Hello Stefan,
the problem with "big" antennas for RX is that they overload the
receiver and cause IMD effects. Most harmfull is 3rd order IMD and the
good thing with an attenuator between antenna and RX is that for every
dB attenuation the 3rd order IMD is reduced by 3dB. Therfor you may
want to increase attenuation as long as the external noise level is a
bit stronger (let's say 3 to 6dB) than the internal (RX) noise level.
As long as this is the case the attenuator will not affect the overal
RX sensivitity (although you will get little or no s-meter reading),
but IMD behaviour will improve by 100dB or more.
73, Rik ON7YD - OR7T
Quoting Stefan Schäfer <[email protected]>:
Hi Rik!
Tnx for your explanations. So, perhaps my RX SNR could even be
increased by more attenuation, that would be fine.
I thought the most of us using a separate RX antenna, like the
PA0RDT design or a receiving loop. But now it sounds, that some are
using the TX ant?
Another question: If you say that there has just to be a little
difference in the noise between connected and disconnected antenna,
I think you mean, that the difference has to be as small as
possible? Because, the case that there is a difference, occurs at
all attenuation levels from 0 dB to a maximum, say 60db. So 0 dB
would also fulfill this statement, of course. But if the difference
should be as small as possible, what about the internal noise of the
RX system? Is the part of this internal noise in that case not to
much relevant? Or in other words, doesn't it decrease the SNR, if
the part of the received signal is small compared to the RX-internal
noise? I thought, the level of the received signal should be as
HIGH as possible, so that the input stage is just not overdriven!?
73, Stefan/DK7FC
-----Ursprüngliche Nachricht-----
Von: [email protected]
[mailto:[email protected]] Im Auftrag von Rik Strobbe
Gesendet: Montag, 28. Dezember 2009 15:23
An: [email protected]
Betreff: Re: AW: LF: Influence of a TX antenna to a small rx antenna
Hello Stefan,
there is a easy way to check if you have too much attenuation or not:
Tune the RX to an unused freq (only noise/QRN) and disconnect the
antenna; if you can hear the difference between antenna / no antenna
then attenuation is not too much.
With a 11m high / 25m long lazy-L antenna at a quiet location I could
add 40dB attenuation and still hear the noise increasing when
connecting the antenna.
I guess with a kite-antenna 50 to 60dB might be a good value.
Of course it all depends also on the RX used, I use a Kenwood TS440
that seems to be sensitive at LF.
73, Rik ON7YD - OR7T
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