[Lowfer] Active whip antennas
Bill Ashlock
ashlockw at hotmail.com
Tue Dec 21 16:57:27 EST 2010
"nail at 20 Ft with a wire attached and through a transformer to coax,,,, great idea"
That's one way to do it, Bob, but I see better sensitivity by connecting to a 30 to 50ft wire routed over the top of the tree. This avoids the 'voltage divider' effect in the resistance of the tree trunk.
Bill
> To: lowfer at mailman.qth.net
> Date: Mon, 20 Dec 2010 22:15:56 -0500
> From: riese-k3djc at juno.com
> Subject: Re: [Lowfer] Active whip antennas
>
> thats right you were the tree guy,, nail at 20 Ft with a wire attached
> and through a transformer
> to coax,,,, great idea
>
> Bob
> On Sun, 19 Dec 2010 20:59:02 -0500 Bill Ashlock <ashlockw at hotmail.com>
> writes:
> >
> > Hi Bob,
> >
> > In every E-probe experiment I have run relating to tall trees, the
> > tree acts as a large high-resistance conductor through a resistance
> > of 2K to 5K ohms to ground (the trunk). The E-probe couples nicely
> > to tree because of its high input resistance but the 2K to 5K
> > reduces the voltage level of the signal (unless the trunk is sawed
> > through :) The tree usually offers more signal voltage than a 3ft
> > metal rod at near ground levels and you can prove this by touching
> > the E-probe to the trunk of the tree and see something like a 6db
> > increase in signal voltage. I'm sure there is some height that the
> > E-probe can be mounted away from trees that exceeds the E-probe
> > signal from a tree but that's often academic because it's most often
> > that we have to accept their presence.
> >
> > This leads me onto my 'tree antenna' soap box. I seldom use E-probes
> > because of their IM problems... even the good ones. Instead I shoot
> > a line over the top of the tallest tree and pull up a 30 to 50ft
> > wire. This connects to a 50 to 5 turn transformer (77 ferrite core)
> > at the base of the tree that has the other end of this winding
> > connected to a ground stake. A 75 ohm coax connects to the 5 turn
> > winding and runs from there to the shack (shield isolated from gnd).
> > This is form of 'long wire' antenna but it's the trees conductivity
> > that has the most effect over performance. The ground at the base of
> > the tree also has a large effect on performance since it is located
> > where the signal is received, not inside the typically noisy shack.
> > The typical sensitivity of this antenna at 185K is -8db...... which
> > is plenty for most good communications receivers like the R-75.
> >
> > Bill
> >
> > > To: lowfer at mailman.qth.net
> > > Date: Sun, 19 Dec 2010 19:37:56 -0500
> > > From: riese-k3djc at juno.com
> > > Subject: Re: [Lowfer] Active whip antennas
> >
> > > Bill
> > > did you measure or see any difference if the probe was in a tree
> > > which would make the support ?
> > > I understand that isnt a good idea but it would be one possibility
> > for me
> > >
> > > Bob K3DJC
> > >
> > > On Sun, 19 Dec 2010 19:32:28 -0500 Bill Ashlock
> > <ashlockw at hotmail.com>
> > > writes:
> > > >
> > > > "I wonder if anyone has tried that experiment at a more open
> > site,
> > > > perhaps hauling the antenna up a wooden mast that wasn't a
> > tree."
> > > >
> > > > John and all,
> > > >
> > > > You may remember an experiment quite a few years back where I
> > set up
> > > > my E-probe and potable SVM in an open field where trees were at
> >
> > > > least 200 ft away. I raised the probe mounted on a insulated
> > pole in
> > > > incremental distances and measured the signal output on the SVM
> > from
> > > > a constant signal source many miles away. Turned out that the
> > signal
> > > > strength was very close to being proportional to the height of
> > the
> > > > probe. I was careful to have plenty of coax laying on the ground
> > at
> > > > all probe heights so that the capacity of the shield to ground
> > did
> > > > not vary in any significant degree and was considered very large
> > in
> > > > comparison to the probe's input capacity to gnd.
> > > >
> > > > Bill
> > > >
> > > >
> > > >
> > > > > Date: Sun, 19 Dec 2010 18:43:45 -0500
> > > > > From: w1tag at charter.net
> > > > > To: lowfer at mailman.qth.net
> > > > > Subject: Re: [Lowfer] Active whip antennas
> > > > >
> > > > > > No the output of the preamp will be higher due to the fact
> > the
> > > > antenna is
> > > > > > higher up in the air and in the clear compared to being at
> > > > ground level
> > > > > > and surrounded by lossy obstructions like vegetation and
> > trees,
> > > > plus being
> > > > > > further away from house wiring and AC noise. I have proved
> > this
> > > > myself by
> > > > > > hoisting an active whip up into a high tree branch and
> > observing
> > > > the change
> > > > > > in the signal to noise ratio.
> > > > >
> > > > > I wonder if anyone has tried that experiment at a more open
> > site,
> > > >
> > > > > perhaps hauling the antenna up a wooden mast that wasn't a
> > tree.
> > > > >
> > > > > John A.
> > > > >
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