[Lowfer] Active whip antennas

[Bill Ashlock]

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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