What is the theoretical effect of a small, knotted loop, in an insulated-wire antenna?
I see many linked antennas with small, knotted loops, such as this. Clearly, in the practical world, such a loop is inconsequential.
But I’m unable to figure out the theoretical effect. Could someone help me?
The electrical current obviously must flow through the full length of the loop, but what’s happening with the electromagnetic field? Is it coupling straight through the knot, so the knot simply behaves as a thick spot in the wire?
73 DE K4KPK/Kevin
I would use the inductance formula for a small air-core loop to get a first approximation. I expect it is very small. Inductance would slightly increase the electrical length of the wire, lowering the resonant frequency, but probably not a measurable amount in practical use.
But why is a knot used with that insulator? Weave the wire through the three holes. If that still slips, add some glue or a tie wrap. Maybe use a tie wrap as a stopper around the wire.
A very astute observation for that particular example. There are other cases where a knotted loop can be handy. However, your point is well-taken in that perhaps I should just think a little harder in those cases too.
Probably, most of them can be eliminated with a couple of zip ties and a spot of glue. Hmmm… I’m partial to PFTE antenna wire. I’m going to need to track down a glue which sticks to that.
I’m mostly curious about what happens. Clearly, it has no significant consequence, or people
It is easy for me to visualize how DC would travel through the knot and the loop. Wunder offered the “it works like an air coil with a single loop” for the RF in the loop itself.
Electromagnetic waves are something for which I have no intuition. When they hit the knot+loop, I have no intuition as to whether they basically ignore it and barrel on through in a straight line despite there being no straight-line conductor, or do they follow the conductor?
There are some well known names like dogbone or egg but that is just a piece of plastic. Possibly stamped with a die out of a larger sheet of plastic. Possible some polycarbonate material.
My insulators are made from the transparent plastic used to make CD jewel cases. They were sliced up on a guillotine for speed. But anything non-conductive will do. You probably put all sorts of suitable plastic material into your kitchen (recycling) waste every day.
I’m with Ron on this one. Whatever effect there is, is minor. And my antennas mostly do have knots in the dxwire-like antenna wire at every insulator. On 40m, my linked dipole has knots each side, on 10, 12, 15, 17, 20 and 30m. If they have a cumulative effect, it is pretty hard to measure or notice. My score of over 1700 points to date, demonstrates that it is nothing to worry about. My insulators are made using discarded nylon-like kitchen chopping board, repurposed using a hacksaw and drill. Not very attractive, but again, the results indicate it is good enough.
Current flowing through a wire, even a straight wire, makes a magnetic field around the wire. When the wire is coiled, the magnetic fields add, making a stronger field around the coil (inductor). For current flowing through the wire, the coil is like straight wire, only more so. The magnetic field is larger than it would be if the wire was straight. This increased magnetic field is how electromagnets and electric motors work.
For alternating current, like RF, coiling it makes the electrical length of the wire longer for the same amount of wire. A loading coil has a large enough effect that the antenna can be much shorter, often half the unloaded length or less.
But, because the field is concentrated around the coil, that part of the antenna doesn’t radiate very much, so loaded antennas are less efficient at turning RF currents into radio waves.
Throwing some very approximate numbers into an online calculator, I got an inductance of 2 nH (nanoHenries) for a one-turn coil of that size. A loading coil for a half-sized 20 m antenna would be 300-400 μH, so the knot has around 1/100 of the inductance needed to make a real difference in the antenna length. That estimate could be off by 10X, but it still wouldn’t make a meaningful difference.