Here's some calculations to support that last comment:
Solid dielectric coax Vf=0.66 and foam dielectric coax Vf=0.85,
therefore relative dielectric constants must be 2.29 and 1.38 respectively.
So, if we keep the coax outer diameter the same, the foam inner
conductor must increase by SQRT(2.29/1.38)=1.288; that makes the RF
resistance 0.776 times what it was.
If we assume a loss of 1.8dB in a particular length of solid dielectric
cable, that reduction in RF conductor resistance reduces the losses to
1.5dB - *exactly the quoted figure*.
In other words the increased centre conductor diameter and the
consequent reduced copper losses *exactly* account for the quoted
difference in cable loss; *losses in the dielectric are not a
significant factor*.
73,
Steve G3TXQ
On 26/01/2012 15:57, Steve Hunt wrote:
If you look at the technical data for a Belden solid dielectric RG58
(say 8259) and compare it with a foam dielectric RG58 (say 7807) you'll
see that the centre conductors are significantly different diameter:
0.035" vs 0.044" - they have to be to achieve the same 50 Ohm
characteristic impedance with different dielectric materials.
The difference in loss is almost totally down to the smaller conductor
diameter, not to differences in dielectric loss.
73,
Steve G3TXQ
On 26/01/2012 13:43, Carl Moreschi wrote:
Here is some data taken from the ARRL handbook on losses for various
transmission lines at 14 mhz per 100 feet.
RG-58 hard dielectric 1.8 DB
RG-58 foam dielectric 1.5 DB These two have the same I squared R loss
RG-8 hard dielectric 0.85 DB
RG-8 foam dielectric 0.6 DB These two have the same I squared R loss
Open wire line air dielectic TV type less then 0.1 DB
As you can see from above, the dielectric has significant meaning to the
loss.
Carl Moreschi N4PY
121 Little Bell Dr.
Hays, NC 28635
www.n4py.com
_______________________________________________
TenTec mailing list
TenTec@contesting.com
http://lists.contesting.com/mailman/listinfo/tentec
|