[TowerTalk] Answer to Pete's (N4ZR) Question

[Stan Griffiths]

Honest, I've really tried to stay out of this thread.  Now it is almost
humorous except that a lot of bum info is being pushed onto some pretty
naive newcomers.

Steve Best says the following:

>     Pete:
>     
>     Let's start with the simplest case.  The 50 ohm antenna fed with 50 
>     ohm coax, balun (1:1) and 50 ohm transmitter.  Let's first assume that 
>     the coax and balun are lossless.  If we deliver 1000 Watts power into 
>     the transmission line, 1000 Watts power will be delivered to the 
>     antenna for radiation.  Let's now assume that the coax is 100 feet 
>     long and has 2 dB/100 feet attenuation.

Bad assumption.  Average hams use something like RG8 which has only 1.3 dB
attenuation per 100 feet and that is at 30 MHz, worst case, for most of us
here.  It would make infinitely more sense to make assumptions that are
closer to real life.  How about assume 15 MHz where RG8 has more like 0.85
dB of loss per 100 feet.  Of course, many of us use lines longer than 100
feet, but we also use coax lines with lots less loss than RG8, CATV hardline
for example.  A good typical number to assume for feedline loss without
regard to SWR would be 1 dB, in my opinion.  With the basic assumption
flawed, everything that follows is flawed . . .

  With 1000 Watts power into 
>     the transmission line, 369.04 Watts will be lost in the coax and 
>     630.96 Watts will be delivered to the antenna for radiation. 

Steve Best continues:
     
>     Now lets consider the more complex case of the same antenna connected 
>     to the transmitter with a 600 ohms feeder and a tuner.  I will assume 
>     that the tuner is lossless.  I will also assume that the feeder line 
>     has no spurious radiation.  

Do we really care about the stuff in the next two paragraphs?  I don't think
I do . . .     

>     I will first assume that the feeder line is 100 feet long and lossless. 
>     The steady state input impedance at the input of the feeder will be 
>     63.524 - j310.662 ohms. Using a conjugate match tuner the following 
>     power distribution will occur:
>     
>     The initial power delivered to the tuner will be 284 Watts.  The 
>     initial power delivered to the antenna will be 80.67 Watts.  The 
>     steady state power delivered to the antenna will be 1000 Watts (80.67 
>     watts signal, 919.33 watts echo).  A 600 ohm forward power meter would 
>     read 3521 Watts at the tuner output and a 600 ohm reverse power meter 
>     would read 2521 watts at the tuner output.

Steve Best continues . . .
     
>     If the feeder line has 2 dB/100 feet attenuation (no VSWR or other 
>     affects included so that it is the same level as above matched antenna 
>     example), the input impedance to the feeder line will be 226.373 - 
>     j277.493 ohms.  Using a conjugate match tuner the following power 
>     distribution will occur:

Well, this is where you and I REALLY part company Steve.  Open wire line
typically has 0.1 dB of loss per 100 feet at 30 MHz.  At 15 MHz, the loss is
so low it is off the bottom of the chart.  I would guess maybe 0.05 dB per
100 feet. 

Again, if the basic assumption is flawed, so is everything that follows . . .

>     The initial power delivered to the tuner will be 715 Watts.  The 
>     initial power delivered to the antenna will be 128.13 Watts.  The 
>     steady state power delivered to the antenna will be 250.65 Watts 
>     (128.13 watts signal, 122.52 watts echo).  A 600 ohm forward power 
>     meter would read 1399 Watts at the tuner output and a 600 ohm reverse 
>     power meter would read 399 watts at the tuner output.  Note that the 
>     wattmeters would indicate 1000 Watts steady state power being 
>     delivered into the feeder line.  However, because of the feeder loss 
>     in both the forward and reverse directions, the steady state power to 
>     the antenna is reduced significantly.
>     
>     With line attenuation included in the calculations, the matched antenna 
>     radiates 380.3 Watts (4 dB) more steady state power than the antenna with 
>     600 ohm feeder and tuner.  
>     
>     When attenuation is considered, the matched antenna system radiates more 
>     power than the unmatched/tuner combination.

Maybe so, but most of us don't use resistance wire to make open wire
feeders.  Where do you get this open wire line that has two dB per 100 feet
of loss at 30 MHz?

>     In practice, match your antenna to the transmission line as good as 
>     possible and use the lowest loss line possible (no new revelations here).

Well, I can't argue with that!

>     I can provide the math if anyone wants to see how the calculations are 
>     performed.

The calulations won't mean a darn thing if the underlying assumptions about
feedline loss are way off . . .
     
>     73, 
>     Steve VE9SRB

Now a personal question:  What class of ham license do you hold, Steve?  And
why don't you have a U.S. call since you are apparently in the U.S. and
working for a U.S. company?  Please remind me again what qualifications you
have that should make us believe some of this stuff you are saying.  When it
comes to RF losses in transmission lines, the world of VHF, UHF, and
microwaves is VASTLY different than HF.  Your explanations and comments lead
me to believe you have spent your earlier life well above the HF spectrum.
Most of us on this reflector live in the HF world and if you want to have
meaningful conversations with us, you must come down to our bands and talk
there . . .    

Stan  w7ni@teleport.com


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