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Message |
    
Scarecrow
New member
Username: Scarecrow
Post Number: 9
Registered: 1-2021
| | Posted on Friday, October 22, 2021 - 5:11 pm: | 
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I have come across some old magnet mount cb antennas that have the lead from the base end of the coil soldered to the shield and the center pin. This gives continuity across the center pin and shield. Can someone explain what the purpose of this manufacture would be? TY |
Ke0koy
Junior Member
Username: Ke0koy
Post Number: 42
Registered: 12-2018
| | Posted on Saturday, October 23, 2021 - 2:08 pm: |
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Are they shorted together at the connector or is it shorted through some portion of the coil? I tried to upload a picture of my antenna, but in the age of TB hard drives, this forum unfortunately still has a ridiculously small attachment size limit. Anyhow, on mine, the bottom of the coil is grounded and the center conductor is tapped into the coil about one turn from the bottom. At DC, this is a dead short, but not at 27MHz. In my case, my single coil is serving as two inductors, one in parallel and one in series, forming an L-matching network. Is your situation like mine where there is some length of conductor (inductor) between the center pin and shield, or is yours shorted directly across the connector with the shortest possible wire? |
Scarecrow
Junior Member
Username: Scarecrow
Post Number: 10
Registered: 1-2021
| | Posted on Tuesday, October 26, 2021 - 5:16 pm: |
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ty. Mine is connected from the center pin to shield by approx 3 to 4 windings of the same wire that winds around the insulator 15 or so times, and connects to the antenna.
Thank you for taking the time to explain the 27 MHz part of it to me. yes, i have some length of conductor between the center pin contact (solder) point and the shield.
it is like one small coil goes down the insulator to shield, and the larger coil goes up the insulator to the stinger, both from the same point that is soldered to the center pin terminal on the insulator.
Make any sense?
thanks again for answering me. i have not come across this before. i have fixed corroded contacts in the base of mag mounts before, and always used DC continuity to confirm.
If the coil to shield portion was removed, how would that affect the antenna performance? |
Ke0koy
Junior Member
Username: Ke0koy
Post Number: 43
Registered: 12-2018
| | Posted on Sunday, October 31, 2021 - 12:47 pm: |
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Yes, this makes total sense. It is a matching network so the antenna sees 50ohm. Do not disconnect the coil from shield. Take a look at this video. He gives a simple crash course on smith charts then uses a VNA to demonstrate how capacitors and inductors can be used to adjust impedances (which is what that coil is doing to your antenna). https://www.youtube.com/watch?v=gw1TYWwfvGk |
Scarecrow
Junior Member
Username: Scarecrow
Post Number: 11
Registered: 1-2021
| | Posted on Sunday, October 31, 2021 - 7:42 pm: |
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ok. Thank you so very much for the replies and all the shared knowledge, it is greatly appreciated |
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