Batboard

The Department I work for operates Apco P25 in the 460.00 area. We have horrible rx and no tx in out locker room / bathroom which is located in the basement. I would like to propose that they budget for a passive repeater to help in this situation. We have in house techs that would do the install. I just have to list the equipment and price range and mabe it will be considered.
Thanks Mike
BTW we have one tx site aprox 5 miles from hq and multiple rx sites.

Assuming a 100 WERP signal from the transmitter produces the following:

50 dBm = 100 Watt.
103 dB is the free space path loss for 5 miles assuming clear line of sight.
-53 dBm therefore is the received signal strength with a unity gain antenna 5 miles from the transmitter.
3 dB is the additional cable loss to the inside antenna.
-56 dbm therefore is the signal at the inside antenna.
55 dB is the inside path loss for a distance of 100 feet from the inside antenna. (Portable to inside antenna)
-111 dBm therefore is the signal strength 100 feet from the inside antenna.
6 db loss assumed for the portable radio antenna.
-117 dBm or 0.32 uV therefore is the received signal by the portable.

Increasing the gain of the outside antenna by using a 10 db gain beam antenna brings this up to 1 uV at 100 feet from the inside antenna. Using a beam antenna inside can add an additional 10 dB. Keeping the feed line loss low is important here.

For talkback, the loss is 167 dB. Subtract from this 10 dB for each beam and an additional 10 dB (assumed) for the tower antenna and the result is a total loss of 137 dB. For a 4 Watt (36 dBm) portable, this results in a –101 dBm or 2 uV signal at the tower receiver. Multi-coupler losses etc. at the tower are not included, but should be low.

Passive repeaters work better when there is no signal penetration from the outside at all. Outside signals, because of phase differences may cancel or aid the inside signal.

If I didn’t screw up, this shows that you can make this work.

Nand.

We have used passive antennae for 460 and 800 in past and present and they have done a great job considering the simplicity. NAND did some good calcs for you. Good beam outside, decent feedline, and good indoor antenna. We have used omni's at sites with good rf figs. inbound and outbound.

They put a passive repeater in a 'local' police station over here so the Chief's Nextel would work from his office. It works great. However, in 4 years, they still haven't put one in for the UHF portable radios the other 24 officers carry and don't work in the building or lockup.

Thanks for those numbers Nand. It's nice to be able throw numbers out now as well if someone asks about this. You can say "yea, it will work" until your blue in the face but dB and uV numbers to back it up do help. Of course they are all "general rules" but at 1 or 2 microvolts, there is plenty of room to wiggle! You definately need to take advantage of all the simple things to make it better. Yagis pointed at the site and quality coax are key!

I largely did the calculation to see for myself why some say that it works and others say that it doesn’t help. You are correct in assuming that this is sort of a general example. A lot depends on how well the outside antenna can pick up a signal from the tower antenna. If the outside antenna is not within the tower’s antenna pattern, the path loss could be very high and unpredictable. In that case, a measurement may be needed.
It looks like if all information is available, the calculated signal strength ought to be very close.
There also are bi-directional duplex amplifiers available for this type of use if a passive setup can’t work.

Nand.