VHF Cable - Thru Hull

[myerssl]

All,

I'm mounting the VHF antannae (sp) on the mast and have not decided whether to run a two way connection through the hull (requires two pieces of cable, one from the top of the mast to the deck and another from the inside of the deck connection to the radio), or to run a contiguous cable from the top of the mast thru the deck and to the radio inside.

Willl an additional connection point significantly degrade the signal strength?

Thanks

 

[Don12364]

The usual approach is to bring the cable through a waterproof gland at the base of the mast and then have a short length of cable under the deck and fit a BNC connector. Then a mating connector on the remainder of the cable to the VHF set and hide it all under the headlinings.

This means the joint is below deck in the dry yet it will allow you to take the mast down by undoing the gland. Put a poly bag over the connector when the mast is ashore on a storage rack.
Compared to the losses in the cable as a whole, the loss/mismatch in the connector will be negligible.
The site below is a UK source of glands but the same items must be available in USA - though I didn't fiind them on the WM site.
http://www.seamarknunn.com/acatalog/DeckCableGlands.html

 

[Bill Roosa]

Check your radio impedance

"most" VHF radios these days are 50 ohm output. You absolutely need 50 ohm cable and connectors if your radio is 50 ohm output. Do not by cheap cable or connectors. The signal loss in the VHF range is SIGNIFICANT with long cable runs no mater what cable you use but bad cable or cable with the wrong electrical properties is worse.

If cost is a variable, then consider putting the antenna on the pushpit rail and shortening up the cable run. The difference in range due to more height and bad cable vs less height and good cable is a wash. My personal opinion is VHF is short range comms and I dont need to be talking to someone 100 NM away. The cost to run the cable up the mast just cant be justified from a range standpoint. And when you get demasted and really need that antenna it will be gone. Word to the wise is sufficient, carry a spair cable antenna set or hand held.

To answer your question the biggest "sucker of signal strength" is mismatched cables, radios, and antennas. Included in that are connectors (yes martha there are 75, 50, ... ohm versions for all popular cable sizes) I prefer to us "N" type but that is not standard. A well executed connector will not be noticable. Coiling that extra cable in the overhead will. VHF is really bad for line loss. Some of the intermediately priced cables have 3 dB losses in 25 ft!!!! That is 1/2 your signal strength to those non hams. So keep the runs short even with good cable and don't coil the exces cable.

 

[woodster]

" I prefer to us "N" type but that is not standard. A well executed connector will not be noticable. Coiling that extra cable in the overhead will. VHF is really bad for line loss. Some of the intermediately priced cables have 3 dB losses in 25 ft!!!! That is 1/2 your signal strength to those non hams. So keep the runs short even with good cable and don't coil the exces cable.

so will this type not have the 3 dB loss in 25 ft......and if so is it ok to have 55ft run with one splice connector in the run?....... and what brand cable do you prefer......

regards

woody

 

[Don S/V ILLusion]

You can take a look at this link to get an idea of what the actual loss would be for different kinds of coax. If you use good quality coax and don't screw up the connections with cheap crimp connectors and keep the water out, you will be fine with a mast mounted antenna.

http://www.timesmicrowave.com/cgi-bin/calculate.pl

 

[woodster]

im confused and consider myself pretty knoledgable regarding eltronics . If there is a 3db loss every 25 ft, then why so many people have ant monts on top of 47 ft mast with cable runs over 50-75 ft?..

Remembering that VHF is pretty much line of sight, can you see farther from the rail or can you see from top of mast?
Just use a water tight pass through on the deck next to mast and keep connections inside as much as possible.

Sometimes we get caught up in technical stuff and start splitting hairs.

just my humble 2 cents.

thanks for the input chuck...i was down at fair hope last saterday with a buddy of mine setting up his slip rental and was wondering where you are in that area...didnt have a lap top with us or would have tryed to contact you......

regards

woody

 

[Bill Roosa]

signal loss

Good cable is expensive, VHF is VERY lossy for any cable. So those two facts would drive a "best range on transmit" solution that minimizes the length of cable and maximizes the height of the antenna.
There are lots of other considerations however. Antenna gain, the fact that in a worst case situation (max range required) you will be transmitting a) over open water and b) to a USCG tower that is 150' in the air, routing and cable runs, redundancy when de-masted, lots of things
So a 2 dB loss vs a 3 dB loss is not the way to look at the problem. EVERYTHING factors into having comms when you need them. Convenience comms are not usually over great ranges, emergency comms should address the expected emergency.
So I'd ask; What are you trying to accomplish by putting the antenna at the top of the mast? Other than a lot of work and another potential water leak, and an increased risk of lightning getting into the electrical system and boat heel effecting the range in random ways and listening to everybody in 50 miles talk about silly stuff on the weekends, and potentially loosing all comms when de-masted
Not trying to be an arse here, but just because other boats do it is not always a good reason for you to also. I'd note that when talking single side band (SSB) radio installs most installers will insist on lots of copper ground ribbon all over the boat and connecting all your underwater fittings (say corrosion) so that you have a ground for the antenna to "push" against. That is a real pain to do and pretty expensive and makes your bonding plan a mess. I've found that a wire in the water (I cruise in salt water) with a weight on it works just as well, better on some freqs.

