VHF Antenna Placement - Ideas?

[truenorth]

I'd like to install a 3db VHF antenna somewhere on PocketShip (please set aside whether or not I should, etc .. one is going in, just not sure where). I plan to do some coastal and Great Lake cruising with PocketShip and want the added range. I can't, though, figure out a good place to install it.

Some places I've considered:

* The leading candidate is just outside the boom gallows poles. This will keep it out of the way of lines and rigging and yet still give a bit of a boost to the range. However, placement here limits the range.

* On top of the mast. There are some reasonable options up on high which would give the best range. I'd run the wire down inside the mast and out. I'd have to figure out how to get the anchor and steaming lights to work with the antennae up there. I also think there's some risk when trailering, so it would need to be removable. All downsides.

* On the side of the gaff. Questionable placement but outside the rigging makes it possible. Would add a little extra blocking and by a little I mean like a 3x3 block. Wire would run either inside the gaff or along the side.

* On the side of the tabernacle. I think the line would foul just about every time here.

* On top of the cabin. Probably in the way of lines here, too, plus disrupts the view, and it's just one more thing to step on.

Has anyone installed one? Which did you choose and where did it go?

[chaertl]

Might be maintenance headache but how about inside the mast and run the cable out just above the pivot? No rigging or spars to get caught on that way.


Chris

[John C. Harris]

Bolting the antenna to the transom skirt would be my vote. It's the least-worst choice, it'll be out of the way, the cable and connections are accessible for maintenance, and you can get a pretty tall antenna for reasonable range.

The worst choice would be at the top of the mast. Someone can do the moment calc for me right quick, but the added pounds at the top of the mast will require an additional X pounds of ballast, where X is a shockingly big number. Also, I worked as a professional rigger for awhile, and grew embittered by having to repair dozens of connection faults in masthead aerials. A gigantic nuisance.

When I fitted out my Folkboat I fretted over the VHF antenna, eventually locating it aft on the 8-foot mast that held the wind generator. Presently I discovered the same thing you'll discover: on a small sailboat, a good handheld VHF has the same range as the built-in VHF, and it's a lot more convenient to use. In the years that followed, the built-in VHF in the Folkboat's cabin only got used for weather reports, while the handheld made more sense in the cockpit.

[truenorth]

Thanks, John & Chris. Inside the mast is an intriguing idea. There are fittings that would make it a reasonable and safe install, there wouldn't be much of a moment issue, and the wire runs right where I need it to go (no wires inside the cabin and no more holes through bulkhead 8 either). Actually, maintenance would be very light, because if anything goes wrong, it's a new mast and/or antenna. I also thought inside the boom gallows pole would work but then something called physics kicked in and reminded me radio waves have a tough time with metal.

Looks like it's the transom skirt. I've got a little time to figure out, but if it is aft (which is more and more likely) I want to run the wire now to avoid running it visibly in the cabin.

John, I've done some testing with the antenna vs. a handheld and I got pretty impressive range with the antenna. I was able to call ~6 miles to Omaha Bridge from my home lake using the antenna held over my head but wasn't able to hail them at all with the handheld. Granted, lots of trees and whatever affected the handheld, and the handheld probably wasn't as good as it could've been, but that convinced me I wanted to figure this out.

Any other suggestions from people would be appreciated.

/Larry

[DanaDCole]

One question about locating the antenna on the transom skirt. There is an FCC rule that VHF antennas should be at least two feet away from any passenger. There is the small likelihood that someone might sit at the back of the cockpit and possibly even rest their arm against the antenna. Perhaps if the antenna is mounted dead-center it would always be far enough away from any casual sitters. If someone is working on the rudder or something like that, though, they could be rubbing against it.

This is only a problem during transmit. Perhaps when there is a need to transmit there could be a rule that people should stay at least two feet away from the antenna and warning given, but I don't know if that would satisfy the FCC.

[DanaDCole]

Maybe I'm wrong, and someone smarter than me please correct me if I am, but I am wondering about John Harris's statement, " Someone can do the moment calc for me right quick, but the added pounds at the top of the mast will require an additional X pounds of ballast, where X is a shockingly big number." Moment calculations are pretty complicated, but to my mind a much simpler, leverage formula is needed instead. That formula is C = (A x L1)/L2, where A is the antenna weight, L1 is the distance from the heeling (roll) pivot point (fulcrum of a lever) of the boat, L2 is the distance from the pivot point to the bottom of the hull, and C is the counterweight required to counter-balance the antenna moment. (The below diagrams are not to scale.)

|<------------------------------------------------19' 3"------------------------|--------------------------------->|
[Masthead]<-------------------------------------------------------------->[Roll Point]<-------------------->[Hull]

The total distance from the hull bottom to the top of the mast is about 19' 3" (19.25 ft.). I think most 3dB VHF antennas weigh somewhere around 2 pounds, and most of this weight is at the bottom of the antenna. I don't know the all-important distance of the boat's roll pivot-point from the hull bottom, but conservatively it should be around 2' or 1' at the worst. So I'll run the formula twice for 2' and 1'. In this example A = 2lb, L1 = 17.25' or 18.25', and L2 = 2' or 1'.

