How does the antenna length affect signal transmission/receipt in different bands?

Question

My Canadian Amateur Radio Basic Qualification Study Guide tells me that my antenna length is specific to the wavelength I wish to broadcast on and gives a formula that equates to antenna length = half the desired wavelength. In a number of places it also suggests that the length of the antenna must support a standing wave of the radiated frequency exactly in its length.

Given the propagation effects of the atmosphere, I'm guessing that I am going to need to chop and change between bands in order to talk to people locally and around the world depending on the time of day, phase of the moon and whether it's raining in Helsinki while my Uncle Bob is having his coffee in Copenhagen. Will I need different length antennas to be able to do this?

I have a handheld Yaesu VX-7R radio which appears to do okay on multiple bands, but I clearly don't change the antenna to flick between them. My bedside clock radio also doesn't require a different length antenna for each frequency... so all of my practical experience of radio (which admittedly is virtually nil) tells me that I'm not understanding what the book is telling me. Can I broadcast to higher frequencies using an antenna designed for the 160m band?

Can someone clarify what my book means when it says "exactly in its length"?

Answer 1

It sounds like your study guide is conflating a few issues, and oversimplifying others.

Firstly, in antenna engineering, there are concepts of physical length and electrical length. Physical length is measured with a ruler, and is pretty straightforward.

Electrical length how long the antenna seems to the currents travelling in them. For example, a vertical is typically a 1/4 wavelength long, but because there's a ground plane below it which makes an image antenna, electrically it looks like a 1/2 wavelength. There are also ways to "electrically lengthen" an antenna, for example loading coils and capacity hats¹.

Note that these concepts of "length" don't apply to all antenna types. For wire antennas it makes sense, but what about a fractal antenna or a Vivaldi antenna? These antennas aren't long thin wires, so it's not clear how any concept of "length" would unambiguously apply.

More generally, it is usually the objective to make the antenna system match the characteristic impedance of the feedline (usually but not always 50 ohms), either by making the antenna resonant and selecting an antenna type with the desired radiation resistance (50 ohms), or by adding a matching network to adjust the antenna impedance to match. The reason we care about impedance matching for transmitters is to maximize power transfer from the transmitter to the antenna. For receivers, this is less important.

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Regarding changing frequencies for different propagation conditions, this is true on HF, where the ionosphere plays a significant role in propagation. Ionospheric conditions vary by time of day as well as solar activity.

On VHF and UHF, this is not a concern, because the ionosphere is mostly transparent at these frequencies². Rather, propagation is by line-of-sight, and the difficulties you encounter are largely due to obstructions (buildings, mountains, etc) being in the way. There is some difference in how particular frequencies can penetrate obstructions, but mostly propagation on VHF and UHF is predictable and consistent.

^{1: I haven't included a link to capacity hats because I couldn't find a good reference. Beware of sites on antennas written by hams (like this one): hams love to talk about antennas with authority, but few of them are qualified to do so (including me). Remember that it's amateur radio.}

^{2: notable exception: troposcatter.}

Answer 2

Using the 70cm band as an example, The respective frequencies are 420-450 MHz, yielding wavelengths between 71.4 & 66.7 cm. This varies from +2% to -5% of the nominal 70cm wavelength. Fitting an antenna to the nominal frequency/band plan suffices for frequencies in said band plan.

While I'm not sure about the antenna length being half that of the nominal wavelength, my 2m/70cm handheld txer antenna is about a sixteenth (2^-4) wavelength long for the 2m band, and a quarter (2^-2) wavelength long for the 70cm band.

I'm not sure what "exactly in its length" means.

Answer 3

Half-wave antennas have one major advantage: They do not require a ground plane. No need to mount it on a car, or put down radials in the ground, etc.

So by their definition that would be "the length."

Quarter-wave verticals require ground planes (which should be roughly 5% bigger around than the antenna is tall. For AM broadcasting this means putting down 120 wires from the antenna mount point (every 3 degrees). Then the ground plane acts as a reflector to provide the other half of the half-wave length.

As a practical matter, unless you are Nikola Tesla, you want the feedpoint to be at the middle of a multiple of half-waves so the voltage is at a low while current is at a high. Handheld radios do not have that luxury, hence their definition.

Your clock radio doesn't need a specific antenna length for each station because 1) The stations are nearby and 2) They transmit a lot of power. Receiver sensitivity is not an issue in those situations.

For DX (long distance), you will want a resonant antenna so you capture as much of a weak signal as possible, because receiver sensitivity is finite, and you want to give it as much to work with as possible.

Receiver sensitivity is only half the story; the other is the noise floor. All circuitry produces some amount of noise that gets amplified with the signal. This includes external sources such as power lines. So using a resonant antenna also helps filter out undesired external noise.

For operation on non-resonant antennas you will want an antenna tuner. So yes, a 160 Meter antenna can work other frequencies.

The rule of thumb is that you want to try to use odd multiples of antenna length to get as close as possible to resonance. So, for instance, a 40 Meter dipole will work OK on 15 Meters. But since it isn't a perfect match it always helps to have a trusty antenna tuner to dial it in better. You can tune it by ear, or, if transmitting you can use an SWR meter to find the best match.