Message-ID: <20091228175004.z90hw4yyw0gg8cwc@webmail6.kuleuven.be>
Date: Mon, 28 Dec 2009 17:50:04 +0100
From: Rik Strobbe <Rik.Strobbe@fys.kuleuven.be>
To: "rsgb_lf_group@blacksheep.org" <rsgb_lf_group@blacksheep.org>
References: 	<38A51B74B884D74083D7950AD0DD85E828AD0B@File-Server-HST.hst.e-technik.tu-darmstadt.de>
In-Reply-To: 	<38A51B74B884D74083D7950AD0DD85E828AD0B@File-Server-HST.hst.e-technik.tu-darmstadt.de>
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Subject: Re: AW: AW: LF: Influence of a TX antenna to a small rx antenna
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Hello Stefan,

the problem with "big" antennas for RX is that they overload the =20
receiver and cause IMD effects. Most harmfull is 3rd order IMD and the =20
good thing with an attenuator between antenna and RX is that for every =20
dB attenuation the 3rd order IMD is reduced by 3dB. Therfor you may =20
want to increase attenuation as long as the external noise level is a =20
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 =20
RX sensivitity (although you will get little or no s-meter reading), =20
but IMD behaviour will improve by 100dB or more.

73, Rik  ON7YD - OR7T

Quoting Stefan Sch=E4fer <schaefer@hst.tu-darmstadt.de>:

> Hi Rik!
> Tnx for your explanations. So, perhaps my RX SNR could even be  =20
> increased by more attenuation, that would be fine.
> I thought the most of us using a separate RX antenna, like the  =20
> PA0RDT design or a receiving loop. But now it sounds, that some are  =20
> using the TX ant?
> Another question: If you say that there has just to be a little  =20
> difference in the noise between connected and disconnected antenna,  =20
> I think you mean, that the difference has to be as small as  =20
> possible? Because, the case that there is a difference, occurs at  =20
> all attenuation levels from 0 dB to a maximum, say 60db. So 0 dB  =20
> would also fulfill this statement, of course. But if the difference  =20
> should be as small as possible, what about the internal noise of the =20
>  RX system? Is the part of this internal noise in that case not to  =20
> much relevant? Or in other words, doesn't it decrease the SNR, if  =20
> the part of the received signal is small compared to the RX-internal =20
>  noise? I thought, the level of the received signal should be as =20
> HIGH  as possible, so that the input stage is just not overdriven!?
>
> 73, Stefan/DK7FC
>
> -----Urspr=FCngliche Nachricht-----
> Von: owner-rsgb_lf_group@blacksheep.org  =20
> [mailto:owner-rsgb_lf_group@blacksheep.org] Im Auftrag von Rik Strobbe
> Gesendet: Montag, 28. Dezember 2009 15:23
> An: rsgb_lf_group@blacksheep.org
> 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
>
>
>
>