[TowerTalk] 75 ohm - v - 50 ohm coax

[Jim Lux]

On 2/19/13 6:21 AM, Jim Thomson wrote:
> Another point with respect to loss in that coax. Because its diameter
> is quite large, loss is MUCH lower than the RG8-size coax we usually
> use. It is also lower because the loss in ANY coax at HF is I-squared R
> loss in the copper, and the current in 75 ohm coax is proportionally
> less for the same power level.
>
> 73, Jim K9YC
>
>
> ##  say what ?   What about skin effect at HF ?
> .875” heliax uses a  hollow cu tube.  If it was solid CU center
> conductor, loss’s would NOT decrease.

Sure it would. as long as the wall thickness is more than, say, 5 skin 
depths, the AC resistance is going to be entirely determined by the 
frequency and OD of the tube.

There's two factors at work in the loss.. IR loss is one (goes down as 
impedance goes up)  and cross sectional area.
Z goes as log(D/d):  75 ohm has an inner d of 1/3.5, 50 ohm has inner d 
of 1/2.3  (for air dielectric).

So, for the same outer diameter, the 50 ohm line will have a inner 
conductor with 1.52 times the diameter of a 75 ohm line.

The AC resistance of the two lines work out as follows (normalizing to 1 
for the outer conductor)

50 ohm: 1+ 2.3 = 3.3
75 ohm: 1+ 3.5 = 4.5

So the net is this:

Loss for 50 ohm line:   3.3
Loss for 75 ohm line   (50/75) * 4.5 = 3

So the 75 ohm line has 10% less loss than the 50 ohm line.


Skin depth in copper at 2 MHz is about 0.046 mm or 1.8 mil. I suspect 
the inner tube has a wall thickness >0.010

Andrew VXL5-50 says 0.214 dB/100 ft loss at 30 MHz (foam dielectric)
Andrew HJ5-50  says , 0.189 dB/100 ft  (heliax) basically the same loss.

LDF5-75 (foam 75 ohm) is 0.195dB/100 ft
HJ5-75 (heliax 75 ohm) is 0.209 dB/100 ft (more than the 50 ohm stuff)

The data sheet gives 0.0505dB/100 ft for 2 MHz, which is slightly less 
than the 0.054 dB I'd expect from simple square root scaling by frequency.

That implies that the center conductor tubing wall thickness is more 
than sufficient for the current to be fully carried in the surface layer.

it also says that simple models don't work..otherwise HJ5-75 would have 
less loss than HJ5-50

But in this case, we're talking and arguing about mB, not dB.


>
> ##  Fine for your to recommend  the use of 1” 75 ohm coax for a high dipole,
> but do you have any clue how heavy that is gonna be ????   Never mind
> the  “balun” at the feedpoint.
>

I think most people run hardline up to some convenient anchor point, and 
then transition to something smaller and more flexible.  I doubt anyone 
is using 1" hardline coiled through a bunch of ferrite cores to make a 
choke, for instance.