Antenna effective length

vice

nna.

petered

.4 in

fit

hought

iled

't

ngth

out?

An element that is under 1/4 wavelength looks capacitive. If you think of your coil of wire as an inductor in series with short element, you will begin to get a handle on what happens when an open ended coil is used.

It's not necessarily poorly matched, it's just highly inefficient.

That's not an easy question to answer. Depending on how tightly you coil it, it could be just about anything... tightly coiled antennas like this are often only as electrical long as they are "tall" (the height of the coils stacked together) -- which is generally still electrically short -- and the coils just provide for a sort of distributed inductor match, whereas loosely coiled antennas can act like helical antennas where there's non-negligible interaction from turn to turn and the electricla length is a not a direct function of either the total wire length nor the height of the "stack" (Kraus's antenna books goes into lots of detail on this -- he more or less invented the things).

Their are some antenna experts on rec.amateur.amateur.antenna who can undoubtedly give you a far better answer than I can, BTW.

---Joel

A coil of wire having a separation between turns much smaller than the diameter of the turns can behave as a 'normal-mode helix' and this configuration is often used as an antenna for a walkie talkie - the polarisation is aligned with the axis of the helix resulting in predominantly vertical polarisation in this case. The effective length of the antenna is much the same as the physical length of the coil, that is the separation multiplied by the number of turns, but the termination provided by such an antenna to its drive point is affected by the distributed inductance of the coil. A monopole made of a straight wire shorter than a quarter wavelength (with an adequate ground-plane or other counterpoise) presents a reactive terminal impedance, like a small capacitor in series with the radiation resistance. For a monopole of the same length made of coiled wire, the distributed inductance tends to reduce the reactance of the terminal impedance. The trick is to make the reactances cancel (i.e. resonate) at the frequency of interest, then what is left is resistive - the radiation and loss resistances in series.

Radiation from a short monopole is described in many of the well-known text books, such as Antennas by John Kraus (and Ronald Marhefka in the third edition). The important point is that making the element shorter than a quarter wavelength has little effect on its ability to couple to an EM wave (e.g. expressed as volts/metre, some given distance away, per amp flowing in the element) but causes a major reduction of its radiation resistance, making it more difficult to match efficiently to electronics.

Enough?

Chris

...even more on rec.radio.amateur.antenna.

Almost, thanks. Assuming near-direct connection to the transmitter's output balun (such as in a handheld, where a short whip is desired) there should be no additional difficulty, right?

But if you need to run co-ax, you'd need a balun at each end? Just making sure I have this straight.

Clifford Heath, VK3CLF.

petered

Try researching normal mode helical antenna and L.O Krause

Correction on my last, John Kraus of Ohio State University is tha author as another had posted.

petered

Try researching normal mode helical antenna and John D. Krause of OSU.

If the antenna were a balanced dipole then a balun would be needed to connect it to electronics that presents an unbalanced port, particularly if co-axial cable is involved. The point of the balun is to prevent the outside surface of the outer conductor of the coax, or some part of the electronics, from behaving as part of the antenna (although this may still happen to some extent on account of re-radiation from cable or electronics close to the antenna).

However, a monopole over a ground plane, or one of the many common variations on this theme (e.g. a monopole on a walkie-talkie), presents an unbalanced termination and the above issue doesn't arise if it is connected to the electronics through a co-axial cable. There is then no need for a balun at the antenna end of the co-ax.

In addition, as you say, a transmitter may include its own balun to connect a balanced, push-pull circuit to an unbalanced load such as co-ax.

The additional difficulty with antennas much shorter than a quarter wavelength is their small radiation resistance (perhaps a fraction of an ohm). Steps need to be taken to transform this to a resistance into which the transmitter can develop a useful amount of power. The 'maximum power transfer theorem' requires the real part of the load impedance to be equal to that of the generator's impedance. The theorem doesn't apply in reverse ... lower generator impedance always results in greater power transfer.

Chris