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I"m having difficulty in getting a simple 1/4 wavelength, 442 MHz trap to function. A 1/4 wavelength at 442 MHz in RGx-8 coax is 7.4 inches per an on-line calculator. Cutting the coax to this dimension, shorting one end and measuring the other using the VNA calibrated at CW of 442 MHz does not show a high impedance. There is a high impedance visible with "resistance" display and scanning frequencies from 100 MHz to 500 MHz, but it is far from 442 MHz. I cut the length of the coax in increments and am now at a length of 2 1/2" (from the SNA connector to the shorted cable end). This shows a high resistance at 375 MHz (peak at about 600 Ohm). Admitted rookie, and doing this more to learn the nanovna functions than get a working antenna (dual band J-pole). Suggestions on why 1/4 wavelength shorted coax at 7.4 inches doesn't reflect a high input impedance around 442 MHz?
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First, take into account the propagation constant.? The line needs to be physically shorter than free space calculations.? So 400 MHz translates to 3/4 meter wave length.? One fourth of that is 3/16 meter, or .1875 meters.? Multiply by, say, 2/3 prop constant and you get about 1/8 meter.or 4.92 inches of coax.? You are operating at about 10% higher so knock off 10% length and you get about 4.4 inches.
Second, look closely at the quality of the short; it should be close to the cable, ideally no lead length.? Then, with the Smith chart display, sweep from about 300 to 600 MHz and look at the impedance around 450 MHz.? It should be near the right edge of the chart.? Use Port 0 display.
Bob On Tuesday, February 25, 2020, 02:46:40 PM PST, Robert Putala KE8UW <rcp@...> wrote:
I"m having difficulty in getting a simple 1/4 wavelength, 442 MHz trap to function.? A 1/4 wavelength at 442 MHz in RGx-8 coax is 7.4 inches per an on-line calculator.? Cutting the coax to this dimension, shorting one end and measuring the other using the VNA calibrated at CW of 442 MHz does not show a high impedance.? There is a high impedance visible with "resistance" display and scanning frequencies from 100 MHz to 500 MHz, but it is far from 442 MHz.? I cut the length of the coax in increments and am now at a length of 2 1/2" (from the SNA connector to the shorted cable end).? This shows a high resistance at 375 MHz (peak at about 600 Ohm).? Admitted rookie, and doing this more to learn the nanovna functions than get a working antenna (dual band J-pole).? Suggestions on why 1/4 wavelength shorted coax at 7.4 inches doesn't reflect a high input impedance around 442 MHz?
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" Suggestions on why 1/4 wavelength shorted coax at 7.4 inches doesn't
reflect a high input impedance around 442 MHz?"
It is too long. Read the following:
Assume you are using RG8X coaxial cable. The velocity of that coax is 0.82
( ). Therefore,
1/4-wavelength at 442 MHz is 0.82 X physical wavelength in free space. At
this frequency, that's pretty short, so go to a 3/4-wvelength which will
function the same as a 1/4-wavelength 'trap'.
Free space wavelength at 442 MHz:
c = Wavelength X Frequency so
1-Wavelength at 442 MHz = (2.997925 E^10 cm/sec) / (442 E^6 MHz)
= 67.8 cm = 26.7-inches
In RG-8X with a velocity factor of 0.82, one-wavelength is:
(26.7 inches) X 0.82 = 21.9-inches
3/4-wavelengths accounting for the velocity factor is then:
0.75 X 21.9 inches - 16.4-inches
1/4-wavelengths accounting for the velocity factor is then:
0.25 X 21.9 inches = 5.5-inches
Take your pick of 1/4 or 3/4 wavelengths in coax.
It's always better to *understand* what you are doing than to trust some
JAVA online applet!!
