VHF antenna position

[Impaler]

My masthead antenna has given up on me. I've double checked the connections at deck and at the set itself. Rather than going to the expense of dropping the mast I thought of fitting a 1 metre or thereabouts one to the pushpit. Can anyone say how much this would reduce the range? I only sail within site of land so long range is not a big deal for me.

 

[shmoo]

Careful you don't mount it where someone could get toasted. A gold finger ring or watch bracelet could get significant amounts of induced rf. A stand-off taking the twig itself well outboard of the rail may do it.

From observation of the performance of my AIS antenna, which is is on the pushpit, you may find the polar diagram rather lumpy i.e. works well for stations astern but not ahead.

 

[st599]

My masthead antenna has given up on me. I've double checked the connections at deck and at the set itself. Rather than going to the expense of dropping the mast I thought of fitting a 1 metre or thereabouts one to the pushpit. Can anyone say how much this would reduce the range? I only sail within site of land so long range is not a big deal for me.

How would you enforce the legal safe operating distance? A 25W RF burn is quite serious.

Assuming 20m mast and 2m pushpit, radio horizon would drop from 11 miles to 4 miles.

 

[carise]

How would you enforce the legal safe operating distance? A 25W RF burn is quite serious.

What is the safe "legal" operating distance? Am concerned as regularly go on others boats where the antenna isn't that far away, i.e. often within arms length or 2. (mine's ok at top of 7m mast!!).

 

[Impaler]

What is the safe "legal" operating distance? Am concerned as regularly go on others boats where the antenna isn't that far away, i.e. often within arms length or 2. (mine's ok at top of 7m mast!!).

Likewise, I've been on RIBs where it's within arms distance.

 

[KellysEye]

Why do you need to drop the mast? Just climb it and check that everything is OK. Before doing that borrow another VHF set and try it on the current aerial

 

[NorthUp]

RF burns?
consider a 5w hand held- at 2 inch range!
Radar frequencies are a hazard, but VHF?

 

[Sandyman]

I dont quite understand when you say ''My masthead antenna has given up on me''.

You say you have checked the connections, but have you checked the mast cable and internal cable leading to the mast ??

You seem to be going straight to a short term, possibly expensive, fix without investigating the original problem which may be quite simple to fix.

Again I dont understand why you would want to ''drop'' the mast as you put it. If the mast cable is defective it no great problem replacing it mast in place, with or without a mouse in place, you can use the duff cable as a pull through for its replacement.

Are you sure the VHF itself is working ?? Do you have an emergency aerial you could use to test it ??

 

[Impaler]

I dont quite understand when you say ''My masthead antenna has given up on me''.

You say you have checked the connections, but have you checked the mast cable and internal cable leading to the mast ??

You seem to be going straight to a short term, possibly expensive, fix without investigating the original problem which may be quite simple to fix.

Again I dont understand why you would want to ''drop'' the mast as you put it. If the mast cable is defective it no great problem replacing it mast in place, with or without a mouse in place, you can use the duff cable as a pull through for its replacement.

Are you sure the VHF itself is working ?? Do you have an emergency aerial you could use to test it ??

The set is fine, I've tested it. I have no idea how you could fit a new cable and antenna without dropping or lowering or whatever the mast. I have no intention of climbing the mast either. I have checked all the connections so the only alternative is either rely on my hand held or fit a lower antenna. As most motor boats have fairly low set antennae I can't see it being a big deal. Also if one day my mast gets tired of living and keels over I'll still be able to cry for help.

 

[Sandyman]

The set is fine, I've tested it. I have no idea how you could fit a new cable and antenna without dropping or lowering or whatever the mast. I have no intention of climbing the mast either. I have checked all the connections so the only alternative is either rely on my hand held or fit a lower antenna. As most motor boats have fairly low set antennae I can't see it being a big deal. Also if one day my mast gets tired of living and keels over I'll still be able to cry for help.

