Ja, då får jag väl öppna lekstugan igen eftersom solen är på väg ner och de lägre HF banden börjar öppna sig för mer sofistikerade QSO,n.
För er som är mer intresserade av fakta har jag sökt med ljus och lykta för att få en del av mina påståenden verifierade och jag har hittat några som kanske bringar ljus i det jag säger!
Men först av allt så vill jag rätta till en freudiansk felsägning som smög sig in i mitt inledande inlägg!
Jag skrev där "Med SWR = 0 så ingen eller ringa utstrålning!"
Detta skall ju vara "Med SWR = 0 i effekt, så ingen eller ringa återstrålning!"
Och med det menas SWR1:1
Uttrycket "För högt SWR så brinner skiten upp!" menas " brinner slutsteget upp."
Nu lämnar jag det och återgår till referencer som är väl accepterade för mina övriga inlägg i tråden!
1: Mitt påstående som inte var välformulerat där jag säger: "Varje gång man försöker att anpassa ett missfoster till antenn så sker förluster!
Att behöva använda en sk antennanpassningsenhet eller matchbox är ett lysande exempel på att antennen är felkonstruerad!
Varje watt som kompenceras går upp i värme, var? I anpassningsenheten! den watten kommer aldrig fram till antennen."
Kommentar: ARRL Handbook 25:15
REASONS FOR USING AN
ANTENNA TUNER
If you use a linear amplifier, however, you may find
that it can’t load some coax-fed antennas with even moderate
SWRs, particularly on 160 or 80 meters. This is usually
due to a loading capacitor that is marginal in
capability. Some amplifiers even have protective circuits
that prevent you from using the amplifier when the SWR
is higher than about 2:1. For this situation you may well
need a high-power antenna tuner. Bear in mind that
although an antenna tuner will bring the SWR down to
1:1 at the amplifier—that is, it presents a 50-Ω load to
the amplifier—it will not change the actual SWR condition
on the transmission line going to the antenna itself.!!!
For example, assume that the load at 1.8 MHz is
12.5 + j 0 Ω. For this example, the output capacitor C3 is
set by the program to 750 pF. This dictates the values for
the other two components. At 1.8 MHz, for typical values
of component unloaded Q (200 for the coil), 7.9% of
the power delivered to the input of the network is lost as
heat. For 1500 W at the input, the loss in the network is
thus 119 W. Of this, 98 W ends up in the inductor, which
must be able to handle this without melting or detuning.
The T-network must be used judiciously, lest it burn itself
up or arc over internally.
ARRL 9:1
The term broadband antennas has frequently been
used to describe antenna systems that provide a wideband
impedance match to the transmitter. This is something of
a misnomer, since most antennas are good radiators over
a wide range of frequencies and are therefore “broadband
antennas” by definition. The problem is getting the energy
to the antenna so it can be radiated. Antenna tuners solve
this problem in some cases, although losses in transmission
lines, baluns and the antenna tuner itself can be excessive.
Also worthy of mention is that antenna directional
properties are usually frequency dependent.
ARRL:
—Matching the dipole with a complementary RLC
network greatly improves the SWR characteristics,
nearly 1:1 across the 3.5-MHz band. However, the
relative loss at the band edges is greater than 5 dB.
9-4 Chapter 9
An extreme degree of bandwidth broadening of an 80-
meter dipole is illustrated in Fig 6. This approach is not
recommended, but it is offered here to make a point. The
broadening is accomplished by adding resistive losses. From
network theory we obtain the RLC (resistor, inductor,
capacitor) matching network shown. The network provides
the complement of the antenna impedance. Note that the
SWR is virtually 1:1 over the entire band (and beyond), but
the efficiency falls off dramatically away from resonance.
The band-edge efficiency of 25 to 30% shown in
Fig 6 means that the antenna has about 5 to 6 dB of loss
relative to an ideal dipole. At the band edges, 70 to 75%
of the power delivered down the transmission line from
the transmitter is heating up the matching-network
resistor. For a 1-kW output level, the resistor must have a
power rating of at least 750 W! Use of an RLC
complementary network for broadbanding is not
recommended, but it does illustrate how resistance (or
losses) in the matching network can significantly increase
the apparent antenna SWR bandwidth.
The loss introduced by any broadband matching
approach must be taken into consideration. Lossy matching
networks will usually provide more match bandwidth
improvement than less lossy ones.
ARRL:
FEED-LINE IMPEDANCE MISMATCHING
The simplest broadband matching network is a transformer
at the junction of the transmission line and the
antenna. See Frank Witt’s article entitled “Match Bandwidth
of Resonant Antenna Systems” in October 1991
QST. Observe that the impedance of the series RLC resonant
antenna model of Fig 2 increases at frequencies away
from resonance. The result is that more bandwidth is
achieved when SWR at resonance is not exactly 1:1. For
example, if the antenna is fed with a low-loss 50-Ω transmission
line, the maximum SWR bandwidth is obtained
if the effective RA = 50/1.25 = 40 Ω. The improvement in
BW2, the 2:1 SWR bandwidth, over a perfect match at
resonance is only 6%, however. This condition of RA =
40 Ω can be achieved by installing a transformer at the
junction of the feed line and the antenna.
If the line loss is larger, you can gain more benefit in
terms of bandwidth by deliberately mismatching at the
antenna. If the matched-line loss is 2 dB, the improvement
rises to 18%. To achieve this requires an RA = 28.2
Ω. Again, the improvement over a perfect match at resonance
is modest.
It is interesting to compare the 2:1 SWR bandwidth
(at the transmitter end of the line) for the case of a lossless
line versus the case of 2-dB matched-line loss. BW2 for
the lossy line case is 1.95 times that of the lossless line
case, so substantial match bandwidth improvement occurs,
but at the cost of considerable loss. The band edge loss
for the lossy-line case is 3 dB!
This example is provided mostly as a reference for
comparison with the more desirable broadbanding
techniques below. It also explains why some installations
with long feed lines show large match bandwidths.
Så, har jag glömt något om missmatchning och förluster så hoppa gärna på mig!
Nu är lekstugan stängd!!!!
/J