can transmitters broadcast at other frequencies?

if the antenna is 1 meter long, can it broadcast at higher and lower frequencies?

Your topic says transmitters but the content says antenna.

If you mean will a 1m long antenna work at any frequency, yes, but there is an optimal length for each frequency. It will work when the the length isn't optimal - but not optimally

Brian.

that doesn't tell me if the antenna will broadcast at other frequencies. that does not answer my question. will a 1 meter long transmitter antenna broadcast at higher or lower frequencies?

An antenna is the part of a transmission system that converts a power source to electromagnetic radiation. As stated, the length of the wire and the length of the wave (1/frequency) are related but the antenna itself doesn't control the frequency at all. It is the oscillator circuit providing the power to the antenna that decides the frequency.

You can feed low frequencies to a 1m antenna and you can feed high frequencies to a 1m antenna, it will still work to some degree. For a given source, if you change to a 0.5m antenna or a 2m antenna the frequencies will be exactly the same. Only the efficiency changes.

Brian.

so if i feed .5 meter to a 1 meter transmitter antenna, the antenna will still broadcast radio waves, but the efficiency will be lower, and if i feed 2 meters to a 1 meter transmitter antenna, the antenna will still broadcast radio waves, but the efficiency will be lower?

Hi,

Why do you ask again and again.

The previous posts still answered:
From post#2:

...yes, but...

Click to expand...

It will work...

Click to expand...

From post #4:

it will still work...

Click to expand...

Klaus

In terms of simple theory, as best I understand the most efficient broadcast is at a frequency which generates standing waves in the antenna.

The standing waves are easiest to produce at a harmonic or subharmonic.

Most any sort of antenna can emit radio wave transmissions (aka photons). It's hard to quantify efficiency. My homemade waveform generator goes up to several hundred kHz. It has no antenna yet it causes noises to come from a nearby radio when I tune them around the same frequency. I'm not sure about the nature of the coupling, whether (a) magnetic or (b) electromagnetic.

And who knows what might result if I attached an antenna? Whatever the length, it would be much less than 1 wavelength or 1/2 or 1/4. At 1 MHz a wavelength is 1000 feet. Can my little waveform generator create standing waves in a few inch length of antenna? I doubt it.

Without specifying the antenna type (e.g. λ/2 dipole, λ/4 monopole, different directional and wideband types), there's no simple relation between frequency and antenna length. Also different antenna types have different behavior at off-band frequencies.

The question could be answered more clearly if you ask about a specific antenna type.

Frequency isn't measured in meters. In some contexts the free space or "vacuum" wavelength is used to specify a radio frequency, in the present thread it causes mostly confusion, I believe.

As already stated, there are "electrically short" (length << λ) and also "electrically long" (length >> λ) antennas, they do transmit, the latter more easily than the former.

KlausST said:

Hi,

Why do you ask again and again.

The previous posts still answered:
From post#2:

From post #4:


Klaus

Click to expand...

i am trying to make sure i understand the response. i don't want to be wrong about anything?

Given the OPs apparent lack of understanding of the basic principles, I'm not sure if this will help or not.
Basically you want the antenna to resonate at the frequency (or harmonic) that you are transmitting or receiving. An antenna might look like a piece of wire but it acts like a capacitor in series with an inductor and resistor that are in parallel (as a very simple, 3-element model - better models for any particular type of antenna exist by adding in more inductors and capacitors but the principle is the same). The equivalent inductance and capacitance will resonate at a given frequency when their (complex) impedances (which are frequency dependent) are equal [which means that the imaginary parts cancel].
Now, when a piece of wire is too long for the frequency to want to transmit at (i.e. has too much inductance) you can add in capacitors; similarly if it is too short then you can add in inductance - which is why you sometimes see the bottom of an antenna with a twisted section in it.
Why is resonance important? Because at resonance, the most energy that you feed into the antenna (or is being picked up by the antenna) will be transmitted (or passed back to the receiver). If the frequency you are trying to send or receive does not match the resonant frequency, then energy is lost - for a transmitter that really means it is sent back down the feed to the transmitter (read up on SWR) and for a receiver it means that it is not picking up as much as it could, resulting in a weaker signal.
This is why everyone has been saying that, yes the 1 metre long antenna will transmit, but depending on the frequency you are trying to transmit, the efficiency could well be way down.
Susan

so if i connect the antenna in series with inductors, the transmitter antenna will be shorter and still transmit longer wavelength radio waves than the antenna length?

