Message-ID: <4E42CABA.3020804@freenet.de>
Date: Wed, 10 Aug 2011 20:15:22 +0200
From: wolf_dl4yhf <dl4yhf@freenet.de>
User-Agent: Mozilla/5.0 (Windows NT 5.1; rv:5.0) Gecko/20110624 Thunderbird/5.0
MIME-Version: 1.0
To: rsgb_lf_group@blacksheep.org
References: <4E418609.6020500@iup.uni-heidelberg.de> <67A6F7BF45BF4A0193A3DCB53000A283@PcMinto> <008401cc56ce$2f1fb2c0$0401a8c0@xphd97xgq27nyf> <12C475F3F4C84B818461753F2E8A60A6@PcMinto> <4E41AECB.90808@iup.uni-heidelberg.de> <8D68749D37B94275855FDBA46A3F6C97@PcMinto> <4E427DFB.50801@iup.uni-heidelberg.de> <CAA8k23SzJFi2tGfkN0fSRMi9bCwa_63uWiAZSV3P+dxcKfuSpg@mail.gmail.com>
In-Reply-To: <CAA8k23SzJFi2tGfkN0fSRMi9bCwa_63uWiAZSV3P+dxcKfuSpg@mail.gmail.com>
Subject: Re: LF: Re: HB9ASB...
Content-Type: multipart/alternative;
 boundary="------------030805010806020909040607"
Sender: owner-rsgb_lf_group@blacksheep.org
Precedence: bulk
Reply-To: rsgb_lf_group@blacksheep.org
X-AOL-SCOLL-URL_COUNT: 0  
x-aol-sid: 3039ac1d60d74e42cb2c5f9a
X-AOL-IP: 195.171.43.25
X-AOL-SPF: domain : blacksheep.org SPF : none


This is a multi-part message in MIME format.
--------------030805010806020909040607
Content-Type: text/plain; charset=windows-1252; format=flowed
Content-Transfer-Encoding: 8bit

Hi Andy and group,

Thanks for the detailed description. Knocking on open doors here.

Actually not a passive 180° combiner, but this 'active differential 
antenna' did a great job on VLF here, before it was - guess what- 
replaced with a E-field monopole antenna, extra counterpoise (remote 
earth point), and galvanic decoupled DC feed:

http://www.vlf.it/cr/differential_ant.htm


My own version of it, and the setup used at the moment (at the bottom of 
the page):

http://www.qsl.net/dl4yhf/vlf_reception/VLF_Receivers.html

P.S: the 200 kHz "wide-spaced" DC/DC converter with toroid core has been 
replaced with a (kind of) 'Austin Transformer', using an electrically 
shielded 50 Hz toroid transformer.

Cheers,
    Wolf .





Am 10.08.2011 17:30, schrieb Andy Talbot:
> I'm surprised so many seem to misunderstand how high impedance E-field 
> probes work.  There is too much hearsay and false comments being 
> peddled.   Stop thinking of the feeder as 'part' of the antenna as if 
> it were all one entity and consider each stage.... And forget that 
> thing called Ground completely.
> It starts with a high input-impedance amplifier which must have two 
> connections - ignore any common reference for now, it just has two 
> input pins.  A voltage is imposed across these from any antenna with 
> two ports,  like a short dipole.   This input voltage is buffered, 
> amplified,  and sent to the two output pins from which it travels down 
> a feeder - balanced, coax, or whatever -  and into your receiver.   So 
> far we haven't made any connection between input and output, and they 
> could (and ideally would) be independent and isolated from eachother
> However, they're not.   One input connection is usually common with 
> one output - usually the  0V DC supply pin and the reference  (and 
> please note, I am not referring to this as ground; ground is taboo, a 
> dirty word,  and will not be mentioned)    Which means the other 
> hot-side input pin now has a voltage imposed on it with respect to the 
> reference.  This will probably come from a probe antenna which is 
> coupling to the E-field of a radiated signal.   Now, bear in mind any 
> antenna must have two output ports, so where is the other side?   As 
> the reference input pin is connected to the output reference pin, any 
> other connection to this point will form the other 'half' of the high 
> impedance short dipole antenna.
