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antenna gain

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#1
Hello all:

Looking for some technical and experienced insight.

This is a public safety system that operates at 154.1800. We are having to replace the antenna and are 340W ERP.

The repeater currently has a DB224 but I am pushing for a DB228 to help receiving the handhelds. Adding the DB224 to make a DB228 is +3dBd. However, I am told that the +3dBd will not make that much different on the receive side. I have always been informed +3dBd is twice the signal. Insight, please?
 
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#2
Very simply, sometimes 3dB or twice the gain, just isn't enough.
'Gain' comes in several flavors. Meaning that gain may have directional properties that don't 'fit' the situation (that's both in horizontal directions and vertical directions). Changing antennas on a repeater can certainly improve things, but so can getting that antenna higher. In most cases, that higher will make more difference than antenna gain.
The other side of that coin are the antennas in use with the outlying stations. Making improvements there is almost always 'better' (also harder or impractical).
There are a number of possibilities, none of them easy or cheap or even practical in some cases. Your idea about a 'larger' antenna on the repeater is one of those possibilities, and probably the most common improvement.
Good luck.
- 'Doc
 
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#3
Almost every company in the tower or antenna business now has coverage prediction software. Get the salesman to have the plots of your current coverage and the the predicted improvement with the more expensive replacement printed up to prove it is worth the extra cost.
 
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#4
There was recently another thread on here explaining the science behind gain, I can't find it now, but basically it said 3db of gain will give you 1.414 times the coverage. I will try and find the thread when i get home

Sent from my ADR6425LVW using Tapatalk 2
 
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#7
Another way to compare is a radio "s" meter is usually calibrated at 6 db per "s" unit. The additional 3 db gain from the antenna would only be 1/2 "s" unit. Everything else being equal the additional 3 db will hardly be noticed.

BB
 
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#8
There was recently another thread on here explaining the science behind gain, I can't find it now, but basically it said 3db of gain will give you 1.414 times the coverage. I will try and find the thread when i get home
A 3 db power gain is equal to a voltage gain of 1.414. That does not necessarily translate to 1.414 times the coverage. There are too many variables involved that would make a blanket statement like that very misleading.

To put it in other terms, a signal at the verge of useability at, say, 0.25 microvolts would be at 0.355 microvolts with a 3 db gain. You can see how insignificant that is with an FM receiver by measuring sensitivity with a signal generator, and then changing the generator level by 3 db. Yeah, you can hear the difference. Just barely. But 3 db might be noticed at the very fringe of coverage.
 
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#9
They want more to print out a propagation chart than they do for the antenna! It is mounted at the very top of a 350', so increasing the height isn't going to happen.

It will be mounted in an omni position AFAIK, but it sounds like the gain isn't enough to justify the extra $800.

I have no idea about the stress or wind load. Got me on that one...
 

kb2vxa

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#10
This being a public safety radio system and if I read you right the antenna has to be replaced. You're barking up the wrong tree here, we're not the professionals who will be assigned the job. Since the antenna is one of the determining factors regarding compliance with the terms of license, unauthorized changes may result in a violation so leave it up to the shop to deal with it. All that you read here is moot for that reason, next time don't ask a handyman to do a contractor's job.
 
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#11
I'm not asking about transmit power, how it changes the ERP, or changing the height-to-tip. I'm asking how it affects receiving. Don't infer something I"m not asking.
 
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#12
I'm not asking about transmit power, how it changes the ERP, or changing the height-to-tip. I'm asking how it affects receiving. Don't infer something I"m not asking.
You specified in your original post that the station has a 340w ERP. Replacing that antenna infers that will impact transmit as well as receive performance. The more information you get from the answers provided, the better you'll be able to make a rational and informed decision about what to do.
 
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