[Lowfer] [EXTERNAL]Re: Active Whips

Thu Jan 24 13:45:57 EST 2013

Ive been typically adding a high inductive reactance at Mf/lf about 10 to 15ft below the "probes" on the feedline- as well as the other mitigations mentioned'
Iid love to live in a bonded rebarred and concrete house - really helps cutting down the outer coax/loop/noise if you typically connect the outer to a metal bonded window frame, old style radiator etc- but site specific - I have a stick built wooden transparent ro everything ranch house, and other by specific height and distance do i get the better results. On dx expeditions ive had best results at the corner of the house above and out 5 feet or more above what ever you can get....but again its very site by site.

Id hoped to be listening from ve5 but with temps approaching -40c and not being able to get my baby bottle antennae above camp accommodation flat roof level the results as i expected were a bust. Internet and cell coverage was also 0/0 - yep in the boonies

Sent and miss typed 
from my iPhone 
Laurence KL1X /ve5 enroute @ Calgary then p seattle and north to Alaska....

On Jan 24, 2013, at 7:10 AM, "jrusgrove at comcast.net" <jrusgrove at comcast.net> wrote:

> 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
>> ______________________________________________________________
>> Lowfer mailing list
>> Home: http://mailman.qth.net/mailman/listinfo/lowfer
>> Help: http://mailman.qth.net/mmfaq.htm
>> Post: mailto:Lowfer at mailman.qth.net
>> 
>> This list hosted by: http://www.qsl.net
>> Please help support this email list: http://www.qsl.net/donate.html
> 
> ______________________________________________________________
> Lowfer mailing list
> Home: http://mailman.qth.net/mailman/listinfo/lowfer
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:Lowfer at mailman.qth.net
> 
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html