Click to go to the Home Page.Marine VHF Radio FAQ Page

Commonly asked questions concerning VHF radio quality, troubleshooting and installation.

Can I cut the coax on my VHF antenna?
Yes, if you need to. However, you should leave at least three feet of coax. Note that this is not true for all antennas. CB antennas ' coax should not be cut.

How can I check the antenna to make sure it is working?
You should transmit to a friend's receiver, and have the friend assess your transmission for quality from a variety of known distances. Note than the US Coast Guard frowns on radio test calls made to them. To help you measure the efficiency of your antenna/radio system more scientifically, Shakespeare makes an antenna / radio tester. The ART-2 will test the output power of your transceiver, as well as the VSWR of the antenna.

Will metal or other objects that are close to the antenna affect it?
Yes they will. It is best to locate the antenna at least three feet from any metal objects or other antennas.

Can I mount the antenna beside the radio?
No, it must be at least three feet away from the radio, so transmission will not interfere with the radios reception (usually resulting in a squeal in the speaker).

Do I need a ground plane for the antenna?
No, Shakespeare VHF marine antennas are designed with independent ground planes built in.

What is the antenna's wavelength?
It can range from 1/4 wave to 5/8 wave, depending on the length and style of antenna. Please see the antenna specifications section for information on your particular antenna. Go To Shakespeare Antenna Web Site.

How high must the antenna be?
There is no particular height requirement, but remember, the higher the antenna, the better the reception.

What range can I expect from the antenna?
Here is the formula to determine range:
Calculation for Range of an Antenna:
6db Antennas: Square Root of Height (in feet) above water x 1.15= Range in miles
9db Antennas: Square Root of Height (in feet) above water x 1.52= Range in miles
Remember to perform the calculation for BOTH vessels, then add the results for the range between two vessels.

What is the receiving zone for my antenna?
On all Shakespeare VHF Marine antennas the receiving zone is 360 degrees or omni-directional.

What is the half power beam width for my antenna?
On all Shakespeare VHF Marine antennas the beam width is between 24 and 26 degrees.

What is the difference among the many styles of antennas?
Basically, the price, type and size of its elements, plus the way the antenna's fiberglass is wrapped, and the coax that is used determine the basic cost factors of an antenna. To this, add the relative cost of the finish applied to the antenna.

Do I lose anything by using a Combination antenna?
Yes, usually - depending on the type of combination antenna. Usually reception performance in the VHF side is somewhat sacrificed. Shakespeare recommends using separate antennas if space is available for them.

Is it better to use a dedicated AM/FM antenna or a band separator?
Shakespeare recommends using a separate antenna for this purpose. Band separators were designed for people who don't have room for two antennas.

What is gain, and how is it measured?
Gain is an increase (or decrease) in "effective radiated power" from an antenna, usually stated in + or - dB (deciBels). As a rule of thumb, you can add 4 times the radio's output power for 6dB antennas and 8 times the radios output power for 9dB antennas.

Can I paint the antenna, and with what?
Shakespeare's antennas can be painted any color you wish. But, make sure not to use any paint containing metallic chips or lead bases.

Can I repair the antenna if the fiberglass becomes frayed?
Yes. Simply wash the antenna with soap and water first, and allow it to dry completely. Next, paint the antenna with polyurethane or a lead free, non-metallic paint. Then lightly sand the su rface with 400-grit sandpaper. Additional coats of paint may be added, but are not really necessary.

 

 

Click to go to the Home Page.Marine VHF Antennas

Boat Rigging & Equipment
Performance - Troubleshooting - Installing VHF Antenna Systems

 

    The finest VHF radio you can buy will give you dissapointing performance with a poor antenna installation, while a good antenna will squeeze the absolute best out of a marginal transceiver.

