Many many years ago when I worked at radio shack, we were taught that an antenna has to be at least as long as the wavelength it receives, (which was why antennas were either several feet long, or had a coil at the base). However, Wikipedia shows that some of the lower frequencies (including the AM range) have wavelengths of several kilometers, so obvious that’s not the case for all frequencies.
Can somebody explain this apparent contradiction? While we might have been taught marketing propaganda, that seems unlikely given the hobbyist nature of our clientele back then.
also the antenna can be other lengths besides those fractions with the use of capacitive and inductive elements either as part of the antenna or part of a tuner/matching circuit.
A half-wave dipole is an efficient antenna. It has a total length of half of a wavelength, and has an impedance that’s close to real (as opposed to complex). If you have a transmission line with the same impedance, most of the energy received by the antenna will go into the transmission line, instead of being reradiated away from the antenna. That means you’re maximizing the power received.
If you had a wire that was 1/2 wavelength long, its would naturally oscillate (electrically, not mechanically) at about that frequency, analogous to how a guitar string vibrates at a certain frequency depending on how long it is. Shorten the string by fretting it, and it vibrates at a higher frequency.
For a car antenna, the wire antenna is half of the dipole, and so should be about 1/4 wavelength long. ETA: For FM, it is close to 1/4 wavelength.
For AM, your antenna is much smaller than 1/4 wavelength. It will still receive energy, but its impedance will have a large imaginary part, so much of the energy it receives is scattered away. Adding coils to the antenna adds an imaginary part with the opposite sign, making the antenna impedance be closer to real. It would be better to have an antenna 1/4 wavelength tall, but then you’d have to build a tall tower to essentially be the antenna.
An antenna works best when it’s equal to the wavelength of the frequency being received. An antenna which is not matched to the signal wavelength still works, just not as well. This can be to a degree overcome with better, more sensitive receiver electronics, especially when the signal is very strong in the first place.
Antennas which are a quarter or half the wavelength also work fairly well, if not quite as well as a full-wavelength antenna.
It is also possible to add additional electronic components to the antenna to change the electrical resonance of the antenna, making it act like a longer antenna. This is commonly done to keep antenna lengths manageable for low-frequency signals.
ETA: Zenbeam did a much better job of explaining it than I did.
a loading coil is part of an antenna that has a straight element.
a coiled antenna in a ferrite bar has the whole antenna in a coil.
the coil of both has inductive properties.
a loading coil on a straight antenna gives some straight antenna properties (the pattern of the signal) which is important for two way communications. for a AM broadcast receiver the coiled antenna is a design compromise but it works OK for that function.
So does it still follow the rule of being some fraction of the wavelength?
Is there is a simple explanation for why that matters. I’m guessing it’s going to be analogous to normal harmonics, but that seems bizarre.
Slightly off topic. What about reflectors? I’ve noticed that HDTV antennae seem to have them. Is the distance between the receiving element and the reflector following a similar rule?
You’d want the reflector to be about 1/4 wavelength away from the driven element. The reflector shorts the electric field right at its surface, and will roughly double the electric field 1/4 wavelength away (and the field would go to near zero again at 1/2 wavelength).
I have a question on that. I’ve seen the design of an antenna with 1/4 lambda wire attached to the center of a square sheet with sides 1/4 lambda with the idea being that it makes a really strong dipole antenna.
Why wouldn’t you use a disk instead of a square so that every measurement from edge through the lead wire is 1/2 lambda?
This is untrue. Full wavelength radiators have several problems. The peak radiation lobes are two cones that seldom provide gain in desirable directions.
The most basic antennae are a single quarter-wavelength radiator operated against earth or a conductive ground-plane, or two quarter-wavelength radiators operated against each other (AKA half-wavelength dipole) . Nearly all AM broadcasting uses 1/4 wavelength radiators, either singly or in phased arrays to provide directivity. FM and TV broadcasting usually use dipole based arrays, as do many cell installations. The short wavelength of cellular service makes some fairly exotic antenna configurations practical that are seldom if ever seen at lower frequencies.
By using inductive loading coils, shorter radiators can be made to behave as half or quarter wavelength radiators, albeit at the sacrifice of efficiency and bandwidth.
It is possible to feed a 1/2 wavelength radiator at one end instead of the middle if suitable impedance transformation is made to match it to a typically low impedance feed line.
When a full-wavelength radiator is desired, it is usually split electrically into two in-phase half wavelengths. This requires a phase inverting network (either LC circuit or a quarter wave stub) at the midpoint. This would be known as a colinear array. This provides a donut shaped radiation pattern like a dipole, but with better directivity…kind of a squished donut that got thinner but bigger as compared the simple dipole.
