antenna-theory.com

[rchadwick]

This is something I always wondered, but now have an application. Suppose I wanted to make my own dual band antenna. Could I simply cut two 1/4 length wires, and connect them both to the transmission line in parallel? For that matter, could I cut 5, 10, 20 lengths and make my own wideband antenna? I assume multiple elements may interact with each other. Assume the lengths were arranged in a circle?

While I'm generally curious, my application is for receiving, and possibly transmitting (50mW max) across 200mHz-1GHz. Length needs to fit inside a plastic box, 3 inches or shorter. Not looking to moonbounce, would just be happy to be a little more efficient than simply a random length wire.

My related question... I know I'm not fitting a 200mHz 1/4 wave antenna in 3 inches. I'm assuming 1/8 wave is still better than random. Is there a point where length isn't important? Is a 1/16 or 1/32 wavelength antenna still better than random?

Thanks!

[admin]

Let's look at 3 half-wavlength dipoles,

(A) 0.5meter (300MHz),
(B) 1 meter (150MHz)
(C) 1.5meters (100MHz).

If you put them in parallel and measure at 100MHz, the parallel combination would probably radiate the same as just antenna (C) by itself. The smaller/higher frequency antennas would still have a relatively large impedance.

However if you measure at 300MHz, both antennas (B) and (C) would be in their healthy radiation region. In particular you'd be right on the 3rd harmonic of (C). So the impedances would be comparable, and energy would flow. It's not clear then if the overall impedance match would be good or terrible as a result. Any specific direction radiated from the antennas would also potentially add constructively or destructively. In addition, the antennas would couple to each other, causing a detuning effect you'd want to measure.

In regards to minimum length, if you are well below a half wavelength, 1/16th is always better than 1/32nd, which is notably better than 1/64th. No matter how low you go, there is an increase in efficiency the larger it gets.

[chaicustard]

I like to think of an antenna as a transformer that idealy converts 50 ohms to 377 ohms ( impedance of air).
A single length of 1/100th wavelength cant do this, BUT we can always at some RF components before hand to help convert the impedance . Sometimes we add bits onto the antenna ( like in a J-Pole feed ) to help .

It is possible to build a circuit that impedance-matchs a really high impedance down to 50 ohms..In your case, it would be a wide-band antenna tuner.
by the way, your starting idea reminds me of a LPA ... a set of increasing-length -wires protruding from a feed-line...
https://en.wikipedia.org/wiki/Log-periodic_antenna#/media/File:Log-periodic_monopole_antenna.png

[CarolynGonzalez]

Thanx for sharing link