Passive Antenna For Building

[janopey]

Hey all I was asked a question about getting 800MHZ (local Fire Depart) signal into a High Rise. I was thinking about using a Passive setup. I would hang two Yagi 800s (16 db gain) on the outside pointed at their closest tower. Running the Best Cable I can find inside to an Omni Directional Antenna (6-9 DB gain) These are only needed in the Stair Wells but as you know its usually Steel stairs or concrete and make signal PEN bad. Any thoughts? I also have looked at using an AMP on the line one TX one RX. Suggestions would be nice as this may be implemented for multiple buildings with same issue.

Thanks

Johnathan

 

[Thunderknight]

My experience with passive antennas is they only work well up close to the inside antenna.

Regarding amps, such as bi-directional amplifiers.
Read this, please: http://www.coopergeneral.net/das-gone-wild/

It is SO important they be designed correctly or they can be a disaster to the RF spectrum. Both to make sure they provide the correct inside signal level, but also that they don't interfere with others or oscillate.
If it's a wideband BDA, it has to be registered at the FCC website also.

These folks make BDAs and you should talk to one of their engineers before doing anything active. The Most Reliable RF Signal Boosters

 

[prcguy]

I suggest you test it on a small scale before committing to buying all the parts. If a Yagi and some cable to an omni or whatever gets signal into a known dead spot then its probably worth doing. I've seen some attempts at a passive antenna system fail and after a lot of money was spent for nothing in the end. On the other hand I a gave a friend some antenna and feed line stuff to make a passive repeater and he successfully got cell phone coverage inside his apartment where it didn't work before, so there may be hope.

Besides getting as much signal from and to the repeater with the most gain you can get on the outside antenna, it might be better to use leaky line or Radiax cable run the length of the stairwell instead of an actual antenna inside. This is a common practice in long tunnels.

Adding a bi directional amplifier can be expensive because it will need narrow filters to isolate and pass only the handheld side going outbound and the same for the received repeater signal going into the building. How many stories are these buildings? If they are only a few stories high then a Yagi at the top of the stairwell pointing down might suffice.
prcguy

 

[mmckenna]

I've got a few systems at work, specifically for 800MHz trunked systems.

Unless you have a __really__ strong signal outside the building, __really__ low loss coax (or extremely short run), and a really high gain donor antenna, the losses in the system will negate the benefits.

If you are looking at a high rise building, you will likely need more than what you expect. A single antenna at the top of the stairwell isn't going to get very far. Trying to add more antennas will require splitters, and each one will drop your signal by more than 3dB.
To make these things work well, you'll need to have antennas inside the stairwell spaced out pretty well, not just one at the top.
You'll also need some amplification, both directions. Once you add amplification, you'll be adding filters. You'll also have to have some way of automatically shutting down the system if it begins to self oscillate. If the system does start to self oscillate, it will destroy coverage for the radio system over a very wide area.

Since you'd be taking someone else's signal and amplifying it, you'd need to have the system owners permission, or at least make sure they are well aware that your system exists and how to contact you in case it starts causing issues.

Many cities/counties are starting to add these systems as a requirement into the building codes. Often they will be required for new construction. They usually need to be built/operated with the cooperation of the agencies that the system supports (Fire, PD, etc).

If this system is intended for first responder use, I'd really strongly recommend you talk to the department about this.
Then, talk to a company that has designed, built and operated these systems before. They are not something that you can do well without the proper experience and test equipment.

For a couple of high rise buildings, you'd probably be better off looking at a fiber optic based system with a centralized head end. That can save you a load of money in coax, gives you better control, and might let you use existing fiber optic cable that might already exist in the building.

Take a look at these guys. We've been looking at one of their large campus systems with a centralized headend and fiber based distribution. Not cheap, but for a couple of large buildings, it might be what you need: Coverage - Cobham Wireless

 

[Project25_MASTR]

So if you want to figure out passive equipment, the theory is actually quite simple. Figure out the ERP (in dBm) from the repeater to the yagi, then add the gain of the yagi, subtract the loss in the feedline, add the gain in the omni then figure out the receive level at the portable from various points in the coverage area.

You'll find that in very few occasions is it really practical to obtain a reasonable SNR.

A properly engineered DAS is the way to go.