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 it added a ventilated enclosure.

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.