|SJ23 Tech Tip E10, (Updated 2017-06-19) Bob Schimmel|
So you're tired of being out of VHF (very high frequency) radio contact at your favourite anchorage and have decided to install a 25 watt fixed marine radio as your primary means of communications! This is an excellent upgrade especially if you choose a digital selective calling (DSC) radio that has features compatible with Coast Guard and commercial operations. Its always considerate to not interfere with commercial operation. The irony of having a fixed mount VHF radio over a low powered (1 watt) handheld radio is that many people can now reach you at your favourite anchorage, breaking the solitude you seek. The real reason of course is the reassurance that you can call for help in case of an emergency (or someone can call you) and since a sailboat is not exactly the fastest thing on the planet, you need lots of operating range to buy time. To increase the operating range you require lots of ERP (Effective Radiated Power) which requires a high antenna that is clear of obstructions. The low power of a handheld radio held close to your head (which absorbs RF) doesn't have that operating range. It works fine in a harbour that is a few miles across but at a remote anchorage you are usually out of range to home base. Having said all this, there is a limit to the operating range of a VHF radio when communicating over the ground. This path is the most difficult one for a VHF signal to travel through as the propagation is limited by line of sight (horizon) for distance, by interfering obstacles along the ground and by electrical noise. You can expect 20 to 30 miles from a good antenna installation. Occasionally VHF will skip (bounce off an object or atmospheric reflections) to travel a greater distance, but don't count on it for reliable communications. For example; I once talked 500 KMs (almost line of sight) using my screw driver for an antenna on a general mobile VHF radio, but this is in all other respects a freak of communications. On a sailboat you are striving for dependable communications that works when you need help. This requires clean power and a first rate electrical installation with the antenna and associated RF ground being the most important.
WHERE TO INSTALL a FIXED MOUNT VHF RADIO - Most radios are installed where a person can read and operate
the front panel with ready
access to the microphone. It is usually possible to satisfy these ergonomic
requirements by installing the
power cables and coax in an organized fashion.
Regardless of how tempting it may be, DON'T
mount the radio above the stove as the steam will hasten the demise of the
electronics. It sure hurts the hands to reach for the microphone through
the steam and you may need
to use the radio if the cook has an accident! Duh!
First rule in first aid, don't create a second
casualty! A VHF radio is a rather delicate thing so
mount it securely in a dry location.
COCKPIT SPEAKER - Install a remote speaker in the cockpit so you don't miss that important message. I have missed so many calls (and raft ups) due to wind or outboard noise that I now consider a cockpit speaker almost mandatory. There are better reasons of course (URGENT, DISTRESS or SAFETY calls) but a personal call will do for now! A cockpit speaker can also make it possible for a second set of ears to understand a garbled or marginal signal on a better quality speaker than what the VHF has. Consider the poor sailor calling MAYDAY. If the speaker is installed under a cockpit seat you can hear it through the cracks. If you want it louder then install it under the tiller, inside a tough vinyl case with the speaker pointed down. Best to use a membrane speaker since they are water proof. DO NOT install it behind a grill mounted in the cockpit foot well. The speaker cone can't stop knee deep water in the cockpit! A wireless mic/speaker combination is a real nice feature when you are locked to the tiller or when the cook has command of the galley. I'd be the last person to kick the cook out of the galley! Oh the joy of it all. TOP
I originally installed a cockpit speaker in the aft end of the starboard locker,
thinking it would be loud enough
with the lid closed. I
quickly discovered how well the lid mutes speech. Add some wind or run
an outboard and speech is
completely drowned out. Apparently even a
sailboat can be a noisy place! To solve this I installed the speaker
on the backing board of the outboard bracket, pointed
forward and upward to a vent
hole on top of the transom (shown here). The beauty of this
location is that it is well protected from the rain and I can hear speech,
most of the time. By crawling under the cockpit I could just reach the backing board by lying
over the cockpit drain hoses. But
this was only possible by using a drill to drive the mounting screws
(Thank you Mr. Roberts) and a temporary
plywood "bridge" to support me over the drain hoses.
