need opinions on antenna downtilt

[eboe]

ok here's the goegraphic info.

using the traditional HAAT number is not going to be very helpful because HAAT is calculated using measurements at distances out to 10 miles. the required coverage area of my repeater does not exceed 2 miles from the repeater site. that being said...

it's a coastal city so obviously ground elevations start at sea level. the highest elevation in the city is 85 ft AMSL. very casual topography with no cliffs or valleys or anything like that. elevations rise gradually with minor fluctuations as you go inland.

the repeater is on a building that's over 300 ft tall and it's located on a site that's 25 ft AMSL. simple math will tell us that the highest piece of land is still over 240 ft below the antenna and less than 2 miles away.

the antenna info...

looking at an RFS antenna model number 455. it's a 10dB gain antenna with a 7 degree beamwidth of the primary lobe. from the beam pattern chart, the secondary lobes below horizontal seem to come in at about 4-5 dB gain. so it doesn't appear to be a slouch up close.

my observations....

let's say we have this 7 degree beamwidth antenna with NO downtilt. the primary lobe will cover from 3.5 degrees above horizontal to 3.5 degrees below horizontal, correct? if i was standing next to my repeater antenna and i looked up at an angle of 3.5 degrees i'd see airplanes. i'm not in the air-traffic control business so that seems silly to point a transmitter up there.

all my locations are down below the horizon. much below. i did some trig calculations and if you were to draw a line down from the repeater antenna to the ground at a 3 degree down angle, you'd intersect the ground approximately 6200 feet from the repeater site depending on the fluctuations in terrain.

so with a 3 degree down angle, the center of the primary lobe would be aimed 6200 feet away from the antenna. now it's a 7 degree beamwidth so it will still cover from 0.5 degrees above horizontal, down to 6.5 degrees below the horizontal. 6.5 degrees below horizontal calculates out to about 2900 feet from the repeater antenna. so from 2900 feet on out to the horizon, we're covered by the 10 dB gain lobe. from 2900 ft in to the site we still have the 4-5 dB lobes.

i asked for a quote on this exact set-up from a local moto dealer and of course they have better ideas. some other decibel products antenna with about 8 dB gain and a similar beamwidth but NO downtilt. the reason given was because they only recommend downtilt on sites such as a campus or industrial park.

take a look at the picture below for an illustration of my point. the yellow area is a crude representation of the 7 degree beamwidth of the 10 dB gain lobe. doesn't the bottom version make more sense than the top version? am i so stupid that everything i ask for must be 2nd guessed? or is my logic correct on this? please be honest with your answer because if i'm stupid i should really know so i don't hurt myself with scissors or something.

[Will]

10 Db gain at the horizon is too much.

A 3 to 5 db gain antenna would work better and save some cost that you could put into better feed line for less loss over all.

Hard to tell if this is a VHF or UHF system and is it a repeater.

[Max-trac]

We should all have it so easy.
With no cliffs, canyons etc. whatever you put up should play very well.
A downtilt would be fine, but overkill unless you have some very close-in dense buildings... IE I see a quarterwave ground plane doing a fine job....
Always love seeing a 20ft Station Master though!

[RKG]

For this application, I'd seriously consider a unity gain base station antenna. Frankly, you could probably get away with a coat hanger in terms of RF, but you want the weather-resistance of a high quality base station antenna in any event.

Be careful about electrical downtilt claims. You should insist on a lab-grade analysis of the effective downtilt over the range of the antenna's advertised bandwidth. I have found that both advertised gain and tilt data are nominal figures and that there is a good deal of variance within the published bandwidth.

[psapengineer]

I'd also use unity or at max 3db gain.

If the costal community is long and narrow I'd consider using 1 or 2 folded dipoles (like from Telewave) with a harness and spacing them 1/2 wavelength off of the mast (point the dipoles in the narrow direction so the pattern is left and right along the long direction), to control the pattern.

Check out the patterns on Telewave folded dipoles such as this one:

http://www.telewave.com/pdf/TWDS-7045.pdf

found at

http://www.telewave.com/pricelist/antdipoledat.html

good luck, Bob

[RFdude]

Having been in the frequency coordination business, seeing the application described is not that unusual. Some folks overdo their coverage.

For 2 miles of range, a loaded wet noodle antenna will work well (okay, this is an oversimplification, but you get the point). The path loss is a more significant factor in this equation than than antenna gain or pattern.

Why do you need to be on a 300' building with a 10 dBd antenna? It is the wrong tool for the job. Although you don't state what kind of coverage you are trying to achieve, it would appear campus coverage is what you want. Others have summarized well... a unity gain antenna with a few watts of power will work well. For 2 miles of coverage, it would seem that antenna gain and height isn't a factor. Going high might introduce more issues for you on your receive side (co-channel, other interference, etc).

RF Dude

[eboe]

ok i forgot to mention that it's uhf. and i did mention that it is a repeater antenna. currently we have a db404b i think is the correct model number. it's not cutting the mustard. the way the antennas are mounted on the roof is on a railing around the perimeter. 3/4 of the db404 is below a 4 foot diameter 1" boiler plate chimney stack that protrudes from the center of the peak. so there's a height disadvantage there. now i guess i could put the dipole on a 15 foot mast, but i don't think that's such a good idea. so other than the building casting a shadow on the pattern, i also have a MAJOR problem penetrating buildings. i have people down in basements/boiler rooms all over the city and reception is dicey.

