Posted by
N2EY on
Nov 17, 2007; 12:21pm
URL: http://elecraft.85.s1.nabble.com/Transmission-line-loss-tp366525p366526.html
In a message dated 11/17/07 12:02:32 AM Eastern Standard Time,
[hidden email] writes:
> 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...).
For comparison, check out the online loss calculator at:
http://www.ocarc.ca/coax.htm> 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.)
According to the online calculator, the matched loss of that coax is 1.009
dB/100 ft. It shows the additional loss due to 5:1 SWR as 1.13 dB, so the total
loss is 2.139 dB and about 62% efficiency. Insignificant difference between
your calculations and the online calculator. .
> 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.
The online calculator shows Belden 9258 (which is RG-8X type) as having
slightly higher matched loss (2.188 db/100 ft @ 50 MHz) which makes the SWR loss
1.784 dB and the total loss 3.972 dB. Efficiency almost exactly 40%. Again, an
insignificant difference between your calculations and the online calculator. .
>
> 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.
All the numbers I have are close to the numbers you got. But there are other
factors to consider.
First is the actual coax length. IIRC, you need 128 feet, not 100 feet, and
that pushes the losses up a bit. In practice the coax length will probably need
to be greater - maybe 150 feet when all is said and done. For example, you
want to leave some slack at both ends of a buried cable.
Second is the cost of a dB. If the difference between a K3/10 and a K3/100 is
ten dB and the price differential is $380, that's $38 per dB - but only on
transmit. If better coax reduces the overall loss by, say, 2 dB, and the cost
differential is $50, that's $25 per dB. And it works on both transmit and
receive, not just transmit.
Remember too that a lot of the 'cost' of the installation will be in the work
it takes to put it up.
Third is maxing out performance. While an S-unit of loss will normally not be
noticeable most of the time, when things get tough it can be the difference
between a QSO and no QSO. This is one big reason the big guns work stations the
rest of us don't.
Fourth is purely esthetic - some folks like knowing that they have the
highest efficiency reasonably possible.
73 es GL de Jim, N2EY
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