VK2RK
Active member
My thoughts on the Folded Dipole array discussion
At the club meeting discussion took place regarding the construction of a folded dipole array to replace the supposed faulty antenna.
Having had several years of experience at Motorola in the design and maintenance of communal repeater sites I make the following observation in the use of stacked folded dipoles arranged in a manner to provide a quasi non directional propagation.
Gain
The theoretical gain by stacking 2 dipoles is 3dB
Propagation patters
To obtain a 360 deg. Pattern four dipoles are required 90 deg. from each other, this will have an uneven pattern the is influenced by the distance from the support mast, but will still result in an uneven pattern.
Then if stacked the pattern width of each dipole is reduced making the omni pattern effect worse, resulting in uneven gain in the propagation circle.
Supposed sturdier construction
it is argued that the advantages of such a system provides adjustability, and a grounded system that has less inherent noise, this is true but also has disadvantages.
It is argued that it offers better lightening protection, not true, a direct strike will damage any antenna system.
Then the phasing harnesses are very critical in the manner of construction having to follow the root of the sum of the squares, then the fact that there will be 5 T connection up in the air as part of the phasing system, and no matter how good the connections are these will be subject to the weather and temperature extremes.
Is it worth it
Using the stack provides 3dB gain less than a collinear.
Commercial practice
Stacked dipoles in the design of a base repeater are often used if wanting to control the propagation pattern in particular directions.
Mt. Wombat is not such a site, it has very good omni propagation characteristics, and lends itself to a collinear antenna.
The cost associated with the self-construction of a folded dipole array is considerable and does not provide definitive performance metrics, meaning that to tune the system the antenna will require to be accessed and changes made, we only have subjective methods that at best provide educated guesses to the obtained results to the changes made. I am sure that the response will be that we can do this on the ground, WRONG the moment it goes up the pattern will change.
The rigging costs are large so I think is to get the existing collinear down, fix it and put it back up.
We have spent a considerable amount of money in the first place for the antenna, due to our inaction we have blown our warranty options, and now we are embarking on an experimental path, I don’t think it is a good idea to build our own array system when we don’t have the tools to measure and provide turnkey metrics before a very expensive installation.
At the club meeting discussion took place regarding the construction of a folded dipole array to replace the supposed faulty antenna.
Having had several years of experience at Motorola in the design and maintenance of communal repeater sites I make the following observation in the use of stacked folded dipoles arranged in a manner to provide a quasi non directional propagation.
Gain
The theoretical gain by stacking 2 dipoles is 3dB
Propagation patters
To obtain a 360 deg. Pattern four dipoles are required 90 deg. from each other, this will have an uneven pattern the is influenced by the distance from the support mast, but will still result in an uneven pattern.
Then if stacked the pattern width of each dipole is reduced making the omni pattern effect worse, resulting in uneven gain in the propagation circle.
Supposed sturdier construction
it is argued that the advantages of such a system provides adjustability, and a grounded system that has less inherent noise, this is true but also has disadvantages.
It is argued that it offers better lightening protection, not true, a direct strike will damage any antenna system.
Then the phasing harnesses are very critical in the manner of construction having to follow the root of the sum of the squares, then the fact that there will be 5 T connection up in the air as part of the phasing system, and no matter how good the connections are these will be subject to the weather and temperature extremes.
Is it worth it
Using the stack provides 3dB gain less than a collinear.
Commercial practice
Stacked dipoles in the design of a base repeater are often used if wanting to control the propagation pattern in particular directions.
Mt. Wombat is not such a site, it has very good omni propagation characteristics, and lends itself to a collinear antenna.
The cost associated with the self-construction of a folded dipole array is considerable and does not provide definitive performance metrics, meaning that to tune the system the antenna will require to be accessed and changes made, we only have subjective methods that at best provide educated guesses to the obtained results to the changes made. I am sure that the response will be that we can do this on the ground, WRONG the moment it goes up the pattern will change.
The rigging costs are large so I think is to get the existing collinear down, fix it and put it back up.
We have spent a considerable amount of money in the first place for the antenna, due to our inaction we have blown our warranty options, and now we are embarking on an experimental path, I don’t think it is a good idea to build our own array system when we don’t have the tools to measure and provide turnkey metrics before a very expensive installation.
