Re: 80 meter folded dipole VS loop
- --- In firstname.lastname@example.org, Andy <ai.egrps@...> wrote:
>Yes, but there is a big difference....
> > Thanks for the reply, from looking at images and diagrams of folded
> > dipoles they looked like a squashed loop, but after reading some info at
> > Wikipedia I see they don't make a connection forming a loop (they just
> > come close) = solved that mystery.
> Actually, most folded dipoles are wired exactly like a loop. If you start
> with a loop and squash it flat, you would end up with a folded dipole
In a loop the difference in the wave causes the current to flow in one direction on one side of the loop, and the opposite direction on the other side of the loop, because the two sides of the loop are being intersected, and magnetic wave cut on a different part of the wave.
But with a "folded dipole" both sides of the "loop" formed are too close together, and the signals are going in the same direction. The other half of the "folded dipole, that is, the half not connected to the feedline, is wasted. It has the same currents and voltages as its parallel portion all along the wire. So no current flows from the feedline half to the other half and vice versa. The voltages out on the ends are the same, and have zero current flow.
I have made folded loops and then cut them at the ends, and it makes no difference. The other piece of wire is wasted.
> > Actually, most folded dipoles are wired exactly like a loop. If youIndeed there is. What I described above is just the physical topology, that
> > start
> > with a loop and squash it flat, you would end up with a folded dipole
> > exactly.
> Yes, but there is a big difference....
the wires of the folded dipole do trace out a flattened loop. But as an
antenna, the folded dipole behaves nothing like a loop and everything like a
dipole ... with the two exceptions that (1) its feedpoint impedance is
quadrupled, and (2) it has DC continuity.
Like you say, the current directions are those of a dipole.
> The other half of the "folded dipole, that is, the half not connected to...
> the feedline, is wasted.
> I have made folded loops and then cut them at the ends, and it makes noI'm not sure exactly what you're saying, if it's how I'm reading it ... but
> difference. The other piece of wire is wasted.
it isn't wasted. Yes, you can cut the bridges between the two wires and
THAT won't make much difference, but it's not the same thing as removing the
other (now disconnected) wire. That other wire has a significant effect; it
is quite tightly coupled to the fed dipole wires. It causes the terminal
voltage across the other wires to double, and the current to be halved, for
the same power. So the feedpoint impedance quadruples. Big change.
Does it improve antenna gain, or reception? No. But it is most useful to
move the impedance upwards. For a low horizontal HF antenna, that is an
important matching technique. Also for FM radios, to transform the 72 ohm
dipole impedance up to the radio's 300 ohm input impedance.
And as a side effect, it widens the antenna's bandwidth a bit, just like a
cage dipole does, because the antenna is effectively fatter.
It's worth remembering that the folded dipoles we're talking about shouldn't
be confused with T2FD's, which employ a terminating resistor(s) to achieve
very wide SWR bandwidths while sacrificing efficiency. It's a different