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Old 25-Jul-2014, 8:34 AM   #7
tripelo
Senior Member
 
Join Date: Jan 2010
Location: Dallas, TX
Posts: 172
Stack Bay Antennas w/o Reflector, Twin Lead or Coax?

Quote:
Originally Posted by GroundUrMast View Post

Twin-lead...
... not too close to metal...
...1 to 1.5 turn per foot needs to be maintained...
...stand-off insulators to ensure that the wire stays...
...use it as a part of a phasing harness ...

...Coax for
...the down-lead and distribution...
...run in contact with metal mast, conduit, pipe and duct work...
Thanks GroundUrMast, all good points.

Quote:
Originally Posted by pips View Post
...my intent is to build 4 sbgh with no reflector and stack vertical down the mast on top of a 50 foot tower...
Pips, it seems you are trying to optimize performance, if so:

Without reflectors to help isolate active elements from effects of the transmission lines, it seems difficult (maybe impractical) to vertically stack four such antennas using either twin lead or coaxial cable.

Keep in mind, all four transmission lines (twin leads or coaxial cables) need to be the same length. If stacking four vertically, this means that the two center antennas will have to have leads/cables going downward and then turning upward and vice-versa.

Transmission lines should be gradually bent, no sharp turns. This means that at the turns, the transmission lines will be routed horizontally.

Additional caveats apply to twin lead; at all times it should be kept separate from other portions of itself, or any metal.

Horizontal twin leads or coaxial cable*, in front or behind, can disturb the pattern and the gain of a reflector-less bay antenna.

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If one can accept less than optimal performance, then compromises can be made (Guidelines and rules-of thumb do not matter as much).
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* Induced currents in the coax shields are the primary sources of degradation. One could load the cable with a few selected ferrite cores at the horizontal portions. The ferrite would inhibit the induced currents, thus mitigate most of the potential degradation.


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Last edited by tripelo; 25-Jul-2014 at 9:21 AM. Reason: clarify
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