Co -ax

Don S/V ILLusion · Feb 13, 2021

"30% weight savings" over what? Small diameter coax choices are abundant. I can't tell from the advertising what the loss is at VHF compared with equal weight known brands and I'd prefer to know what I'm buying.

dlochner · Feb 13, 2021

First, all coax cable is not the the same. The radio guys here can go in to more detail about the differences. In real simple terms the characteristics of the coax need to match the frequencies being transmitted and the power of the radio.

Why not just use a handheld? Range and clarity.

Handheld radios transmit with 5 or 6 watts, fixed mount VHFs at 25 watts. More power means better clarity at distance and potentially greater range.

VHF radios are line of sight, at 6' above the water the line of sight distance is about 3 miles. At the top of the mast, about 25 miles.

Cell phones are a poor choice in many situations because only one other party can hear the conversation. If you want privacy, then phones are fine, if you are in trouble you'll want as many potential rescuers as possible to hear your call.

shemandr · Feb 13, 2021

If you are doing around the buoys racing in a confined area with other boats in a competitive fleet, I think the hand held is fine. And, you could lose that anchor light and tri-color if you have one and the associated wiring. You can always leave messenger webbing so you can run wires if your needs change. Do you really need masthead wind instruments? It's a question you can ask. Dingy sailors don't have electronic ones and they seem to find the groove. I've known boats on which spare halyards were removed to save weight. With modern cordage I'm not sure that makes sense. But all of those wires and lines add weight aloft. If it's just 20 lb's it, can make a difference.

dlochner · Feb 13, 2021

If the concern for weight aloft is about racing, then there are lots of things to do that are just as important or more so to improve boat speed. Assuming a masthead about 35' above the waterline, each pound at the top adds about 35 foot-pounds of leverage. In really light air this can be an advantage as it helps to heel the boat to allow gravity to shape the sails. Otherwise, I think a really clean smooth bottom and faired keel will reduce drag and increase speed more than the weight of an antenna and a few feet of wire. The shipping weight of a Metz 6db antenna is only 1.5 lbs and the antenna itself weighs much less. (the shipping weight includes its physical size which is about 3 feet long.)

If you are not racing and just cruising, the priorities are different. In that case you want your VHF to broadcast to reach as far as possible and any speed loss is negligible.

shemandr · Feb 13, 2021

So if the 20 lb number weight reduction is real that’s 700 lbs of leverage. I’m not a calculator but that could be as much as one crew member one the rail. That is significant.
Smooth bottom and faired keel probably more important but there’s a reason carbon fiber masts are on so many sailboats.
I totally agree that cruising has different priorities. Our delivery last June in the ocean was made so much less stressful having an AIS range of around 25 miles due to an antenna at the masthead. We also had water making, solar panels and a crawl in fridge.
“...loving the sunshine ...”

Brian D · Feb 13, 2021

Not sure what the intent is for the use of this cable, or desired use. The OP did not stipulate that.

I would think that stuff is rather expensive. It is designed for aircraft/helicopters so it must meet FAA guidelines which mean, like anything with the term marine attached, is probably cost prohibitive.

I looked at the specs and it is rated for .1 dB/m loss @400 MHz. This is the thicker stuff of 7mm Ø. The smaller cable has less loss. Normally the lower the frequency the lower the loss per 100 meters.

My thinking is since the specs say dB/m and not dB/100m or dB/100ft, then it is high loss cable. Not sure though who the target aircraft is, private single engine or passenger. But again, seeing how it is rated at dB/m it is probably for very short runs.

All this is JMHO.

dlochner · Feb 13, 2021

shemandr said:
So if the 20 lb number weight reduction is real that’s 700 lbs of leverage. I’m not a calculator but that could be as much as one crew member one the rail. That is significant.

You're right! However, one would have to put a lot of crap on the mast head to reach 20 lbs. While I haven't weighed the items, at the top of my mast is a TriColor, wind instruments, windex, and a Metz 6db antenna. If all of that weighs 3 lbs, I'd be surprised.

If you want to win sailboat races the first thing you need to do is learn to sail fast, have good tactics, a great start, and skillful crew work. Unless the sailing skills improvement, boat improvements only lead to one less excuse for losing.

