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. 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.
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.
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.
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:
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.
In practice, match your antenna to the transmission line as good as
possible and use the lowest loss line possible (no new revelations here).
I can provide the math if anyone wants to see how the calculations are
performed.
73,
Steve VE9SRB
______________________________ Reply Separator
_________________________________ Subject: Re: [TowerTalk] Mismatch Loss and
Tuners
To: <towertalk@contesting.com>
Author: Pete Smith <n4zr@contesting.com> at InterNet Date: 8/24/98 6:52
PM
Steve wrote :
[much deleted]
SUMMARY - A matched antenna will significantly outperform a
mismatched antenna with a tuner. A tuner cannot introduce "reflection gain"
to change this fact.
This whole discussion is a semantic thicket. To ease my brain, let's
consider a specific case. A 1/2 wave dipole, at its resonant frequency, fed
with 600-ohm open wire line, matched to the transmitter with an appropriate
LC circuit (including balanced-to-unbalanced transformation) at the
transmitter end. Assume that the antenna has a feedpoint impedance of 50
ohms resistive.
Compare to the same antenna fed througha balun with 50-ohm coax, matched to
a 50-ohm transmitter. Will this second antenna "significantly" outperform
the first? By how much? Where do the losses occur?
73, Pete Smith N4ZR
n4zr@contesting.com
"That's WEST Virginia. Thanks and 73"
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