Cell phone antenna

[DevilsAdvocate]

What is the value of a little antenna in a cell phone? The cell phone I have now has a retractable antenna that can come up about an inch. I never use it because it doesn't seem to make any difference in reception.

I've been looking for a new cell phone. One that I like has a cone on one side that comes up about 3/4 inch, which I think will just be jabbing me all the time the phone is in my pocket.

So it got me thinking: How can a 3/4 inch antenna actually do anything for reception? Wouldn't moving the phone an inch higher do about the same thing? Are the waves the phone picks up so delicate that an itsy-bitsy stick of metal has to deform the shape of the phone with a jabby point? Most cell phones don't have this feature, but many do? What gives? (Since this is the JREF forum, feel free to rant on about phony cell phone antenna boosters while you answer my other questions.)

 

[LTC8K6]

Wavelength of the signal. Higher frequency = shorter antenna, basically.

Remember those long CB whip antennas, or the really long wire used for shortwave? Low frequencies.

 

[DevilsAdvocate]

Wavelength of the signal. Higher frequency = shorter antenna, basically.

Remember those long CB whip antennas, or the really long wire used for shortwave? Low frequencies.

I atill don't get it. I didn't get it with old TVs either. You move the bunny ears one way and get no reception, tweak them another way and reception is clear. At least with the bunney ears there are substantially long pieces of metal moving about. I can't quite grasp how a tiny piece of metal can pick up something that it otherwise would not simply by being raised three quarters of an inch.

 

[Unk]

I can't quite grasp how a tiny piece of metal can pick up something that it otherwise would not simply by being raised three quarters of an inch.

I think that the antenna isn't just that 3/4 inch. The major part of the antenna is in the phone. The part you're pulling out is just telescoping it to a longer length. You may not hear any difference in reception, but the antenna is also transmitting a signal and that extra length might make a difference there.

 

[LTC8K6]

For the best reception, you want your antenna to match the wavelength of the frequency you are using, or a fraction thereof. thus the term "1/4 wave antenna", which is 1/4 of the wavelength of the frequency. The common CB whip is a 1/4 wave antenna.

Wavelength (in feet) = 984 / frequency (in megahertz)

25mhz CB - 984/25=~39 feet 39 feet is too long for a vehicle so we use 1/4 wave or about 10 feet.

2.4Ghz cordless phone - 984/2400=.4 feet or around 5 inches. Too long for our sleek phone so we can divide by four for 1/4 wave and get about 1.25 inch.

You can by now guess that a 5.8Ghz phone is only going to need about a 5/8 inch bud.

I had a handheld CB once that used your body as part of the antenna system.

By adjusting the TV antenna, you are just aligning it with the direction of the strongest signal.

Things like buildings and trees can block signals and reflect them, which is why sometimes a surprisingly small adjustment changes the reception greatly.

 

[Walter Wayne]

Well increasing the length will get you better reception, simply because you are increasing the area you are sampling the electric field in. Having not opened up a cell phone I don't know how much of an antenna is already there, so how much improvement you get is a guess.

One problem that could arise is though, is that as the antenna length becomes significant in terms of wavelength it's directivity increases. This basically means that transmission/reception is concentrated more towards certain directions. As an example, an ideal half-wavelength dipole antenna directs none of the energy along its axis, but instead directs the energy broadside. If you cell-phone had a perfect half-wavelength antennna on it, and you held it pointing the antenna straight up, the signal would be directed horizontal out across the ground, and very little straight up.

Since most people hold the phone at an angle, it wouldn't surprise me if they were designed to be somewhat less than a half-wavelength. I imagine that for practical purposes it really only gives significant aid when your getting a very weak signal.

My antennna experience is outside the cell-phone industry, so take that with a grain of salt.

Walt

 

[CaveDave]

I atill don't get it. I didn't get it with old TVs either. You move the bunny ears one way and get no reception, tweak them another way and reception is clear. At least with the bunney ears there are substantially long pieces of metal moving about. I can't quite grasp how a tiny piece of metal can pick up something that it otherwise would not simply by being raised three quarters of an inch.

Good grief, DA, you have asked for FAR more than you probably expect.
I must assume from the way you framed the OP and your other remarks that you have little or no knowlege of physics (specifically electronic theory, electric and magnetic fields and their interaction as electromagnetic waves, the velocity of light, etc), the basics of radio design, the EM spectrum and the various bands that are assigned to various types of service that lumped together under the heading "cellular communication", or the construction of the internals of a cell phone.