If that is what you got from my message then you have missed my main point.
There is a whole heap of engineering that goes into antenna design. Note the use of the word "design" - it is key.
You can't just throw random bits together and expect something to work.
At best you will get nothing. You might even blow up the transmitter. At worst you could kill yourself (depending on the power you are trying to transmit).
Also I get the impression that this is an 'X-Y' problem - you have some underlying thing that you want to do (which you have not told us) and you think that slapping some bit of wire onto some transmitter circuit and so are asking about that. I would guess that even if you do get an answer to your question that it will not help you as it is really not the problem you are trying to solve.
How about telling us the underlying problem - perhaps there really us an answer here for you.
Susan

i am thinking about getting into ham radio. i know i will have to get a license. i will work on getting a license. maybe i will use high frequency radio for long distance. but how short can a transmitter antenna be and still broadcast 30 megahertz radio waves? i know the antenna will be most efficient at 10 meters. but can the transmitter antenna be 3 feet long and still broadcast 30 megahertz radio waves? i know the efficiency will be less. but any way to make sure a 3 feet antenna will broadcast 30 megahertz radio waves? as long as the 3 feet transmitter antenna can broadcast 30 megahertz radio waves over several hundred miles, i will be happy.

Adding capacitive or inductive 'loading' to change the apparent length of the antenna doesn't necessarily make it work better but it does make the transmitter happier. Looking at it in the extreme, if you just used a small coil instead of an antenna it wouldn't work very well at all. On the other hand a walkie-talkie on 30MHz would be unwieldy if it had a 10 metre long antenna sticking out of it. Needless to say that you would have difficulty walking under bridges or through doorways with it!

Loading isn't the perfect solution to making usable antennas but it lets a good compromise be found between the electrical and physical requirements. The other factor in this is called 'impedance matching', without going in to boring math, a transmitter is designed to have a particular output impedance, typically 50 Ohms for radio applications and that means it will deliver most power to a load that also has an impedance of 50 Ohms. For any fixed frequency, if you could magically change the antenna length (think of elastic wire) its impedance would swing from low to high according to length. In fact it would repeat the pattern of lows and highs as the length got longer and longer. To get the maximum transmitter power for an antenna that for practical reasons can't be the best length, a loading inductor is added to make the impedances match each other. It isn't a substitute for the ideal antenna but it makes the best of what is convenient.

Brian.

dl09 said:

as long as the 3 feet transmitter antenna can broadcast 30 megahertz radio waves over several hundred miles, i will be happy.

Click to expand...

This does not always work at 30 MHz, even with much larger antennas and high power. Propagation and resulting range varies a lot at 30 MHz.

betwixt said:

Adding capacitive or inductive 'loading' to change the apparent length of the antenna doesn't necessarily make it work better but it does make the transmitter happier. Looking at it in the extreme, if you just used a small coil instead of an antenna it wouldn't work very well at all. On the other hand a walkie-talkie on 30MHz would be unwieldy if it had a 10 metre long antenna sticking out of it. Needless to say that you would have difficulty walking under bridges or through doorways with it!

Loading isn't the perfect solution to making usable antennas but it lets a good compromise be found between the electrical and physical requirements. The other factor in this is called 'impedance matching', without going in to boring math, a transmitter is designed to have a particular output impedance, typically 50 Ohms for radio applications and that means it will deliver most power to a load that also has an impedance of 50 Ohms. For any fixed frequency, if you could magically change the antenna length (think of elastic wire) its impedance would swing from low to high according to length. In fact it would repeat the pattern of lows and highs as the length got longer and longer. To get the maximum transmitter power for an antenna that for practical reasons can't be the best length, a loading inductor is added to make the impedances match each other. It isn't a substitute for the ideal antenna but it makes the best of what is convenient.

Brian.

Click to expand...

you said adding inductive or capacitive loading to change the apparent length of the antenna doesn't make it work better, but it does make the transmitter happier. i am not sure what that means. i know if the transmitter antenna is less than 1 fourth of a wavelength, the efficiency will be less. but how do i determine the minimum length of a transmitter antenna that is less than 1 fourth of a wavelength and the transmitter antenna will still radiate radio waves, using inductive loading?

