On 10/10/2016 9:03 PM, Ray Benny wrote:
I am building a 90 ft, 80m rotatable dipole. I am near the point of fabricating
the inductors/coils that will go about 23 ft out on the element. I am guying
the element just before the inductor. I have several questions:
1. Does the size of the gap between the center element and element tip make
much difference? Is one inch enough, or should it be almost the same size as
the coil length?
I suppose it depends on how you mount the inductor; concentric or offset. I
suggest making the gap at least the length of the inductor.
2. Is there advantage of winding the coils of copper vs. aluminum? Is it worth
silver plating the copper windings? I'm most likely planning to use 1/4" tubing
either way.
Both materials will tarnish (oxidize). Since aluminum is already oxidized, I
would go with it, since we are both in AZ where, until the big one, we are
removed from salt air. Silver plating is a waste, IMHO, and not necessarily of
any benefit. See: http://k6mhe.com/n7ws/Plating.pdf
3. Once I figure out the total length of the element, a friend is going to run EZNEC and tell me the inductor valve I need. I will use a calculator to compute the physical dimensions of the coils. What is the advantage of building the coils on either 3 inch vs. a 4 inch form?
Is there anything else to consider in the construction of this antenna?
Tnx for your thoughts,
I "built" your proposal in AutoEZ (http://www.ac6la.com/autoez.html) and EZNEC.
I assumed your 90' overall length, inductors at 23' from center. The inner
tubing 1.5" diameter; a 1' length to hold the inductor, 1" diameter and the
remaining length also 1" diameter. I modeled in free space to avoid unknown
ground and height effects.
AutoEZ is too cool for words (I was an early beta tester) and has a built-in
resonating function. I let it set the reactance of the inductor to resonate the
element. I used 3.55 MHz for this and came up with +jX = 413 (18.5 uH). Now
the fun part is to sweep the inductor Q (X/R), by using a swept variable "R". I
can then plot (or tabulate) the gain v. Q. The point of this is to question the
wild claims about the superiority of coil X (no pun intended) to coil Y. I
don't know whether the following table will make it through the list filter but
here it is:
F Q R X SWR GAIN
3.550 50
55.02 -0.71 1.101 0.67
3.550 75
50.41 -0.37 1.011 1.05
3.550 100
48.11 -0.22 1.040 1.25
3.550 125
46.73 -0.13 1.070 1.38
3.550 150
45.81 -0.08 1.092 1.47
3.550 175
45.15 -0.04 1.107 1.53
3.550 200
44.65 -0.01 1.120 1.58
3.550 225
44.27 0.01 1.129 1.62
3.550 250
43.96 0.03 1.137 1.65
3.550 275
43.71 0.04 1.144 1.67
3.550 300
43.50 0.06 1.149 1.69
3.550 325
43.32 0.07 1.154 1.71
3.550 350
43.17 0.07 1.158 1.73
3.550 375
43.04 0.08 1.162 1.74
3.550 400
42.93 0.09 1.165 1.75
3.550 425
42.82 0.09 1.168 1.76
3.550 450
42.73 0.10 1.170 1.77
IMHO, what this shows is that an inductor Q of 200 or so is good enough and
heroic efforts to increase it are met with rapidly diminishing returns, as well
as increased cost, wind loading, etc.
Using K7MEM's calculator
(http://www.k7mem.com/Electronic_Notebook/inductors/coildsgn.html) which
implements Terman's formulas, also, the late Charlie Michaels, W7XC, simplified
some of Terman's tables to determine the AC resistance of a coil. ("Loading
Coils for 160-Meter Antennas", QST, April 1990) Using these references, I did
some playing.
Arbitrarily, I selected 12 AWG bare wire and a 1:1 length to diameter ratio.
Skipping some of the arithmetic and assuming I did it correctly, for 18.5 uH the
program calculates among other things that 18 turns requires ~174" of wire on a
3" dia form. The DC resistance is therefore ~0.11 Ohm. From Michaels, the AC
resistance ~1.62 Ohm @ 3.55 MHz. So Q ~ 250. Good enough.
What your design has going for it is that it's not too heavily loaded. Shorter
lengths would require much more loading.
FWIW,
Wes N7WS
Ray,N6VRLocated in central AZ
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