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Repeater antenna db gain vs altitude


WRTZ750

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So I'm putting up a repeater antenna on a short 10 foot mast on my garage for personal comms.  I want to play radio close to the ground before I jump "all-in"like the big boys with big towers and big costs.

I sit in the Mississippi River valley with a bluff directly behind my house and another across the river on the Iowa side.

Would a higher gain antenna at such a low mast height, like the db420-b be beneficial or would other lower gain/lower dollar antennas work just as well since I'm not achieving the "height is might" addage?

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For a DB404 you can buy it right here on the site. I've scored some decent ones on ebay, but you need to be looking at the stuff. Talley and Tessco are my go to for DB420 and other antenna's but your going to pay a good price there. 

 

https://shop.mygmrs.com/collections/antennas

 

https://www.talleycom.com/

https://www.tessco.com/

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DB404 is a great antenna but the vertical radiation angle is only 27 deg. Is that high enough to get over the bluffs?  You might want an antenna with unity gain like the Tram-Browning BR6140 which has a much higher radiation angle. 

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1 hour ago, WRTZ750 said:

Is there a way to use Radio Mobile or another line of sight program with elevation/GPS overlay to help me calculate the angle from antenna to the top of the ridge?

Trigonometry. 
 

But unless there’s a repeater or someone you’re trying to reach at the top of the ridge it might not matter. A radio signal that is aimed at the ridge will keep on going in a straight line, not curve back down where someone on the other side of the ridge will receive it. 

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11 hours ago, WRTZ750 said:

About 175 yards behind the house we're headed up the bluff incline.  In this area there's about 100 feet of elevation change.  That's immediately behind the house.  I'm hoping to get the antenna on top of the ridge eventually.

100 feet of rise in 525 feet of run is a 10.78° angle above horizontal. 

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If you are down in the valley, you want an antenna with a higher angle of radiation to get up to the hills. If you are in the right location, you may be able to take advantage of knife edge defraction. I am able to do that where I live with one hill and am able to get a signal from my place into a valley 20 miles away to a mobile. It is not perfect as far as 100% coverage but it does work. With UHF, whether mobile or base, you have to pay attention to your terrain and the angle of radiation on your antenna. High gain is not the end answer to everything. This isn't HF. I have encountered many Hams applying HF to UHF with many thing and it doesn't work the same way. I have spent quite a bit of money being misled, until I started doing my own investigation and learning how different signals are affected by terrain.

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Think of the signal as a beam. I'll use two of my antennas as examples. I have a Comet GP9 (9db) and a Comet CA-712EFC (6db). Yes the GP9 has higher dB but it also concentrates the signal in a narrower "beam" when compared to the CA-712. The GP9 works better for longer distances over flatter terrain while the CA-712 works better getting around different terrain. 

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Good point on the ca-712.  I'm not sure if one day I get this repeater on top of the bluff that a high gain antenna would not transmit down to the river banks below the ridge but rather, transmit across the river.  This would leave a dead zone below the level of the antenna, I'm thinking.

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6 minutes ago, WRTZ750 said:

Good point on the ca-712.  I'm not sure if one day I get this repeater on top of the bluff that a high gain antenna would not transmit down to the river banks below the ridge but rather, transmit across the river.  This would leave a dead zone below the level of the antenna, I'm thinking.

That’s pretty common. Some repeaters use phased antennas that direct the pattern downward. 

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2 minutes ago, Sshannon said:

That’s pretty common. Some repeaters use phased antennas that direct the pattern downward. 

You can see that in the antenna specifications referred to as  "Down Tilt" angle. The purpose is to direct a bit more power closer in to the base area of the antenna. Otherwise the signal strength can be a bit poor close in but much better once you get some distance out from the repeater site.

Both of the antennas in the attached datasheets have no down tilt. However look at the beam width specification. The higher gain antenna has a VERY narrow one, 14 degrees, compared to the lower gain one at 27 degrees. Something to consider before buying. 

DB408-B Product specifications.pdf DB404-B Product Specifications.pdf

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I use a CA-712EFC on a 15-ft pole down on the St Joe river at near water-level with mountains all around the valley.  This is connected to my Retevis RT97s portable repeater.

 

With a HT standing directly under the antenna, I have no trouble getting keyed up.

 

With the 20w mobile in my SxS running a 1/4-wave mag mount, I have been around 15 miles away in a draw a full mountain range away and had no trouble getting into the repeater.

 

I know this can all vary from trail to trail.  I was initially worried about the gain of the Comet and through I wanted a different antenna with less gain, but so far it's been working well for our camping and trail riding comms. 

 

image.thumb.jpeg.51610ece8d934f890a9c7c7f179f5f53.jpeg

 

 

 

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On 1/16/2024 at 10:06 AM, WRQI583 said:

If you are down in the valley, you want an antenna with a higher angle of radiation to get up to the hills. If you are in the right location, you may be able to take advantage of knife edge defraction.

Knife edge refraction works best with hard surfaces, esp on UHF (GMRS is in the UHF range). Large, hard deep rocks are more likely to give knife edge than soft soils. 

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On 1/14/2024 at 9:04 PM, WRTZ750 said:

So I'm putting up a repeater antenna on a short 10 foot mast on my garage for personal comms.  I want to play radio close to the ground before I jump "all-in"like the big boys with big towers and big costs.

I sit in the Mississippi River valley with a bluff directly behind my house and another across the river on the Iowa side.

Would a higher gain antenna at such a low mast height, like the db420-b be beneficial or would other lower gain/lower dollar antennas work just as well since I'm not achieving the "height is might" addage?

This is a great question. A little background: I worked at Motorola for over 30 years and asked this very question to some of the RF guys there. The resounding answer is to "go high baby". Well, in a case like yours where your high-gain and low height option is not really very high, then yes some extra maybe 10-20ft with a somewhat lower gain will be better. Now if you were already at 100ft with high gain and could go up another 20ft with low gain, this could be a different story. That would depend on the surrounding terrain. But in short "go high baby". 

As evidence of how this works, I too ran a repeater during the above mentioned time period, that being a 450MHz ham band repeater at my house. I lived on a small hill in the Chicago suburbs, maybe a 40ft above average terrain. Sounds like not much, but in the Chicago suburbs that's all you get for a hill :)  I chose to go high baby, and put a 6dBi gain antenna as high as I could get away with, about 60ft above the hill. I was afraid to put a larger higher gain antenna at that height due to tower strength limitations. And wow, did it have great coverage for being just a home brew repeater. In the direction where the land sloped down slightly it easily had a 40 mile mobile coverage and I had people 80 miles away using it from simple base stations. Now granted this was one high performance repeater: 25 watts TX (yea, not huge I know) but the receiver was souped up by me, being an RF receiver guy, it had less than 2dB noise figure counting the duplexer loss (for those who don't get noise figure, 2dB in that case is outstanding). 

Hope this helps.

 

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