Which frequency band is suitable?

hi.
for an AMR like system for water meters, i want to choose a frequency band. currently i want to use RFM23B (Based on SI4431-revB1) and in my country available ISM bands are 434MHz and 915MHz. and water meters are typically 50cm under ground. with these situation, which frequency band is suitable?
(the TX deviation is +-38.4KHz and FHSS is not required).

As your water meter is installed under ground, from such position any VHF or UHF transmission will not work. But the antenna location must be above ground; then you can use any suitable frequency band for which transmitters and receivers are available.

Thanks. but there are many version of water meters that have AMR capability and have not any part above ground. and also i previously designed such thing and i get an RF range in about 15m (with RFM12B (SI4421) and 868 frequency band) and i want to increase this range.
i want to know that with which frequency band offer more operation range. i think that with 434MHz band i may have more range (with respect to 915MHz band). is it true?

The communication range of RF depends on frequency, Tx power, Rx sensitivity, Transmission line loss, antenna gain/loss, and directional or antenna beam.
So, first try to increase your antenna gain and make it directional on both Tx and Rx.
If it is not enough range, you can make a repeater by using your exixting Rx and a pair of new 433Mhz to extend to range. And you can repeat this sheme until reaching your required range.

thanks. i only want to know that in the same situation (antenna type, Tx power and same loss), in which frequency (434 or 915MHz) i can get more RF range?

As Pico and myself have explained, the answer is not so simple and straight as you may wish.

First, locating a wireless device including antenna under ground or water makes no sense. The antenna simply must be located above ground to function at all. Using antennas with a different directivity usually adds to range extension but you must point it to the other end, and you must be sure there are no obstacles in the way. At VHF and UHF, 430 to >`1000 MHz, those are important things.

The best way you can find by experiments- no easy way by sitting at a computer. Become a radio amateur, this has been the best way to learn the basics!

Thanks jiripolivka. I'm not experienced in RF and i have a lot of question in my mind.
i read that SubGHz bands offer more communication range with respect to 2.4GHz band (and by the reason they used for AMR) and i think that in closed area (like residential buildings), increasing frequency (434 to 915MHz) reduces RF range (with the same situation).
i read that WSNs like Zigbee support SubGHz bands to increasing RF range.

is there any relationship between RF range (in closed area) and frequency?

Yes, the propagation loss in the free space is a function of frequency. In real conditions there is no such thing as free space. You should learn by experience, no textbooks can help you .
Run experiments, and learn from them. Radio waves can create an interference field, with a deep minimum just at the receive-antenna location. Then you can pump a lot of power to a transmitter, without any effect. The solution is to relocate the transmitter antenna, or point is to another direction, to avoid obstacles causing reflection... which causes the interference.
RUN EXPERIMENTS!

You can also use the propagation model in the antenna textbook for 1st design, and make margin as 20dB or more, then tune in experiments.

Thanks for your answers. i found a white paper that explained why in lower frequency we could have more range.

I think some information must be wrong. Is really the radio and antenna fully below ground from RF view? At 433 MHz do I then not see any realistic possibilities to penetrate ground in a range of 15 meters as no alternative signal path exist. Attenuation trough ground will be to big for a normal low power RF system.
Even if the radio is placed say 1 m deep in a 1 m wide dry concrete manhole will no reflections reach a receiver placed 15 meter aside as nothing will reflect signal in that direction (moon bounce? . If communications should be possible across normal ground with a certain moisture during these circumstances must RF frequency be very low, so low that magnetic coupling is possible between RX and TX. Compare submarine RF communication which often operate below 100 Hz. As TX antenna must be relative big to have any efficiency at such low frequencies must it be much simpler to use a direct wire between TX and RX locations if it only is 15 meter that is needed. If I would try to communicate across normal ground for say 1 meter with low power TX had I done a try around 1 MHz and used a tuned loop antenna as TX antenna. For 15 meter distance and I had selected 1 kHz carrier and no antenna, just a balanced RF output connected to two metallic rods buried with a meter distance.

Is it something like this, placed in a manhole?

like the first photo E kafeman. but i test it in 868MHz and i get RF range about 10 to 15m.

---------- Post added at 02:54 ---------- Previous post was at 02:48 ----------

currently i use RFM22B (SI4432-rev B1) in a wireless system that one node is in -1 floor and another is in 2nd floor in a building. it respond good.

---------- Post added at 02:56 ---------- Previous post was at 02:54 ----------

in the above system the Tx power is 20dBm and i use 434MHZ band.

So "under ground" is more like a basement and there is no need to penetrate real ground?

The water meter is in about 50 to 70cm depth. the manhole have an iron lid.

If the manhole is RF-shielded is it hard to have a communication in MHz-range at any distance. Why not an external antenna? A patch-antenna can be just 10 mm high and molded in plastic to withstand weather and cars.

because nothing should be out of manhole, no wire, no antenna and etc.
i think that i should test it. (although i tested it with RFM12B, i should test it again with RFM22B).

Not even waterpipes? ;-)
A bit out of ideas, slot antenna maybe? Cut a slot in the iron lid and fill the slot with plastic. No one will notice it. If it not is a too big slot will it not reduce lid strength.
Slot antenna: Antenna-Arrays
RF-isolate the iron-lid were it is resting at the edge of manhole. and tune the whole lid as an patch-antenna is possible but requires measurements for the tuning.
Use the water-pipes for LF signaling.

I am not experienced in RF. and i don't know about slot antenna. also i can not change the lid (because it need an strong cutting tool).
most of water pipes are made of plastic and i think that i can not use them for any communication.

Slot antenna size is simple to calculate, length= 0.5xwavelength. The slit can be done by drilling several holes or using a iron saw.
Do some antenna testing on a thin metallic sheet or a PCB to find out if it works good enough, before starting drilling in the real stuff.
For water-pipe solution, it exist commercial products that is sending ultrasound several kilometers as a kind of radar, checking for leakage.
No need for metallic pipes, but it must be water in the pipe.