Antenna selection question

[emtunderwood]

If you had your choice of antenna with these options on VHF which would you choose and why, Exposed Dipole at 6db omni 16 degree vertical beamwidth, Exposed dipole bi-directional at 8.5db 16 degree vertical beamwidth, Collinear omni stick at 6db 16 vertical beamwidth, 5db 18 degree vertical beamwidth, or 4.5db 22 degree vertical beamwidth pros and cons antenna looking from around 1600ft at tower centered and need to talk 20km far end of needed talk area with some mountains 1200ft elevation below antenna and down as far as 600ft asl. What are your thoughts?

 

[mmckenna]

Depends a lot on the terrain around the tower. The numbers are close on many of the antennas you listed. Vertical beamwidth is good if you are trying to cover rugged terrain close in around the base of the tower. Gain is nice, but if most of it is out at the horizon, it's not always helpful.

I've had really good luck solving coverage issues in deep canyons by using lower gain single bay dipoles. They have a wide vertical beam width that brings the pattern down lower to the ground. But only needing to cover a few square miles, high gain wasn't necessary.

A good dipole or stacked dipole is pretty rugged and good in remote locations where you may have high winds, ice loading, etc. Cheap fiberglass antennas don't always work well in that environment. Then again, neither do cheap dipoles. Getting quality antennas that will last is pretty important. The numbers don't mean much if the antenna won't last through a harsh winter.

 

[emtunderwood]

Choices are:
SD2352-SF2PASNM Sinclair | Dual array, exposed dipole, 6 dBd/8.5 dBd, field adj, 138-174 MHz

SC281-HF3LNF Sinclair | Collinear omni, 5 dBd, HD, 148-168 MHz

SC251-HF3LDF Sinclair | Omni antenna, 3.5 dBd gain, low PIM, PIP rated, HD, 148-168 MHz

Whichever one will be supported at the base and at the top and the dipole will be supported top, middle and bottom.

Just trying to figure out which would be better for 20KM range and to get into some low lying areas around N 38 38 13.5 W 81 17 05. Everyone is saying something different when it comes to antenna selection.

 

[mmckenna]

Looking at the path on Google Earth going out 20KM shows some pretty rugged terrain, lots of valleys, dips, ridges.

One thing to look at is what others are using and what their experience is. Especially on that tower. Getting someone with propagation prediction software would be a good investment.

Getting as high up on that tower as you can will help. I'd probably look at the SD2352 with a bit of downtilt to bring the pattern a bit down off the horizon.

 

[emtunderwood]

We are 305ft up on the tower and good for 180w. I have some good prop software from a friend that works with motorola but the antenna files have to be purchased or each degree manually inputed. The system he works with throughout the state is UHF and has just the antennas in their system. How much downtilt do you estimate or feel would be necessary?

Commscope shows:

Radio Horizon =33.675 mile /54.183 Km

Down Tilt (deg)	Main Lobe (mi/km)	Lower 3dB (mi/km)	Upper 3dB (mi/km)
0°	33.675 / 54.183	0.764 / 1.229	over Horizon
1°	6.152 / 9.899	0.678 / 1.091	over Horizon
2°	3.075 / 4.948	0.609 / 0.980	over Horizon
3°	2.049 / 3.297	0.552 / 0.888	over Horizon
4°	1.536 / 2.471	0.505 / 0.813	over Horizon
5°	1.227 / 1.974	0.465 / 0.748	over Horizon
6°	1.022 / 1.644	0.431 / 0.693	over Horizon
7°	0.875 / 1.408	0.401 / 0.645	over Horizon
8°	0.764 / 1.229	0.375 / 0.603	33.675 / 54.183
9°	0.678 / 1.091	0.351 / 0.565	6.152 / 9.899
10°	0.609 / 0.980	0.331 / 0.533	3.075 / 4.948
11°	0.552 / 0.888	0.312 / 0.502	2.049 / 3.297
12°	0.505 / 0.813	0.295 / 0.475	1.536 / 2.471

 

[mmckenna]

I'm in the hills at work, but we're on the side of the hill above the ocean. About 800 feet above sea level. 6º downtilt works well for us on our 800MHz system.
From the looks of your chart, 8db seems to be about the maximum you'd want to go. 6dB might be a good place to start.

Putting all that data into the the software would be a good idea. Other than that, you'll be doing a bit of educated guessing, but still guessing at that.

 

[emtunderwood]

0, 2, 4, 6, 8 electrical tilt is available for SD2352

 

[mmckenna]

I'd probably do 6º or maybe 8º No point in blowing power at the horizon if it's outside your licensed area.

Your FCC license should have an ERP listed. Make sure whatever you choose, you stay inside the limits of your license.