Dave - W?LEV
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Show quoted text
On Tue, Feb 25, 2020 at 10:46 PM Robert Putala KE8UW <rcp@...> wrote: I"m having difficulty in getting a simple 1/4 wavelength, 442 MHz trap to
function. A 1/4 wavelength at 442 MHz in RGx-8 coax is 7.4 inches per an
on-line calculator. Cutting the coax to this dimension, shorting one end
and measuring the other using the VNA calibrated at CW of 442 MHz does not
show a high impedance. There is a high impedance visible with "resistance"
display and scanning frequencies from 100 MHz to 500 MHz, but it is far
from 442 MHz. I cut the length of the coax in increments and am now at a
length of 2 1/2" (from the SNA connector to the shorted cable end). This
shows a high resistance at 375 MHz (peak at about 600 Ohm). Admitted
rookie, and doing this more to learn the nanovna functions than get a
working antenna (dual band J-pole). Suggestions on why 1/4 wavelength
shorted coax at 7.4 inches doesn't reflect a high input impedance around
442 MHz?
--
*Dave - W?LEV*
*Just Let Darwin Work*
*Just Think*
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On Tue, 25 Feb 2020 at 22:46, Robert Putala KE8UW <rcp@...> wrote: I"m having difficulty in getting a simple 1/4 wavelength, 442 MHz trap to
function. A 1/4 wavelength at 442 MHz in RGx-8 coax is 7.4 inches per an
on-line calculator. Cutting the coax to this dimension, shorting one end
and measuring the other using the VNA calibrated at CW of 442 MHz does not
show a high impedance. That length does not make sense to me. lambda= 300 / f = 300 / 442 = 678 mm in vacuum. You want a quarter-wave, so divide by 4 to get 170 mm =* 6.7 inches in vacuum*. You need to make it *shorter* than that, for a cable, not longer, so I don't know what calculator you use, but 7.4 inches is rubbish. Now what's the velocity factor of RG-8? According to it can vary from 0.66 to 0.84, depending on the dielectric. So let's go in the middle, and call it 0.75 So then the length of the stub would need to be 0.75 * 170 = 118 mm, which is 4.6 inches.
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I should have said your stub is too long. No wonder it shows what you are after at a much lower frequency. Dave - W?LEV On Wed, Feb 26, 2020 at 12:09 AM David Eckhardt via Groups.Io <davearea51a= [email protected]> wrote: " Suggestions on why 1/4 wavelength shorted coax at 7.4 inches doesn't
reflect a high input impedance around 442 MHz?"
It is too long. Read the following:
Assume you are using RG8X coaxial cable. The velocity of that coax is 0.82
( ). Therefore,
1/4-wavelength at 442 MHz is 0.82 X physical wavelength in free space. At
this frequency, that's pretty short, so go to a 3/4-wvelength which will
function the same as a 1/4-wavelength 'trap'.
Free space wavelength at 442 MHz:
c = Wavelength X Frequency so
1-Wavelength at 442 MHz = (2.997925 E^10 cm/sec) / (442 E^6 MHz)
= 67.8 cm = 26.7-inches
In RG-8X with a velocity factor of 0.82, one-wavelength is:
(26.7 inches) X 0.82 = 21.9-inches
3/4-wavelengths accounting for the velocity factor is then:
0.75 X 21.9 inches - 16.4-inches
1/4-wavelengths accounting for the velocity factor is then:
0.25 X 21.9 inches = 5.5-inches
Take your pick of 1/4 or 3/4 wavelengths in coax.
It's always better to *understand* what you are doing than to trust some
JAVA online applet!!
Dave - W?LEV
On Tue, Feb 25, 2020 at 10:46 PM Robert Putala KE8UW <rcp@...> wrote:
I"m having difficulty in getting a simple 1/4 wavelength, 442 MHz trap to
function. A 1/4 wavelength at 442 MHz in RGx-8 coax is 7.4 inches per an
on-line calculator. Cutting the coax to this dimension, shorting one end
and measuring the other using the VNA calibrated at CW of 442 MHz does not
show a high impedance. There is a high impedance visible with "resistance"
display and scanning frequencies from 100 MHz to 500 MHz, but it is far
from 442 MHz. I cut the length of the coax in increments and am now at a
length of 2 1/2" (from the SNA connector to the shorted cable end). This
shows a high resistance at 375 MHz (peak at about 600 Ohm). Admitted
rookie, and doing this more to learn the nanovna functions than get a
working antenna (dual band J-pole). Suggestions on why 1/4 wavelength
shorted coax at 7.4 inches doesn't reflect a high input impedance around
442 MHz?
--
*Dave - W?LEV*
*Just Let Darwin Work*
*Just Think*
-- *Dave - W?LEV* *Just Let Darwin Work* *Just Think*
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I appreciate the responses. My primary purpose in this effort is to learn about antennas, feed lines and now the Nanovna. Now retired, I have time to work on the fun stuff!
