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Author Topic: Can't Tune 40M???  (Read 2603 times)
WT3O
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Posts: 16




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« on: July 03, 2014, 12:56:15 PM »

Well, I have been using my FT-857 for over a year now and find myself using the radio on my commute to/from work. Everything works well, except that I cannot get my ATAS antenna to tune 40M. I can operate 20M-70cm without a problem, but tuning 40M just results in the antenna moving up and down without finding the sweet spot. I thought my grounding might be an issue, so I bonded my vehicle (Honda Element) with the trunk, tailgate, doors and hood.

Does anyone have any ideas why I can't tune 40M? Do you think I need even more bonding?

73 de WT3O
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K0BG
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« Reply #1 on: July 03, 2014, 01:21:14 PM »

The ATAS operates with just two SWR settings—under 2:1 and over 2:1. Typically when it won't tune it is an issue with common mode, although this doesn't rule out a problem with the innards.

Common mode current with the ATAS series has always been an issue. In fact, it is mentioned in both the 857 and ATAS manuals. If you go to my web site, and look under Common Mode, you'll find the answer to curing it.
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KA3NXN
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« Reply #2 on: July 08, 2014, 09:26:56 AM »

Do you have a shunt coil installed at the base of the antenna? This might be your problem.

Jaime-KA3NXN
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K0BG
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« Reply #3 on: July 08, 2014, 03:00:48 PM »

Shunt coil? The only reason to use a shunt coil is to impedance match the antenna. In the case of the ATAS, the overall system losses are rather high, exacerbated by the clip mounts most folks use. As a result, the input impedance is almost always over 50Ω.

The antenna is already DC grounded by its innards, so even a high impedance choke isn't warranted.

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