Range in electronics

[Fernanda]

I have a friend asking me the following questions:

How come his 2.4 GHz phone only has 100 meter range while his GPS transponder at 1.5GHz has its range in miles...?

So I answered...intesity.

Though I was sorta shaky on my answer...he then asked one further:

How can I tell one phone from another? That is, what tells me that a phone with the same 2.4 GHz frequency as another has more range?

My first answer was wattage...but then again, it\'s sort of a dumb answer because it a cordless phone.

Any ideas?

[Fernanda]

I have a friend asking me the following questions:

How come his 2.4 GHz phone only has 100 meter range while his GPS transponder at 1.5GHz has its range in miles...?

So I answered...intesity.

Though I was sorta shaky on my answer...he then asked one further:

How can I tell one phone from another? That is, what tells me that a phone with the same 2.4 GHz frequency as another has more range?

My first answer was wattage...but then again, it\'s sort of a dumb answer because it a cordless phone.

Any ideas?

[errandir]

With a cordless phone, the most important parameter I think would be the SNR, and CSO/CTB. There are also related parameters, like NPR and CNR. I have tried only one 2.4 GHz phone and it blew ass (Uniden). My suspicion is that the manufacturers, in a rush to keep pace with the artificially increasing carrier frequencies, sacrificed the overall quality of the electronic components in their phones.


Thus, you could compare a 900 MHz phone with a NF of 5 with a 2.4 GHz phone with an NF of 14, and you can be sure that the 900 MHz phone will out perform the 2.4 GHz phone hands down (note, I just pulled the NFs out of my ass).


Furthermore, the higher in frequency you go, the more of a problem it is to get just the proper shape of your filter band. In general, the higher the frequency, the wider the band (disregarding the response distortion). This increases ingress noise, and, with more and more devices operating in these ranges (and some that have been doing so for decades like the TV), the ingress is probably another huge factor.

[errandir]

With a cordless phone, the most important parameter I think would be the SNR, and CSO/CTB. There are also related parameters, like NPR and CNR. I have tried only one 2.4 GHz phone and it blew ass (Uniden). My suspicion is that the manufacturers, in a rush to keep pace with the artificially increasing carrier frequencies, sacrificed the overall quality of the electronic components in their phones.


Thus, you could compare a 900 MHz phone with a NF of 5 with a 2.4 GHz phone with an NF of 14, and you can be sure that the 900 MHz phone will out perform the 2.4 GHz phone hands down (note, I just pulled the NFs out of my ass).


Furthermore, the higher in frequency you go, the more of a problem it is to get just the proper shape of your filter band. In general, the higher the frequency, the wider the band (disregarding the response distortion). This increases ingress noise, and, with more and more devices operating in these ranges (and some that have been doing so for decades like the TV), the ingress is probably another huge factor.

[C-Protected]

ok my question is a follow up. What would be the measure of the intensity for a wave ? As far as I understand, intensity is different than power, since more power ( a light bulb with 75 watts for example) does not mean more intensity (a laser bean is much more intense than a light bulb, but it uses much less power, in factt much less than 1 watt for those pens we see around.) So what would be the measure of intenity of a frequency ?

[errandir]

Actually, it\'s basically the same thing. Intensity is just power normalized to some area. Since the laser beam has a very small cross-sectional area, it has a much higher intensity for some given amount of power. If you get right up next to the filament of the 75 W bulb, the light is very intense, but think about the consequence: you have shadowed the rest of space where that light (and the power thereof) would have gone, so it makes a kind of intuitive sense. If you allow the light to spread out and hit more points of space, then you dilute it, and it becomes less intense. It is highly analogous to the difference between mass and density.

When talking about E&M radiation, it is usually more appropriate to consider intensity rather than power. Along these lines, antenna engineers are intimately concerned with the directivity of the radiation. You may have seen a polar plot of this; it looks like a bunch of lobes and is usually symmetric about some axis. The distance a point is on the lobe from the center of the plot represents the intensity of the radiation at that point. A consequence of physical E&M radiation from an antenna is the existence of nodes in this pattern. So, if you put the receiver at an angle to the source corresponding to a node, the receiver won\'t pick up a signal. (you could look at this in the perspective of the source as well). There is a reciprocity between transmission and reception for antennas, so, this would also be the case if you orient the receiver so that the source is at one of the nodes.

A simple example of this is a dipole. If you have a cordless phone with a monopole antenna, you can demonstrate this. Point the antenna on top of the phone straight at the base unit. This should dramatically reduce the signal. Then, try pointing the antenna on the base unit directly at you (the phone in your hand) and the same thing should happen. One thing to point out, though, is that directivity is a far-field characterisation, so you should do this demonstration across the room. (far field means that the source and receiver are separated by much more than a wavelength.) This should demonstrate that the best relative orientation for the antennas of the phone and base unit is parallel to each other (and forming a requalr trapazoid).

In the directivity picture, this is because they have a directivity lobe that points out perpendicular, and a node at the two ends of the antenna. Physically, this is due to the freedom the electrons have to move up and down the antenna but not from one side to the other. (there is also a driving point issue, but that is a bit lengthy of a discussion.) There is a formula to calculate the coupling efficiency of two antennas that takes into account their orientations. I think it\'s called the \"Friis Transmission\" Formula.

So, to get to your specific question of what this all means for a particular frequency. Well, you can\'t literally consider just one exact frequency, but you can consider a very narrow band, and then discuss this band as referring to its center frequency, which is what one usually does. But what is really going on is a frequency band, and sometimes one must consider this fact. So there is another quantity that gives the power density in terms of frequency (W/Hz, or, usually in engineering speak, dBm/MHz). This is called the spectrum. Thus, for a spectral intensity of 1 W per Hz at 900 MHz with a band width of let\'s say 1 kHz (extremely narrow band), you would say that you have a 1 kW 900 MHz signal. But this only applies in the electronics. When you send this out into space, you have to consider the spatial distribution, or directivity, as mentioned above.

As you can see, considering the E&M waves in free space becomes quite complicated, in that, if you want to consider the effect of different frequencies, you have to consider the spectral density AND the directrivity. At first glance, this may seem like a trivial multiplication, but, upon further consideration, you must realize that the directivity depends on frequency, so there is an inherent spectrum for the antenna as well as the spectrum of the signal as generated by the electronics. This propogates to the receiver, which also has an inherent spectrum. Then, you have filtering inside the electronics, which is there to shape the spectrum further. Of course, this filtering is to REshape the signal so that it has some desired spectrum. This filtering can be passive or ACTIVE. It is the fact that active filtering is usually necessary that causes the most difficult of the problems that I mentioned in my other post.

[Edited on 10-6-2003 by errandir]

[C-Protected]

thanks for your long response, errandir. I would like to take the GPS satellite for an example. The frequency is 1.575 Ghz. Here is an interesting site: http://gpsinformation.net/main/gpspower.htm
It mentions that the power used is 500 watts, but there are other technical figures that are beyong my knowledge for now. If someone could explain what the figures mean, that would be greatly appreciated.

[errandir]

I did not see any figures on this page.