 

[Don12364]

Woodster

The commonly fitted cable which comes attached to proprietary antennas is RG58U. This is a solid polythene dielectric cable which has 52 ohms impedance is less than 1/4" diameter and quite robust.
Most cable makers supply this and there is little to choose between them.
At 150 MHz the attenuation is 6.1dB per 100' - so, for your 55' = 3.3dB. A thicker cable will have less attenuation but will make for more difficulty in feeding through the boat and mast.

However line of sight is vastly more important than radiated power at VHF frequencies.
Two pushpit mounted antennas each 5' above water level will have communication only up to approx 5½ miles regardless of transmitter power so 5 watts will do just fine - and please those on the same channel.
Two antennas each 50' high will give 17 miles. Remember this as your VHF radio is likely to be your primary means of communication for a Mayday safety shout.

For info I have tested the range of a VHF transmitter and antenna which I did for the UK's first nuke sub many years ago. Using only 5 watts we had clear voice communications with a helicopter at 45 miles, so power is not the main issue here.
BTW - the chopper was flying at 2,000 feet!

 

[Bill Roosa]

my point Exactly

You should design the communication system, not just do what others do. Line of sight is important but the USCG has tall towers.
What do you want your VHF radio to do for you? If your abandon ship plan is to use a handheld then it really does not matter how your VHF is set up and you are accepting risk using a low power, low antenna height configuration for your emergency communication plan.
Can you use long lengths of bad cable with lousy connectors, and still get out a signal, sure but will that serve you in an emergency?

Design your system to be there when you REALLY need it.

I would note that there are lots of examples of folks leaving their boat in a life raft never to be seen again but their boat is found floating several months later. The boat survived.

BTW, Donalex is correct, power is not as important to range as antenna height with VHF. It does get you through noise however and can offset losses in your antenna system.

Things I look for in an emergency;
I want the antenna to still be there or be able to erect one in short order with what I have left onboard.
I want to get out a decent signal (power) so I can talk over the crazes talking about how bad the fishing is
I want some sort of removable battery operated system I can take on the life raft as I step up from my boat to the life raft.

An all band (0.1 - 400 mHz) Ham moble fits the bill nicely

 

[myerssl]

Thanks for all of the input, it is helpful.

I moved the antannae to the mast for the additional range. Often I sail near coastal, (up to 15 - 20 miles out) and the prior setup gave me about 4 - 5 mile range.

As far as help is concerned, I do want to be able to reach the CG, but I also like to stay in touch with local dive operations (mate on one of the boats during the weekend), when I'm out there. Most of the dive boats are within 10 - 15 miles of my sailing area.

I've decided to limit the number of connections and use one cable RG8x with a connection at the antannae on top of the mast and going through the deck (appropriate water tight fitting) and doing another connection at the radio. Total run will be about 45' and will have a signal loss around 2.7 db.

Again, thanks for the help!

 

[Don S/V ILLusion]

if you are still following, I would suggest you compare RG8x with some far more preferable types. RG8x is among the worst coax types you can choose in terms of loss and among the various mfgs of coax, some are really bad.
Use the coax calculator in the link I attached earlier if you want to verify this.

 

[Bill Roosa]

Dito what Don said

My ham book has the charts for RF loss vs freq and in order of increasing (best first) line loss at 150 MHz I get (db/100 ft):
Open wire "window" line 1/2" and 1" 0.38 db/100 ft (not a coax line BTW )
7/8", 50 ohm "hardline" 0.7 (good stuff but VERY hard to work with)
1/2", 50 ohm "hardline" 1.0 (hard to work with)
RG-17, 17A, 18, 18A 1.4 (your first best choice)
Belden 9913 1.6
RG-8 Foam Diel 2.1 (your choice not too bad)
RG-59, 59A, 59B, 58 Foam Diel 4.5
RG 58, 58B 6.0
RG-58A, 58C 7.0
RG-174, 174A off the chart at 10 guess would be 80ish on a log chart

The order is the same for all freqs BTW

 

[Al - s/v Persephone]

Just a note on antenna height.

While having the USCG able to pick up your transmission from a long distance via a high-site antenna is comforting, in that scenario your best source of rapid help is from another vessel.

(You'll often hear USCG Pan-Pan broadcasts requesting same, pending their ability to get an asset on-scene.)

That means you should be sure you maximize the probability that some power boater with a deck-mounted antenna can hear you.

(My own setup is a sleeve-dipole model that the USCG mounts on their vessels, mounted at the top of a 58ft mast and fed with RG8. I do have to listen to a lot of chatter - including USCG traffic all the way from Sector SE New England to Sector Northern New England, in addition to my local and Sector Boston traffic. I consider it just an ongoing radio check.)

Fair winds,
Al - s/v Persephoine