Topband: In search of resonance

[Tree]

You might be dealing with AM BCB being detected by the meter - and masking
what you are looking for.


On Thu, Jan 30, 2014 at 9:14 AM, Tom W8JI <w8ji at w8ji.com> wrote:

> 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. ***
>
>
>
>
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