Are you saying that the "raise the reflector off the ground and point the
antenna straight up" method that I have used on:
204BA @ 73'
153BA @ 88'
103BA @ 95'
TH3-JR @ 45'
TH3-MK3 @ 47'
C-3
WARC-7
2m 4el @ 75'
6m 5el @ 47'
hbr W2PV designs:
10m 4el @ 95'
12m 4el @ 75'
15m 4el @ 88'
20m 4el @ 95' (41' boom)
doesn't work? Or just doesn't work within a gnat's eyebrow of perfection...
The original design for my 4 element W2PV design yagi's is for 12 meters. I
scaled that design first to 10 meters, then from 10 to 15 meters and then
from 15 to 20 meters. Then after I realized what I had done I scaled the
20m design back to 12m as a check and the check was good.
I designed the beta match using MATCH, a program that came with Yagimax.
The only change I made between design and use was to lengthen each driven
element half by 4" and move the beta shorting bar out 6". These adjustments
were made with the antenna pointing straight up.
A note: the W2PV design is very narrow banded. SWR, F/B and forward gain.
My antennas are all adjusted for 25 KHz up from the bottom of each band for
resonance (best SWR) and are dedicated for cw use. If I want to work ssb I
go to the C-3.
All of my antennas work great and are very, very competetive in any pileup
or contest I enter. The only adjustment I have to make now is if I want my
monobanders to be effective with domestice stations I have to run the tower
down to raise the signal launch angle.
I quote my previous posting: "For all intent and purposes the antenna is
blind to everything behind the reflector."
That's my story, and I'm sticking to it.
de Paul, W8AEF
----- Original Message -----
From: "Jim Lux" <jimlux@earthlink.net>
To: <paul@w8aef.com>; "Neal Sacon" <neal@sacon.net>;
<towertalk@contesting.com>
Sent: Tuesday, June 14, 2005 7:07 PM
Subject: Re: [TowerTalk] TA-33 Jr. SWR curve
> At 10:18 AM 6/14/2005, paul@w8aef.com wrote:
>>For all intent and purposes the antenna is blind to everything behind the
>>reflector. All you gotta do is get the reflector off the ground.
>>
>>de Paul, W8AEF
>
>
> This is not true. The reflector has RF current flowing in it. Put that
> element close to a dielectric and the current distribution and magnitude
> will change because of the dielectric loading. It's no different,
> conceptually, than using insulated wire or bare wire for a dipole.
> There's
> an easily measureable difference.
>
> Running a quick NEC4 model of a 3 element 20 yagi, over 13/.005 ground
>
>
>
> at 20 m off the ground, the feedpoint impedance runs like this:
> 14.0 18.57-j3.01
> 14.02 18.2-j1.05
> 14.04 17.84+j.953
> 14.06 17.48+j2.99
> 14.08 17.12+j5.06
>
> At 30 cm (1 ft) off the ground, it's like this
>
> 14 20.02 -j3.93
> 14.02 19.56 -j1.98
> 14.04 19.09 -j0
> 14.06 18.63 +j2.01
> 14.08 18.16 +j4.06
>
>
> Sure.. there isn't a huge change in the SWR, and the resonance shifts by a
> whoppin 10 kHz, but you are changing the impedance by several ohms out of
> 17-18. From a sending power from the transmitter standpoint, you'll
> probably not see a huge change. A 3 element Yagi just isn't all that high
> Q a device anyway.
>
>
> However, if you're trying to adjust element lengths by looking for the
> resonant frequency of the elment. so that you get a particular F/B, then
> those small half ohm changes in the reactive component are important.
>
> Running the same simulation, but looking for where the resonance on the
> back element is.
>
> at 20 m off the ground, the back element has a fairly noticeable resonance
> at around 13.21 MHz
>
> But, close to the ground (1 ft), the resonance isn't very pronounced, and
> is somewhere around 12.5 MHz.
>
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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