Radiation Direction From Conductor

Thread Starter

andrewdero

Joined Aug 9, 2019
12
If the length of a circuit’s conductor is much shorter then the wavelength I’ve read/been told that the radiation from the that conductor is negligible for circuit analysis. The explanation being the voltage along the line at each point will be essentially the same. As I understand in any circuit there will be an electric field between the positive and negative conductors in the circuit and since the magnetic field is around the conductor the propagation of the EM wave will be along the direction of the circuit towards the load (basically the description of the direction of the poynting vector). However, if the length of the circuit’s conductor becomes closer to the wavelength the voltage potential can no longer be considered constant along the conductor. Since there is a voltage potential across the wire does this mean we now also have a small electric field parallel to the direction of the conductor and thus an EM wave will be radiated perpendicular to the conductor? I know there are a lot of other factors to do with radiation patterns and they can be very complex. I am just looking if I have the right general idea for the direction an EM wave is emitted when the conductor approaches the wavelength.
 

nsaspook

Joined Aug 27, 2009
8,389

One thing to remember about the charges in the conductor. They don't actually physically move from one end to the other at high speed. What happens is slight shifts in the positions of massive numbers of free charges to give a field gradient across the conductor.
 
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nsaspook

Joined Aug 27, 2009
8,389
I fell like I understand the idea of how a dipole radiates. I get confused when thinking of how a power line for example emits EM. At the right length would it begin to act like an end fed dipole?
Almost all the emitted energy from a power line is from near-field induction. 60Hz has a electrical wavelength of 16,400,000 feet so a 'really' long wire would start to radiate.

For high harmonics the power line can be a traveling-wave radiator.
 
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Thread Starter

andrewdero

Joined Aug 9, 2019
12
so a 'really' long wire would start to radiate.
Sorry if this sounds like a repeat question just want to make sure if got it right. So at that length for 60 Hz mention we would have essentially just made a very long end fed antenna? Is this essentially the idea of how random wire antennas operated?
 
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G4WKW

Joined May 27, 2021
1
I looked at this thread with some interest.

Regarding end-fed long wires, you might look at the June 2021 edition of RadCom (published by the RSGB), there is an article on page 18 "Antennas - End-Fed Wire Antenna Revisited - Part 1". It demonstrates the relationship of the actual wire length with differing wavelengths (generally the HF amateur radio bands), but the principles apply generally.
 

nsaspook

Joined Aug 27, 2009
8,389
I looked at this thread with some interest.

Regarding end-fed long wires, you might look at the June 2021 edition of RadCom (published by the RSGB), there is an article on page 18 "Antennas - End-Fed Wire Antenna Revisited - Part 1". It demonstrates the relationship of the actual wire length with differing wavelengths (generally the HF amateur radio bands), but the principles apply generally.
Former radio professional, started as Radioman in the US Navy, worked my way up to Technician with a specialty in secure field communications. We very seldom used resonant HF antennas because we were frequency agile. This usually meant a end-fed antenna or whip of a length that had neither high or low impedance on any of the bands we would use with the coupler to provide a 50 ohm impedance to a typical transceiver. If there was some tuning issues we would just make the antenna either longer or shorter to help the coupler depending on site conditions and grounding. Good grounding conditions usually dictated one site over another on land. On a ship at sea grounding was never a problem.

Chi181004m27.jpg

A classic old training film.
 
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