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Catalina/Capri 25/250 Sailor's Forums

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VHF antenna question

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AADIVER
Admiral

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USA
966 Posts

Initially Posted - 01/29/2007 : 19:10:26

If I mount a 3 or 4 foot Shakespeare antenna on the stern rail of my 250 and connect it to my hand held VHF, how much increased range will that give me? Enough to warrant the expense or fuhgetaboutit and pop for a masthead retrofit and a 12 volt VHF?

Frank Farmer
Long Beach, CA
aa.diver@verizon.net
http://mysite.verizon.net/aa.diver

PRETTY PENNY
'01 C-250 WK, Hull #558

Edited by - on

Ericson33
Admiral

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USA
892 Posts

Response Posted - 01/29/2007 : 19:24:06

I think that VHF is line of site, thats why you mount it on the top of the mast. Not 100% sure, but thats what I heard.

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Nautiduck
Master Marine Consultant

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USA
3704 Posts

Response Posted - 01/29/2007 : 19:33:49

You'd be better off just standing up. The external antenna does not add power. It is all about height when it comes to VHF antennas. Suggest you get a full size VHF with mast mount antenna and use the handheld as backup and use in the dinghy.

Edited by - on

ilnadi
Captain

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452 Posts

Response Posted - 01/29/2007 : 19:48:56

Yes, VHF is line of sight. If I was not too lazy (plus a few glasses of wine) to walk up and find the books, Could tell you the range difference 30ft makes, but it is a bit.

Plus, VHF communication is all about transmit power. Most radios, from 12V permanent mount to handhelds have about the same receiver. The transmitter will be different because the 12V has access to more power and more size to dissipate heat. That is why (especially if you have a mast mount) you will hear the CG from the next state (they transmit typically 150W, while handhelds transmit .5 or 1W and 12V transmit 3/5W) but never hear the other side of the conversation.

Now, a base-loaded 2-foot antenna will have more gain than the whip on the handheld, but also you'll get losses from the connectors, so it may wash out.

If you want to talk to the horizon, and most importantly if you want to be heard, get a mast mount and a 12V radio.

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AADIVER
Admiral

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USA
966 Posts

Response Posted - 01/29/2007 : 22:16:02

Thanks, y'all, that's the consensus of opinion. A friend gave me his Furuno radio, so I'll wait for next haul out to have it installed top o' the mast.

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Champipple
Master Marine Consultant

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USA
6855 Posts

Response Posted - 01/29/2007 : 22:22:47

without getting really whacked out the distance - line of site using a modified pythagorean theorem to account for the curvature of the earth is

The square root of (twice the Height times the radius of the earth).radius = 6.38x 10^6 (this is with one receiver on the ground.) With a reciever off the rail (we'll call it five feet) you are transmitting at about 5 miles line of site.

For a mast mount - (We'll call 35 feet with the actual antenna and the water to deckline)its about 13 miles.....

Raise the antenna 35 feet on the receiving end and your looking at 26 miles...

Edited by - on

Steve Milby
Past Commodore

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USA
5895 Posts

Response Posted - 01/29/2007 : 22:36:31

If you want a practical demonstration of the difference between the masthead antenna and a handheld VHF, turn on both types in a busy harbor and listen to both. The handheld will only pick up a few of the most localized transmissions. There'll be much more chatter on the VHF with a masthead antenna. It'll pick up many more transmissions that you won't hear at all on the handheld. At first, you'll wonder if the handheld is even working.

Edited by - Steve Milby on 01/29/2007 22:40:30

Dave Bristle
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Djibouti
10005 Posts

Response Posted - 01/29/2007 : 23:00:18

There are two principal variables in VHF transmission--the height of the antenna and its gain. The height allows it to "see" further over the curve of the earth. A higher gain focuses the transmission into a flatter pattern that reaches further in a plane perpendicular to the antenna. The "rubber ducky" antenna on a handheld has "zero gain", and transmits a pattern somewhat like a sphere. An 8' whip has a gain of 6 db. A 3' mast-top antenna has a 3 db gain. I don't recall the ratio of distance to the gain of the antenna, but a 6 db rail mount antenna will generally give you similar range to a 3 db mast-top unit using the "high" power (25 watts) of a fixed mount VHF. (Handhelds are generally 1 or 5 watts.) That's why powerboats usually use the 8' fiberglass whips while sailboats use the 3' stainless sticks at the masthead.

The Coasties can pretty much pick up any signal within 50 miles--even a handheld if there isn't too much chatter--and transmit a signal that you'll hear on your handheld from that distance. That's because their antennas are hundreds of feet up and they transmit at a power level that's illegal for the rest of us. BTW, VHF can actually reach over the horizon to a degree for the same reason waves in the ocean can bend around a point. So the Coast Guard equipment has receive and transmit range well beyond line-of-sight.

Edited by - Dave Bristle on 01/29/2007 23:18:58

Ben - FL
Admiral

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880 Posts

Response Posted - 01/30/2007 : 02:03:05

I was sitting on Chick-a-pea at her slip at Amity Anchorage one day listening to two cruisers off shore conversing as they headed south. So I hailed one of them and she answered me. I asked her location and she was off of St. Augustine. It had to be twenty-five statute miles. That was with my ship's radio, not the hand held. I was surprised! I don't know about VHF, but CB antenna have to be tuned so the signal does not back feed back into the unit. Anybody that is an electronic wiz might know something about this.

Edited by - on

Admin
Forum Admin

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460 Posts

Response Posted - 01/30/2007 : 05:57:40

I think this is a sparky question....

