Where do you mount your fixed VHF antenna

dclark · Post by **dclark** » Fri Oct 29, 2004 7:48 am

I have a fixed mount radio with a mast head mounted antenna as well as a hand held. For close range stuff it really doesn't matter, the hand held is more convienant. But I do get a lot more range from the mast head and it's made a differance a couple of times. I've been with a group on several occassions and acted as a relay.

Dimitri-2000X-Tampa · Post by **Dimitri-2000X-Tampa** » Fri Oct 29, 2004 7:59 am

Good point about relaying. If I were ever going to go seriously offshore, I think I would want to bring along my aviation band radio with me too. You can get airplanes to relay your messages around the curvature of the Earth I suspect....now that's a high antenna!

TonyHouk · Post by **TonyHouk** » Fri Oct 29, 2004 9:12 am

Hey All,
I have both a masthead and a aft side mount. When I am cruising around with the mast down I use the aft antenna. When the mast is up I use both. The aft mount is connected up to the radio with a tee fitting. I have not had any problems running both antennas. I thought at one point my radio was not working after I installed the aft antenna, but I was able to contact someone with it. It is another option for everyone. Happy sails, Tony

Terry · Post by **Terry** » Fri Oct 29, 2004 12:00 pm

MarkStanton wrote:

I don't really know our transmission range but obviously vastly superior to a lower mounted aerial.

Curious? Does the antenna transmit as well as receive? I only have the hand held and have found it is just fine as long as the boat I am communicating with is in my line of sight. Otherwise it doesn't seem to cut it, also it is only a 1 watt to 5 watt power, I believe if I had the 25 watts I could send and receive further, but then again, is the send/receive distance dependent on the height of the antenna or is the send dependent on the 25 watts and receive dependent on antenna height? Can anyone clarify this?

Frank C · Post by **Frank C** » Fri Oct 29, 2004 12:37 pm

Terry wrote: ... hand held ... is just fine as long as the boat I am communicating with is in my line of sight. Otherwise it doesn't seem to cut it, also it is only a 1 watt to 5 watt power, I believe if I had the 25 watts I could send and receive further, but then again, is the send/receive distance dependent on the height of the antenna ...

I recall that the West Marine advisor has a good explanation of this. The short answer is that antenna height is more important than wattage exponentially. It's akin to the increasing force of wind on sails, as wind speed increases.

and just below, Moe wrote:On a sailboat, I'd have a 25W radio, with a 3-4.5db antenna, mast-mounted, for best performance. As you probably discovered playing with the calculator, antenna height has less impact than often attributed to it, so if practicality demands, a lower mounting is okay too, if you don't have anything metal at the same height as the antenna within 2-3 wavelengths. --- Moe

EDIT: Oops .... Moe, you forced me to chase down the West Marine stuff that I was recalling ...

Moe, since I only use a handheld VHF, I get a headache reading the details. I do rely upon most everything you post (no sarcasm), but I also recommend reading this link to WM Advisor, good info and well-written. Rather than contradicting, I believe it's emphasis is a little different ...

excerpted briefly, WM Advisor wrote:
* recreational boats are limited to 25 watts of output
* ... increase your power, it wouldn't help much. VHF radios operate on a line of sight principle between stations, meaning that the signals do not bend around obstructions or over the horizon (they do a little, but think of them as traveling about as straight as a beam of light.)
* This means that antenna height, more than any other factor, is responsible for determining how far you can transmit.
* ... a very small transmitter can communicate over vast distances. For example, the signal from an EPIRB (emergency beacon) is less than one tenth of a watt, yet it is picked up by satellites orbiting 528 miles above the earth.
* wattage of the radio is much less important than antenna height in determining range.
* Antenna gain is important, however. ... the higher gain antenna is the one the receiving radio will hear.

Moe · Post by **Moe** » Fri Oct 29, 2004 12:37 pm

To see the effect antenna height has on line of sight distance, try this Java calculator. For example, for station 1, use the boat's antenna height, and for station 2, use a 100' antenna on land.

The received power at any given point falls off with the square root of the distance. For example, at double the distance, received power is 1/4 of that at the original distance.

FM receivers drive the IF into limiting to eliminate noise, with is largely AM, so their signal to noise ratio is better. However, there's a "knee" in their S/N curve, where the recieved signal is lower than the IF amps can boost to limiting. At this point, the FM S/N goes bad very quickly. What that means that at longer distances, having adequate power might not mean just a clearer intelligence (voice) reception, but may be the difference between reception and no reception of the information being transmitted.

Besides radio amplifier power, there's antenna gain. From a vertical antenna, the radiation pattern is like a doughnut. Simply put, more gain on the antenna smashes that doughnut, making it extend out further, but get thinner. With too much gain, that means if the antenna ISN'T vertical, as it isn't with a heeling sailboat, the radiated signal can be aimed downward, short of the receiving antenna, or upward, overshooting the receiving antenna.

