I just posted the following update to the QRZ.com forum, which includes
a picture of the dipole and dielectric sleeves:
Dave AB7E
*****
As best I can tell, NEC2 and NEC4 simply produce errors for partial
dielectric loading ... and maybe even for uniform loading. I have no
idea why that might be.
I decided to do an actual test to see what is going on. I made a dipole
out of #12 (2mm) bare copper wires 6 inches long on either side of a BNC
panel mount connector. I used a BNC to SMA adapter to connect to a good
quality nanoVNA for the measurements. I suspended the dipole several
inches above a wooden workbench and used a 12 inch length of coax to
reach the VNA. The 20mm long, 10mm diameter dielectric sleeves are 3D
printed (15% infill) with PETG which has a relative permittivity of
about 2.5. I took SWR readings with no sleeves and also in pairs (for
balance) at various positions along the wire. I also cut one of the 20mm
sleeves in half to give me two 10mm long sleeves.
By the way, I realize that the SWR minimum does not necessarily
correlate with resonance, but I think it's good enough for this purpose.
Here are the results, which were very repeatable:
no sleeves = 432 MHz
20 mm sleeve at ends of wires = 420 MHz
20mm sleeves at roughly 3/4 position from center = 425 MHz
20mm sleeves at roughly 1/2 position from center = 427 MHz
20mm sleeves at roughly 1/4 position from center = 430 MHz
20mm sleeves as close as I could get to center = 432 MHz
2x20mm (40mm total) sleeves starting at ends of wires = 413 MHz
4x20mm (80mm total) sleeves starting at ends of wires = 407 MHz
10mm sleeves at ends of wires = 425 MHz
I did not see any anomalous behavior like that shown by NEC2 and NEC4.
NEC5 seems to get it right. Both Dan's NEC5 calculated results and my
experimental results show pretty much the same profile as well as
roughly the same percentage of shift in the resonant frequency. From
that I conclude three things:
1. NEC2 and NEC4 results for the effect of insulation are probably suspect.
2. It's interesting how effective relatively little dielectric loading
is at the ends of the wires. Obviously that's where the E-field get very
high, but still ...
3. It might be practical, at least for UHF antennas, to make them easily
(and fairly significantly) tunable by varying the dielectric loading.
Dave AB7E
p.s. The dielectric sleeves were 3D printed with 3 perimeters and 15%
infill, and the holes were just barely larger than diameter of the #14
wire. That means that the portion of the sleeve closest to the wire
would have close to the rated 2.5 Er for PETG, but much of the rest of
the sleeve was just air. I suspect that the effective Er was closer to
2.0 or maybe even less.
As best I can tell, NEC2 and NEC4 simply produce errors for partial
dielectric loading ... and maybe even for uniform loading. I have no
idea why that might be.
I decided to do an actual test to see what is going on. I made a dipole
out of #12 (2mm) bare copper wires 6 inches long on either side of a BNC
panel mount connector. I used a BNC to SMA adapter to connect to a good
quality nanoVNA for the measurements. I suspended the dipole several
inches above a wooden workbench and used a 12 inch length of coax to
reach the VNA. The 20mm long, 10mm diameter dielectric sleeves are 3D
printed (15% infill) with PETG which has a relative permittivity of
about 2.5. I took SWR readings with no sleeves and also in pairs (for
balance) at various positions along the wire. I also cut one of the
20mm sleeves in half to give me two 10mm long sleeves.
By the way, I realize that the SWR minimum does not necessarily
correlate with resonance, but I think it's good enough for this purpose.
Here are the results, which were very repeatable:
no sleeves = 432 MHz
20 mm sleeve at ends of wires = 420 MHz
20mm sleeves at roughly 3/4 position from center = 425 MHz
20mm sleeves at roughly 1/2 position from center = 427 MHz
20mm sleeves at roughly 1/4 position from center = 430 MHz
20mm sleeves as close as I could get to center = 432 MHz
2x20mm (40mm total) sleeves starting at ends of wires = 413 MHz
4x20mm (80mm total) sleeves starting at ends of wires = 407 MHz
10mm sleeves at ends of wires = 425 MHz
I did not see any anomalous behavior like that shown by NEC2 and NEC4.
NEC5 seems to get it right. Both Dan's NEC5 calculated results and my
experimental results show pretty much the same profile as well as
roughly the same percentage of shift in the resonant frequency. From
that I conclude three things:
1. NEC2 and NEC4 results for the effect of insulation are probably suspect.
2. It's interesting how effective relatively little dielectric loading
is at the ends of the wires. Obviously that's where the E-field get
very high, but still ...
3. It might be practical, at least for UHF antennas, to make them
easily (and fairly significantly) tunable by varying the dielectric loading.
Dave AB7E
p.s. The dielectric sleeves were 3D printed with 3 perimeters and 15%
infill, and the holes were just barely larger than diameter of the #14
wire. That means that the portion of the sleeve closest to the wire
would have close to the rated 2.5 Er for PETG, but much of the rest of
the sleeve was just air. I suspect that the effective Er was closer to
2.0 or maybe even less.
As best I can tell, NEC2 and NEC4 simply produce errors for partial
dielectric loading ... and maybe even for uniform loading. I have no
idea why that might be.
I decided to do an actual test to see what is going on. I made a dipole
out of #12 (2mm) bare copper wires 6 inches long on either side of a BNC
panel mount connector. I used a BNC to SMA adapter to connect to a good
quality nanoVNA for the measurements. I suspended the dipole several
inches above a wooden workbench and used a 12 inch length of coax to
reach the VNA. The 20mm long, 10mm diameter dielectric sleeves are 3D
printed (15% infill) with PETG which has a relative permittivity of
about 2.5. I took SWR readings with no sleeves and also in pairs (for
balance) at various positions along the wire. I also cut one of the
20mm sleeves in half to give me two 10mm long sleeves.
By the way, I realize that the SWR minimum does not necessarily
correlate with resonance, but I think it's good enough for this purpose.
Here are the results, which were very repeatable:
no sleeves = 432 MHz
20 mm sleeve at ends of wires = 420 MHz
20mm sleeves at roughly 3/4 position from center = 425 MHz
20mm sleeves at roughly 1/2 position from center = 427 MHz
20mm sleeves at roughly 1/4 position from center = 430 MHz
20mm sleeves as close as I could get to center = 432 MHz
2x20mm (40mm total) sleeves starting at ends of wires = 413 MHz
4x20mm (80mm total) sleeves starting at ends of wires = 407 MHz
10mm sleeves at ends of wires = 425 MHz
I did not see any anomalous behavior like that shown by NEC2 and NEC4.
NEC5 seems to get it right. Both Dan's NEC5 calculated results and my
experimental results show pretty much the same profile as well as
roughly the same percentage of shift in the resonant frequency. From
that I conclude three things:
1. NEC2 and NEC4 results for the effect of insulation are probably suspect.
2. It's interesting how effective relatively little dielectric loading
is at the ends of the wires. Obviously that's where the E-field get
very high, but still ...
3. It might be practical, at least for UHF antennas, to make them
easily (and fairly significantly) tunable by varying the dielectric loading.
Dave AB7E
p.s. The dielectric sleeves were 3D printed with 3 perimeters and 15%
infill, and the holes were just barely larger than diameter of the #14
wire. That means that the portion of the sleeve closest to the wire
would have close to the rated 2.5 Er for PETG, but much of the rest of
the sleeve was just air. I suspect that the effective Er was closer to
2.0 or maybe even less.
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