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Author Topic: Current choke: where?  (Read 825 times)
K3GHH
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Posts: 42




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« on: December 25, 2010, 02:01:44 PM »

I'm hardly a newcomer, but questions about current chokes have come up before on the Elmers forum.

I just received a DXE coax-coax current choke for Christmas. It sounds, from the instructions, as if it should be mounted between my roof-mounted vertical and its RG-8 feedline... right? Sure would be handy just to mount it here in the shack, but I suspect that would defeat the purpose of the choke as I understand it.

John K3GHH
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AC5UP
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Posts: 3864




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« Reply #1 on: December 25, 2010, 02:11:03 PM »

Sure would be handy just to mount it here in the shack, but I suspect that would defeat the purpose of the choke as I understand it.

The purpose of the choke is to prevent antenna currents from getting too friendly with the feedline and adopting it as part of the antenna. Best place to reduce that tendency is at the feedpoint of the antenna. Your suspicions are well founded..............
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NO6L
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Posts: 179




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« Reply #2 on: December 25, 2010, 08:26:07 PM »

A little more specifically, these are the same thing as a W2DU current BALUN, except the two pigtale wires are replaced with an SO-239. They are used for the same purpose, but in a different location, to block RF energy from undesired locations.

Say you erected a dipole and some of the feedline runs in line with the wires, they will act as antennas or parasitic elements and distort the antennas pattern. It will also likely inject some of your RF right back into the shack. Insert one of them in line to break it up into smaller non-resonant lengths. They can also be used at the entrance to the shack and if it has a ground strap, that would be grounded, too.

There ya' go
and 73
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WA3SKN
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« Reply #3 on: December 26, 2010, 04:46:35 AM »

If you want to keep the RF off the coax, it is a good idea to place the filter between the antenna and the coax.  Placing the filter at the radio defeats the purpose!
73s.

-Mike.
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W5DXP
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« Reply #4 on: December 26, 2010, 09:13:54 AM »

Your vertical is a standing-wave antenna and common-mode signals are reflected from the open-circuit at the top of the antenna. If you install an ideal choke at the transmitter, the same thing happens. The common-mode signals on the coax braid encounter a high impedance at the choke and are reflected back toward the antenna forming standing waves on the coax braid. Thus your "antenna" extends all the way from the top of your vertical back to the transmitter. Best to interrupt that standing wave function with a choke at the antenna feedpoint.
--
73, Cecil, w5dxp.com
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
WB6BYU
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Posts: 13243




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« Reply #5 on: December 26, 2010, 10:23:18 AM »

We had problems with the OCFD antenna at the county EOC picking up a lot of noise and not being resonant
where we expected it.  We got two feedline chokes and put one at the rig AND one up on the roof about
15' away from the feedpoint (where there was a splice in the coax.)

Adding the one at the rig didn't make a lot of difference (but we left it there anyway.)  Adding the one at the
antenna brought the resonances back where we expected them to be and reduced the noise pickup.

If you only have one choke, put it at the antenna end.
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K3GHH
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Posts: 42




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« Reply #6 on: December 27, 2010, 06:29:19 AM »

Thanks, everyone! I'm going to try to use a female-to-female adapter at the antenna base to avoid a short coax jumper, if I can fabricate something to mount the choke on the 3' tripod that supports the vertical.

Unfortunately I couldn't sneak in this little task before our east-coast snowstorm. It just brushed us here in Baltimore but roof climbing is going to wait a while anyway.
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