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Author Topic: Repeater antenna selection  (Read 1576 times)

AF6D

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Repeater antenna selection
« on: May 12, 2019, 07:43:25 AM »

We have a desirable yet relatively low elevation repeater site (Sierra Peak) of 3800 ft compared to our current 6550 ft main site. We have other sites but they do not cover the area in question. This 3800 foot site is directly over a main freeway in Southern California (SR91) and even using a 6dB gain antenna it's in the donut hole. It's notorious for it. Our current main site  fills in this donut hole area  with only one spot  that can get noisy if you are on an HT. Using a 6dB gain antenna also has a drop off out in the coastal area and weakens just before it.

It is my belief that given the relatively low elevation considering other sites that do not cover south to north as we need for fill in I believe that a 9dB gain antenna would be more appropriate. It's not going to get down in the donut hole anyway but it's going to hit rock solid into the mountains and foothills 50 miles to the north which is our donut hole currently. That's the purpose.

Penetration into buildings is desirable as is mobile operation. Our current site at 6550 ft uses a 5.25dB gain antenna deliberately and with relatively good coverage with the exception of its donut hole. We are already linked to another site further west with very good coverage but it has the same donut hole and does not cover the mountain area or the desert area that we cover behind it.

I recognize there are far more experienced people than I. What are the ramifications of using 6 vs 9dB in such a scenario? I realize that I can't show you a topo map but it's predominantly flat with bumps here and there. I understand that gain is obtained by narrowing the beam width of the aperture. There's mountainous country behind the site and we have no expectation of covering this area from this site. Our existing site gets it. We're contemplating using a side mounted db224 with an elliptical pattern although the manufacturer specifies it as a cardioid pattern. In practice it's somewhere in between. A 6dB station master has a 70 degree vertical beam width while a side mounted db224 has a 16 degree vertical beam width. Calculations using prediction software suggest a 9dB would be the better choice but what does reality suggest?

Still learning and not ashamed to ask questions.
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N2AYM

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RE: Repeater antenna selection
« Reply #1 on: May 12, 2019, 08:04:42 AM »

Yes - a 4 bay dipole array with all 4 elements on the same side (9DBm) is your likely best solution.
I have such a configuration on my repeater pointed in the direction i need to cover and it works
well this way and favors the direction i need covered. You did not mention what band your is for.
Mine is for 220 and it works excellently.
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AF6D

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RE: Repeater antenna selection
« Reply #2 on: May 13, 2019, 06:32:57 AM »

Thanks for your reply, Daniel. I thought I had mentioned to me but it looks like I deleted it while trying to navigate posting using a cell phone. It doesn't usually work out too well with a cell phone. Because it's too many tears I thought that's building penetration would be better family lower-level site with a more focused antenna.

Our top of mountain site  has more than a 270 degree coverage area.  It gives us full coverage of the coastal area  to the south and desert coverage to the north. We already have db224 side mounted on an 85 ft tower at 6350 ft at our backup site. My home. It is pointed at the farthest location from our site and except for the donut hole that is only modest below it, it kicks butt! But it doesn't get out into the desert like our top of mountain site does.

What altitude is your 220 site at?
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N2AYM

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RE: Repeater antenna selection
« Reply #3 on: May 13, 2019, 06:44:37 AM »

Thanks for your reply, Daniel. I thought I had mentioned to me but it looks like I deleted it while trying to navigate posting using a cell phone. It doesn't usually work out too well with a cell phone. Because it's too many tears I thought that's building penetration would be better family lower-level site with a more focused antenna.

Our top of mountain site  has more than a 270 degree coverage area.  It gives us full coverage of the coastal area  to the south and desert coverage to the north. We already have db224 side mounted on an 85 ft tower at 6350 ft at our backup site. My home. It is pointed at the farthest location from our site and except for the donut hole that is only modest below it, it kicks butt! But it doesn't get out into the desert like our top of mountain site does.

What altitude is your 220 site at?

My site in NJ is at approximately 36 feet above sea level
and the antenna is about 160 feet up on the tower. The
4 bay dipole array is pointed to Philadelphia and covers
a reasonable area in Ocean and Burlington Counties and
in the opposite direction it covers southern Monmouth
county except behind several high hills. Most of the surrounding
terrain is flat except for trees and buildings.
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WB0DZX

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RE: Repeater antenna selection
« Reply #4 on: May 13, 2019, 10:10:45 AM »

More gain at a high elevation without intervening terrain does somewhat increase RANGE and fringe COVERAGE. The downsides, which increase as site elevation increases, are:
1. more gain equals more lobes with the attendant deep(er) nulls. Side mounting increases the number of lobes and deepens the nulls. Seldom is this attempted to be reflected in pattern diagrams.
2. an increase in dead and marginal areas closer in and, which apparently would not apply in your case, especially in the 3db lower lobe behind the 9db lobe. The 3db lobe angle of radiation is still just as high as the 9db lobe and
3. the good possibility of "shooting over the top" with the best fringe COVERAGE in the mountains above the 50 mile point. Those mountains are not able to fill in via multi-path. The signal level from a handheld at 50 miles will also be very weak, even if the 9db reciprocal gain was boresighted on it. Earth curvature is a significant factor at 50 miles. Refraction would not help out; anyway not applicable in your case.
 
