[TowerTalk] 450 ohm feedline and coax

[Jim Lux]

On 12/23/14 9:16 AM, Jim Brown wrote:
> On Tue,12/23/2014 9:07 AM, Tom Osborne wrote:
>> HI All
>>
>> I've seen that you should keep the 450 ohm twinlead at leas the width
>> of the cable away from anything metallic.  Does that include coax cable?
>
> YES! And although I haven't seen any math to support it, I'd want wider
> spacing than that.


The math would be the strength of the magnetic or electric field of the 
balanced line at some distance away.  If it were perfectly symmetric, it 
wouldn't unbalance, but it would change the characteristic Z.

So the question might be "how far away do you have to be before the 
field drops to some specified fraction of the field at the wire's surface.

At a distance (d>>spacing) the field drops off as the 1/cube, so I guess 
the real question is "when is it >> enough"

Let's see, the field due to a current in one wire is mu0*I/(2*pi*r), 
where I is the current and r is the distance.

if the wires are separated by distance 2d, let's take the worst case, 
moving along a line that is in line with both wires, with wire 1 at +d 
and wire 2 at -d

distance from wire 1, r1= x-d
distance from wire 2, r2= x+d

The currents are opposite, so the field is:
B = mu0*I/(2*pi) * (1/r1 - 1/r2)
   = k * (1/(x-d)-1/(x+d))
   = k * ( (x+d)-(x-d))/(x^2-d^2)
   = k * 2d/(x^2-d^2)

x/d	B	dB
1.5	1.60	4.08
2	0.67	-3.52
2.5	0.38	-8.38
3	0.25	-12.04
3.5	0.18	-15.00
4	0.13	-17.50
4.5	0.10	-19.67
5	0.08	-21.58
5.5	0.07	-23.30
6	0.06	-24.86
7	0.04	-27.60
8	0.03	-29.97
9	0.03	-32.04
10	0.02	-33.89


when x/d is 2, that means you're looking at the field that is 1/2 
"t-line width" away.  x/d = 10 means "5 widths away"

I put the field in dB (20*log10(B)) for comparison.. usually, you figure 
that some effect that is 20 dB down is not worth worrying about.

Well, this would imply that going from x/d = 2 to x/d= 5.5 gives you 
about 20 dB improvement.

x/d of 5.5 means about 2-3 widths away.

Obviously, the "length" alongside is also important.







>
>> I want to run some twinlead out to an antenna and where it would come
>> into the shack is a coax cable and it would be laying right on top of it.
>>
>> Just wonder it that would cause any problems.
>
> Yes, it could cause problems.


>
> 73, Jim K9YC
>
> _______________________________________________
>
>
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk at contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
>