> Now, the feeder dropping down from the amplifier / probe assembly (or 
> across, or up and over, or buried in the soil [see, still didn't say 
> the taboo word] ) is connected to the input reference pin by virtue of 
> the amplifiers internally linking them, and must therefore form the 
> other half of the dipole.   This is bad.   We have a hi-Z dipole, with 
> one short element in the air where it should be, and the other element 
> being one conductor of a probably long length of feeder with its end 
> connected to we know-not-what.
> So what do we now have?  A long assymetric dipole plus amplifier 
> assembly, stretching from some arbitrary shack connection up into free 
> space.   The voltage at the not-middle of this is the stuff that is 
> amplified and fed to the receiver.    So, as the bulk of the dipole is 
> close to the shack expect the majority of signal received to be 
> locally generated noise.
> That is the case for a completely unscreened and un-earthed (still 
> didn't use the word :-)  system.  Picking up any locally generated 
> E-fields at high levels onto the dipole.
> Now place the receiver and the rest of the local world with all its 
> noise and QRM generators in a screened room and make the feed coaxial 
> through a bulkhead connector into teh screened room and see what 
> happens.   All the QRM is contained within the screened room and 
> cannot pass outside.    The antenna is now a strange sort-of dipole 
> with one end in free space, and the other connected to a solid mass 
> made from the outside of the screened room.   It can't pick up any QRM 
> from inside the screened room, and all it will now receive are signals 
> generating an E-field between the probe and the solid mass.  Perfect!
> The shack and the rest of the world is not in a screened room, so 
> there lies the problem.   However much you don't want it to be, with 
> this setup the *feeder will always form the majority of the antenna 
> because there is nothing else there that can be* the other half of the 
> dipole.   And if the bottom end of the dipole intrudes into the rest 
> of the world, it will pick up stuff it shouldn't
> Now, lets control the antenna structure all on its own and pretend the 
> feeder is not there at all.  A true dipole would be nice although 
> impractical for now, so lets go back to the classic monopole which is 
> one half of a dipole, and the other half formed by the reflection of 
> this in a mirror, which is usually the surface of this planet called 
> Earth.   Mount the amplifier assembly on a conductive pole with the 
> bottom end firmly connected to the surface of the mirror.  Now, we 
> have a dipole formed of the complete assembly (mast plus probe) with 
> its reflection.  It is not fed in the middle , but assymetrically near 
> the top where the short probe forms the other side from teh feed 
> point.   The total length of the dipole is now twice the total height 
> of supporting mast and probe.   As we're using a high impedance input 
> amplifier, if it nad a truely infinite input Z and zero 
> capacitance,  the actual feed point wouldn't matter, it would always 
> get the same voltage imposed wherever the non-symmetrical spilt 
> occurred.  As Z and C is finite it does make a difference so longer 
> probes help with practical amplifiers - but this is digressing.
> The antenna so far is ideal, and is no more prone to picking up local 
> QRM than any other antenna would be. But now we have to connect this 
> thing called a feeder, which introduces another arm to the dipole (a 
> tripole now perhaps ) which will ultimately go close to bad places.   
> If we could isolate the feeder by inserting a high common mode 
> impedance this would do the job, but it would have to be a near 
> infinite common-mode impedance, so is definitely not on.    A  
> transformer coupling would help, but even that has high (relatively 
> speaking) capacitance across its windings, so will still leak common 
> mode rubbish, gettign worse at higher frequencies.  And there is still 
> the DC power issue
> One solution is to bring the feeder down INSIDE the pole which must be 
> firmly connected to the reflecting surface of the mirror, maintain it 
> buried under the mirror's surface for as long as possible before it 
> sees the Badlands and hope that burying it will decouple any local 
> QRM.   Or make the feeder non conductive like optical fibre.  But 
> there is still the DC power issue.
