Post by whitemorph on May 10, 2009 0:45:10 GMT -5
Hello everyone, I’m a neophyte, having just purchased a vector analyzer and I need some advice from someone with experience in applying the vector analysis data to the actual antenna adjustments.
I have a Cushcraft Ringo Ranger ARX-2 vertical. I recently added the upper element extension kit (ARX-2K) to an old stubby Ringo. The extension kit (upper element) is isolated by a plastic coupling and the upper element is electrically coupled to the lower element by a hair-pin loop (“phasing stub”). There is no decoupling assembly below the antenna (it’s not the “B” model). The Ringo is mounted on a 24-foot tall non-conductive fiberglass pole in the middle of the yard. It is fed by 48 feet of RG8X. The desired transmitting frequency is 146.200 MHz; and the associated receive frequency is 146.800 MHz.
The single-frequency Vector Analyzer readings (@ 146.200 MHz) measured at the transceiver end of the feed line are:
SWR: 1.28
Z: 48.69 Ohms
R: 47 Ohms
X: –11 Ohms
Serial: 46.88 Ohms –j 11.64, 91.8 pf
Parallel: 50.36, –j 200.5, 5.4 pf
Rectangular: –0.02, –j 0.12
Polar: 10.12, Angle: –97°
Return loss: 18db @ –96°
The SWR sweep reveals: 1.32 @ 144 MHz; 1.23 @ 148 MHz (lowest SWR: 1.23 @ 148 MHz)
Possible Antenna adjustments are:
Tap point (“tuning rod”) on the base ring (a one-turn loading coil)
Lower element length
Hair-pin (“phasing stub”) length
Upper element length
At present, the antenna dimensions are meticulously adjusted to 146.500 MHz according to the Cushcraft chart that came with the ARX-2K kit. However, all the vector analysis data seems to indicate that the antenna is capacitive reactive at my target frequency, and thus is too short; and the feed point tap (“tuning rod”) may need to be adjusted a few ohms higher. Is this generalization reasonable?
If this were a quarter wave ground plane, the resolution would be fairly obvious and simple. However, the Ringo is a bit more complex.
First question: Is there something in these numbers that will help me determine whether I should lengthen the lower element or the upper element or the hairpin phasing stub between them — or is this still determined by trial and error? If so, how do I know when I have adjusted one of the elements correctly while the other two may still be out of adjustment? And which element should I start with?
Next question: It seems like 18db is a lot of return loss. What can I do about that—or will that go away once I get the antenna adjusted correctly?
Final question: In reading about antenna theory, I have noted that there is a correlation between length and degrees, because some charts give element length in degrees. Please explain this correlation and whether it has any relation to the degrees in the polar expression.
As a matter of interest, I chose to set up the Ringo for 146.500 MHz as a happy compromise in the middle of the repeater offset for the 146.800(–) repeater. This repeater is located at a distance of 169 miles from my QTH in an easterly direction. The repeater is at an elevation of 5,000 feet, but my signal has to hop several 2,000-3,000 ft mountains and the curve of the earth in between. And yes, I have already had a QSO through this repeater. My rig is a 45 Watt Icom 228H powered by a lawn tractor battery maintained by a 10 amp power supply — albeit my signal was scratchy but readable. My goal is to get into this repeater with a respectable signal. I may have to revert to a beam antenna, but I want to push the Ringo to its limit first.
Regarding the environment around the Ringo; the elevation at my QTH is 600 feet. I am centrally located in the north end of a valley. The valley is about 5 miles wide (east to west) shadowed by 3,000 foot mountains to the west and north, and 2,000 foot mountains to the east. The valley fans out to the south and gradually climbs to an elevation of 3,000 feet to meet the northern slope of Mt. Hood in Oregon.
In my less-than-ideal yard, there is a huge 50-foot oak tree a few feet to the North that overshadows the Ringo, a solid row of 30-foot high arborvitae a few feet to the west, and a 30-foot high two-story house a few feet to the east of the Ringo. The view to the south of the Ringo is unobstructed.