    A basic rule of thumb is: the higher the antenna, the better the reception. VHF signals are line-of-sight only. So, the higher your antenna is mounted, the farther it can "see" to the horizon. You can calculate the theoritical range of an antenna using the following formula:

Formula For Determining VHF Antenna Range

Square root of the height (in Feet) above water x 1.42 = range in miles.
    So if your antenna is mounted on a 36 foot mast;
    the formula would be:
    (the square root of 36, which is 6)
    6 x 1.42 = 8.52 miles range.

    The calculation must take into account the antenna installation on both vessels. However, theoretical range is only part of the equation. You need an antenna with a gain rating appropriate to your boat and the type of boating or sailing you plan to do.

    The farther you venture from shore, the more range you need from your radio. So your antenna has to perform the "reach" with antenna gain, a function of antenna height and length.

GAIN
    Gain is an increase (or decrease) in "effective radiated power" from an antenna, usually stated in plus(+) or Minus(-) dB (decibels). VHF antennas come in gain categories, like 3-dB, 9-dB, etc., which are a measure of how efficiently the antenna deals with the signal it is fed from the radio. Gain depends greatly on antenna length. Generally speakink, you can add four times the radio's output power for 6-dB antennas and 8 times the radio's output power for 9-dB antennas.

Powerboat Antennas
    Antennas with a high dB rating concetrate signal energy perpendicular to the antenna shaft in a relatively flat wave form. This provides a more concentrated signal, but can also cause fading as the boat pitches and rolls. However, higher dB rated antennas are best for powerboats. Tall antennas have higher dB ratings. A 6-dB antenna is 8 feet tall, while a 9-dB antenna is 18-23 feet tall.

Sailboat Antennas
Shorter antennas are rated at 3dB and transmit energy in a less concentrated and broader pattern. This makes 3dB antennas ideal for sailboats, since the less concentrated signal works better when the boat and antenna are heeled fron vertical. Mounting a 3dB antenna at the top of a sailboat's mast will usually compensate for its nominally shorter range than a higher dB rated unit. A short antenna mounted atop a sailboat's mast can give performance equal to or better than a long antenna on a small boat.

Stainless Steel or Fiberglass?
The best sort of antenna for masthead mounting is a stainless steel whip model, for lightness and low windage. Fiberglass and polyurethane-coated fiberglass antennas are more attractive for deck and roof mounting and provide the torsional stiffness necessary for longer antennas. VHF antennas should be mounted vertically (i.e., not raked back).

Installation
When installing on thin surfaces, use a wooden (or other) reinforcing plate under the mount to strengthen the mounting. The base of the antenna mount itself makes the best template for drilling holes. Antennas more than eight feet tall should be supported by stand-off brackets mounted 3' to 5' up from the base. Use bulkhead pass-through grommets and other hardware when running coax cable to prevent chafe and exclude moisture.

Effect of Antenna Height on VHF Range

    Transmitting             Receiving Antenna Height
  Antenna height                5'            25'        250'
5 feet                              5 mi         8 mi      22 mi
10 feet                            8 mi       11 mi      25 mi
50 feet                           10 mi      13 mi      28 mi






Potential VHF antenna mounting Locations:

On the gunwhale or rail.
Surface (deck, roof, or bulkhead) mounting,
flat or vertical.
Mast mounting on sailboats, or attached to some mast-like structure on powerboats.





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Hand Held VHF Radios

Recently, there has been a substantial migration to handheld VHF sets. Today's hand-helds can provide higher performance than full size units of a decade ago, and they offer the advantage of portability and battery-powered independence from the boat's main electrical system, making them usuable in dinghies and other small boats.

    However, for top performance and to conserve battery life, it's ideal to operate a handheld VHF from the ship's power and attached to a full size antenna whenever practical. Handheld VHF's are limited to a maximun of six wattsoutput, compared to a fixed mount unit's 25 watts, but it should be noted that antenna height limits VHF range more than wattage.