DC POWER -
Use the correct or larger gauge
power wire and
don't chintz out on the quality of the wire. See Tech Tip E02
for suitable wire gauge guide. Use fine stranded wire to withstand
vibration and tinned wire to prevent corrosion. Install the shortest
length of wire to the power panel, keeping a bit of slack to absorb vibration and facilitate installation. Use a ferrite bead over the power line to block RF noise. Use an inline fuse on the positive battery wire to protect against fire. Locate the fuse where it is easy to see & access. Label the fuse and power wire. It is doubtful that a fuse on the negative lead can protect the radio against a ground surge. A fuse blows too slow and since lightening has already breached the air gap to the clouds, another inch gap in a fuse is not an obstacle. For this reason I'm not installing this fuse.
ANTENNA - If
you want the maximum possible operating range from your radio then install
the antenna at the mast head with no metallic objects around it.
Height and a clear line of sight is everything when it comes to signal
propagation and reception. It is also the safest place since you will
not be exposed to RF radiation.
Through field tests on
Panache I've determined that a
half wave length VHF whip (about 32" long) mounted on the pushpit has an operating
range of about 5 miles (at best) and the same antenna mounted on the mast head
has a range greater
than 10 miles.
We couldn't determine the maximum operating distance since the beaches I sail
between are only 12 miles apart! Communicating to the other side of a 200'
high island covered in spruce trees is next to impossible.
While a half wavelength whip doesn't require a ground plane to radiate, if the radio is electrically grounded (bonded) to water it can radiate at the optimum. Unfortunately mine wasn't bonded at the time but it still demonstrates the point.
There are several advantages of a center fed dipole antenna over an end fed whip antenna:
CO-AXIAL CABLE - The signal loss through the 50 Ohm coaxial cable to a mast head mounted antenna is minimal regardless of where you install the radio in the cabin. 35' of coax on an SJ23 just isn't long enough to worry about signal loss. However, don't kink, clamp, pinch or bend the coax tighter than its minimum bending radius. Install coaxial cable with a stranded center conductor to survive the flex and vibration in the mast. Double braided (shield) cable is superior to single braid. Foil with braid is probably the best you can get, especially if it has a bleed wire to connect to ground. Use a continuous length of cable inside the mast so you don't have to service an intermittent connection that you can't access. See Tech Tip E09 to install co-ax inside the mast. It is best to use light weight RG58AU or RG8X cable that has a minimum bending radius of 3".
In 2013 I replaced my RG58AU mast cable with RG8X and in 2015 I replaced the cabin portion with RG8X. There was a significant reception improvement with each installation. While the voice reception with the RG58AU was 5x5, the digital functions were marginal. The digital functions now work flawlessly with the RG8X cable, operating at 1W over an impressive distance of 7 KMs. TOP
CO-AXIAL CONNECTORS - The PL259 antenna connector (designed for WWII) is inferior both physically and electrically to most other coax connectors made today. The weakness of this connector is that it loosens with vibration and opens the ground path. Even a little bit of looseness opens the outer conductor, creating a huge impedance mismatch for transmitting and weakening the shielding for receiving. I have seen so many loose PL259 connectors that it is the first thing I look for. It is amazing how many "lousy" radios I have "fixed" over the years by simply tightening a loose connector. The reason why a loose connector is a major problem is that a lot of transmitted power is reflected back to the transmitter, with the possibility that the reflected power will damage the transmitter (unless the radio is protected for high VSWR). In addition, a lot of receive signal strength is lost. This connector is inexpensive, robust and field serviceable, which is why it is popular. To tighten a PL259 connector ensure the two teeth on the plug mate with two recesses on the jack and tighten the barrel snug. Check the barrel often. My VHF radio shown above is equipped with a PL259 chassis connector. It would be difficult to replace it with a TNC or N-Type connector, but not impossible. Fortunately this connector is within easy reach to check the tightness so not much of a problem. The TNC or N-Type connectors are far superior with their excellent impedance matching, minimal signal reflection, minimal signal loss and maximum shielding. They are more expensive and they stay snug with vibration. I have never seen an N-type, BNC or a TNC connector come loose or cause a problem. Try to avoid using a 900 adaptor though, as they attenuate more signal than a straight through connector.
