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146MHz 3 PETAL CLOVER LEAF ANTENNA

146MHz 3 Petal Cloverleaf tower mounted antenna. September 2016


Requiring a horizontally polarised 2 metre band antenna for SSB (Single Side Band) work and also needing to develop a horizontally polarised omnidirectional (HPOD) antenna for both the 2 metre band and 70 centre-metre band for a proposed beacon site, I decided that a 3 Petal Cloverleaf or sometimes referred to as a Big Wheeled antenna could be constructed as a robust unit that would be easily replicated and require little maintenance.

Construction

The antenna is constructed around a standard male N connector with a 100mm diameter aluminium radial mounting disk with a 15mm centre hole to attach to the N connector with the standard coax gland screw cap as shown in Figure 4 and Photos 2 ~ 7. The elements have been attached with 3mm diameter pop-rivets to the mounting disks

The smaller upper disk is fitted to a standard male N connector's coax centre pin and positioned in the N connector and secured with two part epoxy. A small rubber grommet has been fashioned to fit tightly in the end of the N connector body with a smearing of marine grade silicon to make to assembly water proof. 

The antenna mounting is a standard antenna mirror mount bracket with a female to female N connector bulkhead socket fitted for a range of similar antennas that are constructed around male N connectors. See: Generic Antenna Mount.

Photo 1 146MHz 3 Petal Cloverleaf assembled.

 

Figure 1 3 Petal Cloverleaf general layout.

  

Figure 2 Cloverleaf element dimensions.

  

SECTIONS FREQUENCY MHz
144.5 145.5 146.5 222 430 435 440
mm mm mm mm mm mm mm
L1 398 395 392 252 121 119 118
L2 417 414 411 271 140 138 137
L3 161 160 159 105 54 54 53
L4 908 902 896 591 305 302 298
L5 161 160 159 105 54 54 53
L6 408 405 402 262 131 129 128
L7 417 414 411 271 140 138 137
TOTAL SECTION LENGTH 2036 2022 2008 1316 666 658 650
RAD 1 127 126 125 82 43 42 42
RAD 2 127 126 125 82 43 42 42
RAD 3 383 381 378 249 129 127 126

Figure 3 Cloverleaf element dimensions table.

 

Figure 4 Cloverleaf antenna hub plates dimensions.

 

Figure 5 Cloverleaf hub assembly with element attachment.

 

Figure 6 Cloverleaf matching stub

 

Photo 2 3 Petal Cloverleaf hub plates.

 

Photo 3 3 Petal Cloverleaf lower hub plate assembled.

 

Photo 4 3 Petal Cloverleaf lower hub plate assembled.

 

Photo 5 3 Petal Cloverleaf upper hub plate assembled.

 

Photo 6 3 PetalCloverleaf hub plates assembled.

  

Photo 7 146.5MHz 3 Petal Cloverleaf hub plates and matching stub.

  

 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 146.5MHz 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 stub length an SWR of 1.7 was realised at 146.50MHz with a resonant frequency at 146.357MHz. The SWR was low across the full 2m band with only maximum of 2.0 at 144MHz. The phase angle shown in purple displays a shallow zero angle crossing at 146.357MHz indicating the resonant frequency of the antenna.

Figure 7 146MHz Cloverleaf AIM 4170C antenna analyser plot.

 .

AIM 4170C antenna analyser explanation;

SWR Standing Wave Ratio.
Zmag Total Impedance.
Phase Phase angle between voltage and current.

 

Details of the generic antenna mount. See: Generic Antenna Mount.

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Page last revised 27 September, 2016  
 

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