On a sailboat, I'd have a 25W radio, with a 3-4.5db antenna, mast-mounted, for best performance. As you probably discovered playing with the calculator, antenna height has less impact than often attributed to it, so if practicality demands, a lower mounting is okay too, if you don't have anything metal at the same height as the antenna within 2-3 wavelengths.

--
Moe

Casey · Post by **Casey** » Fri Oct 29, 2004 8:50 pm

Kmclemore

Yes, it is the one that I have. I have had it 6 yrs. It has worked well, when the mast is down and when it appears that corrosion has attacked one of the mast connections. I connect it directly to the radio to check to see if it is one of the connections, or as a safety.

Moe · Post by **Moe** » Fri Oct 29, 2004 10:02 pm

Frank, I take no offense at you posting the article that differs. It's good to challenge anything that contradicts what you've read elsewhere.

If you're considering bridge to bridge communications, certainly antenna height is the limiting factor. But when one of the antennas is disproportionally high, such as 100 or more foot towers, and the distance greater, the boat's antenna height has relatively smaller influence on the total distance. Play with the calculator and you'll see what I mean.

WM is also correct about low power transmitting very long distances, but the receiver usually doesn't have to try to understand voice. That's why the HAM radio community insists Morse Code is still useful, and why we often transmit data digitally (like really fast Morse Code). When you get out to 20-25 miles, IMHO, even 25W is getting marginal for reliably transmitting an intelligible message.

WM is contradicting themselves by placing power at a lower priority and antenna gain as important. They combine to establish effective radiated power. It's better for a heeling sailboat to have high power with low gain, while a powerboat can better get by with lower power and higher gain (except in heavy seas, where the powerboat is rolling all over). Keep in mind that almost all marine antennas will be rated at least 3db gain, so you subtract that from the greater gain of a better antenna, to find the difference. For example, a 6 db antenna (which is a bit high for a sailboat), has a 3db advantage. That's twice the effective radiated power, not enough to make a 5W handheld equal to a 25W fixed unit with 3db antenna.

I've read the WM article and seen the height issue discussed many times before. I don't disagree that antenna height is important, but from my perspective, getting an understandable message to those tall USCG towers is what I'm most concerned with.
--
Moe

Dimitri-2000X-Tampa · Post by **Dimitri-2000X-Tampa** » Sat Oct 30, 2004 6:38 am

Yea, I'm concerned too...which is why the latest mod I just finished is a special steel cage that is mounted to the cabin top. On either side of the cage are these sliding doors which have weights controlled by gravity and affected by the heel of the boat. Perhaps I could draw a nice diagram and post it in the mods section.

I feel that the two antenna's on my main VHF, battery powered hand-held VHF, hand held aviation band, and 2 cell phones we carry are not really enough redundancy in communications, so if the boat exceeds 45 degrees of heel, the door on the leeward side automatically pops open releasing 2 carrier pidgeons (for redundancy of course), which have tags around their neck with the boat name on it. The pidgeons are specially trained to fly direct to the nearest USCG facilities..usually in St. Petersburg, FL.

I'm working on an update to the mod which will use a small label generating printer interfaced via NMEA to my GPS. Actuated by the same heeling system for the doors, the label will be printed with the exact lat/lon coordinates of the boat. A small piece of twine is attached to the label and goes into the cage and tied to the birds foot. Early testing showed some problems with the twine getting caught in the door opening mechanism.

Send me an email if you would like some more details...after working out the kinks, this will most certainly be the next "must-have" mod for Mac enthusiasts everywhere!

Steve M · Post by **Steve M** » Sat Oct 30, 2004 7:29 am

One more thing to consider. The Coast Guard and other Gov. services use a lot more power and have very high antennas. Case in point. I can hear from Miami...Key West(they may have repeaters) and Ft Lauderdale and they can hear me.
Whip ant. is mounted on Port side aft. Radio 25W.

craiglaforce · Post by **craiglaforce** » Sat Oct 30, 2004 7:32 am

I know sound dbs are a logarithmic scale. So I would suspect that antenna gain of 6 is a good bit more than twice the radiated power of a 3 gain.

WIth my stern rail mounted 6 db gain 8 foot fiberglass antenna I can easily talk to other boats about 6 miles away, and the coast guard towers much further, probably 25 miles at least.

One of these days I will probably put a small 3 db antenna on top of the mast, but just don't really use the radio that much anyway, so the stern rail has been fine for me. I put my radio on the steering console, which is covered when I'm not on the boat. It is a very handy location for single handing.