[as a side note, talk-in is even worse, but i've already looked into voting for that]

i can't see a quarter wave (or coat hanger/wet noodle LOL) with a couple watts doing better than the dipole with about 20-25 ERP. i got myself coordinated for up to 200w ERP. the feedline will definately be replaced with some nice heliax. the idea behind the high gain and downtilt is to basically bore a hole into concrete basements 1-2 miles away. i figured it's either this or a million dollar simulcast system for 12 square miles which seems a bit dumb even to me. i appreciate the advice to have an analysis done on the effective downtilt, thanks.

as for why such a tall building... the site wasn't chosen by me, but it is the prime site in town. tallest building for sure. even though the landscape is generally flat, we do have some industry and apartment buildings around so if you're below them you ain't goin nowhere. for comparison, we have an exact same repeater set-up for a different channel on a 3 story building that's clear of any nearby taller buildings. 10 times worse. it's our emergency/crisis channel and it's so bad, they want me to relocate it to the 300 ft building which they also hate. luckily, co-channel interference is not a problem on the input.

experience has shown me that in this city, taller is better. and judging by the rest of the antennas on the railing, bigger is better. mine is the runt of the litter up there. quite a few fiberglass whips up there. maybe they've already figured out what i'm grappling with right now.

here's a pic of the way my dipole is mounted on the left side of the building. i put a stationmaster on the right just for S&G's. you can see how a shadow would be cast by the building and the metal chimney pipe, but even on the clear side of the building we have problems in the basements. i realize i may be asking a bit much from a radio wave to get through a brick, concrete and steel building all the way down past the heating pipes into the boiler room, but that's what we need. anything less is unacceptable.

you may say it's overkill, overdone, overunder or not needed or there's a cheaper way, but the question is this:

will it do the job?



PS - the reason i'm a doubtful thomas is because the company that changed my specs on this antenna is the same company that changed the last set of specs on an MC2500 deskset to an MC3000 which doesn't even do what i need it to do.

[RFdude]

450 MHz is probably the best band to be in for close city work. Nice compromise of antenna gain and building penetration.

By the numbers, if you are able to direct your 10 dB of gain exactly where you want it, you will have... well... 10 dB of gain over a unity antenna. And you can't do any better. Realize that downtilt casts a main lobe that itself has a beam-width limit. You will still have "hot" spots in close coverage and would be dependent on reflections, refraction, etc for successfull communications. Only a low gain antenna will provide uniform downward energy. Thats the easy part.

The Europeans have a good approach in mountainous areas to downtilt. They phase together three panel antennas (each in 120 deg different direction), each with 10 dBd gain and each individually directed towards the desired downward angle. Look at SCALA for instance. A lot of good info out there for this as well. But it is expensive including the three way transformer.

The difference between street level and your boiler room in the bowels of a building could be 40, 50 dB or more which is more significant than what your antenna gain can provide. Hopefully, you aren't trying to use the radio inside the boiler when scraping the soot of the boiler tubes! So antenna gain helps to a limited extent, but isn't the biggest factor in your path profile.

Are you paging anyone using this setup? If you get into voting receivers, you will indeed need high power in your central transmitter. But without voting, maintaining a TITO (Talk-In, Talk-Out) balance can also help the portable radio user know when he can get in by listening to the repeater. This includes testing for signal and placing the radio on a ledge (hot spot) where the repeater can be heard while working. If your repeater has 100W ERP, that 5W portable will hear the repeater well, but never have a feel for how they can be heard in the reverse direction. Really depends what level of coverage you want (95% in-building, or can we state 95% boiler room).

To get through 50 dBd of concrete and steel, you may need other methods. A passive repeater mounted at every boiler room with poor coverage. This assumes that the boiler room is your only room of interest. Another method that will work well without going to remote voting receivers is to use a vehicular (or suitcase) repeater.

[Dan562]

Useless your system does not require any Talk-in (Subscriber to Repeater) coverage in and around the downtown metropolitan city area, then there should not be much of an issue installing a +12 dBi antenna on top of the building. I would suggest to install a heavy duty mast pipe with a 12~15 foot length 2~3” O.D. x ¼” Wall Chromemoly pipe to mount to the railing and mounting the existing +6.0 dBi or the new +12 dBi antenna clearing the top of the building’s mechanical structure.

If you mount the new RFS +12 dBi antenna to the same railing level there’s a great probably that the new antenna will degrade the omni-directional transmitted radiation signal pattern and will create more problems with an “Umbrella Radiation Signal Effect.” I can see the antenna but I can not access the repeater. Believe me, this happens more times than not.

Case in point, the Illinois State Toll Highway Authority uses 250~300 foot communication towers along it’s highway system. Mounted at the top of these towers are +9~12 dBi vertical gain antennas and anytime the State Police and/or their Maintenance Subscribers units would get within a few miles radius of the towers the mobile Subscriber units could not gain access the UHF repeater systems.

They went back to the drawing board and decided to incorporate additional tower and UHF repeater sites placing the new tower sites so effectively the antenna radiation patterns provided 100% signal over lap areas. At the time, there wasn’t any Simulcast transmitting capabilities available so their radio engineering group licensed several individual UHF repeater pairs with banks of analog audio voting comparators at their main dispatch center. The entire communication system back bone is using a Microwave Relay system. All Subscriber personnel were instructed that at certain mile markers to automatically select different channels F1 through F4 on their mobile control heads so they could access the system with reliability.

I don’t believe in your situation, you have the finances or liberties to an unlimited radio communications budget. I would stick with a lower gain antenna ... +6.0 dBi to +10 dBi exposed dipole array antenna with a very heavy duty vertical mast pipe to extend the antenna mounting base above the existing building mechanical penthouse or dome structure.