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

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/marine-electronics/antenna-gain-and-vhf-transmission-range

Theoretical or practical?

psneeld said:

Theoretical or practical?

Click to expand...

Practical

Depends on traffic and noise.

I usually vote db.

ofer said:

Practical

Click to expand...

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'

MYTraveler said:

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.

Click to expand...

is your 9 DB mounted lower than the others? how far from the waterline?

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.

ofer said:

is your 9 DB mounted lower than the others? how far from the waterline?

Click to expand...

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.

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.

backinblue said:

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.

Click to expand...

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.

psneeld said:

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.

Click to expand...

i like to hear from you about DB as it relates to high radio traffic areas.

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.

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.

Tom

ofer said:

i like to hear from you about DB as it relates to high radio traffic areas.

Click to expand...

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.

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...

HTurner said:

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.

Click to expand...

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.

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.

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.

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.

HTurner said:

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.

Click to expand...

I think these distances are just to the horizon. If you assume the other boat has the same set up, the distance would actually be twice these distances, right ?

Benthic2 said:

I think these distances are just to the horizon. If you assume the other boat has the same set up, the distance would actually be twice these distances, right ?

Click to expand...

Yes, you are correct. The distances I, calculated only go to horizon. I should add this is particular important when considering AIS.

When considering receive a "bigger" antenna in always better. More signal capture area, better signal to noise ratio. The best rule of thumb is largest antenna -- most gain -- at the highest possible height. This allows the greatest transmitting range and best receive possible.


With HF, as compared to VHF there are other factors to consider.

HTurner said:

.....A higher gain antenna will overcome line and connector losses of the system.

Click to expand...

Only for transmit. On receive, system noise figure is degraded by feed line loss, dB for dB. And, it cant be recovered by the receiver.

One point that is worth mentioning is that as you increase gain the propagation wave pattern is squezed down narrower in height so if the boat is rolling at all higher gain antennas can be less productive.

Bud

My practical experience is that recreational boats and the average sea conditions they travel in aren't a factor compared to day to day signal strength issues for several reasons.

I agree usually not a problem for rec vessels that mostly operate in calm waters. It is that unusual time somebody needs to communicate in a squall or a storm and has a choppy or broken communication because of a high gain antenna mounted 25ft up on the bridge rolling 20 to 30 degrees.

3db gain antenna is good and is equivilant to doubling the power, so if you imagine the antenna propagation pattern as a donut with the antenna in the middle, the Addition of another 3db gain would be like squishing the donut height down in half for 6db gain and then in half again for a 9db gain. IMO 9db gain antennas are for very stable platforms, base stations, lighthouses ect... The 3 and 6db gain antennas are best for vessels in the trawler under 50ft size.

Bud

Bud said:

I agree usually not a problem for rec vessels that mostly operate in calm waters. It is that unusual time somebody needs to communicate in a squall or a storm and has a choppy or broken communication because of a high gain antenna mounted 25ft up on the bridge rolling 20 to 30 degrees.

3db gain antenna is good and is equivilant to doubling the power, so if you imagine the antenna propagation pattern as a donut with the antenna in the middle, the Addition of another 3db gain would be like squishing the donut height down in half for 6db gain and then in half again for a 9db gain. IMO 9db gain antennas are for very stable platforms, base stations, lighthouses ect... The 3 and 6db gain antennas are best for vessels in the trawler under 50ft size.

Bud

Click to expand...

agree. Plus, any serious discussion of this "3dB" antenna needs to better describe that antenna. The popular marine "low gain" antenna tends to be an end fed 5/8 wave metal whip using a couple of reactive matching elements. Metz, Shakespeare, Digital all make these. This will have a different vertical pattern than a more simple 1/4Wave element over a nearby ground plane. see here:

https://ham.stackexchange.com/questions/1142/what-makes-a-5-8-wavelength-vertical-desirable

Plus, nearby stray metal, and other vhf antennas nearby will have shocking effects on the pattern.

Excellent discussion. Personally not a fan of splitters at all. Much prefer dedicated antennas. Needs vary depending upon setting. When talking to a ship out on the ocean his antenna height makes yours fairly irrelevant. But gain is helpful for the AIS. When talking with the CG signal strength isn’t much of an issue. When in a busy harbor a wide beam is a help. So to meet the vast majority of issues suggest having two antennas. One high gain but narrow and the opposite. Even if you don’t use switches (cheap coax switches are ok) screwing and unscrewing is easy. That way AIS and vhf both have their own antenna and you can switch as need arises.
We don’t place antennas near each other or sources of interference (some lights etc.). We’re small potatoes so have a 3 and a 6. Can usually get ships at ~18m but always at 12m with the high 6 which is enough to get out of the way and the 3 covers anything I can actually see.
Will say I’ve never asked nor been asked to”state intentions “.

Bud said:

One point that is worth mentioning is that as you increase gain the propagation wave pattern is squezed down narrower in height so if the boat is rolling at all higher gain antennas can be less productive.

Bud

Click to expand...

Here's a good illustration

https://shakespeare-ce.com/marine/antenna-selector/