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Author Topic:   Internal Antenna Under Hardtop
wood duck posted 08-07-2009 10:10 PM ET (US)   Profile for wood duck   Send Email to wood duck  
I am purchasing a 2009 235 Conquest and plan on installing a Garmin 545S which is equipped with an internal antenna. Might I have satellite reception problems with the unit under the hardtop? Currently I have a 205 Conquest and have the same Garmin 545S and have not had any reception problems with the bimini raised.
jimh posted 08-07-2009 11:21 PM ET (US)     Profile for jimh  Send Email to jimh     
Radio signals travel though fiberglass without any significant attenuation. Fiberglass is often used as a material in a RADOME, the housing that protects an antenna from the weather.

If the hardtop on the 235 CONQUEST is fiberglass and doesn't have a lot of metal reinforcements, the top won't create much of a radio shadow. Those tops are supported by some very elegantly bended metal tubing, and that structure might affect radio signals. However, the tubing forms a very open structure with many wide apertures though which radio waves can easily pass.

TransAm posted 08-08-2009 09:00 AM ET (US)     Profile for TransAm  Send Email to TransAm     
I keep the garmin unit in my truck mounted not on the dash, but just above where the drink holder is in my center console. This is directly under my metal roof with no direct line up to the outside other than the side windows. The unit performs remarkably well with no external antenna.
glen e posted 08-08-2009 09:04 AM ET (US)     Profile for glen e  Send Email to glen e     
I recently saw a 545 installed in the rear stateroom of a 35 express. It had to go thru the rear deck, the hardtop and the tower setup. Worked perfectly. Get the flush mount kit for the unit too. Lets it pop out so you can take it home/store it. It's all of 30 bucks.
20dauntless posted 08-08-2009 07:01 PM ET (US)     Profile for 20dauntless    
I have a Raymarine antenna mounted inside the cabin of my C-Dory and reception is just fine. I don't think you'll have a problem.
Blackduck posted 08-10-2009 02:44 PM ET (US)     Profile for Blackduck  Send Email to Blackduck     
While you are all most likely right in that the thing will most likely work just fine, the antenna's ability to receive will be dampened by the fiberglass panel. Just take your remote antenna and stick it inside your fiberglass console, you will see the bars drop in height, and some may vanish. I'd go with a remote, if possible.
jimh posted 08-12-2009 08:43 AM ET (US)     Profile for jimh  Send Email to jimh     
Laminated fiberglass has a relatively low dielectric. Circuit boards used for electronic assembly are typically made from a laminated cloth and resin sheet product. Inasmuch as microwave circuits are built and assembled on these laminated fiberglass sheets, it seems unlikely that such material can be much of a conductor of microwave radio waves.

The dielectric constant gives some measure of a material's loss characteristics for radio wave propagation. Here are some values of the dielectric constant for various materials:

Vacuum = 1 (by definition)
Air = 1.00054
Teflon = 2
Polyethylene = 2.2 (commonly used in coaxial cables as the insulator)
Paper = 3.5
Water = 88

Cloth and resin laminates at microwave frequencies tend to have dielectric constants between 2 and 10, depending on the materials used.

You can say that a fiberglass hard top will have more attenuation than air, however it is not much of an attenuator for microwaves. If there were a layer of water on the hard top, the water would probably have more effect than the fiberglass.

Another factor which could be influential is the state of cure of the fiberglass. Uncured resin is likely to have much higher attenuation. It appears that measurement of dielectric constant is one technique used to assess the state of cure of laminates and resins.


jimh posted 08-12-2009 10:33 PM ET (US)     Profile for jimh  Send Email to jimh     
Most people now have an intuitive sense of the dielectric constant of materials at microwave frequencies from their first-hand experience in the use of microwave ovens for heating food. We all know that if you put a ceramic bowl filled with water into a microwave oven, the water will absorb the microwave radiation from the oven and become warmer, while an empty ceramic bowl will remain unaffected. This is because the dielectric constant of the water is much higher than the dielectric constant of the ceramic bowl.

To test the dielectric constant of your fiberglass hardtop, saw off a portion of the hard top in a size that is convenient to fit into your microwave oven. Experiment with different settings on the microwave oven to see if the fiberglass top becomes warm during microwave heating. If the fiberglass top remains near ambient temperature, you can be assured that it will not be affecting the reception of GPS radio signals very much.

Replace the sawed out portion of the hard top and fix it in place with a quality epoxy adhesive. Once the epoxy has fully cured it should also be nearly transparent to microwave radio waves.

pglein posted 08-14-2009 03:31 PM ET (US)     Profile for pglein  Send Email to pglein     
Newer GPS's seem to get reception inside a fiberglass cabin, or even inside a steel car. However, it was not long ago (3-4 years) when the cutting edge units were not capable of getting reception under anything more dense than a Sunbrella bimini top.

My Garmin Nuvi 760 was the first unit I've ever owned that would work in my car or inside the cabin of my fiberglass boat without the use of an external antenna. Sadly, despite this capability, they still sold it with a suction-cup mount that requires attaching it to the windshield, and it was spotted by an oportunistic thief one night and I no longer have it.

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