Message-ID: <50732867.7070201@telia.com>
Date: Mon, 08 Oct 2012 21:24:23 +0200
From: "Johan H. Bodin" <jh.bodin@telia.com>
User-Agent: Mozilla/5.0 (Windows NT 5.1; rv:15.0) Gecko/20120907 Thunderbird/15.0.1
MIME-Version: 1.0
To: rsgb_lf_group@blacksheep.org
References: <50731B6C.2060506@iup.uni-heidelberg.de>
In-Reply-To: <50731B6C.2060506@iup.uni-heidelberg.de>
Subject: Re: LF: active antenna output termination?
Content-Type: text/plain; charset=ISO-8859-15
Content-Transfer-Encoding: Quoted-Printable
Sender: owner-rsgb_lf_group@blacksheep.org
Precedence: bulk
Reply-To: rsgb_lf_group@blacksheep.org


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 a=
ntenna
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 oh=
ms
resistor in series with the feedline as it will reduce the total TX outpu=
t power
by 3dB. Half the output power will be wasted in the added resistor so onl=
y 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 inte=
rnal
source resistance. If it happens to be R ohms, self resonant case, you wa=
nt to
load it with R ohms to maximise power transfer (power matching). If it ha=
ppens
to be R-jX ohms, you usually want to load it with R+jX ohms (conjugate ma=
tch) to
remove reactance for max power transfer (loading coil comes to mind...).

Doubly terminated lines are useful in some applications, for example anal=
og
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 cabl=
e is
not perect.

73
Johan SM6LKM

----

Stefan Sch=E4fer wrote 2012-10-08 20:29:
> LF,
>=20
> Another question just comes up here:
>=20
> 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.
>=20
> On LF and MF we say that the wire length is always short compared to la=
mbda. But
> what happens on a wideband RX antenna, which works up to several MHz?
>=20
> The length of the coax between my RX and the output of the active anten=
na is
> about 30m. This is lambda/4 for 2.5 MHz....
>=20
> So why do many active antenna circuits have an output impedance that is=
 nearly 0 ?
>=20
> No problems to be expected from this?  A 50 Ohm cable which is terminat=
ed with
> 50 Ohm at the end (here the RX) appears to the TX (here the output stag=
e of the
> active antenna ) as an endless long cable or simply as a 50 Ohm load. S=
o nothing
> will be reflected. Matching on the output stage is not interesting beca=
use 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 app=
ears as
> the TX for the RX and the antenna is the RX for the TX or in other word=
s both
> are source and sink.
>=20
> Have i answered my question by myselfe or are there other things that m=
ust be
> mentioned?
> I'm asking basic questions which may appear trivial. But i bet not ever=
yone can
> answer them :-)
>=20
> ...just measured the output impedance of my active antenna and found th=
at it is
> nearly 0, i.e. the output voltage does remain nearly constant when conn=
ecting 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...
>=20
> 73, Stefan/DK7FC
>=20
>=20