Why do so many mount vhf antenna near helm?

[towdog]

It seems like most antennas in smaller boats are mounted on the starboard side near the helm. I know mounting it here provides for easy access for raising it up and down, but I thought you were supposed to mount it at least 3 ft away from the radio? And as far away from humans as possible? I've got a Four Winns 240 Horizon and am trying to find the best place to mount it. I was thinking about maybe just moving it a bit forward and mounting it on the bow rail.<br /><br />I am just wondering if the 3ft rule is that important since I don't often see people obey it.<br /><br />Thanks.

 

[Silvertip]

Re: Why do so many mount vhf antenna near helm?

On smaller boats used for fishing the bow or stern would location would interfer with lines. On larger boats or boats that aren't used for fishing any location would work but the higher the better. On my boat the radio is located in a small bow compartment and the antenna is at the helm. Why? Because when fishing, thats where my pedestal seat is, the radio is handy, and the antenna is out of my way.

 

[waterinthefuel]

Re: Why do so many mount vhf antenna near helm?

Towdog, I may be mistaken, but I do believe the rule is to keep the antenna wire at least 3 feet long. The distance between the radio and antenna, as far as I know, doesn't really have much of a rule. My little boat's radio and antenna probably aren't 18 inches apart, but I have the full 16 feet of cable wrapped up and stored under my steering thingy, and my radio works great. I use it every time I go fish, and nobody ever has a problem hearing me.<br /><br />I certainly agree mount it as high as practical. To figure out the estimated range of your radio, use this formula:<br /><br />Square root of the height in feet above the water of your antenna x 1.42. <br /><br />If your antenna is 4 feet above the water....the square root of 4 is 2 so 2 x 1.42=2.84miles. As you can see, height is the key. If you put an antenna on top of your home, say 25 feet up, 5 x 1.42=7.10 miles. Add that to your 2.84 miles and you should be able to talk to your base station out to about 10 miles over water.<br /><br />I would only suggest the purchase of a rather expensive antenna tester by Shakespere. It shows SWR and transmit power. I found out my radio on high only puts out about 17watts, as opposed to the advertised 25. My father's Uniden MC210 puts out the full 25, in fact, its closer to 30.

 

[byacey]

Re: Why do so many mount vhf antenna near helm?

VHF radios operate basically line of sight between the antennas, so the higher up the antenna, the less reception problems due to hills, trees and other terrain or buildings. With radios of more than 100mw (1/10 of a watt) It's a recommended to keep the antenna away from people. The RF field is known to produce cancer and sterility at close proximity. This field strength diminishes by the inverse square of the distance from the antenna to the person.

 

[waterinthefuel]

Re: Why do so many mount vhf antenna near helm?

If you want to get technical, Bill is right. I just don't transmit enough for it to matter. I also always use 1 watt due to the small distance I ever travel from the dock.<br /><br />As long as you aren't hugging up to it I don't think it matters where it is. Ok, technically it does, but it won't be Cherynobl if you sit next to the antenna.

 

[18rabbit]

Re: Why do so many mount vhf antenna near helm?

Originally posted by waterinthefuel:<br /> I just don't transmit enough for it to matter.

How do you know? The effects of radiation are accumulative over your lifetime. Perhaps the one short broadcast you make from your boat’s VHF could be the straw that breaks the camel’s back.<br /><br />There is a reason why that warning is required to accompany the documentation of 25w fixed-mount VHF radios. (I believe it was 3 meters above a person’s head or 5 meters above the deck). I would definitely heed the warning. Doing so can’t hurt anything. And I would be especially cautious if children are ever on a boat that ignores the warning. Those kids have a lifetime of accumulated radiations ahead of them to deal with.<br /><br />Perhaps the question of following the FCC guidelines for VHF antenna height is better answered with a question; do you wear a seatbelt when you are in your car? Most cars don’t crash, and you certainly don’t hop into your car and take off with the expectation of crashing, but we wear our seatbelts just in case. And so it goes with VHF antennas.<br /><br />Follow the guidelines, mount the antenna up there.

 

[Boatin Bob]

Re: Why do so many mount vhf antenna near helm?

