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Old 05-27-2009, 07:47 PM   #67
williaty
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Quote:
Originally Posted by DeathSStar View Post
Chicken and ROFL's to the guy who said no ground will cause the radio to fry. It will work like **** but the radio won't "fry". I guess all the no-ground plane antennas that are used in the industry must cause a whole lotta damage.
Quote:
Originally Posted by cMags View Post
The guy who said no ground will fry the radio wasn't talking about a CB - the quote was one I pasted from another conversation about HAM. I know nothing about HAM and I doubt the radio will fry immediately, but I have heard that not grounding properly in some cases can cause damage.
You're both a little wide of the mark. The same rules about antennas apply to Citizens Band (CB), Amateur Radio Service (Ham radio), and professional radio. If the SWR is greater than 1, some portion of the output power of the radio is being reflected back at the output stage of the radio. If enough power is reflected back, the output stage will burn up ("fry"). Most modern radios include protection devices that automatically reduce the output power if they detect a high SWR.

The connection between this and ground is that an ungrounded antenna will have a very high SWR. Period. Again, we run into the problem of using the same word for electrical ground and RF ground. Having no electrical ground will always result in a high SWR.

The antennas "used in industry" alluded to by DeathStar are more properly called "No Ground-Plane Antennas". All this means is that they have been creatively designed to have a low SWR even when they're not attached to a large metal sheet to act as the RF ground plane. For instance, you have to use a No Ground Plane Antenna to put an antenna on top of your fiberglass motor coach. What these antennas do is to use the outer braid of the coax for a specific length to act as the "other half" of the antenna. In this usage, the bit of feedline that's radiating as the "other half" of the antenna is usually called a counterpoise. The presence of the designed-in counterpoise gives the antenna enough of a RF ground to reduce the SWR to the point that the radio can operate at full power without hurting itself. The drawback is that these No Ground Plane antennas are hugely inefficient and will always perform worse than an antenna above a proper ground plane.
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