A number of my scanner antenna's have 300 to 75 ohm transformers, for my RG-6 to attach to. Is there a way of easily determining whether I have them installed correctly, and the center conductor of my RG-6 is being fed by the upper section of my dipoles, and the shielding layer is being fed by the lower section of my dipoles? THANKS, for any insight.
Greetings, Please clarify.300 to 75 ohm transformers.
- Thread starter colbrz1319
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The "RG6" coax side of the transformer is 75 ohms. The other side is 300 ohms. The 300 ohm "twinlead" side should go to the dipoles.
BB
BB
Possible misunderstanding.
I do have the 300 ohm "pigtails" attached to the vertical dipole. My question is,....How do I know which of the [2] pigtails feeds the center conductor of the coax, and which of the [2] pigtails feeds the outer braid of the coax. My goal is to have the center conductor "feeding" my radio from the upper element of the vertical dipole, and the outer braid conductor "feeding" from the lower element of my vertical dipole.Is there an ability using a volt/ohm meter to determine how the 300 to 75 ohm transformer is "deciding" which path the upper element signal travels, down my coax, and which path the lower element travels, down my coax. SORRY, if I confused folks earlier.
I do have the 300 ohm "pigtails" attached to the vertical dipole. My question is,....How do I know which of the [2] pigtails feeds the center conductor of the coax, and which of the [2] pigtails feeds the outer braid of the coax. My goal is to have the center conductor "feeding" my radio from the upper element of the vertical dipole, and the outer braid conductor "feeding" from the lower element of my vertical dipole.Is there an ability using a volt/ohm meter to determine how the 300 to 75 ohm transformer is "deciding" which path the upper element signal travels, down my coax, and which path the lower element travels, down my coax. SORRY, if I confused folks earlier.
RFI-EMI-GUY
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Ideally you don't want either element on the Balanced 300 ohm side favoring the center pin of the 75 ohm Unbalanced side. There are various different configurations but for a simple one See this:
http://www.warpspace.com/glowbugs/oz1ioc/fm_balun/balun3.jpg
and
http://www.ham-radio.com/k6sti/balun20.gif
and
http://www.hdtvprimer.com/ANTENNAS/AirCoreCurrentBalun.gif
Last edited:
prcguy
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A dipole is inherently balanced and in most cases it will not make any difference which side you connect the braid or center conductor of the coax. Some TV transformers are true transformers with an isolated primary and secondary while others are not.
An ohm meter might show no DC connection between the coax and twinlead or it could show continuity between the center conductor and a specific side of the twinlead or it could show they are all shorted together DC wise.
prcguy
An ohm meter might show no DC connection between the coax and twinlead or it could show continuity between the center conductor and a specific side of the twinlead or it could show they are all shorted together DC wise.
prcguy
Did as you directed with the ohm meter, one pigtail at a time to the threaded "f" connector end. Both pigtails showed continuity to the external threads. So, i then tested each pigtail to the center conductor of the "f" connector, and again it displayed continuity to both pigtails, one at a time. Scratching my head???
As Prcguy said it doesn't really matter with a balanced dipole, but in the real world there should be lower resistance from one leg to the other. The leg with the lower resistance when measured to the threads should be the ground side. But again a dipole is balanced, so as long as the elements are equal it does not matter.
prcguy
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Some TV transformers are an autotransformer where all leads will show a DC short between them but within the specified RF frequency range it does what its supposed to do. I would just connect it any way you want and use it, there should be no difference how its connected to the dipole.
prcguy
prcguy
RFI-EMI-GUY
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See the second drawing I posted to my edited comment above. There are several variations. If this is a normal 75 ohm to 300 ohm BALUN, it should make no difference to you. BALUN means BALanced to UNbalanced. In this case the 300 ohm twinlead side is always balanced.
I remember Bob Grove's First SCANNERBEAM in the 80's and 90's, directing buyers on the correct installation of his 75/300 transformer, so the center conductor was for the "sky pointed elements" from the "ground pointed elements" of the shielding, of his coax. Just figured I'd ask.
RFI-EMI-GUY
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Maybe the original ones used a transformer (UNbalanced-UNbalanced) and not a BALUN, in which case some interaction with the mounting mast might be reduced through polarity of the driven element.
I could not find instructions for the original, but did find for the ScannerBeam II. As you will see it has a BALUN and no mention of the polarity is made.
Instructions for ScannerBeam II
http://www.scannermaster.com/v/vspfiles/files/pdf/sb-2 scanner beam ii antenna.pdf
Ok, thanks to all, for your advise.
prcguy
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Many "TV transformers" are not baluns but simply impedance transformers, they offer no balanced to unbalanced properties. It varies with the different mfrs.
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