# Different antenna lengths for different frequencies?

I just want to understand a few things which I couldn't get clarity from a lot of websites searched through google as I could see differing information.

My questions :

1. How does a single FM antenna length works in the entire FM frequency band range, when there are theories which state that the antenna size should always be close to wavelength/4 for proper reception & transmission?

For example : The FM frequency band ranges from 88MHz to 108MHz. How will an FM antenna length will match all the frequencies from 88MHz to 108MHz? Does the antenna length scale itself within that range or how does it work?

1. Why does low frequency signal have high range but high frequency does not?

Does FM and AM require different antenna sizes?

Not only will they be different lengths but usually completely different types of antennas. For instance, a medium/long wave (circa 100 kHz to a few MHz) receiving radio will use a ferrite rod antenna like this: -

And this antenna is totally unsuitable for FM transmissions in the 88 MHz to 108 MHz band (and above). FM radio will use a monopole like this: -

Different types of FM radios may use a dipole like this: -

Or even a folded dipole like this: -

You can even use Yagi Uda antennas like this: -

How does a single FM antenna length works in the entire FM frequency band range, when there are theories which state that the antenna size should always be close to wavelength/4 for proper reception & transmission?

It's a compromise - it will tend to be perfectly "on-song" at 98 MHz and slightly less "on-song" at 88 MHz and 108 MHz. The signal it pulls from the air will be about the same amplitude at either end of that spectral range but the impedance characteristic of the antenna will vary across the range. Good receiver design ensures this won't be a problem: -

Does the antenna length scale itself within that range or how does it work?

No, it's a compromise.

Why does low frequency signal have high range but high frequency does not?

The transmitted range is exactly the same but, because a receiving quarter wave antenna (for example) will be proportionally smaller at higher frequencies, the "net" that captures radio energy is also proportionally smaller and hence it captures less emitted power. Think about an LED and a photosensitive detector. If the detector only has a small aperture to let light in, it will pick up less light power than a detector that has a large surface area.

• Thank you for your answer. But after a little bit contemplation on it, I had this doubt : There are many modules in our mobile device. Like we may have separate antenna for FM band, WiFi band, GPS and so on. So, when an incoming signal of say freq within the WiFi band, hits the FM antenna, what happens? Does it just ignore or the antenna doesn't get hit by the signal? And same vice vera for a FM band freq signal, hitting a WiFi antenna? May 10, 2020 at 16:07
• A commercial FM band radio broadcast will see a WiFi antenna as a short piece of metal with high impedance. Look at the impedance graph in my answer. I can't speak any more about what goes on inside mobile devices because I just aint too sure. It wouldn't surprise me if the actual FM radio stations aren't picked up at some other unholy transmit frequency but, there's one thing I do know; going back a few years (and it may still be true), the actual FM antenna was the headphone lead and the clever tech picked-up the FM broadcast signal from that wire (using clever circuits). May 10, 2020 at 16:16

But since AM, FM and all work in different frequencies, do they have separate types of antennas or similar type of antennas with different lengths?

Generally, you can use the same antenna style and just scale it – that is, until either things get mechanically hard to do, or side effects that you could ignore on one frequency ruin your day.

Also, antennas fulfill different use cases, and that defines what shape they have and what tradeoffs, e.g. in terms of efficiency versus size, are made

Why does low frequency signal have high range but high frequency does not?

That's simply not the case – waves propagate to infinity, and the area power density always drops with the square of distance; see Friis' transmission equation for details.

What's different is that you also get the square of frequency as loss over distance, but fun fact: you also get the square of frequency in effective antenna area per mechanical antenna dimension, so, if you keep your antennas the same size, the effects cancel out (as long as you can use that larger effective area, which usually requires knowing the direction the wave travels).

What people like to forget: a MW broadcasting tower has in the order of 100 kW of power. Your bluetooth device has at most 10 mW. That's a difference of 10⁷, and that alone yields means you get $$\\sqrt{10^7}= 10^{3.5} \approx 3160\$$ times less reach at the same receive power if everything else was the same.

• With respect to lower frequencies having more range: I don't think it is purely Friis or transmit power. I believe a lot of the lower frequencies used in AM and FM also rely on surface waves and atmospheric guiding or the signal, on top of the vast difference in transmit power. May 10, 2020 at 9:49
• well, yeah, but then again, your satellite TV receiver receives ca 10 GHz from a geostationary orbit, and that's 35,000 km away from you – at least (and that'd be the case if you're standing right on the equator, directly below the satellite). You'll have received few HF transmissions that went as far, and exactly zero of these with a usable bandwidth in the gigahertzes :) May 10, 2020 at 9:56
• Very true, but that 10 GHz link might use dishes on the home-side that are almost 40 dB, and on the satellite itself you might get far more gain. That almost 100 dB in antenna gain (and on top the lower thermal noise because space is cold) makes up a lot of pathloss. But all of this is beside the point, and your answer is correct, I just wanted to add an additional point. May 10, 2020 at 10:00
• no you can't. As I said twice now, the length has to match the frequency. Bigger isn't better. May 10, 2020 at 11:11
• you know what? that's a pretty different question from the one you're asking in your question, and you've asked 5 questions since you said "last question", and that's not what we should be doing in the comments. Please edit your question to reflect what you're actually asking. I will stop replying to the comments here! May 10, 2020 at 11:18

One thing others have failed to mention is that most modern automotive broadcast receiver antennas today are integrated antenna modules. Most new car antennas resemble a wing or fin they actually contain 3 to 5 antennas internally. Regardless of whether they are some sort of whip or a patch type most all are voltage probe type antennas. Meaning that they are active antennas. They usually have a preamp on each Frequency band. Some are Gain type preamps others are unit gain and the preamp is for isolation and impedance matching. Now a days the GPS, Bluetooth, or other 2.4gig antenna, and Cellular antenna all have separate coaxes coming from the antenna housing and still another coax for FM & AM combined. So in times past back in the simpler times you had roughly a 30" whip which was roughly a 1/4 wavelength antenna on the 76~108 MHz band, and since there was no practical way to have a 1/4 wavelength antenna at 500~1700 KHz the 30" antenna did the best it could. Back when I was a kid car antennas were 3 sections all telescoping out to about 60 inches this was mostly for AM .

• Those are all interesting tangential tidbits, but you have not actually answered either of the specific questions which were asked Jun 24, 2020 at 2:55

The math for gathering energy from the "ether", using a single-wire antenna, will show the gathered_energy proportional to the "antenna length" squared. That is, proportional to the AREA of ether affected by the antenna.

Thus a vertical whip antenna resonant at 1MHz will gather 100X the energy gathered by a vertical whip antenna resonant at 10MHz.