At 01:03 PM 6/14/2005, paul@w8aef.com wrote:
>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...
What I was trying to say is that it DOES make a difference if there's
ground behind the reflector. It's not like the reflector is a perfect mirror.
The change in feedpoint impedance isn't very big, so if you're doing this
to adjust the matching network, the resulting errors will be small.
On the other hand, if you're doing this to try and match "free space"
resonant frequency of each element with what you measure, the ground's
going to have a pretty significant effect.
It also is pretty distance and ground properties dependent. Move it so the
reflector is 10 feet away from the ground instead of a foot, and it might
be fairly close (might being the operative word).
The other thing is that if you are trying to adjust your antenna to match a
manufacturer's recommendation, then Steve's prime directive holds. Whether
what you measure happens to match a model is unimportant. You're trying to
match what a manufacturer measured on their "first article" in a similar
test environment, where presumably, the loading effects are similar.
>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.
This is the matching network adjusting thing, where the ground effects are
less. The driven element is already farther from the earth anyway. The
earth effects will predominantly affect the reflector because it's closest.
If you're looking at a reflector that's, say, 1/5th of a wavelength from
the driven element (12-14 ft on 20m), the coupling's pretty low (about -8
to -9 dB). Changing the loading on the reflector is not going to have a
huge effect on the impedance seen at the driven element.
In numbers..
If the 3 el yagi is 20m up, the current in the reflector is about 150
degrees out of phase with the current in the driven element, and the
current is about 0.37 relative to the driven element.
If it's just a foot of the ground, the current is 149 degrees out of phase,
but the magnitude is now 0.21 (relative to the driven element).
That 50% change in the current in the reflector will result in change less
than 5% in the impedance of the driven element. It's like hooking a
several hundred ohm resistor across the feedpoint.
>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.
But you're depending on mechanical dimensions to set the F/B and forward
gain, and your ground adjustments don't change that. It's like putting an
antenna tuner in the line. Changing the impedance seen by the transmitter
doesn't change the antenna pattern, which is what you are really interested in.
>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."
The feedpoint impedance (and hence the SWR) is relatively blind to what's
behind the reflector.
The pattern certainly isn't blind to what's behind there.Compare the free
space pattern of the antenna to the pattern with the antenna sitting over
the ground (pointing vertically). It's quite different.
Which gets back to understanding where the limitations are for testing
antennas near the ground. Adjusting the matching network, just
fine. Adjusting element lengths, less fine.
>That's my story, and I'm sticking to it.
>
>de Paul, W8AEF
_______________________________________________
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.
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|