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Author Topic: 10M mobile antenna probelms  (Read 1003 times)
NV1P
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Posts: 73




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« on: October 17, 2011, 06:40:13 AM »

I recently bought a Jetstream mobile stick antenna, and I'm having a problem getting it to tune. When hooked up to my analyzer it reads resonance at 34MHz, no matter where the whip section is adjusted to. I think the issue may be the mag mount I'm using isn't providing enough coupling to the car. I didn't really think about it when I bought the mount, and it's a 1 magnet job which is probably only designed for VHF/UHF. Any suggestions on what to check?
When tuning is there a narrow range where I would see any change in the resonance pattern? What increment should I use when moving the whip in and out? I don't want to drill any holes in my car. I was looking at a licence plate mount, but I'm not sure if I should invest in one if its the fault of the antenna.

Thanks,

Sam
NV1P

http://www.cheapham.com/magnet-mount-3-8-x-24-antenna-base-with-coax-cable/
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K7RBW
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Posts: 387




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« Reply #1 on: October 17, 2011, 06:45:45 AM »

I bet if you varied the length of (or even just moved) your coax, the frequency changes.

I have a similar mag-mount and got equally bizarre results until I grounded the antenna. It turned out in mine (after I took it all apart), only the center conductor was connected to the antenna. The shield wasn't terminated anywhere to anything (except the radio at the connector).

I added a ground wire to the shield in the antenna base and connected that to the car body and everything started working as I would have expected.
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W8JX
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Posts: 5665




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« Reply #2 on: October 17, 2011, 06:52:56 AM »

When hooked up to my analyzer it reads resonance at 34MHz, no matter where the whip section is adjusted to. I think the issue may be the mag mount I'm using isn't providing enough coupling to the car.

I think before I would get to upset over this I would check it with different test equipment/bridge as it may be analyzer too.
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K0BG
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« Reply #3 on: October 17, 2011, 07:09:28 AM »

I agree. Antenna analyzers are great devices, but they're easy to blow up.

When you use a mag mount, the ground losses increase drastically. The resonant point will move some, but certainly not several megahertz.

There is another issue that you should be aware of. Almost all hamstick type antennas, allow the end of the whip to extend down inside the portion of the antenna where the coil is located. This makes tuning the antenna nearly impossible, especially if all you have is an SWR bridge.
« Last Edit: October 17, 2011, 07:11:01 AM by K0BG » Logged

W8JX
Member

Posts: 5665




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« Reply #4 on: October 17, 2011, 07:29:59 AM »

There is another issue that you should be aware of. Almost all hamstick type antennas, allow the end of the whip to extend down inside the portion of the antenna where the coil is located. This makes tuning the antenna nearly impossible, especially if all you have is an SWR bridge.

Yes if you need to shorten it to point that whip is inside of this coil, you need to shorten/cut whip. It is easier to cut it with a grinder or cutoff wheel than a saw too.
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