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
On 1/26/2012 8:21 AM, Carl Moreschi wrote:
>
> Loss due to dielectric loss tangent (tan) can be very important.
>
> This term is proportional to frequency, so the higher you go, the
>
> more likely it will dominate overall loss (metal loss is only
>
> proportional to SQRT of frequency).
>
>
>
> Some of the power that is fed into a transmission line is lost because
>
> of its resistance. This effect is called ohmic or resistive loss. Another
>
> effect called dielectric loss, adds to the losses caused by resistance.
>
> Dielectric loss is caused when the insulating material inside the
>
> transmission line absorbs energy from the alternating electric field
>
> and converts it to heat (see dielectric heating).
>
>
>
> Carl Moreschi N4PY
> 121 Little Bell Dr.
> Hays, NC 28635
> www.n4py.com
>
> On 1/25/2012 10:37 PM, Jim Brown wrote:
>> On 1/25/2012 5:59 PM, Carl Moreschi wrote:
>>> he reason open wire line has less loss is that the dielectrics for true
>>> open wire line is air.
>>
>> FALSE! It's the ratio between the RESISTANCE and the IMPEDANCE of the
>> line. High impedance lines result in lower current for the same power,
>> and below UHF, virtually ALL loss in a transmission line is I squared R
>> loss in the conductors.
>>
>> The only time that dielectric loss matters at HF (or even VHF) is when
>> the dielectric is WET. Window line exhibits high dielectric loss when
>> it gets wet. Coax exhibits high dielectric loss when the INSIDE of the
>> coax gets wet (and the copper corrodes, which increases the resistance,
>> which increases the loss.
>>
>> 73, Jim Brown K9YC
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