Simple Longwire/Dipole Antenna design

Question

In a couple of days I am going to a mountain cabin and I were thinking about making myself a semi broadband antenna for LF/HF (300KHz - 20MHz or so) for receiving only.

I was thinking about creating either a simple longwire (10m) or a dipole (2x5-7m). It seems like I wont be able to get hold of any ready made baluns/chokes, or any ferrite cores/toroids to make a balun myself by that time. Seems like no one here is selling stuff like that.

Since I wont be able to balance the antenna, would a good option be to use two coax cables as feed lines? Using the center core of the coax for both legs of the dipole? And then connecting the shield of each coax to earth together with one of the cores?

PS: I am a total beginner with radio/HAM/Antennas, so a lot of the terms, wordings, theory behind any of this. eg. Connecting one of the dipole halfs to earth and nothing else makes no sense to me.

Answer 1

Dipole

For a receiving antenna, you do not need a balun. You will experience more noise from whatever equipment is located at the radio end of the feed line, and the antenna pattern and impedance will be less predictable because the feed like will to some degree be acting as part of the antenna, but it will still work. Even transmitting antennas can be constructed without baluns and are often used this way in low-power portable operation.

Construct your dipole elements of wire, and at the feed point (center) connect one wire to the coax center and one wire to the coax shield.

Single element

If you prefer using a single wire (possibly more convenient to erect), connect the shield to the actual earth using a short wire to a metal stake. This should happen at the feed point — the point where the coax ends and the antenna wire begins — not the radio's end of the coax.

This design does not need a balun, though it might benefit from impedance matching (again, not critical for receiving), which would be more precisely an "unun" (both the antenna and the coax are unbalanced devices in this case).

Another note

Since I wont be able to balance the antenna, would a good option be to use two coax cables as feed lines? Using the center core of the coax for both legs of the dipole? And then connecting the shield of each coax to earth together with one of the cores?

This might make a balanced line; I'm not sure. But for it to do anything useful you would also need a balun at the receiver end. Otherwise, the receiver itself connects the shield side of its antenna port to (its) ground, so one of your two coax lines would be just acting as a wire tied to ground on the outside of the shield, so it has no effect, or at least the line is not balanced any more because the two sides are attached to unbalanced things.

In general: the radio's antenna port is not balanced, so to properly connect it to a balanced antenna you will always need a balun somewhere in the system.

(But, to reiterate: a receiver will still work without a balun. Possibly less well, but you'll still receive signals.)

Answer 2

Like Kevin Reid says, you could be fine without a balun, especially at a mountain cabin where there there may be nothing to generate noise. For a receiving application, proper attention to the feed arrangement keeps the feedline from becoming part of the antenna, and since the feedline is usually near computers, switch-mode power supplies, and other sources of noise inside, this can be a huge improvement. However if there's no electricity besides a battery and your radio at the cabin, then none of these noise sources won't be around.

But if you did want to make a balun, there are options that don't require any sort of ferrite core. Many depend on a quarter-wavelength transmission line, but you want a broadband antenna so let's exclude those.

One option is a folded dipole.

^{simulate this circuit – Schematic created using CircuitLab}

Sometimes folded dipoles are made from ladder line since it provides two parallel conductors. The two conductors are shorted at the ends, and one of the conductors is cut in the middle to make the feedpoint, labeled A and B. It could then feed like an ordinary dipole, either with a balanced feedline, or with coax and a balun.

But notice the center of the other wire is ground: the voltages on the left and right are always equal but opposite, just what you want from a coax shield. If you could somehow connect the shield there, but then also connect to the feedpoint on the other side...

Solution: build the folded dipole from metal tubing, with a tee at the ground point where the feedline can enter, then pass inside the antenna to the feedpoint on the other side. If 10 meters of tubing isn't what you had in mind, then just use coax, which is already a tube!

The connections you need to make for such a construction are drawn in blue. Note that it's the shields of the coax that make the folded dipole, while the feedline runs inside the left half of the antenna to connect at the feedpoint at the top. The center conductor on the right half does nothing: cut it off and ensure it can't come in contact with the other parts.