Questions:
1. Which site is 50 miles away from the area lacking coverage? Is the 3800 foot site closer or further?
2. How large (square miles or blocks?) is the area without acceptable coverage?
3. Is the area without acceptable coverage is only SR71 and along either side of it?
 
If the answer to question three is yes:
A. Is/are the affected user(s) using only a low power mini handheld radio and/or stubby antenna? If so, THAT's the problem! No site changes needed!
B. A sidemounted antenna may be on the wrong leg.
C. A lower gain antenna is needed to reduce the donut hole. However, the best fix is a voting receiver, assuming talkout is acceptable. The next best is a site away from the dead/marginal zone, but that could adversely impact existing coverage.
D. "Directly below" may imply the need for a downtilt antenna. Don't mount a poor-man's downtilt antenna - an antenna mounted downside-up - unless the manufacturer rates it weatherproof (with some extent of rain, ice, snow with wind) downside-up.
 
Mike WB0DZX
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N8EKT

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RE: Repeater antenna selection
« Reply #5 on: May 14, 2019, 01:23:20 PM »

Gain creates dead spots beneath the main lobe so lower gain or down tilt are a much better solution for mountain top sites.
If it were me, i would either buy the Comtelco BSLL150XL3-A
3DBD fiberglass antenna.
It has a 40 degree wide pattern and it can withstand 125mph winds.
Or better yet the Hustler HS3-14448 fiberglass
the Hustler has a 36 degree wide pattern and can handle 160mph winds
« Last Edit: May 14, 2019, 01:29:44 PM by N8EKT »
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W9CN

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RE: Repeater antenna selection
« Reply #6 on: May 18, 2019, 08:15:32 AM »

Telewave ANT450D6-9 (UHF) or ANT150D6-9 (VHF) series set to hard cardioid with 5 to 10 degree electrical down tilt (special order)

https://www.telewave.com/product/ant450d6-9-folded-dipole-antenna-406-512mhz/

https://www.telewave.com/product/ant150d6-9-folded-dipole-antenna-138-174-mhz/

Both are 9 dBd in hard cardioid.  Slight gain reduction with the electrical down tilt.
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AF6D

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RE: Repeater antenna selection
« Reply #7 on: June 02, 2019, 01:05:01 AM »

Thank you for your help. I have done extensive modeling and nothing is going to fill in down below on SR91. SR71 is not a issue. A 6dB gain loses it going out towards the beach and drops of on the Pacific Shelf; a 9dB does far better. The HAAT from this site of 3800 feet is only 1880 feet. That's why I believe the 9dB gain without downtilt is the better choice. It's vertical beamwidth is 16 degrees and matches nicely. The difference between antennas is the 9dB has a RX strength at the far limit of 44.8uV compared to a 6dB that only has a 13.6uV. Both give a mobile S9 but one is much stronger.
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W9CN

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RE: Repeater antenna selection
« Reply #8 on: June 03, 2019, 04:20:44 AM »

You could give it a little bit of "mechanical" down tilt when you mount it on the tower. 

And the antenna gain works both ways (TX and RX) so its a win-win.

Glad to be of assistance.  Let us know how it all works out.
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AF6D

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RE: Repeater antenna selection
« Reply #9 on: June 03, 2019, 04:48:59 AM »

You could give it a little bit of "mechanical" down tilt when you mount it on the tower. 

And the antenna gain works both ways (TX and RX) so its a win-win.

Glad to be of assistance.  Let us know how it all works out.

Absolutely. Just use a different mounting assembly. We welded a custom one last time. This one is so low to the area to be covered that the gain really works.  Even without down tilt because neither a 6B or a 9dB antenna get the freeway in the donut hole. There's nothing but mountains behind the site so an omni-directional antenna really serves little purpose. We also link with another repeater that points at this site from 5,300 feet, so a user could choose either repeater depending on location.

I love playing with antennas!
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KB8VUL

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RE: Repeater antenna selection
« Reply #10 on: July 07, 2019, 08:27:26 PM »

With your requirements, I would be checking the clubs bank account and then calling a manufacture and tell them what you are trying to do, with alll the numbers.  Antenna height, ground height at tower, ground height in serviced area and a topo map of the whole thing.  They can design antennas with down tilt in them that will drive the signal down into the valley at the base of your mountain the tower is on.    You are not going to be able to just get something out of a catalog and throw it up on the tower and have it perform optimally.
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AF6D

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RE: Repeater antenna selection
« Reply #11 on: July 08, 2019, 12:10:26 AM »

With your requirements, I would be checking the clubs bank account and then calling a manufacture and tell them what you are trying to do, with all the numbers.  Antenna height, ground height at tower, ground height in serviced area and a topo map of the whole thing.  They can design antennas with down tilt in them that will drive the signal down into the valley at the base of your mountain the tower is on.    You are not going to be able to just get something out of a catalog and throw it up on the tower and have it perform optimally.