Ken Cross · Feb 13, 2021

First ask yourself how much range you need in the worst case scenario. I cary both handheld and installed radio with the antenna
at the top of my mast.

Coax up to the top of the mast has losses that you can easily look up in a table (based on cable length, cable type and characteristic impedance of the terminations on the end of the coax.) Your VHF radio is in the neighborhood of 160 MHz.

What's more critical than the losses in the coax is the ability to get the signal because of the height of the antenna. The actual power you send or receive is much less important than line of sight. Your radio likely only needs 0.1 microvolts to communicate.

Coax losses have the effect of reducing power of both the outgoing and incoming signal, but if you have line of sight you'll still have plenty of signals. (I'm refraining from throwing in the math here). So even if you use a lossy coax going up to your antenna you will still have way more range than you will with the handheld.

The real important thing with your coaxes that it's a 52 ohm coax with proper connectors on both ends. And making coax connections will probably be your largest challenge. Most people have no clue how to properly work a piece of coax.

After the terminations on the end of the coax, the next critical thing is bend radius. All coax have specified limits on bend radius and if you exceed that bend radius you damage the coax (possibly permanently) plus you can reduce your signal to zero. Smaller diameter coax usually allows for a tighter bend radius.

Let me know if you need more detail.

Ken

Ken Cross · Feb 13, 2021

You might also consider going to LED lights which would allow you to put lighter weight (smaller AWG) power and ground wire going up to the light.

jssailem · Feb 13, 2021

@Drinky Crow Why run any cable up the Mast at all.

If you want to reduce weight than eliminate all of the weight from the boat.

No need for cable up in the mast. Need an anchor light raise a battery powered lite up the mast. When not needed lower and store in the cabin. Need a steaming light? Raise a battery powered light up the mast.

Small enough boat you do not need anything but a flashlight shining on the sail. All of this extra convenience stuff is superfluous if you are day sailing and racing.

You can get GPS and VHF from the small lightweight hand held. No need for a battery on the boat. Use a crank start mini outboard 2 cycle maybe 45 pounds. No need for a fuel system for cooking bring a can of sterno . No need for any of that extra chain or even a real anchor. All you need is a 15 lb Aluminum danforth like anchor and 100 feet of rope.

Remove all the doors off the boat and replace with lightweight curtains.

If you are a racer there are a thousand ways to make your boat light.

dlochner · Feb 13, 2021

Drinky Crow said:
Teh mast is lying on sawhorses. This is my chance to sort it out.
There's some thick, old, ugly, black co-ax of unknown manuf. in there now in a conduit along with something like

...^^that^^...

...in a separate conduit...why wouldn't I run something lighter like below instead?

Neither of these wires are coax. The upper photo shows a shielded duplex wire. The lower photo looks like marine speaker wire.

jssailem · Feb 13, 2021

When I consider the coax to be used for VHF radio (a tool I may need to call for emergency help) I look for the cable which promises the lowest loss of my transmission signal. I selected the LRM400UF cable

Low signal loss and flexibility, rated for outdoor marine environment.

Then I avoided adding more than 2 connections from the radio to the antenna.

dlochner · Feb 14, 2021

Let's back up. Are you trying to eliminate a VHF antenna? Or wondering about eliminating the anchor light at the top of the mast? What is the goal?

If you never anchor overnight you do not need an anchor light at the masthead. Even if you do anchor overnight there are ways to show a proper anchor light that don't require a light at the masthead.

A fixed mount VHF radio will work best, i.e., have the greatest range with an antenna at the mast head. It must use coax cable. See the image below. There are different types of coax suitable for different applications.

If your goal is to sail faster, reducing masthead weight is pretty low on the list of things to do. Removing an anchor light or antenna will take a pound or less from the top of the mast. A clean smooth bottom, good sails, less stuff in the boat will make a much bigger difference in performance. And the biggest difference between fast and slow boats in a race are the result of human factors, i.e., skill.

shemandr · Feb 14, 2021

Making good use of your time this Winter!

Co -ax

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SBO Weather and Forecasting Forum Jim & John

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SBO Weather and Forecasting Forum Jim & John

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