Maybe this will help some, it's the FAQ page on the Shakespeare Shakespeare Antenna FAQ website

 

[Boatist]

Re: Why do so many mount vhf antenna near helm?

On my runabout my antenna is on the starboard side as is the radio. Distance is about 4 feet.<br /><br />Reason I mounted it there is I want a location where I can lay it down on the gunnels when traveling and when covered. Port side already had a Loran C antenna. Mine is mounted about 1 foot forward of the Winsheild just before the bow rail starts. In that location it will lay down on the starborad gunnel and is out of the way when trailering or when cover the boat.<br /><br />Personally I would worry more about cell phone held against the side of the head and used much more than I would about a VHF 6 feet away and not used that much.

 

[waterone1@aol.com]

Re: Why do so many mount vhf antenna near helm?

To answer someone else's questions: No, I don't wear a seat belt. I'm just not that cautious....thanks for asking. By the way, compared to the time most kids spend on cell phones and the time they spend glued to a cathode ray tube, I wouldn't worry about them either.

 

[KaGee]

Re: Why do so many mount vhf antenna near helm?

The 3' mounting rule has to due with possibly overloading the receiver with RF, causing feedback. The 3' coax length rule has to do with maintaining proper SWR on the antenna. Not mentioned in the Shakespeare FAQ's is that coax should only be trimmed in 3' increments, with 3' being the minimum. <br /><br />If you are not competent in replacing the PL-259 connector, I would not cut the coax. Coil up the excess and stow it. Notice I said coil and not bundle. Solderless PL connectors suck and will only cause you grief in the long run.<br /><br />Seeing that Marine freqs are Hi-Band, and not microwave, we can let the cancer discussion go to another troll. There may be some validity to that where microwave freqs are used (cell phone) and use is repeatitive.

 

[byacey]

Re: Why do so many mount vhf antenna near helm?

There is evidence that even low frequency such as 60 Hz fields can be damaging. Here locally there was a bunch of testing being done at a school because it was located what some people felt were too close to some high tension power distribution lines. An unusual amount of kids attending there were being diagnosed with leukemia. with this in mind, I wouldn't discount the effects of any kind or frequency of electromagnetic radiation.

 

[18rabbit]

Re: Why do so many mount vhf antenna near helm?

The effects are accumulative. Avoid as much of the radiation as you can as often as you can. As I said, kids are getting exposed to more radiation then any generation before. Contrary to WaterOne1, I would be very concerned about it.<br /><br />I don’t believe the FCC warning is included with every 25w VHF just to exercise the printing press. IT IS THERE FOR A RESON! If anything, the FCC tends to be liberal in its allowances of exposure to radiation.<br /><br />When I inquired into this VHF and cellular radiation issue I went to some folks that operate an EMC chamber. That’s what they do for a living, test for radiation levels. Per those fine folks, cell phone radiation concerns are likely based in politics and they don’t consider it an issue, but not so with VHF. They are very much concerned with their own exposure to VHF. Their recommendation to me was that I shield the cabin top of the boat in addition to following the distance recommendations for mounting the antenna 3-meters above the cabin top.<br /><br />Thing that comes to mind is an Bell helmet advertising campaign for bicycle helmets; if you have a $10 head, buy a $10 helmet. If your person isn’t worth preserving or if you don’t care about the quality of life, don’t follow the FCC guideline when mounting your VHF antenna. It’s that simple.

 

[Chunder06]

Re: Why do so many mount vhf antenna near helm?

I think the reason they stick the antennas close to the helm is some antennas require a ground plane to operate correctly these use the metal work in the helm to shoot the signal back up the antenna especially in glass boats, most modern antennas are ground plane independant and can be fitted any where.

 

[18rabbit]

Re: Why do so many mount vhf antenna near helm?

I think the ground plane (grounding) is important with shortwave radio but not VHF.

 

[Boatist]

Re: Why do so many mount vhf antenna near helm?