> Which shows where the problems lie, and where to start thinking about 
> how to stop them.    Separate dedicated mast firmly connected to the 
> mirror's surface.  Feeder fed down inside it and buried.   If coaxial 
> feeder is used -  connect the braid at both ends to the mirror's 
> surface and bring in to the receiver input port coaxially and 
> screened.   DC likewise - send it up the feeder.
> One solution, but its a difficult bit of hardware to build well, would 
> be a true differential input high impedance amplifier with a proper 
> equal length short dipole.   The differential input - if ideal - will 
> inherently provide isolation between input and output ports but it 
> does have to be a perfect differential input, balanced antenna and so 
> on.   Someone did mention to me once about using two identical active 
> antennas mounted end to end to form a a dipole, with the outputs from 
> each combined in a 180 degree hybrid combiner.  That could prove 
> viable, but don't know if he ever tried it.
> Phew....!
> Andy
> www.g4jnt.com <http://www.g4jnt.com/>
>
> 2011/8/10 Stefan Schäfer <Stefan.Schaefer@iup.uni-heidelberg.de 
> <mailto:Stefan.Schaefer@iup.uni-heidelberg.de>>
>
>     Hi Minto,
>
>     Am 10.08.2011 13:10, schrieb Minto Witteveen:
>
>         Hi Stefan, (et al)
>
>         Well I beg to differ.. :-)
>         What I think happens is this: The outside of the coax picks up
>         electromagnetic radiation like any antenna (including QRM
>         generated by fluorescent lamps and Alinco switching power
>         supplies). This signal travels along the coax to the Miniwhip.
>         (also in the direction of the receiver but that is not
>         important here as the signal is on the outside of the coax).
>         Upon arrival at the miniwhip this signal on the outside of the
>         coax has nowhere to go ­but to the _inside_ of the outer
>         mantle of the coax – it ‘rounds the corner’ at the end of the
>         coax so to speak.
>
>     I think the mechanism is that the unwanted signal on the screen
>     causes a potential difference between gate and source of the first
>     (J)FET. So this causes a current flow in the output stage and so a
>     signal at the RX input.
>     A common mode choke between RX and the antenna ground should form
>     a low pass filter for unwanted signals coming from the shack.
>     Using a common mode choke without a local ground should have
>     little effect, except the coax is some 100m long (between choke
>     and probe) ;-)
>
>     Ah BTW regarding the discussion "the cable to the E field probe is
>     the actual antenna": One could just try what happens if one
>     disconnects the power supply. If the signal is still present then
>     the cable is the antenna, if the signal is gone: The probe must be
>     the antenna. Isn't it?! :-)
>
>
>
>         So how to avoid the QRM that is picked up by the coax to
>         ‘travel back’ via the inside: for the miniwhip it is indeed
>         best (as Roelof mentioned) to short these signals to earth
>         _outside_ the house, preferably as close to the miniwhip as
>         possible. Grounding there would to the trick, aided by a
>         (large enough) common mode choke between the ground point and
>         the house.  The QRM that is picked up in the house would be –
>         after attenuation by the choke - directed into the ground and
>         not up into the pole and the miniwhip.
>
>
>     Yes yes, totally agreed.
>
>         Whatever happens in the house would then be largely
>         irrelevant. Adding a common mode choke close to the rig will
>         do little extra. (it would only attenuate QRM getting from the
>         shack’s earth system to the outside of the coax).
>
>     It would almost have the same effect (when ignoring the C between
>     cable and ground along to the choke near the antenna ground) as
>     placing the choke near the antenna ground, both are in series and
>     increase the current reducing impedance, yes...
>
>
>         Any signals picked up by the vertical coax between the
>         earthing point and the whip will add to the received signal,
>         but at low frequencies it will not be much.
