Topband: A 160 Meter antenne puzzle

[Lee K7TJR]

>I have what I think is an interesting  antenna riddle .
>I was doing some testing today and discovered that touching the 4x4 at the top
 caused the swr to increase from about 1.2 to 1 to 1.7 to 1 .
>What is going on here and should I make an effort to limit the coupling between
 the 4x4 and the antenna?

Well Glenn it is not so much of a riddle as it is an engineering problem.
 My results for a similar element show that at 1.84 MHz the antenna 
element exhibits an end output impedance of about 1 ohm and 84 pf
in series. It is not hard to imagine that a pf or so finger near the antenna 
could change the value. A simple Spice model of this shows that with
95 uHy and 74 ohms in series (resonance) the output will shift with a
 minor change in the element capacitance. My analysis shows a
 change in level of 0.2 dB and a phase change of about 3.9 degrees
occurs when changing the element simulated capacitance by 1 pf. 
These simulations were with a fixed load of 75 ohms resistive.
    Whether this is serious or not to an array is also dependant on
how the combining circuitry is configured. Without some serious
 measurements I dont think anyone can answer your question except
 for some anecdotal observations. 
    I have in the past set up two elements and measured phase delay
 and amplitude differences between them by transmitting an equal
 signal to both. Knowing the amplitude and phase shift difference by 
placing your finger near the element is the only way I can see to
 answer your question. From my experience phase changes are your
 worst enemy. I also NEC modeled a very lossy 4 foot tall element near
the test element and there were severe changes in phase and amplitude.
Meaningless of course but does say that items near the elements can
shift the element output impedance just as you measured. For the
 passive element array to work properly each component must track
in value with frequency, temperature, moisture and more.
   One simple test that might give you a clue as the the magnitude of
the problem is to use both elements and null out a constant signal such
as a BC station by using the back side of the array.  Have someone
 touch near the element as you did while you monitor as accurately as
 you can if the notch depth changes.  The notch should be the most
 sensitive to array value changes.
Lee  K7TJR   OR