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
|