>         So far for theory. Now the proof of the pudding: DCF39 is now
>         > S9+40 dB. My old trusty QRM generator (Alinco SMPS)
>         generates S9+25 at 135.500. When I switch off the miniwhip
>         (cut the power) DCF39 drops down to just above the noise
>         floor. As expected.
>
>     Ah yes, that's what i meant above (should have read your mail
>     completely before answering ;-) ). This is the proof that Mal
>     cannot be right when saying "the coax is the actual antenna".
>
>         But the Alinco signal only drops down some 15 dB and remains
>         the only signal that is audible. This is exactly what I would
>         expect: the QRM travels along the outside of the coax to the
>         miniwhip, ‘rounds the corner’ and comes back via the inside of
>         the coax shield. Further proof that it indeed takes this
>         route: if I disconnect the coax in the shack the Alinco smps
>         signal disappears also (so it is not received via any other path).
>
>     Hm, i rather expect a galvanic coupling i.e. stray currenty on the
>     supply cable of the RX. What happens if you run the RX on
>     batteries? The same dependency?
>     There could be several reasons apply here...
>
>
>         Last year I already bought 3 meters of copper pipe to drive
>         into the ground in the backyard. Bet never got around to
>         finish the job…
>
>     Today it's nice WX here! And in NL?
>
>
>         The main reason the signal strength is much higher with the
>         elevated miniwhip is (I think) caused by the fact that I am
>         surrounded by other houses, gardens, trees etc. Not comparable
>         with an open field…
>
>     Yes.
>
>     73, Stefan /DK7FC
>
>
>
>
>         Regards,
>         Minto pa3bca
>
>
>
>
>
>
>         --------------------------------------------------------------------------
>
>         Ceterum censeo Carthaginem delendam esse
>         -----Original Message----- From: Stefan Schäfer
>         Sent: Wednesday, August 10, 2011 00:03
>         To: rsgb_lf_group@blacksheep.org
>         <mailto:rsgb_lf_group@blacksheep.org>
>         Subject: Re: LF: Re: HB9ASB...
>
>         Hi Minto,
>
>         Am 09.08.2011 22:48, schrieb Minto Witteveen:
>
>             You are right w.r.t. the cable being (a significant) part
>             of the working of the Miniwhip antenna. [...]
>
>
>         I don't think so. There should be no difference between a 5m
>         and 10m
>         long cable. I think about a capacitive divider. The probe has
>         about 3
>         pF, that's one plate of the C. The other one is the cable and
>         metal
>         connected. Once if this part of the C has say >10 * 3 pF, the
>         difference
>         between longer cables become smaller and smaller.
>
>         I think it is just the S/N that rises due to lower becoming
>         noise and
>         higher signal levels. On a flat field without trees and
>         houses, you have
>         excellent reception even with a 2m pole :-)
>
>         73, Stefan
>
>
>


--------------030805010806020909040607
Content-Type: text/html; charset=windows-1252
Content-Transfer-Encoding: 8bit

<html>
  <head>
    <meta content="text/html; charset=windows-1252"
      http-equiv="Content-Type">
  </head>
  <body bgcolor="#FFFFFF" text="#000000">
    Hi Andy and group,<br>
    <br>
    Thanks for the detailed description. Knocking on open doors here.<br>
    <br>
    Actually not a passive 180° combiner, but this 'active differential
    antenna' did a great job on VLF here, before it was - guess what-
    replaced with a E-field monopole antenna, extra counterpoise (remote
    earth point), and galvanic decoupled DC feed:<br>
    <br>
    <a class="moz-txt-link-freetext" href="http://www.vlf.it/cr/differential_ant.htm">http://www.vlf.it/cr/differential_ant.htm</a><br>
    <br>
    <br>
    My own version of it, and the setup used at the moment (at the
    bottom of the page):<br>
    <br>
    <a class="moz-txt-link-freetext" href="http://www.qsl.net/dl4yhf/vlf_reception/VLF_Receivers.html">http://www.qsl.net/dl4yhf/vlf_reception/VLF_Receivers.html</a><br>
