[TowerTalk] tuners and power rating

[Paul Christensen]

Using TLD, I first ran a simulation on 80m using LMR-400 and a 250 ohm load 
for a 5:1 VSWR. I ran the coax length from an electrical 1/4 wave through a 
1/2 wave.

Total loss at 1/4 wave of line:  0.34 dB; 40% conductor loss, 50% SWR loss. 
Z at the tuner is 10 ohms.
Total Loss at 1/2 wave of line :  0.66 dB (percentage of loss attributed to 
conductor and SWR is essentially the same).  Z at tuner is 217 ohms.

So at first it may seem like a quarter-wave section of line has the lower 
system loss.  But, there's a factor missing here:  The tuner.  If the tuner 
has high Q components and a high output C value, a 10 ohm load may still 
result in lower system losses.  OTOH, if the tuner has low-Q components at 
the operating F and insufficient output C, then the system using the 
quarter-wave line may have substantially more loss.

VK1OD tries to address the total system losses in his charts.  I believe he 
chooses typical component values when addressing losses through a T type 
tuner.  To get the real losses requires knowing more about the tuner type 
and Q of tuner components from 160m through 10m.  The L and C values are 
often disclosed by the manufacturer.  But equally important, charts should 
be provided that address Q values, especially Q of L as L changes.  Armed 
with that information, EZNEC (to get antenna terminal Z), a transmission 
line calculator like TLD, and the W9CF tuner loss calculator, we can compute 
accurate loss figures as well as conduct "what if" analysis by changing 
values.

A really cool program would incorporate a basic NEC function (say...just for 
horizontal dipole types), TLD, and W9CF's tuner calculator.  One program to 
vary antenna length/height, line length/type, and tuner component values 
with variable Q.  That would tell the whole story.  Any takers?

Paul, W9AC

----- Original Message ----- 
From: "jimlux" <jimlux at earthlink.net>
To: "Rik van Riel" <riel at surriel.com>
Cc: <towertalk at contesting.com>
Sent: Monday, November 29, 2010 9:25 AM
Subject: Re: [TowerTalk] tuners and power rating


> 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
>>
>
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