[Lowfer] Active Whips

Thu Jan 24 11:00:52 EST 2013

The original AMRAD antenna article went into this. If I may quote from that
article:

"Best LF performance is obtained if the antenna whip is higher than nearby
conducting objects. Imagine pulling a giant plastic sheet over your house
and yard. The whip should be above this imaginary sheet. A more accurate
(and much more complex) way to think of it is to imagine a large metal
sheet several hundred feet above your house and yard (play along). Now
imagine that the sheet is charged with a high dc voltage. If you were to
examine the electrostatic field around and above your house and yard, you
would discover that those points below the plastic sheet are at a 0-V field
potential."

I have seen similar references in other articles. The best performance with
an active whip is supposed to be when it is above the tallest nearby
object, including trees.

I hope to get mine put up this weekend.

73, Zack W9SZ

On Thu, Jan 24, 2013 at 7:12 AM, Douglas D. Williams <kb4oer at gmail.com>wrote:

> Great explanation John. So, when we raise the height of our "active whip"
> antennas, say, an additional ten feet by adding another ten foot long
> section of support pole, and then see an improvement in signal levels, it
> isn't necessarily because the "whip" part of the antenna is higher, but
> because the support pole (or coax shield if the support pole is non
> conductive) is actually part of the antenna, so we have effectively
> increased the length of the antenna by using a longer support pole?
>
> I wonder why some "active antenna" manufacturers caution users that long
> support poles (i.e. placing the antenna much higher than 20 feet above
> ground) are not necessary? Could it be that they are concerned that too
> much antenna length (since the  support pole and/or coax shield is actually
> contributing to the signal level at the receiver)  could overload the
> amplifier and cause IMD?
>
> 73, Doug KB4OER
>
>
>
>
>
> On Thu, Jan 24, 2013 at 3:52 AM, JD <listread at lwca.org> wrote:
>
> > As for JD's situation, he makes a good point. JD is actually using a 40
> >>>> foot long "active whip" to receive. I am using an 8 foot long "active
> >>>> whip". When you get down to wavelengths that span 2000 meters, what's
> the
> >>>> difference? Answer: pretty much no difference.
> >>>>
> >>>
> > And maybe even less difference than that! :-)
> >
> > * Alert::: slightly long and detailed material follows, but hopefully not
> > too overcondensed or too boring *
> >
> > I forget what length the support for your whip ended up being, Doug, but
> > I've heard of some folks up as high as 30-35 feet.  For convenience of
> > comparison, let's just pick a 32 foot support for this analysis.  If a
> > person then mounts an 8 foot antenna up there, does that make it an 8
> foot
> > active whip?  Nope, it's a 40 foot active whip, too.
> >
> > This is because, in the most literal sense, there is no such thing as a
> > monopole.  Every antenna, just as every amplifier input, has TWO
> terminals
> > in reality, or no complete circuit exists.    In my setup, the buffer
> > amplifier is actually receiving signal from a dipole...one leg of which
> is
> > a relatively skinny 40 foot steel mast sticking up into the air, and the
> > other leg of which is a big ball of damp dirt and rock 8000 miles in
> > diameter.
> >
> > The voltage developed across those two terminals by a passing
> > electromagnetic wave of the same polarization as the electrically short
> > mast, is then very nearly equal (provided my circuit is high enough
> > impedance, both reactively and resistively speaking) to the field
> strength
> > in volts/meter times the effective height of the antenna in meters.  If
> > there is not some form of loading to change the linearly tapered current
> > distribution in the electrically short element, that effective height is
> > _half_ the physical height.  So, an EM wave of 1 microvolt/meter
> intensity,
> > having its electric field in the plane of my mast as it happens to pass
> by,
> > will generate a voltage of:
> >
> > (1 uV/m * 12.192 m ) / 2 = 6.1 uV/m approx
> >
> > across my amplifier input terminals.  Not too shabby, assuming QRM and
> QRN
> > aren't generating even more voltage there at the time.
> >
> > Now, our presumed 8 foot whip...if its amplifier has a connection via the
> > support pole, a separate wire, or the coax shield to ground in any way,
> > then IT TOO is receiving its input from a dipole...one leg consisting of
> > the 8 foot whip, and the other consisting of 32 feet of conductor with an
> > 8000 mile ball of damp dirt at the other end.  How can that be the same
> as
> > each other antenna, despite the difference in appearance?
> >
> > Let's look a little closer at what's actually happening in the first
> case,
> > my own "40 ft whip."  There's one point in particular to remember.  The
> > electric field of the passing wave ends just inside the surface of the
> > sorta-conducting big ball of dirt.  So, the total span of the EM wave's
> > electric field being intercepted is 12.192 meters long--from the tip of
> the
> > mast to the surface!   But then, as already mentioned, because of the
> > linear taper of current distribution in the non-ball-of-dirt leg (you
> can't
> > separate the magnetic effects from the electric in a propagating EM
> wave),
> > the _effective_ length of field being intercepted is only half that, or
> > ~6.1 meters.  If you followed what I said about the E field terminating
> > just inside the surface of the dirtball, you see can why the fiction of
> the
> > electric "monopole" is useful in some aspects of antenna engineering;
>  the
> > dirtball, despite its size, contributes almost nothing to the received
> > terminal voltage and hence does not appear in equations involving
> vertical
> > antenna effective height.  But despite that, the circuit is still a
> dipole
> > electrically, because the current induced in the non-dirtball leg still
> has
> > to flow through the dirtball too..
> >
> > OK, now back to the "8 foot" whip.  The antenna input port of the
> > amplifier has an 8-foot conductor on one terminal...and through whatever
> > path, has a 32 foot connection to the dirtball on the other terminal.
>  Once
> > again, our EM wave passes, and once again the electric field terminates
> at
> > the dirtball.  So, the intercepted electric field component spans from
> the
> > tip of the 8 foot whip to the amp, and then from there, 32 more feet to
> the
> > earth side of the conductor, where the field ends.  And once again,
> viewed
> > across the whole length of the dipole, the current tapers linearly from
> tip
> > to dirtball.  The total voltage appearing across the input terminals of
> the
> > amplifier for a 1 uV/m signal is therefore the contribution of the 8 foot
> > leg plus the contribution of the 32 foot-plus-dirtball leg, or:
> >
> > ( 1 uV/m * 2.438 m ) / 2 + ( 1 uV/m *  9.754 m ) / 2 =
> > ( ( 1 uV/m * 2.438 m ) + ( 1 uV/m *  9.754 m ) ) / 2 =
> > ( 1 uV/m * ( 2.438 + 9.754 ) m) / 2 =
> > (1 uV/m * 12.192 m ) / 2 = 6.1 uV/m approx
> >
> > When all the associative and distributive mumbojumbo has been done, a
> > typical 8 foot whip at 32 feet is not "merely" an 8 foot whip, but is
> just
> > the same as a 40 foot whip at ground level, so far as its ability to
> > intercept signal.  An 8 ft whip at 25 feet is equivalent to a 33-footer,
> 8
> > feet at 20 ft is like 28 ft ground mounted, and so on.
> >
> > Could you make an 8 foot whip act like only 8 feet while still way up in
> > the air, or even a 2 inch whip act like 2 inches?  With heroic measures,
> > yes, but why would you want to? :)
> >
> >
> > John
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