[Lowfer] Active Whips

[jrusgrove at comcast.net]

To better frame this discussion we should make a distinction between signal / noise and signal / 
interference ... noise being the 'natural' noise floor ('band noise') and interference being a 
'manmade' interference laden noise floor that masks the 'natural' noise floor.

The tests I've run on numerous e probe antennas were conducted at a closed airport ... flat as a 
pancake with no trees, buildings and sources of line noise for a considerable distance. The 
equipment was battery operated. With the complete absense of line and manmade noise the 'natural' 
background was the limiting factor. As long as the e probe 'band noise' overode the receiver 
background noise by a comfortable margin there was no meaningful difference in signal / noise when 
raising the antenna further and further above ground level. (The exception to this, of course, was 
the PA0RDT Mini Whip with it's tiny antenna element).

Where manmade noise is a factor, as in my backyard, both the placement of the antenna in relation to 
other objects as well as it's height above ground can affect the signal / interference level. With a 
bit of luck it may be possible to locate the antenna in a position to minimize signal / 
intereference levels to the point where signal / noise becomes the limiting factor.

Jay W1VD  WD2XNS  WE2XGR/2



----- Original Message ----- 
From: <ToddRoberts2001 at aol.com>
To: <lowfer at mailman.qth.net>
Sent: Thursday, January 24, 2013 12:14 PM
Subject: Re: [Lowfer] Active Whips


>I do not completely agree that the coax running up to an
> active whip is the same as a longer antenna as far as receiving quiet
> signals goes.
> Using a 30 foot long coax up to an elevated active whip 3 feet long is  not
> the same as a "33 foot long antenna".
>
> Of course the ground return is part of an active whip antenna,
> but there are other things going on when you raise a short active
> whip high in the air versus a ground-mounted vertical or wire of the
> same length as the active whip plus feedline.
>
> When you raise an active whip high above ground into the clear  there  is
> less
> nearby ground clutter
> such as trees, buildings and shrubs to attenuate the signal. That is
> only common sense.
> The E-field is less attenuated high above ground. The Amrad article
> in QST explains that trees and surroundings form an "electrostatic
> blanket" below an average height above ground that will
> attenuate E-field signals on LF and VLF.
>
> Also I have confirmed a definite improvement in signal-to-noise
> ratio using a short active whip mounted high above ground
> versus a ground-mounted vertical element of the same height.
>
> For example when receiving VLF Germany on 23.4kHz at a height of
> 5 feet the line noise level was reading an S-7 and Germany 23.4 was
> reading  an S-3.
> By raising the  3 ft active whip to 30 feet above ground level the
> constant noise level
> dropped to an S-3 while the signal on 23.4kHz increased to about an
> S-8. This is a clear improvement in signal to noise ratio by raising a
> short
> active whip antenna high above ground level. Something you cannot
> do by running a longwire or vertical element of the same length from  the
> ground up.
>
> If this was the same as a 33 ft. ground-mounted vertical then the  noise
> level would probably stay the same at S-7 or perhaps greater with a  longer
> antenna element. Elevating a short active whip definitely improved  the
> signal-to-noise ratio on VLF and helped to isolate the antenna from noise
> running
> along ground level. This is one advantage of an isolated short active  whip
> mounted
> high and in the clear.
> 73 - Todd WD4NGG
>
>
>
> In a message dated 1/24/2013 9:10:54 A.M. Eastern Standard Time,
> jrusgrove at comcast.net writes:
> Good discussion.
>
> Years back I decided to  investigate the much ballyhooed PA0RDT Mini Whip
> antenna with it's tiny
> antenna element. The test involved mounting it at ground level over a
> conductive screen ... along
> with several other 1M and longer e probe  antennas. Bet you can guess the
> results. Trying to point
> out that the  feedline of the elevated Mini Whip was actually part of the
> antenna was met with  great
> criticism ... so I finally gave up the 'crusade'. Folks *wanted* to
> believe in magic ... that the
> tiny antenna element was doing all the work  ... even though their own
> tests showed that it needed to
> be elevated several  meters to be effective.
>
> I use an elevated e probe here for VLF and  general purpose LF/MF work with
> an elevated radial system
> directly under the  antenna ... along with feedline decoupling measures.
> This reduces the effect of
> the feedline to great measure but doesn't completely eliminate  it.
>
> Jay W1VD  WD2XNS  WE2XGR/2
>
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