At 4/20/2006 12:46 PM, you wrote:
>Just about any 5/8 wave antenna mounted on top of a Ford pickup truck. The
>roof metal is so thin that frequent flexing from antenna sway, wind
>resistance, low tree branch contact can cause metal fatigue on the cab
>roof. The antenna NMO mount was installed directly above the cab interior
>light. Nice install but the metal finally cracked about 1/2 inch
>circumference further out from the outer edge of the NMO mount. The
>singing problem was resolved by wrapping thin fishing line around the full
>antenna length at about 1 turn per 1 1/2 inches for its full length. The
>thinner the whip antenna the higher the audio frequency. Observe
>automobile antennas and you will see many factory manufactured AM/FM
>antennas wrapped as mentioned above. Go out and wiggle your mobile
>antenna. Look at the automobile metal near the base. Watch it flex. The
>longer the antenna the more flexing. I solved the problem by relocating
>the antenna using a homemade bracket mounted between the front fender and
>hood near corner of windshield.
>Gary K2UQ
Your mounting arrangement may be mechanically superior, but it's
electrically inferior. Aside from the distorted antenna pattern you'll
get, you'll also get a lot more RF into the cab. I noticed when riding
with a friend who mounted his antenna in the same place you did that I
could not duplex using my HT as the receiver because the RF from his mobile
radio completely wiped out my HT's receiver even when transmitting on low
power on the mobile. I don't have this problem at all in my car, which has
the mobile antenna mounted in the middle of the roof. This is because the
roof acts as a shield: no RF can get inside the passenger compartment until
it loops around the edges of the roof, & by then most of the current has
radiated away (at least at UHF).
I'll take my chances with the roof & keep the RF out of my face, thank you.
Bob NO6B
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