Cell phone areal length

[Thunderchild]

I memory serves speed of light / frequenzy = wave length
wave length / 4 = length of a whip areal. is this correct.
my mobiles areal seems desighned on this basis but if I were to say double the length would it help reception/transmission ? what about making it 4 times the length has anybody ever done this ?

 

[Nigel Goodwin]

I memory serves speed of light / frequenzy = wave length
wave length / 4 = length of a whip areal. is this correct.
my mobiles areal seems desighned on this basis but if I were to say double the length would it help reception/transmission ? what about making it 4 times the length has anybody ever done this ?

You can't just make aerials longer, they are specifically tuned to match the impedance from the transceiver - if you alter the length you alter the impedance, and it doesn't match correctly any more - so performance is reduced.

Essentially 'gain' from an aerial results from making it more directional, although improvements to very short aerials are possible be making them the correct length.

The 'standard' vertical aerials are:

Helical whip
1/4 wave whip
1/2 wave dipole (basically two 1/4 wave whips)
5/8 whip (using a tuning coil to correct the impedance)
Stacked co-linear

It's NOT a simple science, it's a very complicated subject - as any changes would have to be external to the phone anyway, effectively making it non-mobile, you may as well just use the existing aerial mounted on the roof of your house and connected down via coaxial cable.

 

[audioguru]

The little antenna on my Nokia cell phone protrudes out only 1.5cm from its case and I'm sure there is more of it inside, and it works perfectly.

 

[Thunderchild]

yea thats basically what i thought. problem is i have a real problem picking up the umts network at my house. the crap head that located the base station put it so low that all the power and phone cables in the neibourhood are directly in the path between the base station and my phones position.

 

[RadioRon]

Most cellphone antennas are designed to provide good performance in two frequency bands, one around 900 MHz (a bit higher in Europe, a bit lower in North America, for example) and the other around 1800 MHz (again, Europe band is a bit lower in frequency than the North American band, and then there is the UMTS band in Europe slightly higher than the North America band). So, the impedance matching situation gets kind of complicated. If you decided to modify the antenna by adding some additional wire length, I would say that chances are about 98% that you will make it perform worse on one or the other band, and about 2% that you got lucky and it improves in both.

Its also interesting to note that in the lower band, the one around 900 MHz, the so-called antenna is in fact only half of the radiating body. The other half is the phone housing itself (or the conductors inside it, mainly the ground plane on the main circuit board). So Audioguru is surprisingly correct when he says that the rest of the antenna may be inside the phone. Yes, in fact it IS the phone. This is still true but to a lesser extent at the higher frequency band.

The best advice for getting a better connection with an existing phone is to get an external antenna kit (if available) and use it, or hold the phone further away from your body, or get the phone higher from the ground.

 

[audioguru]

Other people have enquired about extending RF service to an area that is shielded from the signal.
I replied to them about adding two antennas that are connected together with coax cable, one in a good location for exposure to the cell phone repeater tower and the other with good exposure to the cell phone.
This idea was meant for the fairly narrow FM frequency band, so might not work with the many frequency bands used by cell phones.

 

[RadioRon]

You've described a classic technique used by many wireless and point to point microwave services called a "passive repeater". It can work very well if the circumstances are right. I have little doubt that one could buy off-the-shelf dual band yagis or similar multiband directional antennas and make it work, even for so-called "world phones" that support up to 5 bands. Of course, I don't happen to know who sells them off the top of my head.

 

[Thunderchild]

thats not a bad idea so how would I design a 1900 MHz yaghi areal. I presume the receiving one and the transmitting one are identical.
If I were to ground the negative of the charger (it is always on charge when I use it as a modem) would that help ?

 

[stevez]

Antenna construction is nicely documented in amateur radio publications. It's not the only place to find info but the publications or journal articles range from simple how-to with very little theory required to highly technical explanations that allow you (if you have the energy and resources) design the antenna for your own purposes.

I use a 5/8 wave antenna on my car for the 2 meter (144-148 mHz) band. If offers some gain over a 1/4 wave when mounted over a ground plane. Some of the energy is is radiated upward is reduced in favor of increased energy horizontally.

Back to amateur radio - there are quite a few articles resulting from experience converting commercial (surplus or otherwise) equipment such as yagis, parabolic dishes, etc.

As Nigel and others have said, it's complicated but you can make progress by understanding some of the basics.

 

[RadioRon]

One way to design a yagi antenna is to start with a simulator to get the lengths of conductors. Here's a good one:
https://www.eznec.com/

You need to be sure that the base station you want is operating at 1900 MHz for such a yagi to be useful. Next, you need to be realistic about your design goals. If you are willing to build a 7 element yagi, which is a reasonable first project, don't expect to get more than about 8 dB or so of gain.

Now, when you apply a couple of yagi antennas to the passive repeater setup, one of them has to be in a really good spot to hit the base station. Usually this means to get it up high in the clear and point it to the cell site. On your roof is a good idea. You will need to have cable connecting the two antennas. This will have to be long enough to reach the high antenna, and then connect to the nearby one that is beaming to the phone. This has to be a high quality thick coax, like RG8U or something like that. And you should keep it as short as is reasonable because the cable also burns up RF energy.

For a passive repeater to work, it needs to provide an improvement of at least 5 dB, because if it doesn't the direct signal from cell site to phone will be interfereing with the signal through the passive repeater (there will be what we call co-channel interference). The signal up above the roof might be 10 dB better than down in the house (this is just a wild guess and could range from 0 to 15 dB), and the antenna might give a gain of 8 dB. The cable will lose you about 6dB for around 55 feet of length. The low antenna will give you another 8 dB gain and then the path loss from the low antenna to the phone might be about 20 dB or so. The net improvement in this example is:
impr = 10+8-6+8-20 = 0 dB so in this example, it won't work. So, we need to get much better signal up on the roof than you would normally geet down at the phone or else its not worthwhile.