Fair enough. Agree about MOBO's & low antenna's. Only yesterday I was looking at a rib which had a Raymarine radar dome 4 foot behind the helmsman's head. !!!!!!! Not sure what power they bung out but I would not be happy to helm one & Tx for long periods.
Cant see it being a problem in your case as long as you dont stand next to it & Tx for hours on end. Which of course never happens in our field. Go for it. Though consider you may lose range so site it as high as possible. Good luck

 

[Salty John]

At 5' height your range will be a little over 3 miles. To that you can add the range of the other party to the transmission. Another yacht with a masthead antenna might have a range of, say, 9 miles; your communication range in this case would be about 12 miles. (Distance is about 1.4 X square root of the combined height of the antennae).

In the USA, where litigation is a great threat, distances from any human whilst broadcasting is quoted at anything from 1.5' to 3' - actually the distance varies with frequency, power and antenna type. I think the risk of any damage when transmitting with a rail mounted VHF type antenna at marine frequencies is very small. I think you'd have to have the antenna strapped to your head and transmit continuously to come to any harm - but don't try it because I said so!

 

[carise]

How would you enforce the legal safe operating distance? A 25W RF burn is quite serious.

QUOTE]

ST599
Sorry to keep on, but it is potentially quite a serious point you have raised- can you please clarify further in the light of mine and others comments about RIBs etc ?

 

[st599]

RF burns?
consider a 5w hand held- at 2 inch range!
Radar frequencies are a hazard, but VHF?

Yes VHF is a hazard. The handheld is designed to be safe. That's why the antenna and connector is encased in a non-conductive sleeve. If you have an exposed conductor, e.g. You have a metal monopole or unscrew the rubber ducky antenna, then you can give yourself a nasty burn which damages nerves.

Close or direct contact with RF transmission lines or antennas may result in RF burns. These are usually deep, *penetrating, *third-degree *burns. *To *heal *properly, these burns must heal from the inside to the skin surface. To prevent infection, you must give proper medical attention *to *all *RF *burns, *including *the *small *“pinhole” burns. Petrolatum gauze can be used to cover burns temporarily before the injured person reports to medical facilities for further treatment.

 

[Channel Ribs]

Close or direct contact with RF transmission lines or antennas may result in RF burns.

RF Transmissions in the marine band?

 

[Cantata]

If it's a particular model of Vtronix (can't remember which) it's likely the connection between the cable end and the antenna itself has become loose or dirty. Not at all drastic to fix.

 

[st599]

How would you enforce the legal safe operating distance? A 25W RF burn is quite serious.

QUOTE]

ST599
Sorry to keep on, but it is potentially quite a serious point you have raised- can you please clarify further in the light of mine and others comments about RIBs etc ?

Yeah sure. There are two problems. 1) direct contact with a metal conductor causing an rf burn. 2) tissue warming caused by exposure.

1) use an encased antenna (flexible whip) rated for the power you use and ensure all connectors are covered by a few layers of self amalgamating tape. Tell people not to touch it and it should be fine.

2) this is governed by an international standard ICNIRP. For each frequency there's an exposure level (actually there's 2, trained people and public) http://rfsafetysolutions.com/RF Radiation Pages/ICNIRP_Guidelines.html
at the marine VHF frequency (~160MHZ) it's 28V/m.
Assuming 25W arrives at the aerial which has a gain of 6dB ie 8.15 dBi (http://en.m.wikipedia.org/wiki/Decibel?wasRedirected=true)

 

[Channel Ribs]

Close or direct contact with RF transmission lines or antennas may result in RF burns. These are usually deep, *penetrating, *third-degree *burns. *To *heal *properly, these burns must heal from the inside to the skin surface. To prevent infection, you must give proper medical attention *to *all *RF *burns, *including *the *small *“pinhole” burns. Petrolatum gauze can be used to cover burns temporarily before the injured person reports to medical facilities for further treatment.

As quoted from here:

http://www.hnsa.org/doc/neets/mod10.pdf

"U.S. Navy Electricity and Electronics Training Series
Module 10—Introduction to Wave Propagation, Transmission Lines, and Antennas"

Not sure that is the most relevant source to the OP's query?

