Topband: In search of resonance
Carl Braun
Carl.Braun at lairdtech.com
Wed Jan 29 23:16:53 EST 2014
Hello to the group
I'm a newbie to 160 having only participated in various contests over the course of the last two years. I used an 80 meter aluminum tube vertical with a Reyco trap affixed to the top with a horizontal "L: wire heading over to the palm tree. I've since converted that antenna into a pair of 40M verticals so I'm focusing my efforts on getting a resonant antenna on 60 and especially 160.
Here's the hardware I'm working with...
90' Tri Ex Skyneedle tower with a Telrex 20 meter 5-element yagi on a 45' boom at 94'.
Inverted L wire runs parallel to the tower and is spaced at 36" up to 85' and then out approx 60' or so.
The tower is grounded via copper strap to three ground rods that are bonded to 1 ½" copper pipe that circles the perimeter of my tower and control panel base. The perimeter copper pipe is a 4' x 8' rectangle that currently has 16 radials screwed to it. Three of those radials are tied into my 40M phased array ground radial system with some of the radials as short as 30' and others as long as 100'. The ground screen for my 40M array uses 100 radials each 60-90' long. I'm adding radials as I have time. Half of the 16 radials are multi conductor rotor control cable that fans out to affix to the radial ring then converge back together for 10' across the driveway and then fan out. I do this to eliminate a lot of individual wires crossing my secondary driveway.
I cannot get the inverted L to provide a dip on my MFJ 259 analyzer anywhere in the 160 meter band. I get dips at 8.2 MHz (R=36 ohms X=0) with reactance on each side of X=0. At 5 MHz R=40 ohms X=0 with reactance on each side of X=0. I cant get any significant dips neat the 80 or 160 band. However, when I approach 1.750MHz the resistance drops to 6 ohms and X is off the scale...at 1.825 I'm at 10 ohms and X is off the scale. Its as if my 140+ feet of wire is resonant on 8MHz.
Also, I've experimented with shunt feeding the tower to see where the thing would resonate but got similar results. ON4UN says my 90' tower and 5 ele yagi should yield an antenna that is 110 to 115 degrees in total length so I followed his guidelines and tapped the tower at 67' with a gamma wire spaced at 36" . The tower had multiple dips at 27 MHz, 20 MHz, 14 MHz and at 7.5 MHz the resistance dropped to 6 ohms...the same low resistance I' m now seeing on the L at 1.750 or so. No dips were observed at 3.5 or 1.8 MHz. I plan to experiment with a gamma wire that goes all the way to the top of the tower to see where it resonates...if its still high in freq maybe I should consider an Omega match
So all of that being said...why cant I find a resonant frequency on 160 with this L? Am I still too long? Is the tower causing that much interaction? I'd rather not cut the L long and insert a variable cap...I want it resonant at 1820 so I can use an unun to match the impedance and then run it into the shack.
Any suggestions on getting the antenna to work?
Thanks in advance for any help
Carl AG6X
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