    <br>
    P.S: the 200 kHz "wide-spaced" DC/DC converter with toroid core has
    been replaced with a (kind of) 'Austin Transformer', using an
    electrically shielded 50 Hz toroid transformer.<br>
    <br>
    Cheers,<br>
       Wolf .<br>
    <br>
    <br>
    <br>
    <br>
    <br>
    Am 10.08.2011 17:30, schrieb Andy Talbot:
    <blockquote
cite="mid:CAA8k23SzJFi2tGfkN0fSRMi9bCwa_63uWiAZSV3P+dxcKfuSpg@mail.gmail.com"
      type="cite">
      <div>I'm surprised so many seem to misunderstand how high
        impedance E-field probes work.  There is too much hearsay and
        false comments being peddled.   Stop thinking of the feeder as
        'part' of the antenna as if it were all one entity and
        consider each stage.... And forget that thing called Ground
        completely.</div>
      <div> </div>
      <div>It starts with a high input-impedance amplifier which must
        have two connections - ignore any common reference for now, it
        just has two input pins.  A voltage is imposed across these from
        any antenna with two ports,  like a short dipole.   This input
        voltage is buffered, amplified,  and sent to the two output pins
        from which it travels down a feeder - balanced, coax, or
        whatever -  and into your receiver.   So far we haven't made any
        connection between input and output, and they could (and ideally
        would) be independent and isolated from eachother</div>
      <div> </div>
      <div>However, they're not.   One input connection is usually
        common with one output - usually the  0V DC supply pin and the
        reference  (and please note, I am not referring to this as
        ground; ground is taboo, a dirty word,  and will not be
        mentioned)    Which means the other hot-side input pin now has a
        voltage imposed on it with respect to the reference.  This will
        probably come from a probe antenna which is coupling to the
        E-field of a radiated signal.   Now, bear in mind any antenna
        must have two output ports, so where is the other side?   As the
        reference input pin is connected to the output reference pin,
        any other connection to this point will form the other 'half' of
        the high impedance short dipole antenna.</div>
      <div> </div>
      <div>Now, the feeder dropping down from the amplifier / probe
        assembly (or across, or up and over, or buried in the soil [see,
        still didn't say the taboo word] ) is connected to the input
        reference pin by virtue of the amplifiers internally linking
        them, and must therefore form the other half of the dipole.  
        This is bad.   We have a hi-Z dipole, with one short element in
        the air where it should be, and the other element being one
        conductor of a probably long length of feeder with its end
        connected to we know-not-what.</div>
      <div> </div>
      <div>So what do we now have?  A long assymetric dipole plus
        amplifier assembly, stretching from some arbitrary shack
        connection up into free space.   The voltage at the not-middle
        of this is the stuff that is amplified and fed to the
        receiver.    So, as the bulk of the dipole is close to the shack
        expect the majority of signal received to be locally generated
        noise.   </div>
      <div> </div>
      <div>That is the case for a completely unscreened and un-earthed
        (still didn't use the word :-)  system.  Picking up any locally
        generated E-fields at high levels onto the dipole.  </div>
      <div> </div>
      <div>Now place the receiver and the rest of the local world with
        all its noise and QRM generators in a screened room and make the
        feed coaxial through a bulkhead connector into teh screened
        room and see what happens.   All the QRM is contained within the
        screened room and cannot pass outside.    The antenna is now a
        strange sort-of dipole with one end in free space, and the other
        connected to a solid mass made from the outside of the screened