I agree the 3 foot minumin distance from the radio is to prevent the transmit frequency from over powering the radio oscilator frequency and causeing feed back or the phase lock loop to run off frequency. Cable length does not change the SWR in a VHF Marine antenna. Antennas are basicly 1/2 wave and tuned to be 50 OHM load. As long as the load is the same as the transmiter then no tunning is required. I had a hard time beliveing this my self until I decided to find out if what the antenna manufactures were saying was true. I started on my boat by trimming coax an inch at a time then 3 inches then 6 inches. My goal was to get the lowest SWR at or near the 156.8 MHZ Channel 16 frequency. I found the most change you can have is about .1. So you can change it from 1.2 to 1 to 1.3 to 1 at the most. Besides my boat I have help lot of other when fishing at the coast. Where we go every year and camp it is mostly fishermen and every one talks to everyone. Over the years have made lots of friends and have help trouble shoot or fix many radios problems. For me in every case it has been the antenna. Most seem to buy cheap fexible antenna that actually use coax cable inside as the radiator. I start by checking the SWR while shaking the antenna some. Then I have them pull their boat over to my area and with a PL258 and a 50 foot of RG8X Coax hook their radio up to my antenna. Their radio will spring to life and have a low SWR. Next check the PL259 and the Coax near the base of the antenna and also where it exits the hull to get to the antenna. Find some cut coax where it goes thur the hull and some where if flex at the base of the antenna will cut in and out. Most of the time it is something broken or wet up inside the antenna.<br /><br />A VHF Marine antenna does not require a ground plane. It has nothing to do with Frequency and everything to do with antenna design. Now if your talking CB or HF Single Side band then it is a different. These antennas can not be long enough to be a half wave antenna due to the low frequency. 1/4 Wave antenna must be tuned by a coil or cutting coax length to get the antenna to match the radio's 50 ohm load.

 

[waterinthefuel]

Re: Why do so many mount vhf antenna near helm?

How do you know? The effects of radiation are accumulative over your lifetime. Perhaps the one short broadcast you make from your boat’s VHF could be the straw that breaks the camel’s back.

How do YOU know? Your microwave puts out more than your VHF radio on it's best day, and you use it daily. <br /><br />

Seeing that Marine freqs are Hi-Band, and not microwave, we can let the cancer discussion go to another troll. There may be some validity to that where microwave freqs are used (cell phone) and use is repeatitive.

Kagee, I couldn't have said it better myself. I rest my case.<br /><br />People mount antennas as high as possible for one reason: range.

 

[bruceb58]

Re: Why do so many mount vhf antenna near helm?

VHF radio waves are going to harm you a lot less than living in a big city with smog.<br /><br />Probably worrying about the VHF radio waves will hurt you more that the radio waves will ever do because of the increased stress you get from all your worrying!

 

[Boatist]

Re: Why do so many mount vhf antenna near helm?

There are many that think 60 cycle high tension lines will harn you over time others say only microwave and some any frequency. I have no Idea but do read all the studies I see, since I use VHF Marine, Ham Radio, Cell Phone, GMRS and FRS.<br /><br />What I can tell you is there are many long term studies going on. They study road flagman who use radio all day long in traffic control. Policemen and Firemen with radios on their side, taxie cab drivers and many more and so far there has really been no Evidence that there is Harm. Having said that we all know you can cook with a high power microwave or fry a bird or person standing in front of a high power radar antenna.<br /><br />If any study does find harm I bet the first thing to go will be kids walkie talkie in the FRS band and the GMRS band. These are used by young kids until the batteries die then they get another set. Power is form 2 to 5 watts now and kids as young as 4 or 5 use them. I still feel if a problems show up will most likely will show up with cell phone users<br /><br />They have been studying this for over 15 years. I guess only time will tell. <br /><br />A local California doctor Dean Odell has read many of these studies on the air but the Jury still out.

 

[swist]

Re: Why do so many mount vhf antenna near helm?

Commercial TV transmitters are VHF (not to mention FM radio). Some of these are hundreds of kilowatts, if not more (I know there's a 50KW limit on AM but I'm pretty sure the limit is way higher for FM/TV).<br />That means most of us in urban area have been cooked by VHF most of our lives. I could be wrong, but I do not recall any study concluding this is a problem.

 

[byacey]

Re: Why do so many mount vhf antenna near helm?

Very people stand within 3 feet of the driven elements on a TV broadcast tower. Microwave ovens are carefully shielded to avoid RF leakage outside of the cabinet. I don't think these are valid arguments here.