 

[st599]

Then the safe distance is about 2.5m.
This assumes the transmitter is on all the time and can be significantly de-rated e.g. If you use it 10% of the time that's an average power of 2.5W so a distance of 0.8m. But if you're having a long chat I'd keep people away from it.
http://www.microwave.gr/content/jsvpm.htm?

 

[William_H]

Antenna

Well maybe not politically correct but I say forget RF burns. If you are really worried cover an exposed wire antenna with heat shrink or similar. I have had many RF burns from proximity to high power transmitters ie 50Kw it stings a bit but nothing more. That burn from touching metal in near proximity to antenna.
In my younger days we didn't think anything of climbing up through the inside of the TV tower antennae transmitting with total of 24Kw to change top light bulbs.Never a burn.

Coming back to the original question. I think you will find a low mounted VHF antenna works fine for your purposes. It certainly does for mine. yes you should have a low mounted antenna as an emergency standby antenna. Yes masts do fall down and thats when you might need some help. You will find some advice on this forum on home made emergency antenna that might solve your problem.

Mine is cable with 17 inches of inner exposed and a 17 inch piece of braid from heavy coax connected to the end of the main cable screen and running down the outside of the main cable. The whole in a piece of plastic conduit which gets the top about a metre above deck. It is clamped to the pushpit.

Re the theoretical radio horizon numbers they must be added to that of the antenna at the other end.

Re st599 wanting to bring the antenna gain into calculations for Rf danger. I don't think so. The gain of an antenna is only produced by the interaction (phasing) of radiation form the whole antenna so gain will not be effective until you get at least a wavelength away from the antenna. Or put another way the additional power comes from the radiation from the top joining forces with the radiation form the bottom so radiation close in to the top middle or bottom will not be augmented by the power form the rest of the antenna.
I think also that the 8db of gain you quote is that theoretical gain over an antenna radiating in all directions not that gain over a 1/4 dipole. Which for a 5/8 wave length is maybe 1 and a bit db over a dipole. The gain being apparent because the antenna radiates more in the horizontal than up or down.

olewill in argumetive mood

 

[carise]

Greetings

So what you're basically saying is that any potential injury is likely to hurt a lot less than my head hurts trying to understand this thread now?????

Paul

Well maybe not politically correct but I say forget RF burns. If you are really worried cover an exposed wire antenna with heat shrink or similar. I have had many RF burns from proximity to high power transmitters ie 50Kw it stings a bit but nothing more. That burn from touching metal in near proximity to antenna.
In my younger days we didn't think anything of climbing up through the inside of the TV tower antennae transmitting with total of 24Kw to change top light bulbs.Never a burn.

Coming back to the original question. I think you will find a low mounted VHF antenna works fine for your purposes. It certainly does for mine. yes you should have a low mounted antenna as an emergency standby antenna. Yes masts do fall down and thats when you might need some help. You will find some advice on this forum on home made emergency antenna that might solve your problem.

Mine is cable with 17 inches of inner exposed and a 17 inch piece of braid from heavy coax connected to the end of the main cable screen and running down the outside of the main cable. The whole in a piece of plastic conduit which gets the top about a metre above deck. It is clamped to the pushpit.

Re the theoretical radio horizon numbers they must be added to that of the antenna at the other end.

Re st599 wanting to bring the antenna gain into calculations for Rf danger. I don't think so. The gain of an antenna is only produced by the interaction (phasing) of radiation form the whole antenna so gain will not be effective until you get at least a wavelength away from the antenna. Or put another way the additional power comes from the radiation from the top joining forces with the radiation form the bottom so radiation close in to the top middle or bottom will not be augmented by the power form the rest of the antenna.
I think also that the 8db of gain you quote is that theoretical gain over an antenna radiating in all directions not that gain over a 1/4 dipole. Which for a 5/8 wave length is maybe 1 and a bit db over a dipole. The gain being apparent because the antenna radiates more in the horizontal than up or down.

olewill in argumetive mood