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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