[Lowfer] Active Whips

[jrusgrove at comcast.net]

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



----- Original Message ----- 
From: "JD" <listread at lwca.org>
To: "Discussion of the Lowfer (US, European, &amp;UK) and MedFer bands" <lowfer at mailman.qth.net>
Sent: Thursday, January 24, 2013 3:52 AM
Subject: Re: [Lowfer] Active Whips


>>>>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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