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Old 06-12-2021, 08:49 AM   #1
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VHF antennas height vs DB

Which would yield longest communication distance?

3 foot 3DB base is 40' above waterline

8 foot 6DB base is 30' above waterline

14 foot 9DB base is 15' above waterline

Thanks
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Old 06-12-2021, 10:30 AM   #2
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My knee-jerk reaction is always to favor height over gain. Especially in boat-to-boat communication (vs communication with a high up ground station), we're more likely to lose communication to the horizon, more so in rough seas. But, obviously this breaks down at a certain point and you can squeeze more out of the higher gain.

Basically, to deal with line of sight, you can take 1.42*sqrt(antenna height) for each of the two radios communicating and add them to get the maximum range. Then, in a simplified model, the decibels help within that range. But, that is a simplification, because there is a field shaoe thing going on, among other concerns.

This article explains the trade-offs nicely, I think:

https://www.practical-sailor.com/mar...smission-range
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Old 06-12-2021, 10:31 AM   #3
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Theoretical or practical?
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Old 06-12-2021, 10:45 AM   #4
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Theoretical or practical?
Practical
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Old 06-12-2021, 11:15 AM   #5
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Depends on traffic and noise.

I usually vote db.
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Old 06-12-2021, 12:51 PM   #6
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Practical
It also depends on sea conditions. A high gain antenna is too directional in heavy seas. I have both a 6db and 9db on my boat. Most of the time, performance is comparable. Sometimes, with the 9db, I can get 80 - 100 nm (I have heard the whole line of sight thing, but there is also ground wave propagation in some conditions -- especially over open sea with no land or buildings to interrupt propogation). Sometimes, the 9db comes in broken while the 6 is just fine.

As to the height aspect, there are rapidly diminishing returns to additional height. Basically proportionate to the square root of height. So, the 40' antenna can only "see" about 15% further than the 30'
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Old 06-12-2021, 04:17 PM   #7
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It also depends on sea conditions. A high gain antenna is too directional in heavy seas. I have both a 6db and 9db on my boat.
is your 9 DB mounted lower than the others? how far from the waterline?
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Old 06-12-2021, 05:01 PM   #8
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You can have 1000 miles line of sight but if trying to transmit in areas of high radio traffic, good luck with anyone hearing or understanding your transmission.

Probably the least understood aspect of practical VHF use.
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Old 06-12-2021, 05:24 PM   #9
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is your 9 DB mounted lower than the others? how far from the waterline?
The tips of the 9db, a SSB and 2 6dbs are all at the same height -- about 40' above the water line. The SSB is 23' as I recall; the others are on extension masts so that they also mount from the same level.
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Old 06-12-2021, 07:15 PM   #10
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Shakespeare has a tool on their website to calculate this. I think my greatest range was a 3db at the top of my sailboat mast. Height wins if you have that option.
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Old 06-12-2021, 07:18 PM   #11
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Shakespeare has a tool on their website to calculate this. I think my greatest range was a 3db at the top of my sailboat mast. Height wins if you have that option.
If range is the only variable, I can see that....I just see height of antenna as rarely best and have been using VHF from boats, ships, land stations and helicopters as a pro for about 40 years.
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Old 06-12-2021, 09:28 PM   #12
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You can have 1000 miles line of sight but if trying to transmit in areas of high radio traffic, good luck with anyone hearing or understanding your transmission.

Probably the least understood aspect of practical VHF use.
i like to hear from you about DB as it relates to high radio traffic areas.
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Old 06-12-2021, 09:55 PM   #13
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VHF FM is line of sight only, baring any tropospheric ducting. The formula to calculate distance is Distance equals 1.41 times the square root of height. Height is in feet. Distance is in miles. 30 ft is 7.22 miles. 40 ft is 8.91 miles. 15 ft is 5.4 miles. This is the max distance expected, no matter what gain antenna is used. Curvature of the earth cannot be overcome with a higher gain antenna. A higher gain antenna will overcome line and connector losses of the system.
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Old 06-12-2021, 09:55 PM   #14
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I have to come down on the side of dB. Most VHF communications in our types of vessels is not necessarily across open water. We deal with a lot of intervening land masses so the signal path is actually reflected energy and not actually line of sight. In reflected energy, height is not as important as gain. I can't think of the number of times I have listened to NOAA weather radio trying to get a forecast and just when my zone comes up there is a rush of white noise and it goes away for 15 seconds. This is the problem with reflected signals, the path to the antenna is varying. One technique I have used is to tune both VHFs to the same NOAA channel and with about 10 feet of space diversity one radio may come through readable while the other isn't. They both have 6 dB antennas. You can get space diversity with height, but for a single antenna I would opt for gain, unless my primary use was across open water.

