[TowerTalk] Q factor question...

[Jim Lux]

At 10:18 AM 3/4/2007, VE2RYY wrote:
>I am  on the way of building the three element parasitic array on 
>160 meter according to ON4UN LowBandDxing book.
>In the array, there is a parasitic element which is a Director...(1.935Mhz)
>This same element is used for Reflector by installing  an inductance 
>of 3.6UH between the bottom end of the parasitic and the ground...
>Now here is the question.
>When making this inductance, do I have to take account of the Q factor .
>I have made one just for fun, with nr 14AWG wire ...etc....
>I get the right inductance needed, but I find that the Q factor is 
>not very high, say about 289 plus  or minus....


Let's look at this in actual impedances...
3.6 uH at 1.935 MHz is about 43 ohms.  So your Q of 290 implies an AC 
resistance of 0.15 ohms.
What's the AC resistance of the element itself?
What's the AC resistance of the ground system under the element?

I suspect that both are somewhat higher than 0.15 ohms (Especially 
the ground resistance), and will dominate the overall system Q.

There would be a slight (and slight is the operative word here) 
change in the phase and magnitude of the current in that element 
(relative to having a "perfect" inductor).  If you had a many element 
array with lots of directivity, then you might have cause to 
worry.  For this, though, the properties of the soil under the 
antenna will have more effect.

If you're really concerned, model the idealized antenna using 
something that does a good job with earth effects and find the 
relative phase and magnitudes of the currents in the elements.  Then, 
build the antenna, then hook current probes to the driven element and 
the parasitic element, and measure the relative phase and 
magnitude.  Squeeze or stretch your inductor until you hit the design points.

Actually, your original question is a great one for 
modeling.  Modeling is very good for doing things like sensitivity 
analysis:  What happens if the series R is 0.10, 0.15, 0.20 ohms, or, 
even, if the series R is 5 ohms...  Does the pattern change a 
lot?  What happens if the soil properties change from 0.005/13 to 0.010/40?

If the design is hideously sensitive to small changes in soil 
properties, then you could spend a lot of time fooling with the 
antennas to account for performance changes which are really due to 
whether it rained last night or the last time you ran the sprinklers.

Jim, W6RMK