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Having a small amount of radio equipment
for this really interesting band, but not having any antennas, it was
time to put this right.
The
antenna is constructed from an old Cushcraft 2m band co-linear ringo
antenna and converted to the 6m band by reconstructing the matching
ring and adapting the 2m band co-linear main element to a single 0.548
wavelength (3.61m) element. The length of the radiating element was
determined from modelling the antenna in MMANA and optimising the
antenna for best horizon radiation pattern and best SWR match.
Construction
The antenna is constructed
from an old 2m band Ringo antenna with only radiator mounting hub and
matching ring mounting hardware being retained. Some of the
aluminium tubing from the main radiating element was reused with an
additional 1.8m length of 10mm diameter aluminium tubing to make the
new 3.61m length main radiating element. The below table along with Figure
1 Ringo
drawing shows element dimensions for a selection of bands.
| Band: |
6m |
4m |
2m |
| Frequency
MHz: |
52.0 |
70.0 |
146.0 |
| Main
Element Length in mtr 'A' : |
3.162 |
2.306 |
1.105 |
| Ring
Diameter in mtr 'B' : |
0.321 |
0.239 |
0.114 |
Figure
1
Ringo
element dimensions
Photo
1
Matching
Ring assembly.
 Photo
2
Main element tube sleeve connection. 
Photo
3
Antenna
mounting and 1:1 chocking balun. 
Photo
4
Complete antenna
installation
Figure
2
Radiation
plot for 52MHz were produced using MMANA antenna modeller and optimiser.
Figure 3 MMANA
antenna model showing the a suitable 3d radiation pattern.
Antenna testing
With
the antenna assembled on an easily accessible test mast well clear of
the ground and surrounding metallic objects, the antenna was connected
to an AIM 4170C antenna analyser to ascertain how the antenna loaded
up.
The
goal here is to find the lowest SWR for the operating frequency of
52.0MHz and ideally have the resonant frequency as close as is
possible to the same frequency
The
AIM 4170C produces a display of all relevant data and most importantly
it can project it's analysis to the antenna end of the coax giving a
truer picture of the antenna.
With
modest adjustment of the matching ring tap an SWR of 1.7 was
realised at 51.3MHz with a resonant frequency at 52.0MHz. The SWR was
acceptably low across about 2MHz of the 6m band with only maximum of
3.0 at 54.0MHz. The
phase angle shown in purple displays a shallow zero angle crossing at
52.0MHz indicating the resonant frequency of the antenna.
Figure 4 SWR
measurements from 45 to 60MHz
Figure 5 AIM 4170C antenna analyser
measurements from 45 to 60MHz
1:1
Choking balun low band VHF
Choking balun
for lower band VHF. (14 ~ 54MHz) FT140-43
Ferrite Toroid Core.
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