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Old 06-13-2021, 06:38 AM   #15
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i like to hear from you about DB as it relates to high radio traffic areas.
I don't know the tech part, but I have been told that the radios receive the strongest signal and sends it to the speaker. So given 2 signals, one stronger than another, both the receive and transmit end, is the one heard. With a higher db antenna, it has the ability to get more of a weak signal, making communication possible versus garbled or faded out communication.

Also as described above, any transmissions with obstructions seem better with higher db as in ICW areas.

The rocking and rolling problem in my experience is overblown for power vessels, sailboats have issues with long lasting heel on a tack.....again just my practical experience. When comms are lousy at sea with any vessel, turn towards the other station if able to reduce any list, heel or roll issues.

To be clear, my observations are just from using hundreds of different radios, antenna, boat combos between all kinds of stations as described before. Often in the rescue business, relay comms are used and then I got to observe who was able to communicate more clearly. I didn't keep a notebook for statistical analysis, just what I generally feel worked best, the most. Plenty of combos and circumstances might fall out of my generalization.

As I have posted in other threads, I prefer multiple radios... one on a lower db, one higher.
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Old 06-13-2021, 07:59 AM   #16
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I think the complexity of the question comes down to this:

IF (big if) you are already in a “line of site situation”, then additional height is not much more effective, and can actually be counterproductive (more coax loss).

The other item is when the COMM becomes competitive. As you know, only the strongest signal wins at the receiver; called “FM capture effect”. Now you want a higher gain and/or transmit power. That puts more power to the receiver listening. Since Tx power is limited to 25Watts for ship installed radios, the only recourse is to concentrate the signal using gain antennas, “and keep the boat from rocking![emoji41]”

OH; one more thing. As I'm going thru my new to me boat, I find a number of opportunities to control radio interference. You can defeat the best radio and antenna installation with unwanted sources of radio noise. Many contributors are switch mode power supplies. Things that contain these are the following:
LED lights, battery chargers, solar controllers, usb phone chargers and invertors. This is just a short list of culprits that are on my boat, right now. And, its a lot of work to tame some of these beasts. One byproduct of higher ship antennas, is that you tend to also increase the distance from these culprits to your vhf. Air conditioner controllers are another source of this evil. You should do a radio survey, listening to a number of channels that are frequently used. 16, WX, 68, 13, etc. You can have interference on just a few, or on all channels, depending on the nature of the culprit source. THe results may be shocking...
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Old 06-13-2021, 08:29 AM   #17
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VHF FM is line of sight only, baring any tropospheric ducting. The formula to calculate distance is Distance equals 1.41 times the square root of height. Height is in feet. Distance is in miles. 30 ft is 7.22 miles. 40 ft is 8.91 miles. 15 ft is 5.4 miles. This is the max distance expected, no matter what gain antenna is used.
Above, I posted (without having actually used a calculator) that a 40' antenna could "see" 15% further than a 30', so I was interested to see that you did the math. However, your results suggest a much larger benefit (almost 25%). So, I got out my calculator -- I think there is a typo in your 30 ft distance, as I get 7.22, and 8.91 is 15% more than that.

Also, from what I understand, it isn't just tropospheric ducting that can create good long range VHF communications -- ground wave propagation does so much more commonly, but that works best over open water, which I suspect is why Ham discussion usually focuses on tropospheric ducting.
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Old 06-13-2021, 08:54 AM   #18
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My assistance towboat had a 9db, 17 foot antenna with the base mounted about a foot above sea level. I usually could easily talk out to an average of 25 miles over water.

It also had a 6db, 8 foot antenna, mounted on a mast about 8 feet (16 total) high. Occasionally I could get traffic about the same distances while in the ocean, rarely on the ICW. Most of the time I used the 9db because I could count on getting through.

The above distances I don't believe take into account the height of the receiving antenna. As posted previously, most antenna to antenna traffic is in line of sight but will be overpowered by stronger signals. Fortunately USCG and assistance tower antennas are hundreds of feet tall.
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Old 06-13-2021, 09:02 AM   #19
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When in inland water, there's another tradeoff that hasn't been mentioned. The big, high db antenna may work better, but once you have to lower it for bridges, it's now useless, while a shorter, worse antenna is still usable. So to some degree, you have to take your cruising areas and expected needs into account when planning an antenna setup.
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Old 06-13-2021, 09:32 AM   #20
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2 tools for 2 different jobs....its a rare cruising boat that doesn't need range.

And if talking height of 3db antennas, they are going to be as high as a tall 9db so if you are going to lower one for a bridge, you will have to have it open it for the higher mounted smaller antenna.....I never found lower and raising my 17s, even haveing to go up on the flybridge that big a deal.

As I have said, would have a cruising boat without both types for the 2 reasons of utility.

People always say .....money should be no excuse for safety.
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