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Author Topic: Mobile installation  (Read 287 times)
AB9DF
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Posts: 87




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« on: March 01, 2008, 01:06:40 PM »

Just wanted to ask a simple question.  I just installed a ball mount on my chevy van and drilled the holes and everything.  I just worked Central america and a contact in the Leeward Islands.  The antenna seems to be working.  I up till now had been using a tri-mag mount hamstick located in the center of my roof.  In comparison, the only thing I have noticed is that using the same antenna ( I am changing over to a Hustler rm 20) a 20 meter ham stick, my swr is now a 1.5 to 1 versus the mag mount which is 1:1.  Based on my understanding, this would indicate that my ball mount is working better than the mag mount.  I am using a Yeasu 897d with the AT897 tuner running 100 watts.  It is my understanding that I should expect a higher swr when using a ball mount (hole drilled) connection versus a mag mount.  I think I am starting to understand this stuff so I just wanted to confirm my understanding.  

Thanks!

Rob AB9DF
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W5FYI
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Posts: 1046




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« Reply #1 on: March 01, 2008, 02:28:07 PM »

Congrats on the new installation, and working the DX. That confirms that the antenna is working. Yes, the more realistic SWR indicates that your antenna is working close to where it should. For a typical 20-meter mobile antenna, radiation resistance would be somewhere around 10 to 15 ohms, for a lossless SWR of up to 5:1. However, losses are inevitable, and I would guess you have around 20 ohms of non-radiation losses, and that is what contributes to your 1.5:1 SWR reading. The same antenna, with 1:1 SWR, would have close to 40 ohms of loss resistance. In other words, the 1.5:1 SWR indicates you are radiating about ten watts more power than you did when you had the 1:1 SWR indication.

Stew, W5FYI
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W5DXP
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« Reply #2 on: March 02, 2008, 05:55:02 AM »

> W5FYI wrote: In other words, the 1.5:1 SWR indicates you are radiating about ten watts more power than you did when you had the 1:1 SWR indication.

This, of course, assumes that both antennas are resonant. If the feedpoint impedance changed from resonant to non-resonant because of the change in mounts and was not re-tuned, then it could be a different story. So the question needs to be asked: Was the antenna re-tuned for lowest SWR after the change, i.e. did the mounting change cause the resonant frequency to change?
--
73, Cecil, w5dxp.com
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K0BG
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« Reply #3 on: March 02, 2008, 06:31:12 AM »

The fact you can work DX, or any station, has no relevance to efficiency or anything else for that matter.

As Cecil alluded to, if the antenna is resonant (assuming no tuner in place) the fact the SWR increased probably means the ground plane under the antenna has improved.

My question is, why use a tuner with the antenna? If you're trying to use a 20 meter antenna on 40 for example, that isn't going to work very well, even if you can work DX stations. There are better ways to impedance match mobile antennas, without the added complexity of a tuner.

ALan, KØBG
www.k0bg.com
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AB9DF
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Posts: 87




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« Reply #4 on: March 04, 2008, 08:24:27 AM »

Appreciate the insight, I will go back and do more analysis.  I was never thinking of taking the tuner out of the picture but now that you mention it, if the antenna is resonant in the center of the band then it really shouldnt be necessary on a mono band antenna.  Thanks again, I am slowly learning but learning none the less

73
Rob AB9DF
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