All of us keep trying to tell you, a three feet antenna will work at all frequencies, so will a 3 inch antenna and a three mile antenna. They will all radiate a signal but for each specific frequency there is a BEST length to use.

Perhaps an analogy will help. Think of a 10 feet (I'll avoid metric units as you seem to be in the US) rope fixed to a wall at roughly your elbow height above ground. You hold on to the other end. The intention is to make a standing wave in the rope, that means you get a high point and low point that stay in the same positions. Now swing your end up and down VERY slowly and see what happens, the rope just tilts, you don't get the fixed shape in the swing. Now move your hand up and down faster and you will find at a certain speed you can get the rope to make a regular pattern, if you go faster the pattern is lost again. Substitute your antenna for the rope and the transmitter for your hand, at a certain frequency, a certain length will give you that standing wave shape. Now repeat the experiment but use a rope 6 feet long. This time you can get the standing waves just as before but you have to move your hand a bit quicker because the wavelength is shorter. When I say wavelength I mean just that, the distance from one end of the standing wave to the same point on the next repeat of the wave. In antenna terms, the standing wave is a voltage or current that peaks at a certain distance along the wire. The frequency at which the waves stand still is called the resonant frequency and what you try to do in a transmitter is achieve resonance at the particular frequency you are using. If you change frequency, it is like moving your hand at a different speed and the standing wave breaks down. Each frequency has a different resonant length. It doesn't mean the rope doesn't move if you wiggle it at the wrong speed just as an antenna that isn't resonant doesn't do nothing, it just works best when it resonates.

The other thing to note with the rope experiment is how much effort is needed to make the rope move, it is harder to do when it isn't resonant than when it is. At resonance the energy in the rope reflects from the wall and your hand and the amount of effort is greatly reduced. In a transmitter the power is like the effort, at resonance less power is wasted so more is available for sending out the signal. when a loading coil or loading capacitance is added to an antenna, it makes the length 'appear' different to what it really is. You cheat the transmitter into thinking the antenna is resonant when it really isn't but in doing so you reduce the losses and make more power available.

The best length is depends on the type of antenna and to a small degree the materials used to construct it. You cannot specify an antenna to achieve a certain distance, many other factors influence how far a signal will travel, most of them to do with atmospheric conditions which none of us have any control over. For example, I have an extremely sensitive radio receiver here and even with a resonant antenna the band around 30MHz is completely devoid of any signals. A little while ago I did a live interview on an Australian radio show where I spoke by telephone but listened back over that same receiver and antenna. I am in Western Europe, the station in Australia is 12,000 miles away. The difference is what we call radio propagation, it depends on many factors, primarily how high some of the upper layers of the atmosphere are and that in turn depends on solar radiation levels and weather patterns along the route.

Brian.

dl09 said:

i am thinking about getting into ham radio.

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Great - now we are really getting to the real problem. And good on you for getting into amateur radio - there are a lot of us around, there are very many aspects (WSPR, digital, moon bounce, fox hunting......) and there are always lots of people who are willing to help when you ask specific questions.

dl09 said:

as long as the 3 feet transmitter antenna can broadcast 30 megahertz radio waves over several hundred miles, i will be happy.

Click to expand...

I know this has already been covered elsewhere.
I'm not sure which country you are based in but there will be both local Amateur Radio clubs you can join and get lots of advice, and also stores that will help you - look at their web sites, possibly for mobile (as they often are smaller) HF antennae.
Again depending on your country, you may have restrictions on the equipment you can use when you get the 'entry' level license (don't let that put you off - this level of license gets you into the hobby and you can grow from there) and some places require that you use commercially available transmitters. Therefore, while you are getting that, you can get an antenna that suits the transmitter - the design aspects have pretty much been done for you so you can start with something that works and later try building something of your own..
Also, depending on your country and the level of your license, you may be limited to the bands that you can use. Here in Australia, the 30m band is restricted to 'advanced' license holders only, although the 40m band and parts of the 80 and 15 m bands are available for all license holders.
Susan VK3ANZ

30 MHz is in the band used for CB radio. It's not likely to travel hundreds of miles. Antennas tend to be a size suitable to fit on vehicles.

Instead the lower frequencies (short-wave and commercial AM) commonly bounce off the ionosphere, traveling thousands of miles.
Wavelengths are longer. They're easier to receive and transmit with an antenna which is correspondingly large.