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Author Topic: Antenna wavelengths  (Read 4337 times)
M5AEO
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Posts: 270




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« on: November 10, 2013, 07:13:12 AM »

We always talk about antennas in terms of half or quarter wavelengths, such as 'half-wave dipoles' or 'quarter-wave verticals'.  I realise that at HF at least, such wavelengths are most practical. But, would an antenna be even more efficient if it was a full wavelength or even multiple wavelengths?  Or do we just run into 'diminishing returns'?  I would be most interested to know the answer, in not too technical terms!

Jonathan, M5AEO, London.


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NJ1K
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Posts: 329




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« Reply #1 on: November 10, 2013, 07:45:59 AM »

Your RF signal on a conductor reverses direction every electrical half-wavelength whether or not the conductor is ¼, ½, or any other length.  Thus, the basis for most antenna elements is ½ wavelength (generally speaking).  A ¼ wavelength antenna works electrically as ½ wavelength as the other ¼ electrical wavelength is indeed a ground plane or some other signal sink.  The RF signal still travels ½ wavelength; ie,  from the tip of the ¼ wavelength element, to ¼ wavelength into the ground plane or signal sink. 

It doesn’t matter whether you feed a ½ wavelength element in the middle, or the end, or anywhere in between, the radiation pattern will be the same.  By combining multiple half-wavelength elements into an array, you can realize more gain/directivity from the array than you would have with a single half-wavelength.  Consider a UHF Stationmaster antenna.  It contains many half-wavelength elements stacked end to end.  Also consider that yagis are multiple half-wavelength elements in a parasitic array.  More gain equals more directivity.

There are many more examples of ½ wave arrays.  The thing to remember is that the RF signal reverses every half cycle (1/2 wavelength).  If a single element is more than ½ wavelength, you will have radiation from part of that element out of phase with other parts of the same element.  This will change the radiation pattern, sometimes desirable and sometimes not.  One such case would be the extended double ZEP, consisting of two 5/8 waves end to end fed in the middle.  Or a 5/8 wave vertical.    Again, the reversal of the signal every half-wavelength will alter the pattern compared to a ½ wavelength element.

Now a single element that is multiple wavelengths long will develop a pattern resembling a porcupine, with huge gain radiating in very narrow spikes with very deep nulls between the spiky lobes.  It is desirable to have a pattern with very smooth well-defined lobes.  This type of pattern will generally provide more consistent signal levels as the direction of arrival changes. 

Hope that helps.
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WB6BYU
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« Reply #2 on: November 10, 2013, 07:55:35 AM »

Efficiency really isn't the issue in most cases - standard wire antennas (ignoring the
ground loss in a vertical) are probably >95% efficient.  What is more important is the
radiation pattern.

A quarter or half wave antenna has maximum radiation broadside to the wire - that makes
it bidirectional when horizontal (like a dipole) or omnidirectional and at a relatively low
angle when vertical.  As you make the antenna longer you have multiple half wave
sections along the wire, and adjacent sections are out of phase with each other.
As a result you have more lobes and nulls in the pattern - higher gain in some directions,
no signal in others.  For a vertical antenna this may result in much of your power being
radiated at high angles that aren't reflected back by the ionosphere.

There are ways to arrange such wires to make the pattern more useful - vee beams,
rhombics, Sterba curtains, and zig-zag log periodic antennas are examples that are
quite different in behavior than a single straight wire.

And many hams use a long wire or doublet on frequencies where it is multiple wavelengths
long, so they certainly can work.  It's just that you have to consider the radiation pattern
on each band to make sure it will accomplish what you want to do with the antenna.
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AC5UP
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« Reply #3 on: November 10, 2013, 08:28:17 AM »

A full wavelength of wire fed 1/4 wave from either end will have a low feedpoint impedance of approximately 70 Ohms (which is affected by the height above ground) much like a 1/2 wave dipole.

What it won't have is a figure 8 radiation pattern. The full wave wire will have a four leaf clover pattern and a broader bandwidth. It will also need more real estate. If that's not an issue this means more of the antenna could be further from a nearby noise source. Or closer to the next available noise source. Personally I like a longer wire as it makes for a better SWL RX antenna but as for TX performance if depends on the band, available support height and mounting direction. I have a hunch a dipole at 50 feet will outperform a full wave wire at 25, but if that can be reversed the choice is a no-brainer...........
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G3RZP
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« Reply #4 on: November 10, 2013, 08:51:12 AM »

Jonathan,

If you go to the RSGB book for the Intermediate Licence, there's a brief description on page 60. To go further, get a copy of the RSGB Radio Communications Handbook and/or an ARRL Antenna Book.
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KA4POL
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Posts: 1995




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« Reply #5 on: November 10, 2013, 09:03:52 AM »

Any antenna is some sort of compromise. Imagine a multiple wavelength antenna for 160 m. So the best antenna for you is the one that provides the best way to transmit and receive at a desired frequency in your location. This may be a vertical, a dipole or a beam.
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M5AEO
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Posts: 270




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« Reply #6 on: November 11, 2013, 08:21:24 AM »

Many thanks for all the helpful answers.
M5AEO


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