A L1 L2 C
C1: |<--------------------------------17.25'------------------------------------>|<------------2'------------>|
C2: |<-----------------------------------------18.25'------------------------------------------>|<----1'----->|

C1 = (2 x 17.25)/2 = 17.25lb
C2 = (2 x 18.25)/1 = 36.50lb

That difference of 1 foot more than doubles the amount of additional ballast needed, but the amount is still manageable, not much more than the weight of a 30 AH battery, and the battery is directly below the mast. Can anyone supply the actual rolling pivot point? If it is higher than 2', the counterweight required would be considerably lower. If it is less than 1' (I don't know how it could be) the counterweight req. would be much higher, for example if it is only 6", the counterweight requirement would be 75lb, but even that much is manageable with, say, a battery and some more lead weights.

All that being said, John is more than correct to warn against the continual additions of items above the heeling point. Anything you do add should be weighed (pun intended) against other items that can be done without or moved lower down. And, I have not included anything about the fore-and-aft (pitch) pivot point, which should also be considered. That point would be further aft, so some of the counterweight should probably be placed further back. Also, the antenna is 3' tall, and even though as I said most of the weight is at the base of the antenna, perhaps the height of the antenna should be figured in somehow. This complicates things, but to my way of thinking it would only add 10% or so to the ballast requirement. To be on the safe side, add 20-25%. Nothing has been mentioned about the weight of the cable either. RG-8X would be OK for such a short run and it doesn't weigh a lot, but still there is that moment-arm thing. There was some mention of putting the cable inside the mast, which would be nice, but after all the careful work sealing the mast against water infiltration I really hate the idea of drilling holes in it.

Again, I am no engineer by any stretch. If there are flaws in my argument (and there probably are plenty) please correct me.

[craig]

Fun workup. I think you proved the point quite well that weight up top is really damaging to stability. Too bad you can't use a halyard to raise the antenna only when you need the range.

Two questions about the math (just because I'm curious):

1. Where is the weight calculated to be placed? Unless I'm not understanding, your calculation assumes the weight is placed directly at the end of L2 (the bottom of the hull). But the weight can't be placed there. It would be 6" or so above that, in the bilges. That should make the added weight need to be even higher.

2. I'm curious too what the boat's CG is. I might play around with this over the weekend. It seems like 2 feet would be conservative. I'd guess with people the CG would be even higher. That would lower the extra ballast you would need.

I used douglas fir mast and spars, which is heavy. Let's hope that whatever the calculations, extra weight up top isn't too detrimental to sailing ability hoho.

[DanaDCole]

You raise some interesting points. My measurement was made on the plans--from the masthead to the inside of the hull, which would be in the bilge, except that there is a platform for the battery there, which would probably be up about 3" from the hull. So we could call it 19' even. (The measurements are not all that exact either because paper expands and contracts, but rather than worry about that I have tried to err on the conservative side.)

Taking into account that the antenna is 3' high with most of the weight at the bottom, why don't we say that the CG of the antenna is about 1 foot up from its base? That gives us an overall length of 20'. This might also make up for some of the coax weight, not all of which is at the top of the mast. That's a great observation that people in the boat raises the CG, hence the roll pivot point, a little higher. But again, to remain on the safe side I think that should be ignored. Anyway the number of people in the boat varies.

So now A is still 2lb, L1 becomes 18', and L2 becomes 2'. Plugging those in, C = (2 x 18)/2 = 18 lb. If all my other assumptions are correct, that is not bad at all. To be even more conservative about it we could assume the CG of the antenna is 2' up from its base: C = (2 x 19)/2 = 19 lb. Not too shabby! Changing the longer length, L1, has much less effect on the result than changing the shorter leg, L2.

This is all assuming I am correct about the leverage formula being what is needed instead of something much more complicated, e.g., moment formulae, where you are dealing with force in Newtons, rather than gravity's pull on weights on a lever. I don't think so, i.e., the force of gravity is measured in Newtons I would think, since he "discovered" it, but I hope somebody more knowledgable "weighs" in. I want to be absolutely sure this is right before I go mounting an antenna on the masthead. The boat might "feel" OK to a novice, but still be prone to capsize, and I do not need that! If I am correct, adding another 20 - 25lb of ballast would not be difficult at all.