I am not trying to put you down for not knowing these things, it's just that without some basic level of understanding of these concepts, giving you a reason for the antenna design would either be book-length, hoplessly over your head, or would require that you just blindly accept on faith that it just has to be that way.

You could google on such topics as radio theory, EM (electromagnetic) spectrum, antenna design, or similar topics, but if the answer LTC8K6 offered left you clueless, you will need to spend many hours reading many tutorials before the light begins to dawn for you. You have picked an extremely complex question to ask.

If I were you, I would pick up a copy of The Radio Amateur's Handbook published by the American Radio Relay League ( http://www.arrl.org/ ) if you want a one-stop education in radio theory, but don't expect to find the answer before you digest the basics.

Good luck on your quest!

Cheers,
Dave

 

[DevilsAdvocate]

For the best reception, you want your antenna to match the wavelength of the frequency you are using...

Thank you. I guess it makes sense once you break it down. It still "feels" a bit odd that such a small thing makes a difference in a world-wide communication.

It still doesn't seem like a good design choice. And it doesn't seem necessary because most phones (of both higher and lower quality/price) don't have the extended nub. Seems like they are adding the extra nub to get higher ratings on reception quality at the price of design quality. Am I wrong on that assumption?

 

[DevilsAdvocate]

Good grief, DA, you have asked for FAR more than you probably expect.

Good grief, CaveDave, you have perhaps attempted to answer FAR more than I probably expect. A technical explanation is not required or expected. LTC8K6 provided an adequate explanation--in far more technical terms than I expected.

Bassed on TC8K6's comments, a nice reply would have been:

Higher frequency = shorter antenna. The high frequency of cell phone means small changes in antenna length can make big differences. The antenna probably runs the full length of the phone. The reason the phone you looked at has an extended protrusion for the antenna is probably because the engineers calculated the optimum antenna length and it was longer than the phone so the added the "nub".

At least that's what I got from TC8K6's post. I didn't get anything from yours. So: nooney nooney bats on you.

 

[Ririon]

Thank you. I guess it makes sense once you break it down. It still "feels" a bit odd that such a small thing makes a difference in a world-wide communication.

It still doesn't seem like a good design choice. And it doesn't seem necessary because most phones (of both higher and lower quality/price) don't have the extended nub. Seems like they are adding the extra nub to get higher ratings on reception quality at the price of design quality. Am I wrong on that assumption?

You can also use it to stir your drink without the phone itself getting wet.

I used to live in California, and sometimes I tell anecdotes from the US to my friends. My second shortest one goes like this: "I had a cell phone with a pull-out antenna." Always gets a laugh.

(The shortest one goes like this: "I had a checkbook." The response is invariably: )

 

[a_unique_person]

I like the internal Nokia antenna. Not long, but a kind of oval shape. How does that work?

 

[CaveDave]

Good grief, CaveDave, you have perhaps attempted to answer FAR more than I probably expect. A technical explanation is not required or expected. LTC8K6 provided an adequate explanation--in far more technical terms than I expected.

Bassed on TC8K6's comments, a nice reply would have been:

Higher frequency = shorter antenna. The high frequency of cell phone means small changes in antenna length can make big differences. The antenna probably runs the full length of the phone. The reason the phone you looked at has an extended protrusion for the antenna is probably because the engineers calculated the optimum antenna length and it was longer than the phone so the added the "nub".

At least that's what I got from TC8K6's post. I didn't get anything from yours. So: nooney nooney bats on you.

I beg your pardon, I have misplaced my mind-reading hat.

I was trying to be helpful.

LTC8K6 had replied already:

Wavelength of the signal. Higher frequency = shorter antenna, basically.
Remember those long CB whip antennas, or the really long wire used for shortwave? Low frequencies.

To which you responded:

I atill don't get it. I didn't get it with old TVs either. You move the bunny ears one way and get no reception, tweak them another way and reception is clear. At least with the bunney ears there are substantially long pieces of metal moving about. I can't quite grasp how a tiny piece of metal can pick up something that it otherwise would not simply by being raised three quarters of an inch.

Which led me to believe you wanted a deeper understanding.

I will now suitably punish myself for trying to help without the benefit of telepathy.