After the co-ax cable is installed inside the mast, attach connectors to the ends. The best connector to use at the mast head is an N-type or TNC but you might have to use a PL259 connector just to match to the antenna. Similarly, at the radio end, install a connector that matches to the radio. Seal the exterior connectors with DR vulcanizing tape (black/white) to seal out water and then cover the DR tape with vinyl electrical tape to protect against UV rot. It is important to protect the antenna connector very well since it is exposed to the weather and not easily accessible for maintenance. All other lower connectors may be protected with two wraps of electrical tape, sufficient to last a season. This is OK on fresh water. On salt water I would go the full moxie with vulcanizing DR and vinyl tape. Make your last wrap going upwards so the outside layers deflect water away, similar to shingles on a roof.
Having sealed Panache's upper connector in the manner described here, I no longer remove the antenna when the mast comes down. The reinforcing bracket is substantial enough to support the antenna in the horizontal position. TOP
The sheath of a VHF radio co-axial transmission line must be connected to a counterpoise (at the top antenna connector) in order for the radio to transmit/receive a signal over its maximum range. A VHF radio will benefit only marginally from an RF ground and is considered overkill in most applications unless you often sail at the edge of normal reception. It may make a difference for that operation. However, an RF ground is essential for a SSB radio to transmit/receive over long range. With SSB you want long range operation. If your SJ23 is equipped with an SSB radio then read on.
Electrical grounding at RF frequencies is vastly different from electrical grounding at DC power. In the case of a SSB radio it means 0 ohms to earth ground (not battery return or negative) at 30-150 mHz, which is quite different than a power ground of 0 ohms at 12 VDC. The impedance of an RF ground cannot be measured with an Ohm meter. Test the performance of the RF ground wiring by comparing the operating range with another radio or measure it with an impedance meter. It is generally recommended to keep your RF ground separate from your DC power return (not ground) so an RF signal doesn't interfere (ground noise) with the instruments, etc. There are two types of wiring for an RF ground:
NOTE - In some installations a good lightning ground may also be a good RF ground, serving a dual purpose of life protection and optimal RF performance. Problem is when the lightning ground is conducting a charge from the surrounding air to the water it might carry enough current to damage expensive electronics. Usually an RF ground is wired with small gauge wire to carry minimal current while a lightning ground is wired with 0:0 wire to carry an ungodly amount of current. Therefore I recommend separating the RF and lightning grounds to simplify maintenance, trouble shooting and potential loss.
GENERAL GUIDELINES FOR A GOOD RF GROUND - Be aware that an RF ground is not really required for a weekend sail since you generally stay within VHF range. However, if you decide to sail to the "working edge" of the VHF operating range you should consider installing an RF ground to improve reception there. It is more effective on salt water than on fresh which why you can talk further over salt water.
Overall, there are probably as many different ways to create a good RF ground as there are people giving advice about it, myself among them. What works on one boat may not work well on another. Shown here is a drawing of the technique I developed to bond a co-ax cable ground to the mast. The mast is also bonded to the compression post which in turn is currently grounded to water via the outboard leg. Good article on RF grounding TOP
EMERGENCY VHF ANTENNA
It's always a good idea to have an emergency antenna just in case the big stick decides to fall down and you want to cancel your theatre tickets. On the other hand if you are the committee boat and two people need to talk to different boats at the same time, use the emergency antenna. Failing that it can always be used as a perfectly good AM/FM antenna.
EMERGENCY RUBBER DUCKY - A "rubber ducky" (flexible rubber) antenna can be more effective as an emergency antenna if it is hoisted up the mast using a 30' long extension of RG58AU coax cable equipped with PL259 connectors. This combination works for the ship radio. If you want to connect a hand held radio instead then carry a PL259 (female) to TNC (male) adapter since most hand held radios are equipped with a TNC antenna connector. Store the adapter and coil of co-ax in a sealed plastic bag to keep it corrosion free. The reason for the 30' length is to raise the antenna as high as possible on a whisker pole or so you can talk from inside the cabin with the antenna hoisted. Remember, height is everything when it comes to RF propagation, especially with a poor antenna like a "rubber ducky."