 

VHF Antenna Quality

Price, type and size of elements, and the way an antennea's fiberglass is wrapped and finished, are the basic determining factors in antenna cost. You tend to get what you pay for. A potential weak point is the ferrule that attaches the antenna to its mount. Less expensive antennas use nylon ferrules, which are weaker and more prone to failure than the chromed brass fittingsfound on higher quality antennas. Better fiberglass antennas also use brass or copper tubes inside the fiberglass tube for strength and durability.


Combining Antennas with AM/FM or Cellular devices

Combination antennas (e.g., VHF and cellular telephone) are available, but reception performance in the VHF side is somewhat sacrificed, It is best to use seperate antennas if space is available for them. Likewise, it is better to use a dedicated AM/FM antenna than band separator on your VHF antenna, anless you absolutely don't have room for another antenna.


The Coax Cable

Your antennas coax lead-in cable should be as short as possible and as large as diameter as you can afford. On average, 50 feet of RG-58 coax have about 3 dB loss, 50 feet of RG-8X coax will have about 2 dB loss, and 50 feet of RG-213 (RG-8U) will have about 1 dB loss. Make sure your coax runs are as short and straight as possible, with no 90-degree bends or kinks.


Table COAX Cable Size VS. Signal Loss

Cable       Diameter    Loss Per    Length Per
Type        (Inches)         100'       3 dB Loss
RG-58       .195           6.1 dB          49'
RG-8X      .245          4.5 dB           66'
RG-8U      .410          2.7 dB         111'


Choosing the proper coax size

RG-8X, RG-8U, RG-213U, or Belden 9913 are all good VHF coax choices. RG-58 is too light for anything but the shortest runs. Marine VHF requires 50-52 ohms impedance coax, so don't try using cheap TV 72 ohm coax or CB coax. It is a good practice to check the coax with an ohm meter before installing connectors. After installation, check again for the same reading. (Note: Some antennas are DC grounded and will indicate a short circuit whentested with an ohm meter. Antennas which do not use DC grounding generally read as an open circuit on an ohm meter).


Coax Connections

Soldered antenna connectors are best, but can be tricky to install. Good-quality crimped (no-solder) connectors are available and can provide satisfactory service if installed with care. For example, Shakespeare's Centerpin PL-259-CP connector is designed to provide excellent conductivity and reliability in a marine environment and is very easy to install without soldering.
Coat exposed coax connectors with silicone caulk (or RW) or waterproof tape to protect them from the elements. Cable splices eat up transmitting power, so avoid them if at all posssible. For attachment to another piece of coax, use an appropriate barrel connector (PL-258) and PL-259 connectors. Try to avoid splicing the coax bare wired.


Splicing Coax Cable

Unlike CB antenna coax, you can safely cut VHF antenna coax to shorten it, but you should leave a minimum of three feet of coax. Make sure to locate the antenna at least three feet from any metal objects or other antennas, and also at least three feet away from the radio, so transmission will not interfere with the radio's reception (usually resulting in a squeal in the speaker).


Maintenance of VHF coax cable and connections

Maintenance-wise, it's a good idea to separate the antenna coax connectors from time to time and check for corrosion. Some authorities suggest keeping the VHF turned on whenever you're aboard to eliminate moisture, although this should not be necessary with guaranteed waterproof (as opposed to "water resistant") models.


Testing VHF Transmitter Output

To test if your antenna and radio are working, you should transmit to a friend's receiver and get the friend to evaluate your transmission for quality from several known distances. Note that the U.S. and Canadian Coast Guards are less than enchanted by radio test calls made to them.

For a quick test to see if the connector and antenna are working properly tune in a weather channel, then disconnect the coax from the radio - making the signal become weaker or disappear entirely. Put just the center conductor of the PL-259 connector in the radio's connector. The signal should return but with poor fidelity and strength. Finally reattach the connector. The signal should return to normal. If instead, it goes away completely there is a short circuit at the connector or in the antenna coax.

If the fiberglass of your antenna becomes frayed, wash the antenna with soap and water, allow it to dry completely and paint the antenna with polyurethane or a leadfree, non-metallic paint. Lightly sand the dried surface with 400-grit sandpaper.

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