 

[emtunderwood]

We also, have the option of dropping our height from 305 to 235, 185 or 160 on the tower.

 

[mmckenna]

We also, have the option of dropping our height from 305 to 235, 185 or 160 on the tower.

I'd not go any lower. If you can run good coax (7/8" or larger) you should be OK on losses. You want the antenna up high to peek down into those valleys.

 

[emtunderwood]

Right now we are running a MTR2000 at 75 watt and its good for about 8-9 miles with the leaves off and with leaves on about 5-6. But yet we can be 30+ mile away and hear it clean with leaves on. Waiting on a new GTR 8000 to arrive mid december.

 

[emtunderwood]

One thing on the antennas, I've spoke with sinclair several times and using the commander that's there currently there and they've said that they need to be top supported and yes we took a lightning hit but this is the reasoning for the post we are changing antennas. They mentioned with VHF and our freqs, the width of the tower and 3ft standoff we needed to be at least 6+ ft off the tower to reduce a null and rx interference. So that's what we are doing, changing antennas to the 2352 and putting the bigger standoffs on the tower, replacing all the coax which is 7/8" ava5-50 already. The motorola guy that works here mentioned using a lower gain antenna(3.5-5db) and using some downtilt, increasing the power to meet our ERP to allow for more penetration and saturation into the low land and valleys.

 

[prcguy]

I've had several conversations with a few antenna mfrs about antenna height and downtilt and they don't recommend any downtilt until you get to about 1,000-1,200ft elevation. I've run many repeaters using 10dB UHF sticks with 7deg vertical beamwidth and had great solid coverage on an 1,100ft tall hill with no downtilt. I've also run several similar antennas in the 5,000 to 5,700ft altitude with a factory recommended 4deg of downtilt with equally great coverage. I would not go over maybe 4deg of downtilt for your 1,600ft hill using wider beamwidth antennas. If you have a specific dead spot in some canyon you can easily ruin your overall coverage trying to fix that one dead spot with too much downtilt.

If lightning is a problem on your hill I would not recommend a fiberglass antenna. I helped replace a VHF Stationmaster for the Coast Guard not long ago that was blown apart by lightning and it was their second failure within just a few years of a fiberglass antenna. We switched to a Sinclair exposed dipole array and expect no more lightning problems.

 

[mmckenna]

The motorola guy that works here mentioned using a lower gain antenna(3.5-5db) and using some downtilt, increasing the power to meet our ERP to allow for more penetration and saturation into the low land and valleys.

That sounds about right.
We fixed coverage issues on our 800MHz trunked system by going from a 9dB antenna to a 6dB antenna with some downtilt. More gain isn't always the solution. But upping your power to make use of all your ERP is a good idea.

 

[emtunderwood]

I've had several conversations with a few antenna mfrs about antenna height and downtilt and they don't recommend any downtilt until you get to about 1,000-1,200ft elevation. I've run many repeaters using 10dB UHF sticks with 7deg vertical beamwidth and had great solid coverage on an 1,100ft tall hill with no downtilt. I've also run several similar antennas in the 5,000 to 5,700ft altitude with a factory recommended 4deg of downtilt with equally great coverage. I would not go over maybe 4deg of downtilt for your 1,600ft hill using wider beamwidth antennas. If you have a specific dead spot in some canyon you can easily ruin your overall coverage trying to fix that one dead spot with too much downtilt.

If lightning is a problem on your hill I would not recommend a fiberglass antenna. I helped replace a VHF Stationmaster for the Coast Guard not long ago that was blown apart by lightning and it was their second failure within just a few years of a fiberglass antenna. We switched to a Sinclair exposed dipole array and expect no more lightning problems.

With the 2352 from sinclair it has a 16 degree beamwidth and our coverage issues start between 5-9 miles away depending on the season, but soon as we go up on the lower hills and even on mountains 35-40 mile away you hear it crystal clear.

On our old tower we only had 419 HAAT above average terrain but it was down in the valley from the tower we are now, and never really had a issue.

 

[mmckenna]

I would not go over maybe 4deg of downtilt for your 1,600ft hill using wider beamwidth antennas. If you have a specific dead spot in some canyon you can easily ruin your overall coverage trying to fix that one dead spot with too much downtilt.

That sounds like good advice.

Looking at the terrain cross section, I think you are going to have some dead areas no matter what you pick. Doing the best you can with your budget is the best you can do. Teaching the radio users how to deal with coverage deficiencies is an important part of the job.

 

[prcguy]

Was your old tower at the same site or on a hill in between your current tower and the rest of the world? If it was the latter, then that lower spot could easily fill in all areas much better than a taller mountain that has to get over another hill down into your main area of interest.

On our old tower we only had 419 HAAT above average terrain but it was down in the valley from the tower we are now, and never really had a issue.