[TowerTalk] tuners and power rating

[jimlux]

Rik van Riel wrote:
> On 11/28/2010 10:56 PM, DJ7WW wrote:
>> A tuner working into a 5:1 mismatch would not improve the antenna and feed
>> line loss anyway.
>> A tuner is the wrong approach to compensate a bad antenna design.
> 
> A 5:1 mismatch can be entirely reasonable depending on the band,
> the antenna, etc...
> 
> For example on 160m and 80m it is common to have high impedances
> at the band edges, because these bands are just so wide (as a
> percentage of the center freq).
> 
> However, a 5:1 mismatch at 2 MHz with 100' of LMR-400 coax only
> results in an SWR loss of 0.4 dB.

It's too early in the morning for me to run the numbers, but in a 
mismatch situation, the coax loss could be much higher:
1) 100 ft of coax is a lot less than a wavelength at 160m.  Loss in coax 
at HF frequencies is dominated by the I^2*R losses, and if the Z at the 
antenna is low, currents will be high To get the "average" mismatch loss 
(figured by looking at the circulating power in the feedline between 
tuner and load), the feedline needs to be an integer wavelength long.

  (Hmm, let's see.. 5:1 VSWR = 5:1 ISWR = Imax*Imin = Iavg^2, so Imax = 
sqrt(5)*Iavg, so loss max = 5*loss avg, for a short feedline with low Z 
load)


> 
> That 0.4dB of feedline loss (.26 of which due to SWR) may be a
> perfectly reasonable tradeoff...
> 
> Now, at 29 MHz that same 5:1 SWR could be a big problem - but
> you don't need to have that kind of SWR on 10m :)
> 
> You can calculate the (SWR related) feedline losses quite easily
> with one of the online coax calculators, eg:
> 
> http://www.saarsham.net/coax.html
>