[Antennaware] Antennaware Digest, Vol 63, Issue 3

[bumerang boom]

It is not the slope of your ground that you think is the variable but the angle of the radials to the radiator. Antenna101 curse teches us all that an angle of   45 degrees will bring the feed point to around 50 ohms.

BB.
WWithout Wax


--- On Wed, 3/4/09, antennaware-request at contesting.com <antennaware-request at contesting.com> wrote:

> From: antennaware-request at contesting.com <antennaware-request at contesting.com>
> Subject: Antennaware Digest, Vol 63, Issue 3
> To: antennaware at contesting.com
> Date: Wednesday, March 4, 2009, 3:00 PM
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> Today's Topics:
> 
>    1. 160M vertical impedance (K9AY)
>    2. Voltages along an antenna (David Gould)
> 
> 
> ----------------------------------------------------------------------
> 
> Message: 1
> Date: Tue, 3 Mar 2009 16:20:40 -0600
> From: "K9AY" <k9ay at k9ay.com>
> Subject: [Antennaware] 160M vertical impedance
> To: <antennaware at contesting.com>
> Message-ID:
> <EACCED6A6F284CDBB832DCD32B6B985E at SUMMIT1>
> Content-Type: text/plain; format=flowed;
> charset="iso-8859-1";
> 	reply-type=original
> 
> I finally took some time to review my modeling vs.
> measurement of the feed 
> impedance of my 160M vertical.
> 
> To review -- my original model showed 39.8 +j48 ohms at
> 1850 kHz, with 8" 
> diameter (yes, diameter) used for the 25G. This was a
> simplified EZNEC model 
> for a vertical, with the antenna at ground level, no
> radials and MININEC 
> ground. The modeled reactance is close to the capacitance
> value I ended up 
> with for matching. However, the measured (estimated)
> resistance was very 
> close to 50 ohms, which is a long way from 39.8 ohms.
> 
> Since I was unable to make the model match the measurement
> by adjusting 
> conductor sizes and lengths, I focused on my initial
> suspicion: the ground 
> slope. Although EZNEC does not have an easy way to change
> ground itself, 
> this is a system with radials that, hopefully, isolate most
> of the fields 
> from lossy ground, so I did the following:
> 
> Used 'High Accuracy' ground, and raised the entire
> antenna model 12 ft above 
> ground. I then let EZNEC create 32 radials based on a
> prototype 110-ft. 
> radial. This appoximates my actual radial system. With
> these horizontal 
> elevated radials, the modeled feed impedance was 40.2 +j37,
> a reasonable 
> match to the original model. Very promising!
> 
> I next re-created the radials with the prototype wire
> sloping from 12 ft. 
> down to 1 ft., representing the maximum slope of my
> terrain. This raised the 
> modeled feed resistance from 40.2 ohms to nearly 46 ohms --
> more than I 
> expected from the fairly shallow slope angle, and much
> closer to the 
> measured impedance.
> 
> However, my ground is not uniformly sloped downhill from
> the tower, so I 
> modified the end heights of the radials to approximately
> match my ground 
> contours. About 1/4 of the modified radial system is
> horizontal, with the 
> rest at various downward slopes. But the resistance only
> dropped to 44.3 
> ohms. It appears that even a small amount of slope in the
> radial system will 
> significantly raise the resistance at the feedpoint.
> 
> While the method may not be perfect, I am now satisfied
> that the slope of 
> the ground is the primary reason for the difference in
> resistance.
> 
> 73, Gary
> K9AY
> 
> 
> 
> ------------------------------
> 
> Message: 2
> Date: Wed, 04 Mar 2009 17:36:29 +0000
> From: David Gould <dave at g3ueg.co.uk>
> Subject: [Antennaware] Voltages along an antenna
> To: <antennaware at contesting.com>
> Message-ID:
> 	<20090304173709.NLNK13254.aamtaout01-winn.ispmail.ntl.com at g3ueg.g3ueg.co.uk>
> 	
> Content-Type: text/plain; charset="us-ascii";
> format=flowed
> 
> First thanks to those who helped a few weeks ago with my
> question 
> about the significant differences between my model and
> reality for my 
> 62ft vertical on 80m.  The concensus was, go with what it
> is 
> measuring at, and ignore the difference in the model.
> 
> Last weekend I top loaded the vertical to bring it resonant
> on 
> 160m,  fed it through an UNUN and had a ball in the CQWW
> 160m SSB 
> contest, making around 40 NA QSOs across 14 states, which
> surprised 
> me, considering that conditions were not that good.   I did
> not seem 
> to be loosing very much compared top my previous 108ft
> vertical over 
> better radials.  (I must have magic soil!!!)
> 
> EZNEC gives a table of currents along the antenna elements,
> is there 
> a way to find the voltages?  I would like to know the
> voltage at the 
> top of the 62ft vertical for both 160m and 80m?  I am
> considering 
> putting either a trap or a relay to give me dual band, so I
> would 
> like to know what ratings any components would need to
> have.   (I 
> have an aversion to traps and reactance in general because
> of the 
> losses and the significant reduction in bandwidth as a 
> result.)   can't see anything in the help, but feel it
> should not be 
> impossible.  I may be missing something obvious...
> 
> 73,
> Dave, G3UEG
> 
> 
> 
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> End of Antennaware Digest, Vol 63, Issue 3
> ******************************************