Rob Putala
KE8UW
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Show quoted text
On Feb 25, 2020, at 5:24 PM, David Eckhardt <davearea51a@...> wrote:
I should have said your stub is too long. No wonder it shows what you are
after at a much lower frequency.
Dave - W?LEV
On Wed, Feb 26, 2020 at 12:09 AM David Eckhardt via Groups.Io <davearea51a=
[email protected]> wrote:
" Suggestions on why 1/4 wavelength shorted coax at 7.4 inches doesn't
reflect a high input impedance around 442 MHz?"
It is too long. Read the following:
Assume you are using RG8X coaxial cable. The velocity of that coax is 0.82
( ). Therefore,
1/4-wavelength at 442 MHz is 0.82 X physical wavelength in free space. At
this frequency, that's pretty short, so go to a 3/4-wvelength which will
function the same as a 1/4-wavelength 'trap'.
Free space wavelength at 442 MHz:
c = Wavelength X Frequency so
1-Wavelength at 442 MHz = (2.997925 E^10 cm/sec) / (442 E^6 MHz)
= 67.8 cm = 26.7-inches
In RG-8X with a velocity factor of 0.82, one-wavelength is:
(26.7 inches) X 0.82 = 21.9-inches
3/4-wavelengths accounting for the velocity factor is then:
0.75 X 21.9 inches - 16.4-inches
1/4-wavelengths accounting for the velocity factor is then:
0.25 X 21.9 inches = 5.5-inches
Take your pick of 1/4 or 3/4 wavelengths in coax.
It's always better to *understand* what you are doing than to trust some
JAVA online applet!!
Dave - W?LEV
On Tue, Feb 25, 2020 at 10:46 PM Robert Putala KE8UW <rcp@...> wrote: I"m having difficulty in getting a simple 1/4 wavelength, 442 MHz trap to
function. A 1/4 wavelength at 442 MHz in RGx-8 coax is 7.4 inches per an
on-line calculator. Cutting the coax to this dimension, shorting one end
and measuring the other using the VNA calibrated at CW of 442 MHz does
not
show a high impedance. There is a high impedance visible with "resistance"
display and scanning frequencies from 100 MHz to 500 MHz, but it is far
from 442 MHz. I cut the length of the coax in increments and am now at a
length of 2 1/2" (from the SNA connector to the shorted cable end). This
shows a high resistance at 375 MHz (peak at about 600 Ohm). Admitted
rookie, and doing this more to learn the nanovna functions than get a
working antenna (dual band J-pole). Suggestions on why 1/4 wavelength
shorted coax at 7.4 inches doesn't reflect a high input impedance around
442 MHz? --
*Dave - W?LEV*
*Just Let Darwin Work*
*Just Think* --
*Dave - W?LEV*
*Just Let Darwin Work*
*Just Think*
|
On Wed, 26 Feb 2020 at 04:07, Robert Putala KE8UW <rcp@...> wrote: I appreciate the responses. My primary purpose in this effort is to learn
about antennas, feed lines and now the Nanovna. Now retired, I have time
to work on the fun stuff!
Rob Putala
KE8UW It will be beneficial if you try to understand the underlying physics/electrical theory, and not rely on online calculators? -- Dr. David Kirkby, Kirkby Microwave Ltd, drkirkby@... Telephone 01621-680100./ +44 1621 680100 Registered in England & Wales, company number 08914892. Registered office: Stokes Hall Lodge, Burnham Rd, Althorne, Chelmsford, Essex, CM3 6DT, United Kingdom
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I¡¯m not sure my use of the nanovna is correct. I started with my own calculation and took into account the velocity factor. First using 450 Ohm ladder line and trimming the length while measuring the resistance over a span of frequencies, a 300 MHz shorted end filter was about one inch long. This is when I tried on-line calculators, incorrectly. I will try the suggested length with RG8x and reply with results. I am in temp housing while a new house is being built, so I have very few tools. Time, but not tools.
Thank you again, I do appreciate the discussion.
