Ready for stupidity? Does the reception of radio waves by a radio or TV diminish the signal strength of those waves?Even by an infinitesimal amount?
Isn’t that like asking whether the amount of light in a room is reduced by the amount that hits our eyeballs?
Well yeah… The eyeballs, and a whole lot else…
Yes,some the waves/photons are abssorbed in exactly the same way they would be is they had struck a brick wall or other solid object, therefore the signal strength is decreased in the same way. If you put a large number of rabbit ears in front of your reciever you would get no signal at all, just as if you put a thick enough metal wall there you would get no signal.
Ok, ok, I knew I was opening myself up for some, “Boy was this a dumb question,” types of responses.
Let me try to salvage some dignity. Is there any wavelength specific loss of signal? Above the non-specific losses y’all are raking me over the coals for. Something akin to light absorbance by colored compounds?
Well now you’re talking about simple filtering. Selectively damping out a specific wavelength (frequency) and leaving others alone is what a filter does, and there are many kinds of filtes used inside electronic devices.
As for whether or not this happens outside of such devices, I am going to say yes. Antennas are “tuned” to work best at particular wavelengths according to their physical & electrical sizes (what the hell does “electrical size” mean, you ask? An antenna can be made to seem longer or shorter - and therefore work better at certain wavelengths - by adding certain electronic components to the adjacent circuitry. An example of this is called “base loading” where an inductor is placed into the base of an antenna to make it act as though it were cut to a much longer physical length).
So if you have a bunch of antennas all tuned to a particular wavelength, they will absorb radio waves of that wavelength, reducing the amount of energy available for other antennas that happen to be tuned to the same wavelength. Have I used the word “wavelength” enough? Wavelength wavelength wavelength.
I don’t think this will ever present a practical problem, because broadcast power is almost always sufficiently high to satisfy whatever load is placed on the signal by receiving antennas.
So the filtering is only a byproduct of the antenna? The tuner connected to it will not influence which wavelength is filtered?
That depends on what you call “tuning”. You can “tune” the antenna, as Attrayant said, both by trimming the physical antenna itself (often done by ham operators 1/16 inch at a time), and by adding (usually) passive components such as inductors and capacitors. These changes will affect the electrical impedance presented to the impinging radio waves, and affect how much energy is absorbed.
Most modern receivers don’t let you tune the antenna directly with a knob (exceping ham and other specialized radios, of course :)). There, the manual tuning is done after the signal is picked off the antenna. The parameters you vary with by tuning with a knob at this point in a receiver doesn’t affect how the antenna appears to the radio signals in the air.
Alright choosybeggar, what are you up to? Don’t make us have to call the Power Puff Girls on your ass.
Most general purpose antennas are broadband, and work fairly well over a wide range of frequencies. A standard rabbit ears, for example, has an effective bandwidth of about 60-70 MHz (I’m speculating here, assuming 6 MHz per channel & 11 channels from 2-13 in the VHF range). It’s the tuner that does the filtering & allows the desired band of frequencies to pass into your television’s front end for further processing.
Not many antennas are very “narrow band” in their electrical characteristics, we find it easier to make a cheap braodband antenna and rely on the tuning circuitry to do the grunt work of filtering out the unwanted frequencies.
You’re going to have to tell us what kind of fiendish plan you’re cooking up before you’ll get any actual blueprints from us.
[sub]Disclaimer: This question arose as a means to enhance my efforts at procrastination.[/sub]
Ok, so there’s a consensus on antennae: they can be tuned to sop up selective regions of the RF spectrum. The effect on overall broadcast signal strength is probably immeasurable, but at least there’d be one.
Can the analogy be extended to tuners, though? Something in the tuner (quartz crystal??) is manipulated to resonate at a desired frequency. The resonance is driven by the signal collected by the antenna, selectively reducing the energy at this wavelength. This creates more “space” in the antenna for the tuned frequency. Thus the antenna grabs just this frequency with higher avidity.
I’m not looking for a big effect, or even a necessarily measurable one. But given the physics of RF reception, is my hypothesis full of shit, somewhat less than full of shit, or without a trace of fecal coliforms anywhere?
A tuner on a modern digital radio will not affect the RF absorption of the antenna. Active amplification stages between the antenna front-end and the tuner decouple the tuner from the antenna, which eliminates tuner effects from changing the antenna impedance (vs. frequency), which means the antenna’s RF absorption doesn’t change with tuner settings.
As I mentioned earlier, older radios, and specialized radios such as ham radios DO have an “antenna tuner” control(s) in addition to a standard tuner. THAT tuner will affect the antenna impedance (in fact, that’s its function). All radios have something like this, but a normal AM/FM radio doesn’t let the user adjust it- the settings are fixed.
So, to sum up- a standard AM/FM radio’s tuner does NOT affect the amount of RF energy absorbed by the antenna. A specialized radio with an “antenna tuner” DOES affect the RF energy absorbed, if you move then antenna tuner settings.
The actual amount of RF absorbed depends on the antenna shape, orientation, and any front-end circuitry is present (include antenna tuner settings, if present).
The energy is very small, but still powerful enough to drive an earpiece- you can use a crystal AM receiver with no external power!
It depends on your radio.
In an old crystal set, the antenna was intimately connected to the tuner and changes of tuning would change the “filtering” of the antenna.
In modern radios there is generally no “tuned circuit”, just an antenna hooked on to the mixer. If there is a low-noise amplifier is in between the mixer and the antenna then there is actually isolation and nothing that happens on the tuner side affects the antenna – it has just the same effect as a piece of wire sticking in the air.