Thanks in advance for the precious time any of you can spare to answer my questions.
I have a Cushcraft Ringo Ranger ARX-2 vertical. I recently added the upper element extension kit (ARX-2K) to an old stubby Ringo. The extension kit (upper element) is isolated by a plastic coupling and the upper element is electrically coupled to the lower element by a hair-pin loop (“phasing stub”). There is no decoupling assembly below the antenna (it’s not the “B” model). The Ringo is mounted on a 24-foot tall non-conductive fiberglass pole in the middle of the yard. It is fed by 48 feet of RG8X. The desired transmitting frequency is 146.200 MHz; and the associated receive frequency is 146.800 MHz.
The single-frequency Vector Analyzer readings (@ 146.200 MHz) measured at the transceiver end of the feed line are:
SWR: 1.28
Z: 48.69 Ohms
R: 47 Ohms
X: –11 Ohms
Serial: 46.88 Ohms –j 11.64, 91.8 pf
Parallel: 50.36, –j 200.5, 5.4 pf
Rectangular: –0.02, –j 0.12
Polar: 10.12, Angle: –97°
Return loss: 18db @ –96°
The SWR sweep reveals: 1.32 @ 144 MHz; 1.23 @ 148 MHz (lowest SWR: 1.23 @ 148 MHz)
Possible Antenna adjustments are:
Tap point (“tuning rod”) on the base ring (a one-turn loading coil)
Lower element length
Hair-pin (“phasing stub”) length
Upper element length
At present, the antenna dimensions are meticulously adjusted to 146.500 MHz according to the Cushcraft chart that came with the ARX-2K kit. However, all the vector analysis data seems to indicate that the antenna is capacitive reactive at my target frequency, and thus is too short; and the feed point tap (“tuning rod”) may need to be adjusted a few ohms higher. Is this generalization reasonable?
If this were a quarter wave ground plane, the resolution would be fairly obvious and simple. However, the Ringo is a bit more complex.
First question: Is there something in these numbers that will help me determine whether I should lengthen the lower element or the upper element or the hairpin phasing stub between them — or is this still determined by trial and error? If so, how do I know when I have adjusted one of the elements correctly while the other two may still be out of adjustment? And which element should I start with?
Next question: It seems like 18db is a lot of return loss. What can I do about that—or will that go away once I get the antenna adjusted correctly?
Final question: In reading about antenna theory, I have noted that there is a correlation between length and degrees, because some charts give element length in degrees. Please explain this correlation and whether it has any relation to the degrees in the polar expression.
As a matter of interest, I chose to set up the Ringo for 146.500 MHz as a happy compromise in the middle of the repeater offset for the 146.800(–) repeater. This repeater is located at a distance of 169 miles from my QTH in an easterly direction. The repeater is at an elevation of 5,000 feet, but my signal has to hop several 2,000-3,000 ft mountains and the curve of the earth in between. And yes, I have already had a QSO through this repeater. My rig is a 45 Watt Icom 228H powered by a lawn tractor battery maintained by a 10 amp power supply — albeit my signal was scratchy but readable. My goal is to get into this repeater with a respectable signal. I may have to revert to a beam antenna, but I want to push the Ringo to its limit first.
Regarding the environment around the Ringo; the elevation at my QTH is 600 feet. I am centrally located in the north end of a valley. The valley is about 5 miles wide (east to west) shadowed by 3,000 foot mountains to the west and north, and 2,000 foot mountains to the east. The valley fans out to the south and gradually climbs to an elevation of 3,000 feet to meet the northern slope of Mt. Hood in Oregon.
In my less-than-ideal yard, there is a huge 50-foot oak tree a few feet to the North that overshadows the Ringo, a solid row of 30-foot high arborvitae a few feet to the west, and a 30-foot high two-story house a few feet to the east of the Ringo. The view to the south of the Ringo is unobstructed.
Thanks in advance for the precious time any of you can spare to answer my questions.
I like to use the Smith chart display in connection with the "Recycle" function to update the display and see things move around, but that assumes you can adjust the antenna while looking at the display (not likely in your situation).