Message-ID: <000b01cda589$e82565c0$0501a8c0@xphd97xgq27nyf>
From: "mal hamilton" <g3kevmal@talktalk.net>
To: <rsgb_lf_group@blacksheep.org>
References: <50731B6C.2060506@iup.uni-heidelberg.de>
Date: Mon, 8 Oct 2012 19:19:58 -0000
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Subject: LF: Re: active antenna output termination?
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Stefan
Use a proper antenna for the job and forget about uProbes, ferrite rods etc
Your 30m coax is acting like one leg of the antenna the other is the probe
Sort of asymetrical dipole
g3kev

----- Original Message -----
From: "Stefan Schäfer" <Stefan.Schaefer@iup.uni-heidelberg.de>
To: <rsgb_lf_group@blacksheep.org>
Sent: Monday, October 08, 2012 6:29 PM
Subject: LF: active antenna output termination?


> 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
>