Week 8: Measurements and Further Construction

Last time, I showed a photograph of my NanoVNA measuring something called SWR. This blog will go over what that means, how I will use it, and how I will measure radiation pattern.

SWR is short for standard wave ratio, and it is calculated by finding the ratio of the impedances of the antenna and the transmission line. Conceptually, however, SWR informs an experimenter how much the transmitting wave is sent back as a reflection. If the SWR is equal to one, all of the energy used to transmit a wave is put into said wave, but if it is greater than one, there is some amount of reflection. A SWR of 1-1.5 is the golden interval for an antenna builder; anything more than 3 is considered unacceptable as most of the wave is reflected back. I will use this measure to show that the antennas being built are acceptable to put under test for radiation pattern.

An important consideration is what happens when you touch the antenna. The human body introduces a huge capacitance to the antenna where a large proportion of the current will travel to. To the NanoVNA measuring the SWR, the impedance of the antenna spikes, and therefore, the SWR spikes too.

The other important consideration is the radiation pattern, which is essential to comparing the helical and extendable helical antenna. It is measured by plotting S21 vs angle on a polar graphical plane. S21 represents the gain of an antenna in decibels, and if it is plotted as an antenna is rotated 360 degrees, a radiation pattern is created, which shows the directivity of the antenna. This, and SWR, is what will be mainly used to compare the helical and extendable helical antenna.