        room.   It can't pick up any QRM from inside the screened room,
        and all it will now receive are signals generating an E-field
        between the probe and the solid mass.  Perfect!</div>
      <div> </div>
      <div>The shack and the rest of the world is not in a screened
        room, so there lies the problem.   However much you don't want
        it to be, with this setup the <strong>feeder will always form
          the majority of the antenna because there is nothing else
          there that can be</strong> the other half of the dipole.   And
        if the bottom end of the dipole intrudes into the rest of the
        world, it will pick up stuff it shouldn't</div>
      <div> </div>
      <div>Now, lets control the antenna structure all on its own and
        pretend the feeder is not there at all.  A true dipole would be
        nice although impractical for now, so lets go back to the
        classic monopole which is one half of a dipole, and the other
        half formed by the reflection of this in a mirror, which is
        usually the surface of this planet called Earth.   Mount the
        amplifier assembly on a conductive pole with the bottom end
        firmly connected to the surface of the mirror.  Now, we have a
        dipole formed of the complete assembly (mast plus probe) with
        its reflection.  It is not fed in the middle , but
        assymetrically near the top where the short probe forms the
        other side from teh feed point.   The total length of the dipole
        is now twice the total height of supporting mast and probe.   As
        we're using a high impedance input amplifier, if it nad a truely
        infinite input Z and zero capacitance,  the actual feed point
        wouldn't matter, it would always get the same voltage imposed
        wherever the non-symmetrical spilt occurred.  As Z and C is
        finite it does make a difference so longer probes help with
        practical amplifiers - but this is digressing.</div>
      <div> </div>
      <div>The antenna so far is ideal, and is no more prone to picking
        up local QRM than any other antenna would be. But now we have to
        connect this thing called a feeder, which introduces another arm
        to the dipole (a tripole now perhaps ) which will ultimately go
        close to bad places.   If we could isolate the feeder by
        inserting a high common mode impedance this would do the job,
        but it would have to be a near infinite common-mode impedance,
        so is definitely not on.    A  transformer coupling would help,
        but even that has high (relatively speaking) capacitance across
        its windings, so will still leak common mode rubbish, gettign
        worse at higher frequencies.  And there is still the DC power
        issue</div>
      <div> </div>
      <div>One solution is to bring the feeder down INSIDE the pole
        which must be firmly connected to the reflecting surface of the
        mirror, maintain it buried under the mirror's surface for as
        long as possible before it sees the Badlands and hope that
        burying it will decouple any local QRM.   Or make the feeder non
        conductive like optical fibre.  But there is still the DC power
        issue.</div>
      <div> </div>
      <div>Which shows where the problems lie, and where to start
        thinking about how to stop them.    Separate dedicated mast
        firmly connected to the mirror's surface.  Feeder fed down
        inside it and buried.   If coaxial feeder is used -  connect the
        braid at both ends to the mirror's surface and bring in to
        the receiver input port coaxially and screened.   DC likewise -
        send it up the feeder.</div>
      <div> </div>
      <div>One solution, but its a difficult bit of hardware to build
        well, would be a true differential input high impedance
        amplifier with a proper equal length short dipole.   The
        differential input - if ideal - will inherently provide
        isolation between input and output ports but it does have to be
        a perfect differential input, balanced antenna and so on.  
