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Author Topic:   VHF Radio Antenna Mounting Location
Basshole posted 08-09-2010 01:13 AM ET (US)   Profile for Basshole   Send Email to Basshole  
Hey all. I just mounted an Atlantic Towers arch on my 1998 Conquest 21 and had to remove my VHF antenna because it was in the way. Some of the cross members of the arch are flat which appear to be ideal locations to reinstall my 8 foot antenna with a swing-up mounting base (or whatever you call it). I especially like the idea of mounting it to the crossmember on the starboard side and fishing the wires through the arch.

Before I drill any holes I wanted to see what you guys thought of that idea. That should place it approximately 3 feet from my Lowrance Broadband radar dome. It will also place it about 4 feet higher than it was before which I hope will improve its performance. I also will need to splice some more coax cable to get it to reach my radio. Is it ok to splice VHF radio coax or will that affect its performance? Thanks in advance since I pretty much know nothing about VHF radios but would like to learn more.

Here is a pic of the boat as it sits. Sorry if the link doesn't work.

www2.snapfish.com/snapfish/slideshow/AlbumID=5229725013/ PictureID=238346091013/a=128576483_128576483/

jimh posted 08-09-2010 08:57 AM ET (US)     Profile for jimh  Send Email to jimh     
Raising the height above the water of the VHF Marine Band radio antenna will definitely tend to increase your radio range.

If you are using a typical 8-foot fiberglass pole antenna, there is a very good chance that the lower 2-feet of the antenna will consist of nothing but fiberglass tube. The actual antenna occupies the upper portion of the tube.

The emission from the RADAR antenna will be in a very narrow beam, and it is likely that the beam will only intersect the radio antenna in the portion of the antenna that is only a fiberglass tube. This should produce little interaction between the two devices.

Coaxial transmission line line RG-58C/U is generally not easily spliced. The usual technique to extend the length of the transmission line is to install connectors and mate the connectors. If two male connectors are installed--the most common technique--a female to female adaptor is used, which is often called a barrel due to its appearance. There is some insertion loss from the added connections, but if they are properly made and quality connectors are used, the insertion loss will be insignificant.

If there were great concern about transmission line loss, you could employ a better quality transmission line for the run from the transmitter, through the boat and up to the arch. Then you would cut off the transmission line from the antenna, leaving just enough to mate with the new transmission line. There are some better grades of 50-ohm coaxial cable available, but they are expensive and generally only available from radio specialty vendors such as Times Microwave.

Buckda posted 08-09-2010 09:16 AM ET (US)     Profile for Buckda  Send Email to Buckda     
I had my antenna mounted to the TOP of my Atlantic Towers Arch and ended up with TWO spliced connections in the line, which I did as described by JimH - using Doodad connectors and a barrel in between.

The reception on this setup was excellent. Be sure your antenna is designed for the extra "whip" load. I use a Shakespeare 5225XT that was designed to be mounted in that location.

The biggest concern I would have about mounting it to the side is that someone may think of it as a hand hold when boarding the boat from a high dock. If that is generally not a problem where you boat/dock, then it seems like a good location to me.

Dave

Basshole posted 08-09-2010 11:13 PM ET (US)     Profile for Basshole  Send Email to Basshole     
Thanks guys, that helps a lot. Very informative replies.
contender posted 08-10-2010 04:36 PM ET (US)     Profile for contender  Send Email to contender     
Basshole, after looking at your boat I think you have come up with the correct location and mount, Just like you stated mount it on the flat surface and run the wires inside the tube alum.. And just like Jim stated it will give you more distance on your radio...Good luck
jimh posted 08-10-2010 06:18 PM ET (US)     Profile for jimh  Send Email to jimh     
An 8-foot tall radio antenna made with a fiberglass tube can have a fair amount of surface area, and when the boat wind is strong, an 8-foot antenna will create quite a bending moment on the mount. If you attach the antenna to your new arch, be certain that the point of attachment can tolerate the stress from the antenna. You might consider using a smaller antenna. The mechanical stress will be much reduced.

When both a 4-foot antenna and an 8-foot antenna are mounted with their base on a tall arch that puts the base about 8-feet above water, the difference in radio range will not be particularly great. If we assume a height above water for the base as 8-feet, and in the case of the 8-foot antenna we assume the center of the antenna will be 5-feet above the base, this puts that antenna at 13-feet above the water. For the 4-foot antenna we assume the center of the antenna is about 2.5-feet above the base, and that puts that antenna at 10.5-feet above the water.

We compare the range to the radio horizon for 13-feet and 10.5-feet:

13-foot: 1.23 x (13)^0.5 = 4.4 miles
10.5-feet: 1.23 x (10.5)^0.5 = 4.0-miles

You may want to give up a very fractional increase in radio range in favor of a substantial reduction of mechanical stress and loading on those welds in your new arch.

contender posted 08-12-2010 04:36 PM ET (US)     Profile for contender  Send Email to contender     
Jim I think the amount torque place on an 8ft antenna vs. a 4ft antenna would be very small. First the boat at best probably does not go over 60 mph. Second the wind resistance on a 1 inch tapered pole is not that much. If Basshole uses a good stainless mount he should have no trouble at all. I have two 8ft antennas mounted the same way on my Contender with no problems, and if you fell there is a problem you can fold it down...take care
jimh posted 08-12-2010 08:20 PM ET (US)     Profile for jimh  Send Email to jimh     
On a 4-foot antenna you can figure the effective area of the antenna multiplied by the wind pressure acts as a moment on the base with a lever arm of 2-feet. Now we consider the 8-foot antenna as being the same, plus an equal area times the same wind constant, but now acting as a lever arm of 6-feet. This simple analysis tells you that the moment on the base is THREE TIMES GREATER with the 8-foot antenna. Maybe you are using non-Euclidean Physics to derive your analysis.
contender posted 08-13-2010 04:57 PM ET (US)     Profile for contender  Send Email to contender     
Jim: How do you get 2ft for a 4ft antenna and 6ft for an 8ft antenna? Would it not be 4ft for the 8ft antenna? I also did not get into any physics for mounting my own antennas. With some common sense and logic I figured by using a stainless mount (for both antennas) it would hold the stress of the antenna with no problem. I mounted them on a flat aluminum plate that I had welded on my T-Top for that purpose....Take care
jimh posted 08-13-2010 10:15 PM ET (US)     Profile for jimh  Send Email to jimh     
The calculations I presented are based on the normal methods of analysis of wind loading on structures. If you have an alternative method for analyzing the wind loading on a structure you should present it.
newt posted 08-16-2010 01:09 PM ET (US)     Profile for newt  Send Email to newt     
I would respectfully submit that the force exerted on the mounting plate will never exceed the load capacity of the fiberglass antenna. The higher the wind load on the antenna, the more the antenna will deform. If the mounting plate can handle the loads at elevation 2 feet, it will handle the loads at elevation 8'. Another way to look at it: if you could grab the antenna and break it off without damaging the mounting plate, then the weak link is the antenna - not the mount.
jimh posted 08-16-2010 01:24 PM ET (US)     Profile for jimh  Send Email to jimh     
The forces on the antenna are distributed along its length, but the turning moment it exerts on the base are concentrated at the surface of the aluminum arch to which the base is fastened. An 8-foot antenna flexing continually in the wind and waves could exert stress on the base which could fatigue the metal or the welds. Based on the experience of a friend who installed an Atlantic Towers arch on his Boston Whaler, I would not characterize the construction of the arch as being built like a battleship, and this is why I offered the caution about bolting on rather tall antennas to the arch top.

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