Cheers,
Dave

 

[Alphaba]

There currently are exciting developments in the technology of cell phone antennas in particular, and antennas in general: the application of fractal geometry to the design (see this popular science paper for instance). It's the first hardware implementation of fractal geometry, and it's revolutionizing the sector of antennas design and making. Both beautiful (see pictures in this page) and super efficient (e.g. true wideband, or multiband antennas with significantly reduced sizes).

 

[casebro]

Even weirder is the 'patch antenna'. Just a rectangular piece of circuit board, with a contact off center. Seems the ratio of thickness between the layers of plating on each side and the size makes for tuning to a particular wavelength. It's how GPS works. Probably other high freq stuff like phones too. I wouldn't be surprised to dissassemble a phone and find nuthin linked to the extendible antenna, it's just a gimmick that the customer can fiddle with.

 

[joe87]

I used to live in California, and sometimes I tell anecdotes from the US to my friends. My second shortest one goes like this: "I had a cell phone with a pull-out antenna." Always gets a laugh.

(The shortest one goes like this: "I had a checkbook." The response is invariably: )

Here in the boonies of rural Pennsylvania, a lot of the small businesses don't accept credit cards because of the 2-5% fee they pay on every transaction. George the plumber will take checks, but he prefers cash.

And my cell phone has a pull-out antenna because I got it in 1999, and most of them were like that then. But the pullout antenna doesn't help much because we don't have decent cell phone reception. This area doesn't have enough population density to support cell phone towers.

 

[arcticbiker]

What is the value of a little antenna in a cell phone? The cell phone I have now has a retractable antenna that can come up about an inch. I never use it because it doesn't seem to make any difference in reception.

It may not make any difference to reception, but it might make a difference to battery life; many cellphones automatically adjust their transmit power to maintain a constant received power at the base station, so reducing antenna performance is compensated by increasing output power.

So it got me thinking: How can a 3/4 inch antenna actually do anything for reception? Wouldn't moving the phone an inch higher do about the same thing?

No, because (as others have already said) it's not just the position of the antenna, but its total size that matters (and most of the antenna is in the phone).

Are the waves the phone picks up so delicate that an itsy-bitsy stick of metal has to deform the shape of the phone with a jabby point? Most cell phones don't have this feature, but many do? What gives? (Since this is the JREF forum, feel free to rant on about phony cell phone antenna boosters while you answer my other questions.)

In general, larger antennas have better performance (all other things being equal), so having an extendable whip can improve performance, but the trade-off is convenience. Most people would like the phone and antenna to be as small as possible (except for the keypad and screen, which have usable limits).

 

[Ririon]

Here in the boonies of rural Pennsylvania, a lot of the small businesses don't accept credit cards because of the 2-5% fee they pay on every transaction. George the plumber will take checks, but he prefers cash.

And my cell phone has a pull-out antenna because I got it in 1999, and most of them were like that then. But the pullout antenna doesn't help much because we don't have decent cell phone reception. This area doesn't have enough population density to support cell phone towers.

In Europe (or Norway at least) you pay a fee in the blood of your first-born for checks. (OK, about $10-$20/check.) Most transactions are by debit card (or cash for small amounts or criminals.) Regular people used their last check in 1985. People will use what is convenient and cheap. I kinda liked the checks.

And my cell phone was bought in 2004.

 

[RationalVetMed]

I will now suitably punish myself for trying to help without the benefit of telepathy.

I knew you were going to say that .

Yuri

 

[macgyver]

Good grief, DA, you have asked for FAR more than you probably expect.
Dave

You mean you're not going to pull out your Smith chart and perhaps discuss stripline design on this forum? Darn, I was hoping for a bit of a refresher on stub matching and RF circuit design.

Basically it all comes down to geometry for the antenna. In the case of a simple monopole like in a cell phone, the majority of the antenna is hidden in the phone case. I'm guessing here, but I suspect for the purposes of asthetics, the antenna is not of optimum length for it's operating frequency, so by extending it a little bit, you are "fine tuning" the antenna for a better match. This would effectively increase it's gain (or perhaps more accurately reduce it's attenuation). Since most cell phones these days are digital, you would not hear a difference in reception, per se, but rather an improvement of operating range and, as mentioned, battery life.

 

[megafly]

I wish the vast majority of Americans were smart enought to feel as silly and backward as I do when people in other countries talk about their technologies.
ie: broadband, banking, cellphones, infant mortality etc.

They might not be such chauvanist nationalists if they actually compared things rationally.