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--- projects:storm:antenna [2015/03/10 21:42] – storm
+++ projects:storm:antenna [2015/03/17 20:52] (current) – storm
@@ Line 1: / Line 1: @@
 ====== Antenna tuning ======
+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.
 ===== Motivation =====
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 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.
+The combined setup looks something like this:{{ :projects:storm:antenna_setup.jpg?direct&500 |}}
 ===== Measurements =====
+I build my antenna according to the [[guides:payload_antenna|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:
+{{ :projects:storm:payload_step2_300-500.png?direct&600 |}}
+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.
+The following figure shows the spectrum for an antenna with a 17.5cm radiating element and 18.5cm radials:
+{{ :projects:storm:payload_step6_300-500.png?direct&600 |}}
+The last figure shows the spectrum for an antenna with a 164mm radiating element and radials of the same length, as described in the tutorial.
+{{ :projects:storm:payload_step10_350-500.png?direct&600 |}}
+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.