| Author |
Message |
    
Hawk1
Junior Member
Username: Hawk1
Post Number: 27
Registered: 11-2003
| | Posted on Wednesday, March 10, 2004 - 12:27 pm: | 
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i always heard to put rf choke coil right at base of antenna for best results,,a guy who has a cb shop told me its best to put the choke 9feet below the base of a omni antenna..im talking about the kind of choke you make by wrapping your coax in loops..why would he say 9feet below....hk |
Ryan
Member
Username: Ryan
Post Number: 72
Registered: 11-2002
| | Posted on Wednesday, March 10, 2004 - 2:26 pm: |
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9' is around 1/4 wave at 11 meters |
Bruce
Senior Member
Username: Bruce
Post Number: 1064
Registered: 9-2003
| | Posted on Wednesday, March 10, 2004 - 2:59 pm: |
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either will work |
Mr_rf
Intermediate Member
Username: Mr_rf
Post Number: 281
Registered: 12-2001
| | Posted on Wednesday, March 10, 2004 - 11:36 pm: |
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Not so fast , Bruce.
The reason for making the loop in the coax is to ward off the potential of the coax becoming part of the radiating portion of the antenna (referred to as the cable becoming "hot" with RF)
Actually, placing the loop 9 feet (or at 1/4 wavelength depending on operating frequency) below the antenna may cause the 9 feet of cable to electrically become part of the radiating portion of the antenna. This is not a good thing as stray currents on the feedline causes VSWR issues, RF feedback (squeals, distorted audio, etc.), and loss in antenna efficiency!!!
Place any loops or ferrite core chokes as close to the base of the antenna as possible, and never wrap the feedline around the antenna mast to form the loops as the mast sometimes becomes part of the antennas radiating structure and wraping the cable around it would cause all sorts of strange induction issues. |
Creator
Member
Username: Creator
Post Number: 53
Registered: 1-2004
| | Posted on Thursday, March 11, 2004 - 12:13 am: |
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what would happen if you ran your coax up the inside of your mast, i was thinking of doing this, i thought the pipe would act as extra shield and reduce tvi.. |
Bruce
Senior Member
Username: Bruce
Post Number: 1065
Registered: 9-2003
| | Posted on Thursday, March 11, 2004 - 12:43 am: |
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Mr Rf
Agreed you can place a loop in the 1/4 wave spot and it can become part of the antenna..... However IF the antenna is made correctly it should be decoupled enough that where you place the coax choke should not matter. Aggreed on never wrap the coax around the mask also i had to place a 2nd choke where the coax came into the shack ...... @ 14 mhz the lengh of coax was becomeing part of the antenna just by being in the main lobes field.... |
Yankee
Junior Member
Username: Yankee
Post Number: 43
Registered: 7-2003
| | Posted on Thursday, March 11, 2004 - 4:47 am: |
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The owners manual for my 2 meter base antenna says to put a single 10 inch loop in the coax to make an RF choke as close to the base of the antenna as possible to keep the coax down lead from becoming a second image of the antenna. My 2 cents worth. Yankee |
Bigbob
Senior Member
Username: Bigbob
Post Number: 1728
Registered: 12-2001
| | Posted on Thursday, March 11, 2004 - 4:59 am: |
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Star duster used that way of feeding their antenna,I think Tech808 should chime in on this one or bruce as they probably have had first hand knowledge of this type of antenna feed,I just meerly saw it.Bigbob |
Tech833
Moderator
Username: Tech833
Post Number: 585
Registered: 8-2002
| | Posted on Thursday, March 11, 2004 - 12:45 pm: |
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Running the cable inside the mast is far more effective than wrapping a coil of coax under the antenna. I will also act as an element is the correct length. Do a search for 'coaxial dipole' sometime.
The starduster had the cable inside the mast for mere simplicity. |
Bigbob
Senior Member
Username: Bigbob
Post Number: 1731
Registered: 12-2001
| | Posted on Thursday, March 11, 2004 - 7:02 pm: |
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I'm sorry Tech 833,I forgot about you because I hardly ever see you on open forum,it is a pleasant surprize. |
Pp1616
Member
Username: Pp1616
Post Number: 51
Registered: 2-2004
| | Posted on Friday, March 12, 2004 - 1:30 pm: |
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so runing your coax in your antenna mast would be a good thing hmmm i have been thinking about doing that and was going to post on the board but forgot to do it.. |
Ozzie
Intermediate Member
Username: Ozzie
Post Number: 123
Registered: 11-2003
| | Posted on Friday, March 12, 2004 - 8:06 pm: |
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Just to confuse the issue a little - some antenna designs RELY on the first quarter wave below the antenna to radiate as part of the antenna. These antennas work without causing any more rf than any other antenna, all else being equal. These antennas are called Controlled Feeder Radiation antennas. |
Bruce
Senior Member
Username: Bruce
Post Number: 1072
Registered: 9-2003
| | Posted on Friday, March 12, 2004 - 8:47 pm: |
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like the ringo ranger 2 |
Tech833
Moderator
Username: Tech833
Post Number: 590
Registered: 8-2002
| | Posted on Tuesday, March 16, 2004 - 1:29 pm: |
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A "Controlled Feeder Radiation" design is simply wasting RF energy through cable reradiation which should have been dealt with in an RF transformer network. |
