Message-ID: <50731B6C.2060506@iup.uni-heidelberg.de>
Date: Mon, 08 Oct 2012 20:29:00 +0200
From: =?ISO-8859-15?Q?Stefan_Sch=E4fer?= <Stefan.Schaefer@iup.uni-heidelberg.de>
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To: rsgb_lf_group@blacksheep.org
Subject: LF: active antenna output termination?
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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