On 1/5/2011 4:42 AM, Rick - NJ0IP / DJ0IP wrote:
Ken,
I don't know if the low angle efficiency falls off faster than the high
angle radiation.
I can't remember ever reading that, but I have probably only read "a drop in
the bucket" of all there is.
Because of extended ground losses, no antenna has strong radiation
exactly at the horizon. It can try but that RF gets absorbed.
I do know that I changed my attitude about verticals since reading Rudy's
papers.
I also know that there is no reason to ever use a vertical with inefficient
ground.
Instead I use a vertical dipole, 30 to 40 ft. overall length, fed in the
middle with openwire, and matched with a matchbox inside the shack.
No radials, yet still has fairly good efficiency.
Good efficiency on 40 and higher bands, but rotten on 160 where a good
center fed dipole would be 240 feet tall.
I can't imagine why people continue to go with the traditional design,
except for the case that they are willing to lay down a complex ground
system (or 4 elevated radials per band).
I have a metal machine shed 48 x 56', I figure a trap vertical at the
middle of all that metal won't need longer radials for the low bands to
work decently. I've used a trap vertical on a 30' diameter steel grain
bin with super results 40 through 10.
73
Rick
A poor ground plane contributes a poor (e.g. resistive) ground in series
with the antenna and so while it improves the bandwidth (and often the
impedance match), it hurts radiation efficiency. But it gets out better
than no antenna at all.
73, Jerry, K0CQ
|