Hi Stefan,
in many cases it is perfectly OK to have a singly terminated transmission line.
If your RG58 coax is terminated with 50+j0 ohms at the end, your active antenna
output stage will see 50+j0 ohms regardless of line length and frequency. Adding
a 50 ohm resistor in series at the antenna will make no difference other than
reducing the signal by 6dB.
Think of a "voltage stiff" class D TX; you don't want to put a real 50 ohms
resistor in series with the feedline as it will reduce the total TX output power
by 3dB. Half the output power will be wasted in the added resistor so only 25%
will reach the load. The efficiency would be worse than class A :-). The antenna
impedance specified for transmitters is not their actual internal output
impedance, it is the impedance (resistance) they want to "see" as a load.
A real "passive" antenna is a different story. It has an unavoidable internal
source resistance. If it happens to be R ohms, self resonant case, you want to
load it with R ohms to maximise power transfer (power matching). If it happens
to be R-jX ohms, you usually want to load it with R+jX ohms (conjugate match) to
remove reactance for max power transfer (loading coil comes to mind...).
Doubly terminated lines are useful in some applications, for example analog
video where a signal can bounce back and forth and create "ghost pictures" if
the RX end of the cable is not perfectly terminated. A well terminated TX end
would absorb the unintentional reflections in case the RX end of the cable is
not perect.
73
Johan SM6LKM
----
Stefan Schäfer wrote 2012-10-08 20:29:
> LF,
>
> Another question just comes up here:
>
> Actually a coax cable must be terminated with its impedance at the ends, i.e.
> 50
> Ohm for a RG58 cable. Otherwise there are standing waves or travelling waves
> which can cause to the RX or TX.
>
> On LF and MF we say that the wire length is always short compared to lambda.
> But
> what happens on a wideband RX antenna, which works up to several MHz?
>
> The length of the coax between my RX and the output of the active antenna is
> about 30m. This is lambda/4 for 2.5 MHz....
>
> So why do many active antenna circuits have an output impedance that is
> nearly 0 ?
>
> No problems to be expected from this? A 50 Ohm cable which is terminated with
> 50 Ohm at the end (here the RX) appears to the TX (here the output stage of
> the
> active antenna ) as an endless long cable or simply as a 50 Ohm load. So
> nothing
> will be reflected. Matching on the output stage is not interesting because
> there
> is no RF beeing radiated from the RX. Is this the correct explanation? This
> would be in contrast to a transceiver and antenna where the antenna appears as
> the TX for the RX and the antenna is the RX for the TX or in other words both
> are source and sink.
>
> Have i answered my question by myselfe or are there other things that must be
> mentioned?
> I'm asking basic questions which may appear trivial. But i bet not everyone
> can
> answer them :-)
>
> ...just measured the output impedance of my active antenna and found that it
> is
> nearly 0, i.e. the output voltage does remain nearly constant when connecting
> a
> 50 Ohm load. So i thought it could help to switch a 47 Ohm R in series but
> this
> lowers the signal levels by 6 dB...
>
> 73, Stefan/DK7FC
>
>
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