Re: Re: LF: VK1OD's analysis of the MiniWhip antenna

[<jrusgrove@comcast.net>]

Marco

The problem is that we're trying to prevent the feedline from appearing at the INPUT of the e probe 
where the impedance is in the megohms. To be effective, the choke would need to exhibit 10's of 
megohms 'choking' impedance ... not very likely in the real world.
If one is not convinced that the portion of feedline above ground level is part of the antenna, I'd 
suggest mounting the mini whip on a ground screen at ground level and checking signal strengths.
Jay W1VD  WD2XNS  WE2XGR/2


----- Original Message ----- 
From: <marcocadeddu@tin.it>
To: <rsgb_lf_group@blacksheep.org>
Sent: Friday, July 05, 2013 11:13 AM
Subject: R: Re: LF: VK1OD's analysis of the MiniWhip antenna


> Hello LF,
>
> I'm happy to join the discussion although I'm probably the least
> qualified..
>
> The insulation, with a suitable choke, of the antenna from
> the feed line is sure a test which can help to determine if in the Mini
> Whip of Roelof or any other active antenna  the feed line is really a
> parte of the antenna.
> This was an easy and ultimate test for magic antennas like EH etc.
> In this case I don't guess it can make a difference due to low
> impedance of the amplifier output which should match the coax and
> further the rx input (under such circumstances the feed line should not
> radiate or receive but an easy test should give the answer).
>
> An other interesting point, according my experience, is the effect of
> "structures" nearby MiniWhip: my MiniWhip is placed about 10m from the
> roof (close to a metal mast) and about 1.5 m on one side there is the
> feed point of a 160m doublet and a 80m doublet, onthe other side there
> is the top hat of the 136/472 TX antenna. The signal level at my rx
> (about 50m coax run after the antennas) changes according the MiniWhip
> is the only cable connected at the receiver input, or when the MF/LF
> antenna is also connected switching from MF to LF on the tx antenna
> affects the received signal..
>
> 73, Marco IK1HSS
> ----Messaggio originale----
> Da: jrusgrove@comcast.net
> Data: 5-lug-2013 16.19
> A: <rsgb_lf_group@blacksheep.org>
> Ogg: Re: LF: VK1OD&#39;s analysis of the MiniWhip antenna
>
> Roelof
>
> Without adequate isolation between the feedline and e probe unit
> itself, the x meters of vertical
> feedline above ground level is part of the antenna.
>
> Jay W1VD  WD2XNS  WE2XGR/2
>
>
> ----- Original Message ----- 
> From: "Roelof Bakker" <roelof@ndb.demon.nl>
> To: <rsgb_lf_group@blacksheep.org>
> Sent: Friday, July 05, 2013 9:09 AM
> Subject: Re: LF: VK1OD's analysis of the MiniWhip antenna
>
>
> > Hello all,
> >
> > Further to this discussion, I have done the ultimate test to
> demonstrate  that an active whip
> > antenna with a 1 meter long whip behaves as a  capacitance at LF.
> >
> > For the test I used the ground wave carrier of semi local NDB ONO at
> 399.5  kHz.
> > As it is the only station at that frequency the carrier level is
> very  stable at daytime.
> > The distance is 59 km, which excludes probably all skywave
> propagation.
> > The antenna was mounted vertical and the carrier of ONO produced a
> signal  level of -69.1 dBm as
> > received with a PERSEUS SDR.
> >
> > Next the antenna was mounted horizontal at the same height as the
> bottom  of the vertical mount.
> > This produce a signal level of -72.0 dBm.
> >
> > I reasoned that the mean height of the 1 meter long antenna when
> mounted  vertical is 50 cm
> > higher. So the antenna was then mounted horizontal 50 cm  higher from
> its previous position. This
> > produced a signal level of -69.9  dBm, close to the value measured
> when mounted vertical.
> > The main point however, is that a horizontal polarized antenna should
> NOT  receive a vertical
> > polarized ground wave signal at (almost) the same  strength.
> >
> > So at LF, there is nothing gained in using a whip instead of a small
> piece  of copper clad PCB.
> > Comments are much appreciated!
> >
> > Best regards,
> > Roelof Bakker, pa0rdt