Re: Planet Rock Saved

Kristoff Bonne wrote:



Adding capacitors and coils affect the ELECTRICAL length of the antenna
(which is what determines the tuning the antenna).
There is a difference here.

You can add 'loading coils' to make the an aerial electrically longer. This is rather like taking a physically longer aerial and then simply winding up part of the length of the whip into a coil so that it is physically shorter. So I presume this would make the electrical length of the aerial = physical length + length of wire wound into the coil. This I presume allows the current running in the aerial to flow as if the aerial was a physically longer straight whip aerial. The radiation pattern, and reception pattern would not be as good as a whip aerial because the aerial is physically shorter, but the currents flowing up and down the aerial would not know the difference.

You can not however make an aerial physically longer in this way without also making it electrically longer. A loading coil can not remove electrical length from an aerial, it can only add it.

This is rather different from adding capacitors (and perhaps coils) to 'match' the aerial to the receiver or transmitter. In this case the aerial it's self is still working in a non ideal way, and not actually providing the correct impedance, but the electrical components between the aerial and the transmitter (or receiver) effectively adjust the impedance so that the receiver or transmitter think they are connected to a correct aerial. This does not fix the performance of an aerial of an incorrect length, it just makes the radio think the aerial is correct. The aerial will still perform relatively poorly compared to one of the correct length.


The physical size of the antenna determines how much radio-energy an
antenna picks up.

Those are two distinctive things. Friis' formula only works on the
PHYSICAL size of the antenna; so it you leave that unchanged, so will be
the path loss.

That's all very well, but if the actual aerial is the wrong length electrically, then other in efficiencies would outweigh the extra signal received by using an over sized aerial.

If you make a antenna the correct size for 200 Mhz and electrically make
it "longer" for 100 Mhz reception, or the other way around; this does
not make any difference.

As explained above, you can make an aerial electrically longer by adding loading coils, but you can not make it electrically shorter. You can only fool the receiver into thinking that it is electrically shorter, by showing the receiver the correct impedance (matching the impedance of the aerial to the receiver). This can not however fix the in efficiencies of using the wrong aerial.

The antenna will always be the same physical size and -therefor- Friis's
formula will give the same value.

Yes, but it would still be electrically wrong, and so you would get inefficiencies. The matching electronics would then hide this problem from the receiver, but would not actually make the aerial work correctly.




Well, getting the tuning correct is more important for a mobile phone
then for a radio-receiver. A GSM is a two-way device, so it actually
transmits.

And transmitting into a badly tuned antenna is usually the best way to
cause problems for your amplifier. (luckely, most devices with external
antenna's that can be unscrewed have some kind of protection-mechanism
for that nowdays).

Or they use the electronics (you mentioned) to match the aerial to the transmitter, effectively making the transmitter think it is connected to a correct aerial.

Now, I can be wrong but -AFAIK- the problem for 1800 MHz is not really
the quality of the antenna. The main problem is that these higher
frequencies have more problem penetrating buildings; so that's why cells
at these frequencies are closer together.

Perhaps.
Building penetration is more of a problem at 1800 than it is at 900. However I seem to remember hearing that GSM900 can work at distances up to about 30 miles (obviously under ideal conditions), while GSM1800 barely works beyond 2-3 miles (if I have my facts correct). Otherwise why did it take the GSM1800 networks so much longer to build up to full coverage, than it to for the GSM900 networks.



And one additional remark.

I know from some people who do network planning for GSM-networks that
most cells in large urban centers (and hence the 1800 Mhz cells) are not
size-limited due to power-constrains, but by the number of users a cell
can service.
To increase frequency-reuse, most of these cells run at reduced power
(and therefor reduced range).

They could presumably do the same trick using 900Mhz.
I don't know how the frequencies are used in your country, but here in the UK 2 of the networks (O2 & Vodaphone) use only 900Mhz, and the other two (Orange & T-Mobile) use only 1800Mhz. No network here in the UK uses both. Actually that seems a bit of a silly way of setting things up, I'd of thought it would be better to use 900 for rural areas, and 1800 for urban areas, but that is not the way it was done. Each network uses just one frequency to all areas.


The sensitity of current-day GSMs is no issue at all, especially not for
1800 Mhz.

Not in urban areas, but I suspect that it was a bit expensive providing GSM 1800 coverage over wide rural areas with low population densities.

Richard E.
.

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