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ContinuousWave: Small Boat Electrical
Shakespeare 5250 Halfwave End Fed VHF Antenna
|Author||Topic: Shakespeare 5250 Halfwave End Fed VHF Antenna|
posted 12-17-2008 12:53 AM ET (US)
I noticed that Shakespeare have introduced a new model to their large line of antennas, the 5250 SKINNY MINI 3-foot VHF Marine Band antenna. This antenna is a base-loaded halfwave antenna, similar to the GAM SS-2 antenna that I have endorsed and used.
The 5250 looks like it would be a compact, low wind resistance antenna that could be mounted atop a 4-foot extension mast on a small boat.
If anyone sees one in person or installs one on a boat, let us know what you think of the Shakespeare 5250.
posted 12-19-2008 07:21 AM ET (US)
How does this differ from the short Shakespeare antenna that is found on many small boats (like my Montauk) and has been around for a while? This newer model has a smaller diameter coil at the base but otherwise seems similar.
posted 12-19-2008 11:02 AM ET (US)
Based on the image of the antenna and not having seen one in person, it appears that the base coil design is smaller in diameter than on the other electrically-similar antennas in the Shakespeare line, which, in contrast, are rather short and fat. From an aesthetical view, the slim design is an improvement. From a mechanical view, there will be less wind loading and perhaps less weight.
From an electrical view, it remains to be determined if the change in coil design has an influence on the antenna radiation efficiency. Design of coils is a complex subject. One informed researcher says of coil form factors:
"In high impedance systems optimum form factor leans towards a longer inductor with a smaller diameter. This is because a longer inductor with smaller diameter has less stray capacitance from end-to-end. A smaller diameter coil can have less stray capacitance to the outside world, current isn't shunted out of the coil through displacement currents."
In this application the coil is used in an high-impedance application in which the end-fed halfwave antenna impedance is likely to be above 1,000-ohms. The coil helps to transform the impedance to match the 50-ohm feed line.
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