Rob Putala
KE8UW
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Show quoted text
On Feb 26, 2020, at 10:07 AM, Dr. David Kirkby, Kirkby Microwave Ltd <drkirkby@...> wrote:
On Wed, 26 Feb 2020 at 04:07, Robert Putala KE8UW <rcp@...> wrote:
I appreciate the responses. My primary purpose in this effort is to learn
about antennas, feed lines and now the Nanovna. Now retired, I have time
to work on the fun stuff!
Rob Putala
KE8UW
It will be beneficial if you try to understand the underlying
physics/electrical theory, and not rely on online calculators?
--
Dr. David Kirkby,
Kirkby Microwave Ltd,
drkirkby@...
Telephone 01621-680100./ +44 1621 680100
Registered in England & Wales, company number 08914892.
Registered office:
Stokes Hall Lodge, Burnham Rd, Althorne, Chelmsford, Essex, CM3 6DT, United
Kingdom
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I have a document where I show the dimensions on my J-pole,
and a document where you calculate an estimate of the required stub length
These may help you get the dimensions right. The build is using TV
twin-lead.
toggle quoted message
Show quoted text
On Wed, Feb 26, 2020 at 12:29 PM Robert Putala KE8UW <rcp@...> wrote: I¡¯m not sure my use of the nanovna is correct. I started with my own
calculation and took into account the velocity factor. First using 450 Ohm
ladder line and trimming the length while measuring the resistance over a
span of frequencies, a 300 MHz shorted end filter was about one inch long.
This is when I tried on-line calculators, incorrectly. I will try the
suggested length with RG8x and reply with results. I am in temp housing
while a new house is being built, so I have very few tools. Time, but not
tools.
Thank you again, I do appreciate the discussion.
Rob Putala
KE8UW
On Feb 26, 2020, at 10:07 AM, Dr. David Kirkby, Kirkby Microwave Ltd < drkirkby@...> wrote:
On Wed, 26 Feb 2020 at 04:07, Robert Putala KE8UW <rcp@...> wrote:
I appreciate the responses. My primary purpose in this effort is to
learn
about antennas, feed lines and now the Nanovna. Now retired, I have
time
to work on the fun stuff!
Rob Putala
KE8UW
It will be beneficial if you try to understand the underlying
physics/electrical theory, and not rely on online calculators?
--
Dr. David Kirkby,
Kirkby Microwave Ltd,
drkirkby@...
Telephone 01621-680100./ +44 1621 680100
Registered in England & Wales, company number 08914892.
Registered office:
Stokes Hall Lodge, Burnham Rd, Althorne, Chelmsford, Essex, CM3 6DT,
United
Kingdom
|
Hi, Rob
Nice to see you here having fun with the nano. Amazing that it isn¡¯t even a year old. Changed your call, I see. I would have too with WA8UZC, I think. When I moved to Illinois, I had to get a new call as back then FCC didn¡¯t let you keep your call when you move out of call zone. WB9PMY.
Gary W9TD
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Test it as it will be used. Connect a "T" to the stub. connect Port 0 or 1
depending on which NanoVNA is being used to one side of the T. connect port
1 or 2 depending upon which NanoVNA. In other words do an S21 test. The
total length of the stub will also include and length of the T to the
center conductor of the pass though of the T.
*Clyde K. Spencer*
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On Tue, Feb 25, 2020 at 5:46 PM Robert Putala KE8UW <rcp@...> wrote: I"m having difficulty in getting a simple 1/4 wavelength, 442 MHz trap to
function. A 1/4 wavelength at 442 MHz in RGx-8 coax is 7.4 inches per an
on-line calculator. Cutting the coax to this dimension, shorting one end
and measuring the other using the VNA calibrated at CW of 442 MHz does not
show a high impedance. There is a high impedance visible with "resistance"
display and scanning frequencies from 100 MHz to 500 MHz, but it is far
from 442 MHz. I cut the length of the coax in increments and am now at a
length of 2 1/2" (from the SNA connector to the shorted cable end). This
shows a high resistance at 375 MHz (peak at about 600 Ohm). Admitted
rookie, and doing this more to learn the nanovna functions than get a
working antenna (dual band J-pole). Suggestions on why 1/4 wavelength
shorted coax at 7.4 inches doesn't reflect a high input impedance around
442 MHz?
|
Dr. David Kirkby, Kirkby Microwave Ltd
Gary W9TD