Antenna Length & Resonance - Please Educate Me

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ultravista

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As a novice in the world of radio, I want to understand antennas. Can someone explain antenna resonance as it pertains to electrical length? I am having a hard time wrapping my head around the concept.

My concept is that antenna ‘resonance’ is akin to a tuning fork humming – the EMF wave excites and vibrates the element. Picturing a sinusoidal wave, the wave’s length matches the antennas element perfectly and it becomes resonant. As the element becomes shorter or longer, the wavelength doesn’t match the electrical length and resonance moves elsewhere.

Let’s talk small lengths, such as a ½ wave 10 meter (28.3 MHz) dipole @ 16.5 feet. What actually happens when the antenna is a full, 3/4, 1/2, 5/8 or 1/4 wavelength? What is ‘resonating’ and is ‘resonance’ a product of the electrical length? If yes, what specifically about the electrical length makes an antenna resonate?

Does a ‘normal’ resonant 1/2 dipole perform with a greater efficiency than a coil loaded dipole? If they are electrically the same length, but a portion of the element is a coil, how and why does it perform in a different way?

How does a resonant loaded dipole (or other) antenna work when it is considerably shorter than normal? For some designs, the coils are very tight with little or no spacing. Why not make a super coiled physically short 160 dipole? One could easily coil a 250 foot long 160 meter half wave dipole into a compact antenna – right?

All of this is to understand how to make an electrically long MW receiving antenna. Instead of a long wire, why not load it up to make it electrically longer for the purpose of resonating more efficiently?

Antenna resonance for Dummies – that’s what I am looking for …
 

W3DMV

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The answers to your questions will consume more pages than this
form can provide.
you might consider purchasing a "antenna handbook" published
by American Radio Relay League. This book covers all the theory
your interested in plus many construction projects. It's about two
inches thick but will answer your questions...
 

ka3jjz

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This is but one of many sources for the ARRL's Antenna book, and as you can see, there are any number of books that are on this subject...

Antenna Books

This is a very complex topic; I'd suggest sticking with the basics understanding things like resonance and how to cut an antenna to be resonant first. You can delve deeper into the theory later.

Mike
 

RFBOSS

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Here is some basic information for a staring point per your questions.

As a novice in the world of radio, I want to understand antennas. Can someone explain antenna resonance as it pertains to electrical length? I am having a hard time wrapping my head around the concept.


....Theory tells us resonance is when the capacitive reactance and inductive reactance are equal. This happens when the antennas electrical wavelength matches the desired operating wavelength/frequency.

My concept is that antenna ‘resonance’ is akin to a tuning fork humming – the EMF wave excites and vibrates the element. Picturing a sinusoidal wave, the wave’s length matches the antennas element perfectly and it becomes resonant. As the element becomes shorter or longer, the wavelength doesn’t match the electrical length and resonance moves elsewhere.

....Yes, mostly.

Let’s talk small lengths, such as a ½ wave 10 meter (28.3 MHz) dipole @ 16.5 feet. What actually happens when the antenna is a full, 3/4, 1/2, 5/8 or 1/4 wavelength? What is ‘resonating’ and is ‘resonance’ a product of the electrical length? If yes, what specifically about the electrical length makes an antenna resonate?


....Again, it is when the capacitive reactance and inductive reactance are equal. This happens when the length of the antenna is correct in terms of wavelength/frequency. If the physical length is such that the capacitive and inductive reactance are not equal (if the antenna is too short it is more capacitive and if it is too long it is more inductive) such as a just a random length antenna, capacitance or inductance can be added to the antenna as needed.

Does a ‘normal’ resonant 1/2 dipole perform with a greater efficiency than a coil loaded dipole? If they are electrically the same length, but a portion of the element is a coil, how and why does it perform in a different way?


....Yes it does, There are losses associated with the coil (inductor) and antenna capture area (larger is better in general terms). The loading can make reduce the bandwidth of the antenna. ....Again, it is when the capacitive reactance and inductive reactance are equal. If the physical length is such that the capacitive and inductive reactance are not equal (if the antenna is too short it is more capacitive and if is is too long it is more inductive) such as a just a random length antenna, capacitance or inductance can be added to the antenna as needed.

.... For example a physically short CB antenna will have added inductance somewhere (base loaded, center loaded or top loaded). The goal is for the electrical impedance at the feed point be as close as resistive (resonant) as possible and this impedance match the impedance of the feed line and the transmitter/receiver.

How does a resonant loaded dipole (or other) antenna work when it is considerably shorter than normal? For some designs, the coils are very tight with little or no spacing. Why not make a super coiled physically short 160 dipole? One could easily coil a 250 foot long 160 meter half wave dipole into a compact antenna – right?


....There are losses associated with the coil (inductor) and antenna capture area (larger is better in general terms). If the antenna is really short, these losses will add up and the antenna may not perform well.

All of this is to understand how to make an electrically long MW receiving antenna. Instead of a long wire, why not load it up to make it electrically longer for the purpose of resonating more efficiently?

