Since I created this article rtl-sdr.com published a similar tutorial (http://www.rtl-sdr.com/rtl-sdr-tutorial-measuring-filter-characteristics-and-antenna-vswr-with-an-rtl-sdr-and-noise-source/) and mentioned rather cheap directional couplers, which I might acquire in the future.
Cutting my payload antenna to length and flying it, without any further testing, seemed a little risky to me, so I dediced to try and test its frequency responce while building.
I do not have a spectrum analyser, frequency generator or antenna tester at home, so I had to come up with an unconventional method.
I am not a ham or antenna expert. Nor did I spend any considerable time researching my method. If you have any comments, please contact me.
Software defined radios are amazing things!! One can simply tune into a wide range of frequencies and measure the signal strength. So it should be possible to operate it as a rudementary spectrum analyser by scanning through the desired frequencies. Luckily other people had the same idea and developed a nice software package: http://eartoearoak.com/software/rtlsdr-scanner
In order to obtain a frequency spectrum our usual transmitters, which only operate at a single frequency, are of no use. Professionally one would use a tracking generator. It is basically a frequency generator whose output frequency is synchronized to the sweep of the spectrum analyser. Again these are very pricey. Luckily generating a heap of wide-band rf-noise is simple. I found a chinese rf-noise generator (50kHz-1.5GHz) on ebay for 25€, which we can use as a cheap substitute for a tracking generator. Its amplifying stages tend heat up rather dramatically, so I added a ventilated enclosure. The below figure shows the noise generator and the output spectrum measured with the above mentioned software.
In a professional setup the tracking generator feeds the antenna via a directional coupler, with the spectrum analyser monitoring the return signal. As a directional coupler is again an expensive component, I am simply using a SMA T-splitter.
I build my antenna according to the standart tutorial, but left an additional couple of centimeters on the radials and the radiating element. The first figure shows the recorded frequency spectrum with 19cm long radials and the same length for the radiating element:
As all elements are longer then optimal a smaller resonance wavelength is expected. At around 375MHz a destinct drop in the return power can be seen. In this frequency range the antenna radiates a significant portion of the input power. All we now have to do is to incrementally cut the antenna elements and monitor the radiation spectrum.
Interestingly the bandwith goes up with the central frequency, while the maximum radiated signal strength slightly drops. To further investigate, I will have to do a proper compensation for the frequency dependence of the noise generator.
It should also be noted, that I see a slight dependence on the orientation of the antenna. I suspect this is due to reflections of metallic surfaces in my rather cramped room. I will test outside and report back.