        Someone did mention to me once about using two identical active
        antennas mounted end to end to form a a dipole, with the outputs
        from each combined in a 180 degree hybrid combiner.  That could
        prove viable, but don't know if he ever tried it.</div>
      <div> </div>
      <div>Phew....!</div>
      <div> </div>
      <div>Andy</div>
      <div><a moz-do-not-send="true" href="http://www.g4jnt.com/">www.g4jnt.com</a></div>
      <div><br>
         </div>
      <div> </div>
      <div> </div>
      <div> </div>
      <div> </div>
      <div class="gmail_quote">2011/8/10 Stefan Schäfer <span dir="ltr">&lt;<a
            moz-do-not-send="true"
            href="mailto:Stefan.Schaefer@iup.uni-heidelberg.de">Stefan.Schaefer@iup.uni-heidelberg.de</a>&gt;</span><br>
        <blockquote style="BORDER-LEFT: #ccc 1px solid; MARGIN: 0px 0px
          0px 0.8ex; PADDING-LEFT: 1ex" class="gmail_quote">Hi Minto,<br>
          <br>
          Am 10.08.2011 13:10, schrieb Minto Witteveen:
          <div class="im"><br>
            <blockquote style="BORDER-LEFT: #ccc 1px solid; MARGIN: 0px
              0px 0px 0.8ex; PADDING-LEFT: 1ex" class="gmail_quote">Hi
              Stefan, (et al)<br>
              <br>
              Well I beg to differ.. :-)<br>
              What I think happens is this: The outside of the coax
              picks up electromagnetic radiation like any antenna
              (including QRM generated by fluorescent lamps and Alinco
              switching power supplies). This signal travels along the
              coax to the Miniwhip. (also in the direction of the
              receiver but that is not important here as the signal is
              on the outside of the coax).<br>
              Upon arrival at the miniwhip this signal on the outside of
              the coax has nowhere to go ­but to the _inside_ of the
              outer mantle of the coax – it ‘rounds the corner’ at the
              end of the coax so to speak.<br>
            </blockquote>
          </div>
          I think the mechanism is that the unwanted signal on the
          screen causes a potential difference between gate and source
          of the first (J)FET. So this causes a current flow in the
          output stage and so a signal at the RX input.<br>
          A common mode choke between RX and the antenna ground should
          form a low pass filter for unwanted signals coming from the
          shack. Using a common mode choke without a local ground should
          have little effect, except the coax is some 100m long (between
          choke and probe) ;-)<br>
          <br>
          Ah BTW regarding the discussion "the cable to the E field
          probe is the actual antenna": One could just try what happens
          if one disconnects the power supply. If the signal is still
          present then the cable is the antenna, if the signal is gone:
          The probe must be the antenna. Isn't it?! :-)
          <div class="im"><br>
            <br>
            <br>
            <blockquote style="BORDER-LEFT: #ccc 1px solid; MARGIN: 0px
              0px 0px 0.8ex; PADDING-LEFT: 1ex" class="gmail_quote">So
              how to avoid the QRM that is picked up by the coax to
              ‘travel back’ via the inside: for the miniwhip it is
              indeed best (as Roelof mentioned) to short these signals
              to earth _outside_ the house, preferably as close to the
              miniwhip as possible. Grounding there would to the trick,
              aided by a (large enough) common mode choke between the
              ground point and the house.  The QRM that is picked up in
              the house would be – after attenuation by the choke -
              directed into the ground and not up into the pole and the
              miniwhip.<br>
            </blockquote>
            <br>
          </div>
          Yes yes, totally agreed.
          <div class="im"><br>
            <blockquote style="BORDER-LEFT: #ccc 1px solid; MARGIN: 0px
              0px 0px 0.8ex; PADDING-LEFT: 1ex" class="gmail_quote">Whatever
              happens in the house would then be largely irrelevant.
              Adding a common mode choke close to the rig will do little
              extra. (it would only attenuate QRM getting from the
              shack’s earth system to the outside of the coax).<br>
            </blockquote>
          </div>
          It would almost have the same effect (when ignoring the C
          between cable and ground along to the choke near the antenna
          ground) as placing the choke near the antenna ground, both are
          in series and increase the current reducing impedance, yes...
          <div class="im"><br>
            <br>
            <blockquote style="BORDER-LEFT: #ccc 1px solid; MARGIN: 0px
              0px 0px 0.8ex; PADDING-LEFT: 1ex" class="gmail_quote">Any
              signals picked up by the vertical coax between the
              earthing point and the whip will add to the received
              signal, but at low frequencies it will not be much.<br>
              So far for theory. Now the proof of the pudding: DCF39 is
              now &gt; S9+40 dB. My old trusty QRM generator (Alinco
              SMPS) generates S9+25 at 135.500. When I switch off the
              miniwhip (cut the power) DCF39 drops down to just above
              the noise floor. As expected.<br>
            </blockquote>
          </div>
          Ah yes, that's what i meant above (should have read your mail
          completely before answering ;-) ). This is the proof that Mal
          cannot be right when saying "the coax is the actual antenna".
