Antenna...E Field

[ms660]

Referring to the attached pic:

It's a representation of a Beverage antenna (aka wave antenna) I can't quite make out what is supposed to be in the white inset, I understand the wave tilt but it's the three arrows that are shown, two of them are shown as Ex and one is unnamed, can anyone confirm what the three arrows represent?

Full article here:

https://archive.org/stream/TheBevera...ge/n0/mode/2up

Lawrence.

[G8HQP Dave]

I suspect those arrows just show the 3 dimensions, and should be labelled Ex, Ey and Ez.

I had a quick read of the article. It seems to me that the Beverage is basically a leaky feeder antenna. Alternatively, it can be viewed as a directional coupler: each point in the antenna couples weakly to space, and all these little bits of field add up in one direction and cancel in the other direction.

[ms660]

I'm trying to find out if that should correlate with the animation shown on Wiki here:

https://en.wikipedia.org/wiki/Beverage_antenna

Rightly or wrongly, as I understand the Beverage antenna so far, the horizontal component of the E field produced by the wave tilt is on the same axis as the antenna wire, if that happens to be so then I'm not sure what the inset in the picture that was posted is supposed to be implying in relation to that?

Lawrence.

[ms660]

Out of interest, Beverage, Rice and Kellog, papers/notes from 1923:

http://ra6lbs.ru/Papers/Bv%201923.pdf

Lawrence.

[Radio Wrangler]

THey keep talking about wave tilt in articles on Beverage antennae but it may be a red herring. It's a travelling wave structure where the wave speed on the wire is very close to the wave speed in space so the two waves are in step with each other and induction effects will add.

I think you could build one in open space with a capacitive body in lieu of ground at the far end, and it would work.

I've never needed graded dielectrics around my directional couplers to make them work.

Perhaps a tilt in the wavefront could be due to the wire acting as a slightly slow wave structure?

David

[G8HQP Dave]

Interesting. They argue that wave tilt is the mechanism, rather than signal potential (which is what I would assume). It may be a combination of the two, as most antennas are affected by both electric and magnetic fields.

[ms660]

What about the wave velocity in the other conductor and what about the horizontal component of the E field?

Lawrence.

[G8HQP Dave]

For signals coming in end-fire the horizontal component of the E field has no effect, as it is orthogonal to the wire.

[ms660]

Wouldn't that depend on which way the horizontal E field is orientated?

So far as I understand it, it's the horizontal component of the E field (created due to the elliptical polarization which was created due to the two different velocity mediums concerned) that creates the signal currents in the wire or am I off beam?

Lawrence.

[ms660]

A bit more digging finds this from Terman:

"The electric vector of the wave produced at the surface of the earth by a vertical antenna possesses a slight forward tilt. As a result the electric vector has, in addition to the normal vertical component a slight horizontal component parallel to the surface of the earth, and lying in the direction of propagation. ... The component of the electric vector lying parallel to the earth's surface that is present because of wave tilt represents a voltage applied across earth's surface. This voltage causes a current to penetrate down into the earth."

That's from The Radio Engineers Handbook starting at page 697, link to the book here:

http://www.itermoionici.it/letteratu...s-Handbook.pdf

That confirms what's been implied in other texts I've read.

He also mentions further reading by several authors including G.W.O. Howe's article in the September 1940 issue of Wireless Engineering entitled Wireless Waves at The Earth's Surface, which I also found helpful, link to the article here:

http://www.americanradiohistory.com/...er-1940-09.pdf

This has given me a better understanding of what's going on, it would be nice to see an animated E field vector for this as that would clinch it.

Lawrence.

[Skywave]

Originally Posted by ms660

It's a representation of a Beverage antenna (aka wave antenna) I can't quite make out what is supposed to be in the white inset. I understand the wave tilt but it's the three arrows that are shown: two of them are shown as Ex and one is unnamed. Can anyone confirm what the three arrows represent?

The lower parts of the E-field are curves. Of the three arrows, the vertical one and the horizontal one are the author's attempt to illustrate that the resulting field - which is curved - can be resolved into two components that are right angles.
If those two resolved components had been labelled Ex and Ey, with the resultant simply labelled as plain E, then I'm sure that you would have understood it immediately. Your puzzlement has arisen because of the author's bad legending.

Al.

[G8HQP Dave]

I was thinking of the horizontal E-field of the incoming wave, which will be transverse. Any tilting will introduce a new field component, which I guess is what they are talking about.

An antenna which relies on resistive loading to work at all seems to be a bad idea. Certainly no good for transmission. For reception you could probably do better, given the space needed for a Beverage. A phased array of short verticals, for example.

[ms660]

Thanks Al, although I accept the mechanism as described in various texts that produces an E field component that lies horizontally in the direction of propagation I'm still having difficulty visualizing the transformation from a purely vertically polarized E field to one that contains the said horizontal component, plenty of animations on the web that show linear, circular, elliptical polarization, no problem understanding them but they don't fit the bill so far as the ellipse effect that takes place for the wave antenna to function.

EDIT: post crossed

Lawrence.

[ms660]

The resistive loading at the far end is what gives the Beverage antenna it's unidirectional properties so far as I understand it.

Lawrence.

[Radio Wrangler]

The Beverage is a directional coupler. a wave travelling along it in one direction produces a signal at one end and little at the other end. A wave travelling in the opposite direction produces a signal at the other end and little at the first end.

The terminating resistor absorbs the signal at the end where a wave from the reverse direction would induce one, and we have a unidirectional antenna.

Take off the termination resistor and the reverse direction wave signal gets reflected back along the antenna and to the receiver. we have a bidirectional antenna.

We could get smart and use a second receiver for the far-end termination. Provided each receiver is a reasonable 50 Ohm load on the antenna, we get two unidirectional antennae for the price of one.

David