Topband: In search of resonance

[Tom W8JI]

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.>>>>>>>>>>>>>>>

Given you have dips on  8.2 and 5 MHz, the MFJ is working normally.

With the tower so close to the Inverted L they are like one big coupled 
system.  I expect the "system" is resonant in the AM BCB, and that is why 
you cannot find the low dip.

I would look for resonance around 5MHz / 3 and or  at 8.2MHz /5 MHz. So look 
around 1650 kHz or.

Another way to find the base frequency for Marconi resonance when a system 
is out of band is to subtract the closest two dips and divide by two. (This 
is what I use in the MFJ 259 firmware to find distance to fault.)

8.2 MHz - 5 MHz = 3.2

3.2 / 2 = 1.6

So you have two different methods pointing to 1.6 MHz as the resonance. This 
tells me your combination antenna is on 1.6 MHz.
It might not move like you think because the tower is fixed at a certain 
frequency, and probably well down in the BCB, but you can try making length 
1.6/1.8 = .889 times what the L is now, or 129 ft.  I wouldn't expect it to 
move perfectly, but it should move.

***Ignore trying to determine resonance with a shunt wire. You are wasting 
time, because that is a complex system. A shunt system consists of a 
transmission line stub mode plus a common mode resonance, so you see the 
combination of the two effects. It will NOT tell you where the tower is. ***