I'll try pinging him

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aeckhart
Master Marine Consultant

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USA
1709 Posts

Response Posted - 01/30/2007 : 08:58:19

Another element in the send-recieve mix besides power, antenna height, and antenna gain is "atmospherics". I'm not an expert in this area but my Army experience tells me that radio signals travel better or worse depending on the atmospheric conditons present when transmitting or recieving. Perhaps Duane, who seems to have a handle on the subject, can fill in the blanks.

As a side note, we often monitor Canadian Coast Guard transmissions across Lake Superior from around Thunder Bay, Ontario. This is a distance of about 95 miles.

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PZell
Admiral

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USA
548 Posts

Response Posted - 01/30/2007 : 09:57:34

The topic has been well discussed. Only thing I could really add is that one wants to keep the power out losses down from the transmitter to the antenna. That basically means the shortest cable run with the fewest connectors (they being properly installed/soldered) possible for our application. A good ground also helps.

Edited by - on

Champipple
Master Marine Consultant

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USA
6855 Posts

Response Posted - 01/30/2007 : 13:25:20

From my old cell phone sales days I know that radio waves do travel quicker over water. I also know that on a clear night I can pick up the curling matches on the radio.

Other than that I merely remembered an old physics formula.

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Dave Bristle
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Djibouti
10005 Posts

Response Posted - 01/30/2007 : 13:46:54

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Ben - FL
 I was sitting on Chick-a-pea at her slip at Amity Anchorage one day listening to two cruisers off shore conversing as they headed south. So I hailed one of them and she answered me. I asked her location and she was off of St. Augustine. It had to be twenty-five statute miles.<hr height="1" noshade id="quote"></blockquote id="quote">
Sounds like they should've switched to low power (1 watt). It's annoying when boats "converse" at high power, taking up a channel for many miles around. Sometimes I hear bunches of sailboat racers conversing with the committee boat at high power--bad form.

I've been told some antennas (like base-loaded stainless whips) are designed to be mounted to a metal surface like a mast, while others (such as fiberglass whips) are designed not to be, and are therefore good on fiberglass hardtops. You should check on which condition an antenna is intended for.

Probably the biggest signal-robber on a sailboat is the plug-in deck connector, which easily becomes slightly corroded. It's a good thing to "shine up" with an abrasive or replace every few years (male and female), and to keep gooped up with some dielectric silicone and covered during winter storage or whenever the mast is down.

Edited by - on

Nautiduck
Master Marine Consultant

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USA
3704 Posts

Response Posted - 01/30/2007 : 13:55:54

There are through-hull devices for coax that do not "plug-in." The whole coax (including its connector)goes through and you use actual coax connectors inside the boat, not outside. West Marine has them. I use them and prefer them to the more traditional plug-in variety which do get corroded very quickly.

Here is a link: http://www.westmarine.com/webapp/wcs/stores/servlet/product/10001/-1/10001/46511/10001/30/29/3

Get the .825 size.

Edited by - Nautiduck on 01/30/2007 14:00:16

dlucier
Master Marine Consultant

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Virgin Islands (United Kingdom)
7583 Posts

Response Posted - 01/30/2007 : 14:38:40

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Duane Wolff
 From my old cell phone sales days I know that radio waves do travel quicker over water.<hr height="1" noshade id="quote"></blockquote id="quote">

Duane,

Whether on land or over water, radio waves travel at the same speed, the speed of light.

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Champipple
Master Marine Consultant

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USA
6855 Posts

Response Posted - 01/30/2007 : 15:07:05

Let's try farther....sorry

ducting effect of tropospheric propagation or something,....

talking out of my u know what here

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JimB517
Past Commodore

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USA
3285 Posts

Response Posted - 01/30/2007 : 16:25:04

Frank, knowing you sail in Southern CA waters in the ocean you should get a fixed 25 watt VHF and either mount a 3 foot whip antenna on top of your mast or mount an 8 foot on the stern rail. Keep the handheld for emergencies.

Keep in mind the US Coast Guard has their antennas up real high. Thye also have high gain and lots more power than 25 watts. You can usually talk to them at least 25 miles out and often more.

I've talked to people in Dan Point from San Diego (over 60 miles) and I've heard people from Marina del Rey (over 100). At times you can get atmospheric "skip" which accounts for these ranges over line-of-sight.

The range on my handheld VHF on 1 watt is about 1 or 2 miles.

Edited by - on

Ben - FL
Admiral

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880 Posts

Response Posted - 01/31/2007 : 09:14:13

One argument I heard for a stern rail mounted fiberglass whip is that if you get dismasted you can still call for help! Of course that isn't going to happen in a Catalina. Now if you have a McGregor... that's a different story.

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Champipple
Master Marine Consultant

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USA
6855 Posts

Response Posted - 01/31/2007 : 09:29:51

I'd venture to say that even if you get dismasted the length of wire running from your radio to the spot where it crosses through the deck is about the equivalent of a stern mounted antenna.

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Dave Bristle
Master Marine Consultant

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Djibouti
10005 Posts

Response Posted - 01/31/2007 : 10:43:43

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Duane Wolff
 I'd venture to say that even if you get dismasted the length of wire running from your radio to the spot where it crosses through the deck is about the equivalent of a stern mounted antenna.
<hr height="1" noshade id="quote"></blockquote id="quote">
But that's coaxial cable... meaning it's shielded--specifically so it won't leak the signal. What does leak is basically going straight up and straight down. A handheld is the answer for dicey situations--every boat on "big water" should have one. Even a stern-mounted whip can get wiped out--all you need is somebody falling against it.

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