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Antenna swr help baffled

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  #1  
Old 03-10-2012, 10:06 PM
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Default Antenna swr help baffled


I installed a new cb setup in my pickup: Larsen nmo27 whip antenna and Uniden Grant LT. The nmo mount was installed on the pickup roof by a professional shop. They also routed the cable to a location under the dash. The nmo mount has a factory 17 foot coax. I installed the radio under the dash. Seemed like a good clean install for all components.

Before I did any tuning I checked the antenna system for continuity, ground, and shorts. Everything checked out okay. So I did my first check with an swr meter and also confirmed first readings on the radio. First readings came back 2.3, 3.0, and >3.0 on channels 1, 20, and 40 respectively. SWRs were high, so I proceeded to tune, with each tune about a quarter inch off the antenna. Here are the readings, after each successive tune. Numbers are in order for channel 1, 20, and 40.

2.1 2.8 >3
2.2 2.4 3
1.9 1.8 2.4
2.4 1.7 2.1

Then since swr on channel 1 exceeded channnl 40, i stopped trimming and adjusted the whip up a bit. So the final numbers are:

2.2 1.7 2.2 on the swr meter and
1.9 1.3 1.9 on the cb.

So in essence i have 2s on 1 and 40, and about 1.5 on 20.

Since everything seemed like a good install i was hoping to achieve better swrs. Am i missing something?? Are these numbers acceptable? Could i have missed something wrong in the system check? Could a ground show good on multimeter but be improved?

After tuning i attached my antenna spring to see if i might get better readings if I tuned with the spring. The swrs went way up again, i suppose because it lengthened my antenna. When i checked the swrs with the spring, it blew the inline fuse twice, so i stopped any more tuning and called it quits.

On a side note, i do have some engine rpm-like noise coming through. Could that indicate a problem? i powered to accessory power in the ignition harness.

Thanks for your help!!! One of these days ill figure all this out. Or maybe im just being too picky and its good to go.


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Old 03-11-2012, 05:39 AM
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Doesn't appear that it will get any better and the antenna by all your readings is very narrow banded.

I would not tie power into a harness unless it's a direct non-switched battery feed but that may not eliminate the alternator whine your getting and you can determine if it's from the power source by disconnecting the antenna at the radio and if the noise goes away then it is coming through the antenna and not your power lead.

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Old 03-11-2012, 06:09 AM
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Ditto
Looks like it's a typical narrow banded antenna, so that's about as good as it's going to get. As for the noise, try the same old 'cures' everybody else does, that's abaout the only thing you can do.
- 'Doc

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Old 03-11-2012, 06:49 AM
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I looked up the spec for the NMO27 antenna, which is 1.5% bandwidth for 1.5:1 SWR and 3% bandwidth for 2:1 SWR.

Here’s the math:

Channel 1 is 26.965 MHz and Channel 40 is 27.405 MHz. Mid-Channel is (26.965 + 27.405)/2 = 27.185 which is Channel 19.

A 1.5% bandwidth is 27.185 * 0.015 = .408 MHz
Equals 27.185 - .204 = 26.981 MHz (approximately Chan 3)
27.185 + .204 = 27.389 MHz (approximately Chan 38)

Thus you should see a SWR of 1.5:1 or better between Channel 3 and Channel 38 when tuned up properly.

Adding the spring lengthened the antenna, so it would need to be shortened (as you determined) to bring the SWR back down again.

I hope this helps you determine what you should expect to see.


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Old 03-11-2012, 06:58 AM
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On reread grounds are very important so if the antenna does not have a ground lug at the mount then one can be made at the radio's cabinet but I always put one there so I don't have to depend on the antennas connection or the negative lead on the power lead.

Covering my bases I suppose, hmm can't hurt!

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Old 03-11-2012, 07:57 AM
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Thanks. Can you please explain narrow banded? The larsen antenna that i have covers 27 to 30 mhz. Theres another brand Laird/Antenex in the same nmo mount style that covers only cb band 26-28. KCOQ, do you calculations hold for my 27 to 30 mhz. Would this be narrow or wide?

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Old 03-11-2012, 08:42 AM
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Looked up the Laird antenna (Model C27 w/o spring or C27S w/spring) and the specs just shows the antenna broad operating range of 26.75 to 31 MHz. It doesn’t state what the SWR bandwidth is, so we can only postulate it is the same bandwidth of the Larsen NMO27 antenna.

Narrow or Wide? Typically the bandwidth is stated as the maximum SWR points; usually the 1.5:1, and sometimes 2:1. I would say the bandwidth is about normal for these antennas. Narrow might be 10 channels wide (i.e., Channel 15 to Channel 23), whereas wide would be about 1 MHz or more.

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Old 03-11-2012, 08:54 AM
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Okay, thanks. Im just frustrated on wnat to do next. I could add the spring and tune again, hoping to get better results. Or get a new antenna, or live with it, and hope my grant lt doesnt get fried.

If i got an antenna that more closely covered the cb band (looks like mine covers 10 m as well), do you think swrs would improve?

Can i tune with a different radio and then replace with my main one, or does it need to be tuned with the main radio. I have an old one that iti wont be the end of the world if it gets damaged.

Just need an experienced person to "tell me what they would do" lol

Thanks a ton.

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