new antenna vs increased power vs repeater replacement

[emtprt]

I am currently in a situation in which a seldom used repeater will soon be used utilized on a frequent basis. The present setup is a simple gr1225 UHF repeater with 25 watts post duplexer and a DB 404 (3.8 db gain) antenna. The antenna is setup for an omni pattern and there is about 100' of hard line run from the radio to the antenna. The antenna is on approx. 380' and mount 60 or 70 feet on the tower. Heading North 1.2 miles drops to 60" above sea level and .75 miles west is nearly sea level. Both areas have poor coverage from the current site.

I know a gain antenna will over shot the site and have considered a Telewave 4 bay dipole which allows downward tilt and a customizable pattern by spacing and positioning the dipoles. This will also increase the ERP closer to our licensed 60watts.

Another consideration is adding a 100watt amplifier and duplexer to increase the power out, the current erp is 60 watts, we are probably around 20-25 right now with the line loss, power output and present antenna.

The last consideration is purchasing a replacement repeater such as an MTR3000 and installing a unity gain or the above stated Telewave antenna. Long term plan to add a voting receiver with a 2.4 or 4.9 rf link.

[Bill_G]

Haven't you and I discussed this before, or is this a different repeater?

[emtprt]

This is another site same setup purchased 10 years ago at all 3 sites. A new console is being purchased allowing all the radios to be added to the system and all the frequncies to be monitored all the time. This channel was used for unit to unit comms but will be transfered for administration purposes and incident commanders to dispatch when operations and dispatch channels are in use.

Mark

[Bill_G]

Send me lat and long and I'll plot it against the other site to see what the coverage should look like as built. You have some gnarley terrain issues that I do not envy having to solve. I have big bumps with big open valleys. You have a wash board. Ugly at UHF.

[emtprt]

Bill
I have been using the software you shared with me and ploted using a low gain antenna with more power which seems the way to go. II really need feed back on the downward tilit option. I only know of one area town using downward tilt and they had good results with a ite 3/4 of a mile North of mine on VHF. I am only worried about covering a 6 mile radius and the software doesn't allow for tilt as an option.

I have one area that is a complete dead zone and would consider a low powered ROIP link for coverage .

Mark

[Bill_G]

You're right - you cannot adjust downtilt in Radio Mobile. You have to import the antenna pattern for the model desired, or use the default omni plugging in the gain for the angle of interest. There are several dipole antennas created by other users that you can download that may fit your DB404. By overlaying plots, you can see the effects of downtilt or lack of it.

I don't suggest a low fill site without multicasting on a different freq, or simulcasting. With such a small area, the possibility for destructive overlap is huge.

[psapengineer]

Hi,

I've had to solve this kind of issue once or twice before here on the west coast; maybe I can help.......

Are you saying that the antenna is on a 380 Ft tower, and it is mounted at 60 ft on that tower? Also, what kind of tower?

Are you also saying that about 1.2 miles away its 60 inches AKA 5 ft AMSL?

Are you essentially trying to cover a coastline? If so, how far in from the shorline is the tower site? Also, is the coast line linear or concave/convex in shape with respect to the tower's location?

At the base of the tower, what's the height AMSL in feet?

Regards, Bob

[Bill_G]

It's the New Haven CT area. Low wash board hills and numerous deep cuts from streams and rivers flowing towards the sound. They don't have distinct mountains and valleys. The glaciers scalped the highest points off, filled in the low spots, and water did the rest. You go up and down 100ft to 300ft over and over mile after mile. Makes for interesting radio prop challenges.

[SlimBob]

Sounds like a tall tower and low gain.

[thebigphish]

I know that area quite well...and I don't envy him at all. The one-shot, one-kill approach figures to be damn near impossible...Voting RX & simulcast TX might be his only option.

[emtprt]

The main systems has 2 transmit and 4 rx sites this is a secondary system that we are improving on slowly.
Repeater replacement is in the planned budget (MTR3000 to replace 1225 repeaters) but could be cut.That is why I am exploring all of the options. Repeater replacement would give us increased tx power and hopefully improve penetration. A voter addition hopefully in 18 months ( ?rf link no phone lines) is my next goal. I spoke to a Raytheon Rep. regarding their voter and they said I could intergrate the old 1225 repeater and and an MTR repeater vs the Motorola voter which could not. I also spoke to our Motorola dealer and they are pushing a 4 bay 6 to 9 db gain Telewave antenna with the adjustable downward tilt and pattern, this reportedly help VHF in an ajoining town.

The hill is 380 feet above sea level. The tower is a 80' Rohn and the current antenna is at 60' on a side bar 6' to 8' off the side. Theer are 2 rivers that create 2 river valleys. The valleys are to the north and and the other is to the west. Both rivers merge 1.2 miles from the antenna WNW and is basically at sea level. The hill creates a wall The hill drops off is 1/4 mile west ans 3/4 miles north of the site (estimated by driving). There is also a slight height increase to the north 1/2 mile away. The hill creates a wall 200 yards WNW of the river merge and continues North and NE for 1.5 to 2 miles. We have another site across the river with a seperate stand alone repeater and we have the same situation with the wall effect but in a Northeast and Northwest direction.

I did investigate a cell site .2 miles west of the current transmitter but they will not allow any private or public safety antennas on their tower. It is a flag pole type enclosed site.


I also applying for a grant to have a BDA installed in a hospital (we had a site on their roof but they had us remove it during renovations and won't allow us to replace it).

[SlimBob]

What about a site on the bluff at the turn in the river?

Or a site on a high point further inland, and one down on the coast that is directional toward land?

[rocketman]

I have a repeater system (UHF) in Southern Connecticut that used to have the same issues. I cured it with several satellite receivers in strategic locations with moderate to high gain antennas and the main site at a very high location with a moderately low gain antenna (5 dB). Problem solved....most spots in the prime coverage area have overlapping portable receive coverage in addition to super distance coverage...covers from Darien to New London along the coast, blankets Long Island to the south, and covers up to Springfield MA and beyond to the North.... running 100 watts into a TX/RX duplexer...have even used the system on the Garden State Pkwy

[emtprt]

Current Tower Rohn 85 foot my antenna is 60 feet on an 8 foot side arm. The Tower is 1.25 miles from the north hill decline and .25-.35 miles from the west declin of the hill.

Any antenna at the mergre of the rivers is basically 3/4 open area that is walled by hills.with an exception of the WNW which has an slight incline 1.5 miles then a large incline. I am basicly covering a bowl area with 2 river vallys.

[SlimBob]

Suggestion? Dipole and a ton (100W) of power.

[emtprt]

I am using a 3.8 gain omni dipole Andrew DB404 planning on replacing the present repeaters ( 3 at 3 locations) with an MTR3000 100watt (trying to get funding).

[psapengineer]

Here are my thoughts on this one:

In this case I would guess that its more about location and antenna height than power output. If I'm correct, what's going on is that there is a lack of fresnel zone clearance into the areas you're having trouble and that is probably setting up diffraction zones with streaks of "cold and hot spots" in the obscured valleys.

Draw yourself a "cross section" of the terrain from the top of the antenna to the dead spots. That will reveal a lot of new information if you haven’t already done so.

In short, in this instance, brute force power increase or antenna pattern may prove to be second order effects as opposed to tower location and antenna height (AMSL) which are likely the first order effect.

Regards, Bob