From: Claudio Pozzi <smtp01@email.it>
To: rsgb_lf_group@blacksheep.org
Date: Tue, 9 Oct 2012 15:26:52 +0200
User-Agent: KMail/1.13.5 (Linux/2.6.32-5-686; KDE/4.4.5; i686; ; )
References: <50731B6C.2060506@iup.uni-heidelberg.de>
In-Reply-To: <50731B6C.2060506@iup.uni-heidelberg.de>
MIME-Version: 1.0
Message-Id: <201210091526.52793.smtp01@email.it>
Subject: Re: LF: active antenna output termination?
Content-Type: Text/Plain;
  charset="iso-8859-15"
Content-Transfer-Encoding: 7bit
Sender: owner-rsgb_lf_group@blacksheep.org
Precedence: bulk
Reply-To: rsgb_lf_group@blacksheep.org


On Monday 08 October 2012 20:29:00 you wrote:
> 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...

The emitter follower transistor have a very low output impedance, no problems 
for receiving purpose but...   if you use a long coaxial cable you connect a 
capacitance of about 100 pF/meter between output and ground. The circuit can 
become unstable and oscillate, I found that a 27 or 33 ohm resistor in series 
to the output help to kill the oscillation and the signal loss is a little 
less than 6 dB (voltage, 3dB power).

I's a good practice to put a few ohm resistor in series at the low impedance 
output devices (OP-AMP, emitter follower, 74HCxxx buffers etc) driving a 
capacitive load to prevent oscillation.

73

Claudio, ik2pii

-- 
Claudio Pozzi  -  Happy Linux User  -  http://www.qsl.net/ik2pii

 
 
 
 --
 Caselle da 1GB, trasmetti allegati fino a 3GB e in piu' IMAP, POP3 e SMTP autenticato? GRATIS solo con Email.it http://www.email.it/f
 
 Sponsor:
 Su MisterCupido.com puoi creare REGALI PERSONALIZZATI. Stampa le tue Foto su: Cuscini, Tazze, Puzzle, Peluche, Magliette, Poster... Consegne in tutta Italia in soli 2-3 giorni
 Clicca qui: http://adv.email.it/cgi-bin/foclick.cgi?mid=12385&d=9-10