Jeff S · Post by **Jeff S** » Sat Oct 30, 2004 8:05 am

I use a formula for radar horizon over flat terrain that takes into account only the height of each anntena without regard to the transmitter power or antenna gain, curvature of the earth (negligible at the range of small VHF transceivers), or any environmentals. It is:

Range (nm)= 1.23 x (sqrt(height of antenna 1) + sqrt (height of antenna 2))

Using this I get the following numbers:

Left Column=antenna 1 (Mac), Rows=Antenna 2

Ant1 10.0 15.0 20.0 25.0 30.0
4.0 6.3 7.2 8.0 8.6 9.2
5.0 6.6 7.5 8.3 8.9 9.5
6.0 6.9 7.8 8.5 9.2 9.7
7.0 7.1 8.0 8.8 9.4 10.0
8.0 7.4 8.2 9.0 9.6 10.2
9.0 7.6 8.5 9.2 9.8 10.4
10.0 7.8 8.7 9.4 10.0 10.6
11.0 8.0 8.8 9.6 10.2 10.8
12.0 8.2 9.0 9.8 10.4 11.0
13.0 8.3 9.2 9.9 10.6 11.2
14.0 8.5 9.4 10.1 10.8 11.3
15.0 8.7 9.5 10.3 10.9 11.5
16.0 8.8 9.7 10.4 11.1 11.7
17.0 9.0 9.8 10.6 11.2 11.8
18.0 9.1 10.0 10.7 11.4 12.0
19.0 9.3 10.1 10.9 11.5 12.1
20.0 9.4 10.3 11.0 11.7 12.2
21.0 9.5 10.4 11.1 11.8 12.4
22.0 9.7 10.5 11.3 11.9 12.5
23.0 9.8 10.7 11.4 12.0 12.6
24.0 9.9 10.8 11.5 12.2 12.8
25.0 10.0 10.9 11.7 12.3 12.9
26.0 10.2 11.0 11.8 12.4 13.0
27.0 10.3 11.2 11.9 12.5 13.1
28.0 10.4 11.3 12.0 12.7 13.2
29.0 10.5 11.4 12.1 12.8 13.4
30.0 10.6 11.5 12.2 12.9 13.5

For example= A Mac Antenna on top of the Mast at 30' would have a lline of sight range of 12.9nm to a ground station with a 25' antenna. A Mac with an antenna height of 5' would have a range of 8.9nm to the same station.

I am not sure what height the USCG antennas are. It is clearly worth putting the antenna up as high as you can. Another thing is that this factors in completely flat terrain- no obstructions. Any obstructions to line of sight would interrupt the range. Having the antenna on top of the mast would help limit the amount of obstructions. Also having the antenna mounted on the aft transom area could limit range in the forward quarter due to obstruction from the boat itself.

Jeff S

One more thing- Water reflects radio waves which will tend to help increase effective range. The above formula doesn't take that into account.

Pouw Geuzebroek · Post by **Pouw Geuzebroek** » Sat Oct 30, 2004 9:36 am

Moe wrote:On a sailboat, I'd have a 25W radio, with a 3-4.5db antenna, mast-mounted, for best performance. As you probably discovered playing with the calculator, antenna height has less impact than often attributed to it, so if practicality demands, a lower mounting is okay too, if you don't have anything metal at the same height as the antenna within 2-3 wavelengths

Moe does this mean that mounting the antenna starboard aft, makes it interfere with the back stay on an X

Moe · Post by **Moe** » Sat Oct 30, 2004 10:25 am

Yes, it does, Pouw. Even on the port side railing the backstay will have some effect on the radiation pattern. What that is, I can't say. By effect, I mean the antenna radiation pattern is no longer completely omnidirectional. It may cause a shift of power to one side, causing a null on the other. That being said, the backstay at a changing angle to the antenna, and thus changing distance may cause one section of the backstay to cancel out the effects of another section. Or it may cause different effects at different heights. The probability of it causing a real-world probelm are low, but I wouldn't worry about it as much on the port side.

Every antenna mounting on a boat is a compromise. Base-loaded whips, like those used on the mast top aren't perfect. A coil at the bottom of the antenna increases the current in the lower part and angles the radiation pattern upwards.

Let me address a couple of other comments:

RF energy, like sound and light, is logarithmic. An antenna that's 3 db greater than than another has twice the radiated power. The 8' marine antenna is really two stacked antennas in what's known as a colinear array. Besides "squishing" the radiation pattern, it also provides twice the antenna surface for receiving.

Different formulas for distance use different coefficients for the curvature of the earth. Jeff's formula, using 1.2 is the most conservative. I believe WM uses 1.4 and the calculator uses an optimistic 1.6.

Jeff, to keep the forum software from messing up your tables by removing extra spaces and using proportional fonts, put the table between

Code: Select all

 [/code ] tags (without the spaces).

FWIW, I was a USAF radio technician once upon a time, so I'm probably more anal about this stuff than most.
--
Moe

Jeff S · Post by **Jeff S** » Sat Oct 30, 2004 1:48 pm

Thanks for the tip on the tables Moe, I was wondering how to fix that.

I teach RADAR use to student pilots from an operator standpoint- less technical than you Moe I am sure- basic things that affect Radar performance and then how to use the Radar. I am reading (slowly) Stimson's book on airborne radars, but the basic principles apply across the board to radio waves.

I think I will eventually put another antenna on the mast just in case- those extra few miles may make a difference, especially if you go offshore at all.

Jeff S

MacGregorSailors.com

MacGregorSailors.com

Where do you mount your fixed VHF antenna

Where do you mount your fixed VHF antenna?

I've got both

Antennas