Topband: 80m rotatable dipole load coil questions

[Wes Stewart]

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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