Thank you but this site sits back away from SR91 below it. Most other repeaters on this site can't hit it and a 6dB can't quite make it to the beach area. With multiple sites on the air we are hardly just throwing something up. We have already worked with the engineers at CommScope and they reminded us that 6dB for example is at the horizon. Dropping to a 3dB gain antenna will limit our reach out to the beach which is why we figure the trade-off of a low-level site using a 9dB gain antenna is the better choice according to modeling done already. Remember that this is a relatively low-level site.

But as far as down tilt, it is our understanding that if one ads down tilt they do so at the expense of hitting the horizon as well. It is 50 watt  mobiles being used for testing and SR71 is not an issue. SR57 an only be hit from Sunset or Johnstone. There are a couple others maintained by county/state agencies that don't easily offer access.

So it comes to down tilt. What is the accurate truth about down tilt and losing it at the horizon? The HAAT is only 1856 feet which if you live in Texas you'd dream of. But it completely misses the coast line.

I look forward to your additional comments on down tilt and the trade off at the horizon.
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KB8VUL

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RE: Repeater antenna selection
« Reply #12 on: July 08, 2019, 10:50:47 PM »

Any time that you modify the pattern of an antenna something suffers.
Now this isn't a change like going from an omni to a beam, but still a change.

Need to hit you with a couple questions first, to get some idea of what to possibly suggest. 
Need  ground level of town wanting to be covered, need ground level and tower height where antenna is to be installed.
That info will give me a better ideal of what will do the best overall.

As far as specifics of downtilt, and gain.
Imagine a coverage pattern of a unity gain omni directional antenna as a donut.
NOw as you add gain to the antenna, you are effectively crushing the donut. 
The donut as you crush it will grow in diameter as it decreases in height, but the other thing that gets missed alot is the crush happens from the bottom up as well as the top down.
So the more gain, the flatter and higher the radiated pattern.
The reason that's important is that you can get to a point that you effectively talk over top a low area in the coverage pattern of the antenna.
So we bring in down tilt.  This makes it like crushing the donut with two funnels with the walls at a specific angle (the down tilt angle).
You effectively tip the entire pattern down by X degrees in all directions.  This can be used to drive signal down a high hill or mountain  and into a valley.
But the coverage distance takes a hit in doing this.

But there is hope.  Depending on a couple things.
The easiest way to 'have your cake and eat it too'  is to run two antennas and a power divider or more correctly a directional coupler to send a specific amount of signal to each of the antennas.
THis is how you can fill a large multi-story building with coverage with one RF source and a BDA.  The antenna system is nothing more than a bunch of ceiling mount antennas and properly selected RF directional couplers. 

So the way that it would work is you have the top antenna as a standard pattern Omni, and then run a vertical with down tilt, or run a panel and put in mechanical down tilt when mounting it. 
If you are wanting to cover one town in a valley this is actually a preferred method.  You want to look at at the antenna beam width and the distance from the tower to the town and ensure that the beam width of the panel or even a corner reflector will cover the town or area in question.   Now the selection of the signal tap on the directional coupler has to be carefully considered.  It sounded like you are trying to get to the coast line with this, so I would with the current setup go take some measurements in dbm of the signal levels from the repeater to the areas of interest for coverage. 
SO lets say that you have an average measurement of -80dbm in those areas.  You can easily get away with a 10db tap, which would send 10 db of the total transmitter signal to the second antenna and allow for coverage in the valley below and the coast line as well. 

There are some requirements to this sort of setup.
First is that the antenna system can't be shared, you will need to run it through a duplexer and not be on a multicoupler system.  I don't believe that will be an issue here but you are significantly modifying the coverage footprint of anyone on that antenna system so others may or may not like the change.

Second is that you will be loosing something in the directional coupler.  And that loss is bi-directional. so it can effect the receive as well as transmit.  You can recover a lot of it with antenna gain, but there is only so much gain that's possible in an antenna.  Transmit power can always be increased but that does nothing for the ability for the receiver to hear, and honestly, wide band, high gain amps in the receive line between the duplexer and receiver are courting a deaf receiver.  Don't trry to do more than about 3 db of amp if at all.

You will need to consider every part of the antenna system and build for minimum loss.  So you are going to need to run 7/8 or larger cable to get the most signal up and down the line.  This of course depends on the height of the antenna on the top of the tower as you can place the directional coupler near the lower antenna and run a short jumper from teh tap port to the panel or corner reflector antenna. 

Lastly, this ain't cheap.  you can do a lot with a proper antenna system, but proper has dollar signs attached to it most of the time.
And of course, if you are trying to cover more than one town at the bottom of the mountain, it becomes increasingly difficult to do.  You can come off the directional coupler into a 50/50 splitter and run two corner reflector antennas, but after that you get to a point of diminishing returns.  But it would still be better than a flat onmi at the top and then a down tilt antenna some distance down the tower  with a 360 coverage pattern.
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