New School Radio System Questions

[thrownout27]

I am an elementary school administrator and we are in the process of setting up a repeater system on VHF with 40 watts (240 ERP) on the main and 5 watts on the mobiles. I live in Western North Carolina with some mountains, but the antenna will be on the building at about 30 feet with not too many blocking obstacles nearby. What kind of range might we be talking about for the system? Thanks all.

 

[mmckenna]

Really depends on the topology. VHF will do a bit better than line of sight, but not much.

240 watts ERP is quite a bit. Sounds like it was designed to cover a lot more than just an elementary school. 5 watt mobiles sounds a bit odd, did you mean portables?

I'd think that if you are within line of site for the system, you'll be able to work it.

For comparison, I've worked 2 meter amateur band repeaters with a 5 watt hand held radio well over 200 miles. Of course I was standing on 9000 foot tall mounting at the time.

 

[SteveC0625]

I am an elementary school administrator and we are in the process of setting up a repeater system on VHF with 40 watts (240 ERP) on the main and 5 watts on the mobiles. I live in Western North Carolina with some mountains, but the antenna will be on the building at about 30 feet with not too many blocking obstacles nearby. What kind of range might we be talking about for the system? Thanks all.

You might want to touch base with the contractor or consultant who designed your. system. They should have propagation study maps that will give you a fair to good idea of your coverage in all directions. Those studies are done using your local terrain and the power and antenna information for your system.

Antenna systems are just as important, if not more so than the base radio power specs. If the antenna can't hear the lower power mobiles, the repeater can not re-transmit out to the rest of the units. Height matters a lot. If you can raise the antenna even just 10', you can significantly improve system performance in both reception and transmission.

I am surprised that you are reporting 5 watt mobiles. That power rating is for handheld portables. Typical mobile installations usually are 25 or 40 to 50 watt units.

Again, too much missing info for any of us to give you useful responses. As I said, you should talk to the consultant or radio contractor who is providing and installing the system.

 

[rescue161]

Go to rfmobile and enter the numbers/info. It will give you a very good coverage map as long as you give it the correct info.

 

[teufler]

According to a Motorola mobile radio calculator, it says, based on your specs, about 23 miles from the transmitter.your 5 watt mobiles will have a hard time reaching back to the transmitter. You will have what we call an alligator transmitter. I have been trying to figure out 240 erp from a 40 watt transmitter. You antenna has to be say 8 to 8.5 dbd gain, this would give you 240 give or take but you have coax loss, say 1 to 1.5. This would be say 7 dbd. 40 watts at 6dbd is 160 erp. Possibly alittle high in your estimate. A lower figure helps match your transmitter to your mobiles or maybe portables. As has been noted, the estimated range is based on level terrain. You could be masked by the hills in western nc so your range night now be as far. I would say the transmitter should work around the town.

 

[rescue161]

He could get 240 ERP with a 9.1 dBd antenna with 1.32 dBd line loss. His problem is going to be the 30 foot antenna height. I ran a calculation using rfmobile and using 5 dBd mobile antennas and another calculation using -3 dBd portable antennas. Neither one looked great. I'm not sure what type of coverage they were expecting.

 

[Mtnrider]

If this is the pending app.......its only 69.9 ERP.....ULS Application - Public Safety Pool, Conventional - 0007063754 - HAYWOOD, COUNTY OF

 

[kayn1n32008]

He could get 240 ERP with a 9.1 dBd antenna with 1.32 dBd line loss. His problem is going to be the 30 foot antenna height. I ran a calculation using rfmobile and using 5 dBd mobile antennas and another calculation using -3 dBd portable antennas. Neither one looked great. I'm not sure what type of coverage they were expecting.

5dBd mobile antenna? A 5/8 wave is only 3dBd and a halfwave will be 2.5dBd if it has a decent groundplane. -3dBd is likely a bit low for a VHF portable, likely closer to -6dBd.

His biggest issue is going to be the repeater antenna at 30'.

