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.
>>
>>
>> RG58 hard dielectric 1.8 DB
>> RG58 foam dielectric 1.5 DB These two have the same I squared R loss
>>
>> RG8 hard dielectric 0.85 DB
>> RG8 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
>>
>>
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