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24-Jul-2014, 3:18 PM   #2
tripelo
Senior Member

Join Date: Jan 2010
Location: Dallas, TX
Posts: 172
Stack 4 Antennas: Twin-lead to Coax

Quote:
 Originally Posted by pips ...if i hook two antennas in parellel on a 300 ohm antenna then the ohm drops to 150 if i add a third in parellel then wouldnt it drop to 75...
The resistance would drop to 100 Ohms if three (300 Ohm) resistive lines were paralleled.

- Four in parallel would be 75 Ohms.

You are writing about an application of Ohm's law: parallel and series combinations. There are web calculators to help.

In some cases, the term 'impedance' can be substituted for resistance.

Quote:
 ...which i could splice and solder the coax and feed straight to tv elimiating the baulan completely?...
If one wishes to accept the consequences (loss, etc.), one can always eliminate a balun.

In this case, if you want the lowest loss, a balun is required. It could be a series of selected ferrite cores (beads) around the coaxial cable near the point of connection to the paralleled twin leads.

This type of balun is often referred to as; a 'ferrite choke balun'.

Use Google Images, "Ferrite choke balun".

Quote:
 ...i would use twin lead to hook them together.is this right or even possible?...
Four antennas (300 Ohms impedance) stacked with feed lines in parallel.

It is possible, several have done so. For some of reasons below, this form of stacking is probably not often used anymore.

Yes, twin lead is appropriate for all the connections to the 300 Ohm antenna terminals.

The twin lead lines should have equal lengths.

There are other constraints for best performance.

Examples,

-The twin lead transmission lines should be perpendicular

* For Yagis, vertical with respect to the antenna elements for some distance, before turning horizontally to meet the other lines.

* For panel arrays (8-bays and such), it is probably most practical to route twin lead rearward to behind the reflector before turning to meet other feed lines. Once behind a reflector, then orientation of twin lead with respect to antenna elements is not as critical.

-The twin lead lines should be kept away from any metal (i.e. mast) or other lines (can use standoff insulators).

- There are probably more.

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Well done, this approach is among the lowest loss methods of combining a stack** of four antennas.

**Stack antennas: Can be two or more identical antennas arranged with respect to each other (horizontally, or vertically, or combination of both) and connected in phase to increase the gain.

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Last edited by tripelo; 24-Jul-2014 at 5:11 PM. Reason: maybe clarify