 

[NDRADIONUT]

It says 45 watts for the mobiles....

http://wireless2.fcc.gov/UlsApp/ApplicationSearch/applAdmin.jsp?applID=9372656# < 80 KM CONTOUR

 

[mmckenna]

From the application:
Eligibility
90.35a RADIOS ARE USED FOR COMMUNICATIONS BETWEEN PUBLIC SCHOOLS AND BUS TRANSPORTATIONS.

45 watt mobile to the repeater. 100 watt mobile on the simplex side. 100 watts on the repeater (69 ERP)

no mention of portables.

Narrow band FM analog.

This is an existing license, first granted in 2012. The latest application is for a modification to the existing.
Dropping wide FM.
Looks like they switched this from a simplex system to a repeater.
New site for repeater.

 

[rescue161]

5dBd mobile antenna? A 5/8 wave is only 3dBd and a halfwave will be 2.5dBd if it has a decent groundplane. -3dBd is likely a bit low for a VHF portable, likely closer to -6dBd.

His biggest issue is going to be the repeater antenna at 30'.

Sorry, I meant to say dBi! The numbers were right, but the little "d" at the end was suppose to be an "i."... :wink:

In all things considered, it was a little less than 3 dBd for the mobile and -5 and change for the portable.

Sorry for the confusion. I'm used to reporting in dBd, but RFMobile uses dBi for their measurements.

 

[kayn1n32008]

Sorry, I meant to say dBi! The numbers were right, but the little "d" at the end was suppose to be an "i."... :wink:

In all things considered, it was a little less than 3 dBd for the mobile and -5 and change for the portable.

Sorry for the confusion. I'm used to reporting in dBd, but RFMobile uses dBi for their measurements.

That makes better sense!

 

[rescue161]

Here is a new map with 69 Watts ERP and 3dBd mobile antennas. <- got it right this time

 

[teufler]

I once ran a dual band co-linear, made by Mahdol, that was 6dbd and 9 dbd on uhf. Talk sucker, in the woods, you can to watch for branches. Also there use to be a single band co linear called a Buck Buster. I think it was made by Cushcraft. Mobiles out in Kansas or the flat lands, ran them the most. Both were about 90" + . These were great mobile antennas, but not a covert antenna. At truck stops, cb'ers were always asking, "what kind of antenna you got?". But at rally time, I would run 150 watts into a dual band co linear, or when I was driving on the interstates, if you could hear a station, you could easily talk back. 30-40 mile simplex was not uncommon. A base could get back to you , but other mobiles, had their normal range.

 

[RFI-EMI-GUY]

Here is a new map with 69 Watts ERP and 3dBd mobile antennas. <- got it right this time

Those 5 watt portables are going to give a much smaller talk back foot print.

 

[RFI-EMI-GUY]

I am an elementary school administrator and we are in the process of setting up a repeater system on VHF with 40 watts (240 ERP) on the main and 5 watts on the mobiles. I live in Western North Carolina with some mountains, but the antenna will be on the building at about 30 feet with not too many blocking obstacles nearby. What kind of range might we be talking about for the system? Thanks all.

A better question might start with where exactly do you need to talk? As mentioned before the portables are only 5 watts and have an antenna factor (loss) of easily -10 dB so your ERP is less than 1/2 watt. On campus this might be just fine, and in the open down the road a few miles may be fine depending upon terrain. If you need this to talk county wide, you need mobile radios, or perhaps a more sophisticated system. Inside large buildings is another concern. Also at VHF man made noise might lower the sensitivity of the repeater.

A Consultant would start with a needs assessment, then progress to a couple design alternatives.

 

[rescue161]

Those 5 watt portables are going to give a much smaller talk back foot print.

That was with mobiles, not portables. The portable coverage area with rubber ducks was significantly less.

 

[buddrousa]

One rule of thumb a $100 radio works better with a $5000 antenna system than a $5000 radio with a $100 antenna system.