If you are making the first proposed balanced, L shaped antenna, or a dipole of any kind, ideally you should feed it with a "bal"un, a balanced to unbalanced RF transformer.
If you are building a one-wire antenna that works against the ground beneath it, use a un-un. For SWL reception you can use the one you have which I think is made out of a minicircuits part. Which are good. They do the job they are supposed to. Just dont use it for transmitting, even QRP. . For QRP, you can make a unun that proabaly would handle up to 5 watts out of a single toroid ring core like a
FT-50-43 For any more than 5 watts, use a larger toroid core.
If you want to use @AW's dipole, I suspect you would have good luck with one like the elecraft BN2, which can be designed to be switchable to allow 1:1 or 4:1 winding ratio. You can make one yourself from two binocular cores, and by all reports its very good. Its very efficient and broadband, it works well from 1.5 to 54 MHz. If the radio you are using has a built in antenna tuner the
balun may help you match to it. It is connected via BNC jacks. Lots of portable-oriented ham radios now use BNCS for quick, portable operation, that is strong and convenient. But you might also want to use SMAs with very low power transmitters of any kind of receive setup. Use good quality, thin, insulated copper wire. multiple turns may get cramped, then use thinner wire. I often use the copper wire from cat5 cable for baluns. At least you know the insulation has low loss. For receive it can be thin wire. Or use enameled copper wire, any size that fits well in there..
The rectangular binocular cores work very well. They aren't at all expensive either. Kitsandparts.com is a very good source for these magnetic cores. They are affordable and fast to ship. The first one that I built used
two of these It works great. More recently I built one using
two of these. It works quite well - for receiving with a dipole- this design is really good for any dipole, but I am debating with myself whether I should use a heavier duty DPDT slide switch. The tiny DPDT switch that I have is supposed to be rated at 10 amps but I just dont see it actually handling that much. Its ebay junk, basically. It would melt. or short out, I am fairly sure. My first clue that it wouldnt be suitable was when I discovered tht these micro toggle switches had muchy higher resistance than the ones I bought at least 35 or 4 years ago when I was first getting into electronics. I know they are quality switches but I would prefer to use a new one. I don't want my switchable balun to suffer a failure thats preventable. Better to get a beefier switch to start out with.
If I was going to build a 9:1 unun to use for a ham station (i.e. transmitting someday) I would use a big ferrite toroid and (teflon insulated?) well tinned copper wire that would be able to handle some power. Probably a
http://toroids.info/FT240-43.php would likely handle up to several hundred watts. (Not the full legal limit, it might get too hot) It would be smart to doube check the temperature with a non content thermometer. If you are planning to transmit as much as a kilowatt into it I would use those FT240 size big toroids, but stacked, in other words, use two of them next to one another treated as one. There is quite a debate online over what materials to use, for different kinds of antennas. (unresonant or resonant) Some suggest using #53 material.. some using #33 (I think) I dont know enough about their performance to make that call! All I know is that #43 is what I see suggested the most. Speaking of the number 43...
A 43 foot vertical wire made out of very strong thin woven insulated grey wire. with a 9:1 unun with a counterpoise and ground will work well as a low visibility general purpose antenna for HF down to several MHz but I would experiment with different lengths and configurations too since you are not transmitting on ham bands - the reason that length was picked. So feel free to use different lengths, and different configurations. Use a un un to connect any monopole, and a balun for any two legged balanced antenna. You can use many kinds of toroid cores thats suitable for RF use. You might even be successful with an iron core magnetic. youhave of suitable ferite material (better for broadband transformers) I have used the most common #43 often for these. Thats the material used in noise suppressing beads which can substitute for an HF toroid core well. Many of us already have some lying around.
Or use any length wire, 43 feet does not resonate on any ham band so that may not be the best length for swls who probably care more for lower frequency reception if tiy have the space, do your best to string an antenna that best uses it.
A unun attempts to match a single wire to an unbalanced feedline like a coax cable. It will smooth our the reception and may improve your reception markedly, reducing noise substantially, anyway that is what its intended to do. If you have a two wire (dipole or loop) antenna use a balun to connect it to your feed line. Like the elecraft schematic. I have had really good luck with that balun design and its easy to make.
To make the unun use three wires and a #43 spit bead as your toroid core, make 12 loops through the split bead Twist the three wires together, use the thinnest strong wire you can find. Or use two split beads and nine loops around. If you like lower frequencies more use twelve or more. The cost of this unun should be a dollar or less. Connect it to your receiver using any suitable coax cable. For HF, RG-174 is fine. Try to use the best ground you can find, or as a counterpoise a wire laid on the ground as long a wire as you can manage. If you use an RF choke around the feed coax put it close to your receiver rather than near the feedpoint. Because the coax cable is the ground side of your antenna.
To make the balun out of split beads instead of binocular cores use four identical split beads (out of the same batch if possible, and use two beads as you would a binocular core. You can use a single or two binocular cores for the switchable balun. The "original" Elecraft BN-1 was made with just a single binocular core, I think.
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