Message-ID: <20091226232910.zvm80doogowog4sg@webmail6.kuleuven.be>
Date: Sat, 26 Dec 2009 23:29:10 +0100
From: Rik Strobbe <Rik.Strobbe@fys.kuleuven.be>
To: "rsgb_lf_group@blacksheep.org" <rsgb_lf_group@blacksheep.org>
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Subject: Re: LF: Influence of a TX antenna to a small rx antenna
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Hello Stefan,

why not use the "big" TX antenna also for RX. With a sufficient =20
atenuator of course.
On 137kHz I alway used my TX antenna for RX, but a 40dB attenuator was =20
required to avoid overload at the RX.

73, Rik  ON7YD

Quoting Stefan Sch=C3=A4fer <schaefer@hst.tu-darmstadt.de>:

> Dear Lowfers,
>
> Yesterday i saw the extreme influence of my 100m vertical tx antenna =20
>  to the small active antenna. Now I have some ideas how to avoid =20
> that  and would be thankful for any comments and hints.
>
> During the QSOs there was much QSB in the RX signals, just as if  =20
> someone is playing with the AF-volume. First I thought it is caused  =20
> by a loose contact in the Converter or in the fiber optic cable but  =20
> later I found, that it was caused by the angle of the tx antenna to  =20
> ground (Wind was good but with some QSB ;-) ).
>
> In my last QSO with IK1HSS I disconnected the TX wire from the  =20
> loading coil and than, the whole signal was almost gone (when  =20
> choosing =E2=80=9Cvisual mode LOW=E2=80=9D at the argo monitor). Switchi=
ng to  =20
> =E2=80=9Cvisual mode HIGH=E2=80=9D and increasing the AF a little bit ga=
ve almost  =20
> the same QRM lines by DFC39 (vy strong yesterday). Now, the SNR was  =20
> much better than before!
>
> In the following pic you can get a impression of the dimension of  =20
> the influence of the TX ant (The RX ant is isolated to the rest of  =20
> the rig by an optic fiber cable and battery supply, so =E2=80=9Cjust=E2=
=80=9D  =20
> coupled by the E field to the TX ant):
>
> [cid:image001.jpg@01CA865F.BA287870]
>
> In the left part the TX wire was disconnected from the coil and mode was=
 HIGH
>
> In the middle part I connected the wire to the loading coil
>
> In the right part I switched to the LOW mode!
>
> I am sure that the TX ant does not only increase the QRM but also  =20
> the wanted signal. So the benefit will not be so strong than between =20
>  left and right part in the pic. But the SNR increases =20
> significantly,  as I saw in the QSO with IK1HSS. So a disconnection =20
> of the TX wire  makes sense.
>
> What are the alternatives?
>
> Placing the RX ant apart from the TX ant to reduce the influence? I  =20
> think with a 100m vertical I can forget that unless I do not want to =20
>  spend 100s of meters of RX cable ;-)
>
> Bringing the ant out of resonance? That is the one and only solution =20
>  I think. Any better ideas?
>
> I could disconnect the loading coil at the lower end to ground. That =20
>  could be done with a relay (My RX/TX sequencer provides an output  =20
> =E2=80=9C12V @ RX=E2=80=9D that could directly be used for this). Probab=
ly there  =20
> will be a residual coupling capacity of the coil to ground and the  =20
> relay contacts and so on, say 30pF but since my ant C is 580pF that  =20
> would be quite enough for deadjusting, I hope so.
>
> As I thought about protecting the contacts and the PA in the case  =20
> that there will be a mistake (contacts have to be closed without a  =20
> voltage at the relay) in the circuit, i thought some Varistors or  =20
> even better some antiparallel diodes could be a good choice. That  =20
> brought me to the idea that a power diode out of a SMPS would easyly =20
>  handle the antenna current with fast enough switching times.
>
> So do you think just two antiparallel diodes (or perhaps 2x2, 2 in  =20
> series, and perhaps with an additional 10=E2=80=A6100k Resistor parallel=
)  =20
> without a relay could be suitable to solve the RX TX ant influence  =20
> problem? Could the TX signal distortion be neglected? When the  =20
> voltage across the diodes (RX case) is less than the forward  =20
> voltage, do the diodes act as an open contact, just with a few pF  =20
> reverse capacity? I think so but I don=E2=80=99t know much about the RX=
  =20
> voltages that are given by such an antenna. There will be some  =20
> experiments=E2=80=A6 :)
>
> Comments welcome ;-)
>
> 73, Stefan/DK7FC
>