Antenna resonance for Dummies – that’s what I am looking for …
When it comes to receiving antennas (and even transmitting antennas), non resonant antennas may work okay. With a transmitting antenna a device (commonly called an antenna tuner) may help match the impedance of the non resonant antenna to the transmitter. At times an antenna tuner can help with a non-resonant antenna, especially if the feed point impedance is very low or very high. This is just some very basic information. As mentioned previously there is a wealth of information on the internet and in books.
 

KC4RAF

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As others' have posted, start with ONLY the simple theory of antennas. Here's why:
Antennas are very complicated. Take for example a transmitter. There are many different components in that radio; resistors, capacitors, ICs, coils, etc. And those parts all have very short theory to them.
Now the antenna is part of that system so that you can transmit. But it has the largest amount of theory per part of a transmitting station!!!
There are many, many books on antennas; but there's only one small pamphlet detailing resistors.
Do as others've posted, start at the part that you feel comfortable with and study that first, then expand your knowledge later.
Who knows, maybe you could design a better antenna as time moves on.

I like the analogy of a pebble dropped in a pond and the ripples it made going out in circles. That's very similar to an antenna sending signals.
 

majoco

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My take on this 'resonance' goes right back to basics.

You feed the centre of a dipole with a low-impedance feeder - the coax cable, usually a nominal 50 ohms.

Now, what about the far end? It's open circuit. What's the only thing that can exist at an open circuit? Only voltage, no current. Now if you imagine a quarter-wavelength of current and it's associated voltage along this leg of the dipole back to the centre, as the voltage is getting lower, the current is getting higher. Simple Ohm's law says that the impedance is dropping until we get right back to the centre where hopefully it's all current and no voltage. Of course that's not quite the case, there's is usually an insulator there or the very low impedance secondary of a transformer, just a few turns of wire.

Now what if the frequency being applied is not the right one for the antenna leg - too high and the low impedance point is somewhere along the wire, too low and the low impedance point has not yet been reached.

In both cases there will not be the low impedance at the physical centre, there will still be some power there which will go back down the feeder and will be indicated on our VSWR meter. Some of your hopefully radiated power is now cancelled reducing your efficiency so reducing the signal by your intended receiver.

Oops - now we're getting complicated again - so what's the final point? For best efficiency, each leg of a dipole must be a quarter-wavelength long or an odd multiple of it.
 
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prcguy

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A lot of good information has been given and there is not enough room here to cover the subject in its entirety. Some good things to understand is that resonance does not necessarily mean the antenna doesn't radiate well. You can make a random length dipole of reasonable size for a particular frequency, feed it in the center with balanced feedline to a tuner and it can work about the same as a resonant 1/2 wave dipole fed with coax. Even if it has a 10:1 or worse VSWR.

Another thing to keep in mind is antennas can be resonant but not provide a workable match to the feedline and a full wavelength dipole (1/2 wavelength on each side) fed with 50 ohm coax would be an example. The feedpoint impedance of that antenna can be several thousand ohms and will be very lossy if fed with a long 50 ohm feedline.

A final thought is the radiation pattern, which will change depending on its length with respect to wavelength. For example a horizontal 1/2 wave resonant dipole will have a radiation pattern that is mostly broadside to the wires with little radiation off the ends, depending on height and surroundings. Make that a full wave dipole and it will have four lobes with a null broadside and four corresponding nulls. Make that a 2 wavelength dipole and now you have 8 lobes and nulls. Not that you would want to make a full or 2 wavelength dipole, but using any dipole across a broad frequency range will run into the same scenario.

You can shorten an antenna by using loading coils but you give up something useful. One thing is efficiency goes down, another is band width gets narrower and feedpoint impedance usually goes down making it more difficult to match with coax. You can make a 160m dipole that is only 20ft long but it may work 30dB or more worse than a full size and will have only a few KHz of usable band width.
prcguy
 

Boombox

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"All of this is to understand how to make an electrically long MW receiving antenna. Instead of a long wire, why not load it up to make it electrically longer for the purpose of resonating more efficiently?"

This is actually done -- some MW loop antennas consist of many feet of wire. Of course, as the others here have said, antenna theory is complicated. When you loop a wire you're dealing with inductance as well as the length of the wire used.

The best thing to do is read some articles on specific antennas used in the bands you want to DX, and go from there. Experiment and build. You can do really well with an antenna without necessarily understanding all the theory.
 

paulears

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He wanted to know so badly, and doesn't seem to have returned?

For what it's worth the ARRL book is still the suggested source material for UK hams - even though it's big, complex and sometimes tricky to digest - but it does tell the story if you work at it!.

Her in my coastal easterly part of the globe, the US built a state of the art, complex and huge antenna farm during the cold war - after many years of trying, they abandoned it because it never did work properly - so not just hams having trouble!
 

ka3jjz

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Hopefully he's gone to find some books to study; he has another thread here with other antenna type questions.

If he does return, everyone should be aware that he is tuning MW and below, so the answers you give should reflect this.

Antenna theory can be very dense; if he starts with the basics, he'll have much of what he really needs

Mike
 
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