Mounting Antennas on Spreaders: To Drill or Not to Drill ?

[DevoDave]

Looking at installing a 3G antenna on the spreaders of a Seawind 1160 (38 foot) catamaran.

Is drilling spreaders something that is or isn't advisable?

What are the alternatives?

[Dockhead]

Thanks for posting this. I have exactly the same question.

I have three new antennae to mount -- a WiFi antenna (with Bullet), a 3G mobile phone antenna for my GlobeSurfer, and a spare VHF antenna for the AIS receiver I am planning to buy. I already have a number of antennae on the pushpit -- satphone, gps, navtex. I thought about an antenna pole on the after deck but baulked at the windage, cost, and clutter. I think the first spreader will be the right place for these. Does anyone know any disadvantages?

[DevoDave]

One piece of advice I have picked up... mount them on the bottom of the spreaders pointing down. That way if a halyard gets caught up by one, it should fall out without too much drama.

Hopefully someone with rigging knowledge will be along shortly to advise if we should pull out the ol' drills.

[Dockhead]

Originally Posted by DevoDave

Hopefully someone with rigging knowledge will be along shortly to advise if we should pull out the ol' drills.

I think there may be ways to mount them without drilling. Depending on how the spreader is constructed, perhaps a clamp would do it?

[boatman61]

Keep em to the wide bit close to the mast and you'll be fine.... as long as its not a 10ft whip arial...
The mount on the underside is a good idea as well..

[deepstar]

Just break in mind that if you ever come to use your spare VHF Antenna - if you have placed the other two antenna's in very close proximity then the electrical equipment connected to the (WIFI and AIS) will be getting a very large amount of RF - which typically does not do sensative electrical things any good.

I realise that you asked a question regarding holes in spreaders - apologies for drifing a little off topic.

[Dockhead]

Originally Posted by deepstar

Just break in mind that if you ever come to use your spare VHF Antenna - if you have placed the other two antenna's in very close proximity then the electrical equipment connected to the (WIFI and AIS) will be getting a very large amount of RF - which typically does not do sensative electrical things any good.

I realise that you asked a question regarding holes in spreaders - apologies for drifing a little off topic.

Good thing to keep in mind. But if I'm using the spare VHF for transmitting on the VHF in an emergency, as opposed to merely receiving AIS which is its main function, I think interference with mobile phone and WIFI signals is not going to be high on my list of things to worry about.

In any case, it's not too hard to obey the required separation distances.

[capt.bobfm]

Physical mounting on spreader arms on most boats presents no problem. The problem is the close proximity to the aluminum mast. What is the wavelength? Can you mount the antenna 1/4 wavelength away from both the mast and the shroud? If not, you'll set up standing waves and at best the antenna will not work efficently,at worst, you'll destroy your transmiter.

[JayH]

Originally Posted by capt.bobfm

Can you mount the antenna 1/4 wavelength away from both the mast and the shroud? If not, you'll set up standing waves ...

Oooh, I've wondered about this. Should the antenna be exactly 1/4 wavelength away? At least 1/4? A multiple of 1/4? Is the mast and shroud the only concern? What about other antenna?

[deepstar]

I would think you want to avoid 1/4 Wave spacing, as this configuration is often used in phased mono-pole's installed on terra firma.

AIS/VHS has a freq of what 162Mhz - which will be about 1.84M - so I would think you need to be at least 55cm apart and closer than 90cm

[capt.bobfm]

If you can set up your mounting so that you have a 1/4 wavelength space between your antenna and another (longer) piece of metal(mast) the mast will act as a reflector element of the antenna and direct your signal in more of an eliptical patern rather than a round omnidirectional whip antenna alone.However,if your shroud (which is also a piece of metal longer than the whip) is there,the signal will reflect back again. Now you've set up a standing wave that's probably not good for you.

[SoonerSailor]

Although nearby metal will affect radiation/reception patterns, and antenna impedances will be affected to some extent, unless the antenna is very close to the mast or shroud, it probably will not cause a problem with the transmitter. If you want to be safe and sure, you would want someone with an antenna analyzer that covers vhf frequencies to check it for you. That is what I would do, but I have and use one already.

You could also use a vhf swr meter to check your install - that would be a bit cheaper than the analyzer.

[Dockhead]

Originally Posted by capt.bobfm

Physical mounting on spreader arms on most boats presents no problem. The problem is the close proximity to the aluminum mast. What is the wavelength? Can you mount the antenna 1/4 wavelength away from both the mast and the shroud? If not, you'll set up standing waves and at best the antenna will not work efficently,at worst, you'll destroy your transmiter.

The longest wavelength used by mobile telephone/3G signals is 37.5cm. A quarter wavelength would be only about 4 inches.

The longest wavelength used by WiFi is about 12cm.

The longest wavelength used by VHF is about 2 meters, so a quarter wavelength would be less than two feet.

I don't think this is going to be a big problem.

Frequency - Wavelength Chart