Passive antenna system!!! HELP !!!!

[n9upc]

A customer had a passive antenna system for 800 Mhz put in there building. The customer is using a digital 800 Mhz smartzone system and has little to no coverage in the building in the basement.

The had someone recommend and put in a passive antenna system which is the following: 1.) An 800 MHz 12 element 11 dB gain yagi pointed at the antenna site, 2.) approx 350 feet of 1/2 hardline ran into the basement of this building where, 3.) an 800 Mhz spliter was place don the line and, 4.) coming off of there is two (2) 3 element 3 dB gain yagi antennas pointed down the hallways to where they need coverage.

Now when you are standing right in front of below the yagi's you have very good coverage with an RSSI reading on the radio of about anywhere from 80 to 94.

Now take 10 steps (for about 10 or more feet) away from the beam and bam you are down to just about 0 RSSI. Now we tried telling the customer that he needs a BDA (Bi Directional Amplifier) in the system to help get better RSSI. Yet they still say no we were told this will work plus they do not want to spend the money on a BDA.

So my question is this: Can there be any improvements on this passive antenna system without the major cost of a BDA???? Me personally I always believe in BDA's but they said they were told by the designer that it would work and they want us to make it work. Of course the original designer is not able to be reached to have questions asked of him.

Any and all help would be great!!!!

[mastr]

The run of cable has about 7 db of loss, and the "2 way splitter" probably about 3.5 db. IMHO, 10 db of line loss is too much to work without a BDA; it is no wonder that the original designer is not to be found....

[Cowthief]

Hello.

Could be a phase issue.
We have a building in downtown Houston, Texas, that had this wild scheme lossy coax, a splitter, radome thing in the basement, the works.
I disconnected the splitter and radome, things got better.
I put the desktop repeater in a utility closet on the second floor, things got good.
The repeater is now in a mechanical room on the second floor, feeding a dipole, coverage is very good.
The best guess was the radome in the basement was out of phase at some points with the lossy cable.

[k4wtf]

When I was working in cellular, we deployed several of the "lossy coax" type installations at customer sites that were not RF friendly. We put a yagi (don't remember the gain but, it was 7 element) pointing at the closest cell site, ran 1/2 heliax inside which connected to the "lossy coax". They worked very well from what I remember. I was always fascinated by the "lossy coax" though. Kinda like irrigation hose for RF.

--
John

[werdnuts]

Sounds like it is time for a BDA.

[jackhackett]

should be simple to figure out if it's phase cancellation, check signal levels from one of the yagis with both connected, unhook the other yagi from the splitter, put a 50ohm termination on that port, and recheck the levels.

[RADIOMAN2002]

While this may sound like a little cheezy. The NYPD put in some homemade BDA's in the subways, in the early 90's. Basically they were 2 antenna's on top of the poles outside the hole in the ground (subway entrances for those non-city types) One was a yagi, aimed at their transmitter, 7/8 coax down to the subway platform where a receiver amplifier. the type you would use on the front end of you repeater reciever was mounted in a metal box, with a 1/4 wave antenna also mounted on the box. 10 ft away was another box with a 1/4 wave antenna on another metal box with another rf amp in it, 7/8 coax to the street, ending on a 5db stationmaster antenna. Worked great for PD, not to hot for EMS at that time. I would imagine you could do something like it, first remove the yagi's in the basement, use 5db mobile antenna's. If that doesn't work, add a RF amp just befor the splitter, with the rf going towards the mobile antenns. Mext add some mobile antenna's run just enough coax to get outside the building at ground leval and reuse the yagi towards the site.

[Jim202]

Passive repeaters don't have a very good track record. Unless you have a very strong donor site, the chances of it working well are low. I have tried it many times over the years with much the same results every time.

My most recent attempt with a pasive, is doing it at the 1900 PCS frequncies right in my own house. Used a pair of 11 DB panel atennas. The house antenna is in the attic about 3 feet over my head. The outside antenna is up on a tower about 60 feet away. Result is that I can't notice any improvement.

In the 30 some years in the 2 way field, I have always been a strong supporter of an in line amp. Just remember that in these type of amplified system installations, you want to have a band pass filter before the amp, to allow only the range of frequencies that you need. Otherwise the line amp will end up trying to amplify a strong signal out of band and not the one your looking for. The strong singnal may saturate the amp and in that case you get nothing with the weak signal your looking for.

Other than in microwave use, passive repeaters generally don't work in the 2 way and trunking range of frequencies. You can't get enough antenna gain to get the re-radiation you need to make them work.

Jim

[OX]

We just installed a BDA in our office for cellular and the results were well worth it. Increased our signal from almost zero to full strength. Granted this is cellular but you should be able to get the same results on other bands with the appropriate equipment.

[commstar]

In my experience, this can work very well. I have done it several times very sucessfully from everything from TV antennas, a mine shaft, several PD, one FD to two mobile antennas on mag mounts on a submarine.

Example: CDF/CYA base at Mt. Bullion,Ca that was too deep in a canyon to get tv stations, they set up on antenna on the hill above and directed the second toward their barracks. Has worked very well for many years.

For whatever reason (i have never taken the time to figure it all out), try to match the components on both ends. and if not matching then equal; if the antenna on one end is 6 db and you want to cophase the other end make sure that the other two yagis equal 6db combined or 3db each. I try for the exact same antennas, connectors etc.

Capacitance bumps are a problem, be aware of kinks in the coax. Use new straight band appropriate, low loss coax right of the reel. No hemorroid coils from the van floor from that "xyz job".

What makes sense in other applications often does not make sense in these parasitic applications. for instance: ** I would try to turn the basement yagi/s to a horizontal orientation instead of vertical.

The smallest tweaking brings big results quite often. for instance if the installation has been up for a couple of years try changing out the connectors on both ends etc.

When you are working with "nothing" the smallest change can work wonders. The BDA is the quickest answer but you just might learn someting messing around with the parasitic antenna/repeater theory.

I look forward to the next one I get to put together.