When is the electrically short really short?

[cyclone24]

Hello

All this while in the literature it is advised to shorten your wires or cables to less than 1/20 wavelength in order for them to NOT act as antennas.

But I also read that sometimes even 1/50 or 1/100 wavelength at a certain fundamental frequency. For example, at 100 MHz, the wavelength is 3 m. Its 1/20 wavelength is 15 cm. 1/50 is 6 cm. Some say even this is not enough.

I would like to know when is it not enough? What are the conditions where the cable < 1/50 can still radiate? Many military specifications are strict about this and play safe by setting the dimensions too small.

 

[vk6kro]

The problem isn't only radiation. It is also inductance.

For example, this site:
http://www.consultrsr.com/resources/eis/induct5.htm
gives a calculator for the inductance of straight wires.

A straight wire 2 inches long and 0.05 inches in diameter will have an inductance of 44 nH.

Doesn't sound like much, but at 100 MHz it will have a reactance of 27 ohms.

This is quite enough to stop many circuits from working properly.
If there was a lead like this in the emitter circuit of a bipolar transistor, or source lead of a FET, it could easily start it oscillating.

At 100 MHz, 2 inches of wire is about 1/60th of a wavelength. This is also about the length of the leads that come with a ceramic capacitor, so these always have to be trimmed leaving only enough wire to solder to.

 

[cyclone24]

Thanks for the reply.

In other words, because of large inductance on the wire, the wire creates reactive fields. These reactive fields when in close proximity of other wires, transfer the RF energy and hence result in emissions. Is this explanation correct? Is this true even in the case of DC circuits?

 

[vk6kro]

You can get inductive coupling like that, or just having a reactance of 27 ohms in series with a 50 ohm load, or the low input impedance of a transistor amplifier, will totally disturb the operation of the circuit.

Having this much reactance in series with a coil that should be resonant at 100 MHz (with a capacitor) could pull the resonant frequency 10 MHz low, which would stop an amplifier working at all.
(I took an example of a 100 nH coil and 25 pF capacitor resonant at 100 MHz. Adding 27 nF in series with the coil drops the resonant frequency to 89 MHz)