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I have a rather beginner question, hope that's okay.

I use my Quansheng to listen over 50 MHz, and I have heard that one can hack the firmware to go down to 18 MHz, but I really want to listen to the LF, MF and HF frequencies so I bought a cheap RTL-SDR with two antenna inputs: 100KHz to 30MHz and 25MHz to 1.7GHz

Now I have to decide what to do for the antenna, preferably without buying them outright, so I have consulted this link: https://www.sdrplay.com/antennasuggestions/ and it looks like a good starting point is this configuration:

100KHz - 30MHz

20 Metre / 66 Foot long fine speaker/transformer copper wire strung from house to nearby tree, insulator at the ends fashioned of garden hose to limit grounding into tree when wet.

25MHz to 1.7GHz

771 whip antenna

OR

Homemade vertical whip antenna

So my question is around connecting those to the box. I can't put the SDR box outside so I can't stick the end of the antenna into the BNC as some people suggest, I could insulate the wire in tape or hose and bring it inside the house through the vent. Or I could wire coax to the outside corner of the house and connect it.

I know my SDR box has a balun on the mainboard, but do I need to put another one at the interface of the coax and the antenna wire?

Do I absolutely need to drive a ground stake to connect to the shield of the coax?

I know my long horizonal antenna should be 66 foot long but how long should the vertical whip be, and can it be constructed in the same way? Would it be better just to locate a stock 771 antenna outside and connect it with coax back to the box, and again would an additional balun be necessary at the connection between coax and antenna?

I'm going for a cheap and cheerful setup if you haven't guessed, so absolute idyllic operation isn't required, just best as can be achieved with about €50.

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spl, I have to make lots of assumptions here, you describe your system in very generic terms. I am going to assume your RTL box has two input connections, both accepting Type F connectors, the kind used by TV cable boxes and 75 ohm coax. Or they could use SMA connectors, a smaller size. Change my answer to suit your conditions.
100 KHz to 30 MHz 66 feet is not a magic number for what you are trying to do. The distance you need is what you have available. You are looking for a broadband antenna, not something for just ham radio frequencies. Speaker coil wire is really fine, and not very strong. Connecting it to a tree will break it when the first good wind moves the tree limb. You want to tie the antenna wire to a thin cord and run this through a pulley, attached to the limb, and down to a small weight. Then when the tree moves, the cord can move, keeping the strain on the wire more even. 22 AWG insulated, stranded hookup wire is stronger, and not expensive. The impedance (the AC electrical resistance of this wire) will change greatly over the frequencies you want to receive. A company called Noolect (nooelect.com) makes a barebones 9:1 balun/unun that can convert the impedance seen on the antenna wire to something more acceptable to your RTL. (or coax). This will give you a stronger received signal. For what you sound like you want to do, if it's not inconvenient, just bring the wire in a window, and connect it to the 9:1 and then through a short piece of coax to the RTL. You can get fancy later, routing it through a vent if it seems to work ok. All antennas have two parts. Two wires in opposite directions for a dipole, or a wire in the air, and the ground ( earth surface) for a monopole ( single wire) antenna. If you are near a window, and can run a 8 foot wire to a stake in the earth to your radio, great. If you have your 9:1 balun outside, and can ground it with a short wire to earth, great(er). The radio wave travels to the antenna wire, through the front end of your receiver, and then to earth. If you can make that path a lower impedance, then the received signal will be stronger. But if you can't, the wave will travel through other paths to ground, just not as strongly. You have at least two other options, Google e probe antennas, and loop on ground antennas. One of those types might strike your fancy. PART 2 The VHF/UHF part I can't speak to very much, locate you antenna up above the roof line if possible, Vhf/Uhf frequencies are very susceptible to shadowing by structures. The Nagoya whip is really designed for ham radio frequencies. A better broadband antenna is called a discone. Tram makes one for about $70. There are others. You might want to research that also. Good luck in your research.

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  • $\begingroup$ You have my sincere thanks for a very thorough answer to what was likely a confused question in the first place, this is really helpful and gets me started! $\endgroup$
    – spl
    Jul 6, 2023 at 10:12

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