Repeater system for pagers?

Question

I work at Shasta Community Health Center. We have a main clinic and 3 satellite clinics in other nearby towns. They just built a new building up in Shasta Lake City. Metal walls, metal studs, metal roof. The signals for the pagers are not getting through, so doctors are not getting important pages. There is signal in the area though, if you walk outside of the building, you have a signal. So, they are planning on putting up a pager repeater system at the building. Here is where I need help though.

They are looking at two systems, the company is recommending a 500 W transmitter, but there is also a 250 W transmitter. The pagers operate on the 900 MHz band. The building is about 70 feet by 140 feet. They want to know if we really need the 500 W or if the 250 W would do the job.

Any help is appreciated. Someone found out that I was an amateur radio operator so they came to me for help. Great...

Anyway, thanks in advance guys.

D. Michael Johnston - KG6RPI

Here is the site with the ones they are looking at. That huge amount of power didn't make sense to me either, I can hit UHF repeaters with my hand held on 6 W that are miles away...

http://www.gtesinc.com/products/rf.shtml

They are trying to get the signal to the pagers inside the building.

Answer 1

Sounds like your building is already built... If you got few concrete floors and/or tens of walls between the transmitter antenna and the pagers at the worst corners of your planned coverage zone, then it may cost less to go with the 500 W transmitter and a higher op-ex electricity bill. That would save the cost of running large low-loss RF cables through floors and mounting multiple DAS.

At 900 MHz of the Motorola FLEX Protocol, the popular Motorola Adviser II pager at 6400 bps needs a field strength of only 13.5 uV/m. Depending on the balun, this pager's receiver should be about 10 dB more sensitive than the -104 dBm of typical cellular phone receivers. In addition FLEX is an FSK (a digital version of FM) which has a good 40 dB spurious isolation as compared to less than 30 dB of code-domain correlation isolation of some CDMA phones. Hence, it's true that very little power is needed.

So, common Ham experience would say that a fraction of a watt of transmitter power would be more than sufficient for a direct air receive power. Suppose that 50 mW of low-end 40 dBc of transmitter power gave plenty of antenna and air link margins for your tiny floor space size. Then, and here may be the overlook, for every wall you would need to add 8 to 15 dB of loss, and for every floor 12 to 27 dB of attenuation. As you know, going from 500 W down to 50 mW is 40 dB: just a couple concrete floors, or a few walls, would attenuate the 500 W down to 50 mW. For Ham's typical open space, going through, say, the trunk of a typical tree could take the RF down by 15 dB. However, unlike in a boxed-up indoor room, there are plenty of paths for an outdoors Ham's EM wave to pass a tree.

Answer 2

500 WATTS? Half a killowatt of RF?

So are they trying to contact the pagers or power them too?

You are licensed for 450MHz, that's twice the wavelength, if you get a pal to stand with an HT where they are going to put the transmitter I bet you can reach every corner of the building with 5 watts.

I'm a ham too and also work for the biggest 802.11 company, we can get round a building like that with less power than it takes to power a small light bulb.

The metal studs don't make much difference, the wavelength is short enough to get through the gaps. 900MHz is about 1 foot wavelength. Welded metal floors are a big issue, our building has floors made of corrugated steel hooked together at the edges and then welded down the seams with an inch or so of concrete to give a level floor. RF does not go through the floors. However, I'm sure it will bounce off things outside the building.

Was it 500 milliwatts? That would be more reasonable and just about what I would expect you'd need.

Answer 3

KG6RPI, my call is WA2QCJ here in Herlong, CA. 250 watts would be enough, but are there any other towers in the area? Has anyone tried "leaky" coax, like what is used in malls and other large commercial or public buildings. You only need to couple the outside to the inside, and back again at your facility. A relatively low power transmitter for use inside coupled with a decent gain receiver should do the trick. One thing on such systems, height is a MUST. To improve radio horizon, get that transmitter and receiver antenna system as high as the FCC will allow.

Answer 4

Radio dude! Tre cool.

Anyhow, to kill this cat, just do a home-brew experiment. Get you a couple of cordless phones and see how signal is in various locations.

You know that 900MHZ will have excellent penetration in buildings and through buildings too. You may run into problems with signal leakage with the higher gain antenna because of signal bleed outside of the facility in that frequency. So, there's a good reason not to get more antenna than you can use.

The cordless phones come in various 'ranges' (read- wattage) so you can try a few on the cheap to get an idea of what sort of problems you have internal to the structure. Watch out particularly for basements, elevators and those fire type stairwells.

I can't imagine you will get any specific answer to this without actual building diagrams and a bit of an engineering study. On the other hand, there's always cheating. Call other facilities of similar size and see what they have installed.

Good luck.