Fw: Re: [Antennas] Vertical impedances

2003年3月14日 09:03:38 -0600

>A 1/4 wave length ground plane antenna with a perfect ground at 
>resonance has an impedance of 36 +j0 ohms
....can't argue with that, but.......
Nearly all the theory (because everyone's location and backyard 
are different) assumes free space patterns, 1/4+ wavelength elevation, 
clear surrounding space, single band antennas, and really good 
ground (all as goals ...your mission is to select the least undesirable 
alternative). While there are hams with 70ft+ towers, flat top 
dipoles 1/4+ wavelength off the ground at 80m, and enough radials 
to wire several houses, this ol' boy ain't one of them and I would 
venture to say neither are 80-90% of the hams. Thank goodness
"half power" is only 3db and that's only 1/2 "S unit".
I have trees and buildings to route around, the ground is mostly
limestone, and a height of 50-55ft is about the maximum (and that's 
used for the beam). There is an inverted "V" as part of the tower
guying system (and a relay at the top to transfer the ladder line 
between the two). The purpose of this activity is a decent antenna
for the lower bands (and I picked up a Butternut at a swapmeet). 
Any additional poles would have to be in the 30-40ft range or a 
vertical at about 20-25 ft off the ground with counterpoise (the
antenna clears the building but not the trees). The Butternut seems 
like a good "trapless" (less loss) vertical which uses the full length 
of it's 26ft pole for all bands, but that's still not 1/4 wave on 40m nor
80m and I have no idea what the radiation resistance or efficiency 
of that antenna is. 
73 Kees K5BCQ