the wires can be found by integrating Amperes law over a cylinder[1]. The resulting equation is a function of the radius of the solenoid, the length, and the number of turns. It is given by

With this effect accounted for and the length of the wire being a quarter wavelength electrically, the matching of the network would be as simple as putting a capacitor in parallel with the antenna that matches its impedance with the input to deliver the maximum power to the load. This value was calculated and in order to account for the error in our build and calculations, we used a variable capacitor in parallel that was within the range of the value we needed. The matching of the network worked well and after adding the capacitor we increased around range for receiving the signal from ~1ft to ~12ft.

the greatest power at a frequency other than the one we had designed it for. The solution for this was a processed dubbed 'wire tapping'. Instead of connecting the source to the antenna at the base, several other points on the antenna were attached to the source and tested. Placing the source at one point gave a different response than the other in terms of frequency and amplitude. After testing many places on the antenna line we found the best place to emit our frequency and the amplitude of the signal was also much larger than the first location. This difference was 1 order of magnitude and gave us a much clearer signal on the other side since we did not have to amplify it as much.

Conclusion: The use of the QWT proved to be effective at transmitting a signal at our design frequency enough to have it heard audibly from the other side of the room after demodulation. The most difficult part of designing the antenna was the impedance matching part because I had little experience with it until later in the semester in EGRE 310: Microwave and Photonics Engineering. After working with my team, other classmates, and the TA I found a much easier was to accomplish this.

References:[1] Fitzpatrick, R. (2006) Self-Inductance [Online] Available http://farside.ph.utexas.edu /teaching//lectures/node82.html

[2] Weaver, R. (2009) Radio Theory [Online] Available http://electronbunker.ca/Radio_Theory.html

[3] Pozar, D. Impedance Matching and Tuning in Microwave Engineering 2nd ed. New York: John Wiley and Sons, 1998