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Author Topic:   VHF antenna
chaimemet posted 11-23-2003 12:35 AM ET (US)   Profile for chaimemet   Send Email to chaimemet  
Have installed a 3' Metz with the ICOM 502 on our M170 03 doing Florida ocean cruising. I have a bimini. I would like to install an 8' or larger antenna. Q- is the extra range worth the hassle? Do I install a Birdsall gromet hole do hickie in the bimini or is there another option? The antenna hookup is on the starboard side of the cockpit console.
InHerNet posted 11-23-2003 06:35 AM ET (US)     Profile for InHerNet  Send Email to InHerNet     
If you were planning on installing the 8 antenna then a good place to mount it would be on the aft section either on the rail or by the steps on either side. Run the cable through the access tube from the aft steps and route it to console. You can lay the antenna down on the outside of the railing and it should not interfere with the bimini.
Maximus posted 11-23-2003 07:51 AM ET (US)     Profile for Maximus  Send Email to Maximus     
Interesting question...

Has anyone done or seen a quantifiable difference between a three foot whip and a 8 foot monster antennae?

I have a three foot shakespere whip mounted on the starboard side of the console railing (ss rail bracket) and have had no problems with VHF communication. I actually got a loud and clear radio check from ~10 miles away (Lynn Harbor to Salem Harbor).

My antennae fits nicely under and out of the way of the bimini.

fredfizzle posted 11-23-2003 09:41 AM ET (US)     Profile for fredfizzle  Send Email to fredfizzle     
I have the 3'Metz mounted to my 2002 Montauk 170 console and have had no problems. Food for thought, the USCG replaced all of their fiberglass antennas on their smaller boats and went with the 3'Metz stainless whip. Cited reasons were durability and superior transmit and recieve capability and compact size. Later.
Moe posted 11-23-2003 03:18 PM ET (US)     Profile for Moe  Send Email to Moe     
The Metz is a shunt-fed 1/2-wave (36" at 156 MHz) antenna. Most other antennas publish a gain spec, but that's relative to a 1/4-wave ground plane... something not practical on a boat. To compare the Metz to them, subtract its 3db gain (that of a 1/2-wave relative to a 1/4-wave) from their rating.

Before going on, let me give you an example comparison of the Metz with a 4.5db rated antenna. Bring up your Windows calculator, and in the View menu, select Scientific. Subtract 3 (db) from 4.5 (db) to find the 4.5 db antenna has 1.5 db more effective radiated power. Divide that 1.5 db more by 10 and you'll get .15 then check the Inv box and click the log button. That tells you that other things being equal (i.e. mounting) this antenna has 1.4 times the effected radiated power of the Metz. To determine what effect this has on distance, check the Inv button again and press the x^2 button to get the result of 1.19. This tells you that, other things being equal, the 4.5 db antenna will transmit the same signal strength as the Metz to a 19% further distance. FWIW, the 4.5 db antenna is typically a 5/8-wave (45").

If you do the same calculation on a typical 6 db 8 foot colinear (two antennas stacked in one tube) antenna, you'll find it has twice the effective radiated power of the Metz, with 40% further distance, when both are optimally mounted. It also has at least twice the antenna length for better reception.

Optimally mounted means that there's nothing, especially metal, above the height of the antenna base within a 2 wavelength (12') radius. Anything, including people, but especially metal, within that range is going to affect the radiation pattern. The closer it is, and the higher up on the antenna it is, the worse it is. On a Montauk, getting close to optimal is hard to do, but getting a long way from optimal is easy. About the worst thing you can do on a Montauk is to mount a short antenna down on the fiberglas console with the windscreen/grab rails inches away from the antenna element.

My point is that mounting location is often far more important than antenna size or gain. That's why short antennas like the Metz often perform as well as, or even better than, larger, higher gain antennas in the real world, where the latter have to be less optimally located because of their size. Most mounting locations are a compromise, some more than others.

One compromise would be to mount a Metz on the top of the console/windshield railing. It would only have problems with adjacent metal when the bimini was up. Another compromise is putting one of those 8 foot antennas on a rail mount on one of the rear side rails, especially with a 1'-2' antenna extension between the antenna and mount. The extensions would get the center point of the 8' antenna up about the same height as that of the Metz on the top of the console rails. Yes, the bimini frame would interfere some with the radiation pattern, but it wouldn't be anywhere as close as the railing to an antenna mounted on the console. The 8' antenna would also have much more antenna length for better reception. It's real hard to beat a good 8' antenna mounted up on a T-top!

A radio geek's installation on a Montauk would probably consist of the antenna cable from the radio running to a female panel-mount BNC connector on the console. When the antenna was not in use, a dummy load would be placed on the connector, to protect the transmitter should the mike accidentally be keyed. The antenna would be mounted on a long pole, tall enough to put the base of the antenna above the bimini (or even the white all-around light), and that pole would connect to a mount or mounts on the console and/or console railing next to where the all-around white light plugs in, so it could go up through the bimini alongside the light pole. There could even be a rachet mount between the pole and antenna to make it easier to lower the antenna for bridge clearance. A 4' 5/8-wavelength Digital antenna would be very effective on top of the pole.

With enough cable, run through the tunnel, across the transom and over to the side rail, an antenna could also be mounted up high on the bimini frame, as I have my Digital 528.

http://www.engr.udayton.edu/staff/lriggins/Whaler/150SportVHFantenna.jpg

Our bimini is almost always up, but this works the same even with it folded up back into the "radar arch" position. The antenna can even be swiveled vertical when the whole bimini frame is folded down and forward. Optimally, I need a 1' extension to get the antenna base up higher than the bimini frame (and to get it above the bow rail when the bimini is completely down), but this is one of those compromises. As it is now, I can rotate the antenna foward or aft and it's short enough to lay next to one of the bimini braces without touching the gunwale. I don't want to give that up.

Hope this helps,
--
Moe

fredfizzle posted 11-23-2003 03:30 PM ET (US)     Profile for fredfizzle  Send Email to fredfizzle     
Chaimemet,

Lots of good info there. Whatever you do, keep it simple and have fun.

jimh posted 11-23-2003 06:50 PM ET (US)     Profile for jimh  Send Email to jimh     
Because of the "knee-effect" of the FM modulation technique, a very minor change in signal strength can often result is a very large improvement in recovered audio signal-to-noise. An extra dB or two could make a significant difference in readability for an FM signal.
triblet posted 11-23-2003 10:30 PM ET (US)     Profile for triblet  Send Email to triblet     
Moe, most modern VHFs will happily transmit into no load
without damage. But RTFM.


Chuck

Moe posted 11-24-2003 02:47 PM ET (US)     Profile for Moe  Send Email to Moe     
Well... it HAS been 25 years since I was an Air Force radio technician and off-duty hobbyist, but I'd NEVER leave an unloaded transmitter powered (or even in a state where it could be powered up) where it could accidentially be keyed.

RTFM is always correct. Assuming the transmitter has no-load protection is risky. Three of the four manuals I checked, including the two brands most likely used by Montauk owners, warn of possible damage:

Icom 402/402S manual: CAUTION: Transmitting without an antenna may damage the transceiver.

SH Intrepid+ manual: Never key the microphone unless an antenna or suitable dummy load is connected to the transceiver.

Simrad RD68 manual: If the RD68 detects a problem with the antenna or antenna connections, the display will show ANT when the PTT key is pressed. To avoid possible damage to the radio the antenna should be checked immediately for any damage or poor connection.

Raymarine RAY53 manual: Shall survive open or short circuit of anttena system without damage(10 min.test)

--
Moe

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