Interesting piece of hardware for using a random-length wire with an SDR-RTL but there's more to this.



First, this is not, technically, a BALUN (BALanced-UNbalanced line transformer), but a UNUN (UNbalanced-UNbalanced line transformer)



The picture I've added shows a standard circuit diagram for a 9:1 UNUN. You'll notice a couple things about this. First, how SMALL the NooElec unit is. Second, one side - the UNbalanced side - has twice as much impedence as the side to ground. This makes a difference. Your VSWR is lower when wired right (I tested it with a SARK 110 vector analyzer, a "random" 36 foot piece of 22 gauge jumper wire - see the other two pictures attached, the one marked CSV_001 is wired correctly, CSV_000 is wrong). While the differences are small in my example, other wire lengths at other frequencies can make a HUGE difference. The side with the paperclip goes to ground.



You can test this out yourself if you look at the circuit diagram. One connector shorts directly to ground. The other side has some impedence. I measured - roughly - 5 Ohms with a Fluke 118B digital multimeter).



From Balun designs dot com:



How long a wire to use? While you CAN use a "random wire", 100 years or so of Hams experimenting and calculating have figured this out for you - it's settled science although things like height of the wire above ground and its proximity to telephone wires or home wiring will alter VSWR too so your lengths may need to be a little different. The effects on receive may be marginal on some frequencies depending on your wire length but can make a huge difference on others; transmitting with very high VSWR can be impossible or damage the transmitter, but, since we're not using this particular UNUN for a transmitter, this is FYI only.



Balun Design dot com has a link to a wire length chart you may find useful (fourth picture):



http://www.balundesigns.com/content/Wire%20Lengths%20for%204%20and%209-1%20ununs.pdf



"unun should be used with the wire lengths listed in the table shown in the above link. Use of the 9132 for "random" wire lengths, other than shown, can potentially create an impedance that is too high for an ATU to match. If a wire length not shown in the table is installed, an antenna analyzer should used to determine the actual feedpoint impedance that will require matching for each band of operation."



"Ununs are intended for use with unbalanced antennas such as verticals and long wires. They will not work with balanced designs such as dipoles or loops and use with a balanced antenna may cause erratic operation and/or damage to the unit."



One side to wire, one side to ground.



Hope this helps.