EMERGENCY DIPOLE ANTENNA - My emergency antenna is a homemade center fed dipole that can really improve the performance of a 1W handheld radio. The antenna is housed inside a PVC tube for electrical insulation and weather protection. The mounting clamp at the bottom fits on the pushpit. See diagram at right. Strip away 47.875 cm of the co-ax vinyl jacket and shield to reveal the insulated center conductor. Seal the end of the center conductor and the end of the shield with marine silicon sealant to prevent corrosion. Then slip five ferrite chokes over the vinyl outer jacket, positioning them 47.875 cm from the end of the braid. The exposed inner conductor defines the length of the top element and the ferrite chokes define the electrical end of the bottom element. They are the same length. Lock the ferrites in place with a light dab of silicon sealant. Slip the antenna inside a stiff 1/2" ID white PVC or fibreglass tube that is 1 meter long. Cap the top, leave a tiny vent hole at the bottom and add a mounting clamp there as well. Extend the co-ax cable to the fixed mount radio or to a hand held radio, leaving several feet of slack for freedom of movement. Alternatively, hoist it to the spreaders or higher, keeping it away from the mast. I intend to install my alternate antenna on the pushpit using it primarily as an AM/FM antenna.
NOTE - Don't install a bare metal whip on the pushpit or where somebody can touch it. If you touch the whip while the radio is transmitting you can get a very nasty RF burn, especially if the cockpit is wet. Use an insulated whip.
It is not too difficult to make an emergency center fed dipole antenna suitable for a sailboat. All you need is some 50 Ohm co-ax cable (RG8X or RG58AU), a choke made by coiling the co-ax, a rigid PVC tube to house the dipole and a suitable co-ax connector to match your radio.
Total length of the antenna in meters is, L=147/frequency in MHz.
For example, a channel 16 dipole (156.8 MHz): 147/156.8 = .9375
To tune the impedance of the antenna:
PRIMARY CENTER FED DIPOLE ANTENNA - You can make a primary mast head antenna from an aluminum tube and a rigid rod if the rod is electrically isolated from the tube and the tube is electrically isolated from the mast when mounted. This antenna design has less wind resistance than the equivalent antenna installed inside a PVC tube as shown above. The rod fits securely inside a phenolic insert set into the top of the aluminum tube. Cut the rigid rod and the aluminum tube 47.875 cm long. The co-ax center conductor connects to the bottom of the rod and the shield connects to the top of the aluminum tube.
radio transmission is
vertically polarized for omni-directional propagation so mount the
antenna vertical with the center conductor pointing up to keep the rain out. At left is
a design I developed (but never implemented) for mounting a 1/2" vinyl supporting tube
to house an antenna mounted on top of
The T configuration should prevent chafe and snagging things. The
co-ax cable exists near the bottom of the vertical tube. It is fastened along the
of the spreader and enters the mast just below the spreader. The co-ax cable
then exits at the bottom of the mast where it terminates in a PL259
connector fastened to a bulkhead connector. (See sketch above) The outside thread
(ground) of the adapter is bonded to the electrically grounded mast to connect to the
lightning ground. The co-ax
cable fed up through the deck in connected to the
bottom end of a bulkhead connector. Ideally there is an antenna tuner
between the antenna and the radio to correct any impedance mismatch of the antenna
regardless of which channel (frequency) you transmit on but is generally not
required for VHF.
The balanced 50 ohm impedance of the dipole should also be matched to the
unbalanced impedance of the radio using a balun transformer. This is
not as important for receiving as it is for transmitting. The mismatch
without the transformer will reduce the sensitivity a bit. If the
antenna is installed on a spreader then the radiation pattern will likely
have a notch (RF shadow) caused by the metal mast. This location it is
a good compromise between reasonable height above water and in the better
position so as not to be the magnet for a lightning strike. Probably
the best use for the mast top is a lightning rod or a tri-light. TOP
- Ever wonder if your installation is working?
GPS ANTENNA - Install
an external GPS antenna on the transom. This spot has the greatest visibility to the sky with
minimal motion. Your body does not affect the GPS signal as long as you
don't cover the top of the antenna. The received signal is extremely
weak, being 10dB below noise threshold. In addition, install it on the
opposite side of the secondary VHF antenna.
TRANSFER a VHF DSC RADIO E/W
MMSI NUMBER - If you transfer ownership of your Canadian
registered VHF-DSC e/w an MMSI number, it must be to a person who has a
Restricted Operator's Certificate (Maritime) with DSC endorsement. In
addition, you must update the Coast Guard in your country to remove yourself
from the registry for your MMSI number and to add the new owner to the
registry. The US equivalent is Restricted Radio Telephone Operator
Permit (RR) or Marine Radio Operators Permit (MP). Talk about
frustrating and time consuming for the Coast Guard to be looking for the