RadioBanter - View Single Post - A short 160M antenna

John S · November 7th 14, 12:06 PM posted to rec.radio.amateur.antenna

On 11/6/2014 7:08 PM, wrote:
In the last installment we got the input impdance up to the order of
2 Ohms with loading and a top hat.

2 Ohms will still lead to lots of loss in the matching device, so let's
see if the input impedance can be further increased.

Those familiar with the folded dipole are aware that the input impedance
of a folded dipole is higher than a dipole, so let's try a folded
monopole and see what happens.

I started with a single folded element spaced at 2 feet from the central
radiator that drops down and connects to a radial.

That brought the impedance up to about 4 Ohms.

If a little is good, more must be better so I added a second folded
element and got about 12 Ohms.

Going for the gusto, I then added 2 more folded elements for a total
of 4 as well as adding a loading inductor into all elements starting
at the 50% level and let the optimizer have at it to optimize the
inductance and inductor height for SWR and gain.

When it finished, here is what I got:

Impedance: 41.32 -9.5 Ohms
SWR: 1.3:1
gain: 1.1 dBi @ 25 degrees
Inductors: 787 uH

So here we have a 0.029 wavelength antenna that is only about 2 dB
down from a full 0.25 antenna and can be fed directly from a 50 Ohm
transmitter without any matching device losses.

As an aside, the differences between zero resistance and copper were
insignificant as one would expect with this input impedance.

The only downside to this antenna is that it is extremely narrow
banded, only about a kHz or so.

Next I added the 7.5' top hat radials and let the optimizer go at it
once more achieving:

Impedance: 58.83 -6.1 Ohms
SWR: 1.5:1
gain: 1.5 dBi @ 25 degrees
Inductors: 577 uH

Not a significant decrease in the inductance as with a single element
and the change in bandwidth was trivial. In this case I don't think
the top hat is worth the bother.

The practical issues with this antenna is getting 5 adjustable inductors
so the antenna is usable over a broader range and as the current in
the inductors is a bit high, they would have to be good adjustable
inductors.

At this point I think the notion that "short antennas are inefficient"
can be regarded as nonsense and the choice boils down to bandwith/size,
pick one.

Next up, a 160M rubber ducky, but only after attending to some roof
leaks revealed by recent rain.

Nice work, Jim.

I can't get a picture in my head of how additional folded elements are
added. Can you elaborate a bit?

Thanks.

John