          <div class="im"><br>
            <blockquote style="BORDER-LEFT: #ccc 1px solid; MARGIN: 0px
              0px 0px 0.8ex; PADDING-LEFT: 1ex" class="gmail_quote">But
              the Alinco signal only drops down some 15 dB and remains
              the only signal that is audible. This is exactly what I
              would expect: the QRM travels along the outside of the
              coax to the miniwhip, ‘rounds the corner’ and comes back
              via the inside of the coax shield. Further proof that it
              indeed takes this route: if I disconnect the coax in the
              shack the Alinco smps signal disappears also (so it is not
              received via any other path).<br>
            </blockquote>
          </div>
          Hm, i rather expect a galvanic coupling i.e. stray currenty on
          the supply cable of the RX. What happens if you run the RX on
          batteries? The same dependency?<br>
          There could be several reasons apply here...
          <div class="im"><br>
            <br>
            <blockquote style="BORDER-LEFT: #ccc 1px solid; MARGIN: 0px
              0px 0px 0.8ex; PADDING-LEFT: 1ex" class="gmail_quote">Last
              year I already bought 3 meters of copper pipe to drive
              into the ground in the backyard. Bet never got around to
              finish the job…<br>
            </blockquote>
          </div>
          Today it's nice WX here! And in NL?
          <div class="im"><br>
            <blockquote style="BORDER-LEFT: #ccc 1px solid; MARGIN: 0px
              0px 0px 0.8ex; PADDING-LEFT: 1ex" class="gmail_quote"><br>
              The main reason the signal strength is much higher with
              the elevated miniwhip is (I think) caused by the fact that
              I am surrounded by other houses, gardens, trees etc. Not
              comparable with an open field…<br>
            </blockquote>
          </div>
          Yes.<br>
          <br>
          73, Stefan /DK7FC
          <div>
            <div class="h5"><br>
              <br>
              <br>
              <blockquote style="BORDER-LEFT: #ccc 1px solid; MARGIN:
                0px 0px 0px 0.8ex; PADDING-LEFT: 1ex"
                class="gmail_quote"><br>
                Regards,<br>
                Minto pa3bca<br>
                <br>
                <br>
                <br>
                <br>
                <br>
                <br>
                --------------------------------------------------------------------------
                <br>
                Ceterum censeo Carthaginem delendam esse<br>
                -----Original Message----- From: Stefan Schäfer<br>
                Sent: Wednesday, August 10, 2011 00:03<br>
                To: <a moz-do-not-send="true"
                  href="mailto:rsgb_lf_group@blacksheep.org"
                  target="_blank">rsgb_lf_group@blacksheep.org</a><br>
                Subject: Re: LF: Re: HB9ASB...<br>
                <br>
                Hi Minto,<br>
                <br>
                Am 09.08.2011 22:48, schrieb Minto Witteveen:<br>
                <blockquote style="BORDER-LEFT: #ccc 1px solid; MARGIN:
                  0px 0px 0px 0.8ex; PADDING-LEFT: 1ex"
                  class="gmail_quote">You are right w.r.t. the cable
                  being (a significant) part of the working of the
                  Miniwhip antenna. [...]<br>
                </blockquote>
                <br>
                I don't think so. There should be no difference between
                a 5m and 10m<br>
                long cable. I think about a capacitive divider. The
                probe has about 3<br>
                pF, that's one plate of the C. The other one is the
                cable and metal<br>
                connected. Once if this part of the C has say &gt;10 * 3
                pF, the difference<br>
                between longer cables become smaller and smaller.<br>
                <br>
                I think it is just the S/N that rises due to lower
                becoming noise and<br>
                higher signal levels. On a flat field without trees and
                houses, you have<br>
                excellent reception even with a 2m pole :-)<br>
                <br>
                73, Stefan<br>
              </blockquote>
              <br>
            </div>
          </div>
        </blockquote>
      </div>
      <br>
    </blockquote>
    <br>
  </body>
</html>

--------------030805010806020909040607--