Message-ID: <5074294B.80807@iup.uni-heidelberg.de>
Date: Tue, 09 Oct 2012 15:40:27 +0200
From: =?ISO-8859-15?Q?Stefan_Sch=E4fer?= <Stefan.Schaefer@iup.uni-heidelberg.de>
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References: <50731B6C.2060506@iup.uni-heidelberg.de> <50732867.7070201@telia.com>
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Subject: Re: LF: active antenna output termination?
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Thanks Johan for the confirmation.

Well, the latest tests and mearurements show that there are strong IM=20
when using no 50 Ohm termination on the RX end. The appear and=20
disappear, just visible all the day (not in the night however!), see=20
http://www.iup.uni-heidelberg.de/schaefer_vlf/DK7FC_MF_Grabber.html

Last night there were some IM but the behaviour was at least stable.=20
This was when using only my high Z input MF filter on the end of the=20
coax in the shack. However when connecting a 50 Ohm coaxial BNC=20
termination load, the band became quiet, i.e. the IM disappeared and the=20
levels dropped by more than 6 dB. This is strange since the probe output=20
behaves like a voltage source. AND, after about 10 seconds, even heavier=20
IM started to appear, continuously, until i removed the 50 Ohm load again.

My guess is that there are some selfe-oscillations starting in the probe=20
circuit, maybe coming from the DC decoupling. However i cannot measure=20
any stronger oscillation on the coax end in the shack...


Do you have some ideas?

Anyway, despite all the pro---- ehm challenges my MF reception is as=20
good as never before and i heared some new stations last night, very=20
exciting.  Uwe/DJ8WX was very strong during his calls to EI...

73, Stefan/DK7FC


Am 08.10.2012 21:24, schrieb Johan H. Bodin:
> Hi Stefan,
>
> in many cases it is perfectly OK to have a singly terminated transmissi=
on line.
> If your RG58 coax is terminated with 50+j0 ohms at the end, your active=
 antenna
> output stage will see 50+j0 ohms regardless of line length and frequenc=
y. Adding
> a 50 ohm resistor in series at the antenna will make no difference othe=
r than
> reducing the signal by 6dB.
>
> Think of a "voltage stiff" class D TX; you don't want to put a real 50 =
ohms
> resistor in series with the feedline as it will reduce the total TX out=
put power
> by 3dB. Half the output power will be wasted in the added resistor so o=
nly 25%
> will reach the load. The efficiency would be worse than class A :-). Th=
e antenna
> impedance specified for transmitters is not their actual internal outpu=
t
> impedance, it is the impedance (resistance) they want to "see" as a loa=
d.
>
> A real "passive" antenna is a different story. It has an unavoidable in=
ternal
> source resistance. If it happens to be R ohms, self resonant case, you =
want to
> load it with R ohms to maximise power transfer (power matching). If it =
happens
> to be R-jX ohms, you usually want to load it with R+jX ohms (conjugate =
match) to
> remove reactance for max power transfer (loading coil comes to mind...).
>
> Doubly terminated lines are useful in some applications, for example an=
alog
> video where a signal can bounce back and forth and create "ghost pictur=
es" 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 ca=
ble is
> not perect.
>
> 73
> Johan SM6LKM
>
> ----
>
> Stefan Sch=E4fer wrote 2012-10-08 20:29:
>   =20
>> LF,
>>
>> Another question just comes up here:
>>
>> Actually a coax cable must be terminated with its impedance at the end=
s, 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 l=
ambda. 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 ante=
nna is
>> about 30m. This is lambda/4 for 2.5 MHz....
>>
>> So why do many active antenna circuits have an output impedance that i=
s nearly 0 ?
>>
>> No problems to be expected from this?  A 50 Ohm cable which is termina=
ted with
>> 50 Ohm at the end (here the RX) appears to the TX (here the output sta=
ge 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 bec=
ause 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 ap=
pears as
>> the TX for the RX and the antenna is the RX for the TX or in other wor=
ds 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 eve=
ryone can
>> answer them :-)
>>
>> ...just measured the output impedance of my active antenna and found t=
hat it is
>> nearly 0, i.e. the output voltage does remain nearly constant when con=
necting 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
>>
>>
>>     =20
>   =20