[Elecraft] Transmission line loss

[Don Wilhelm]

Mike,

Take a look at the loss from 0.40 inch diameter 75 ohm coax that may be 
available free as cutoffs from your cable installing company - it is 
burial rated and will often outperform the 50 ohm coax when you do the 
loss calculations.

You are correct that the matched line loss will vary with frequency, but 
the added loss due to SWR is independent of frequency - it only depends 
on the SWR and the matched line loss.

73,
Don W3FPR

Mike B wrote:
> First of all, I have to congratulate the posters in the earlier thread on line
> loss, etc., for prompting me to actually get my calculator & ARRL Handbook out
> and read up on transmission line loss, dB calculations, etc.
>
> My interest has come up as I'm planning on installing a flag pole vertical in my
> yard, due to CCRs, and the coax run will be around 100 feet outside the house,
> plus 28 feet inside.
>
> Here's my take on line loss, and please correct me if I'm wrong.  For today's
> discussion, let's assume the power is 100 watts, total run is 100 feet, that I
> can tune this vertical up to 50 MHz, and the SWR at the antenna hits 5:1 (it
> won't, as I'll have a remote tuner there, but just for discussion...).
>
> Davis RF-brand "RF-9914F Bury-Flex" has a published matched line loss of 1.1
> dB/100 ft at 50 MHz.  Per the Handbook, a 5:1 SWR at the load will add around
> 1.2 dB of loss, for a total of 2.3 dB over the run, for about 60% efficiency.
> (I'm still fuzzy on the percentage calculations.)
>
> A variety of brands of RG-8X have a matched line loss of 2.0 dB at 50 MHz per
> 100 ft.  A 5:1 SWR adds 1.7 dB, for a total of 3.7 dB loss, for a little over
> 42% efficiency.
>
> Am I reading something wrong here, or is the 1.4 dB difference between the two
> cables correct?  Can you extrapolate that to an S-meter at 6 dB/S-unit?  If so,
> I'd say the 1.4 dB would be barely noticeable on the receiving end at best, and
> the extra cost of lower loss coax isn't worth the money.  Operation at HF would
> be even less of an difference than at 50 MHz.
>
>