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Old 04-30-2011, 07:10 AM
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Default high SWR on short antenna

I set up a bracket on the roof rack for the ham antenna. I got the big bad-arse diamond sg7900 and the a short SBB1. the sg7900 shows like 1.2 swr. , but the stubby antenna is at like 3.2 or more! I don't want to cook my radio. (btw, I got the sbb1, then returned it for a different style/model but still bad swr)

I grounded the bracket to the rack...no difference in swr. I then took my long aligator clips to find maybe a better ground...no difference.

the antenna is mounted near the front of the gamiviti rack - could the signal be reflecting that badly off the front of the rack.

John at HRO said these little antennas like a DC ground. (direct chassis??)

Also...is it dumb to drive to Moab with a 5' tall ham antenna on my roof?
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Old 04-30-2011, 07:24 AM
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Originally Posted by CardinalFJ60 View Post
I set up a bracket on the roof rack for the ham antenna. I got the big bad-arse diamond sg7900 and the a short SBB1. the sg7900 shows like 1.2 swr. , but the stubby antenna is at like 3.2 or more! I don't want to cook my radio. (btw, I got the sbb1, then returned it for a different style/model but still bad swr)

I grounded the bracket to the rack...no difference in swr. I then took my long aligator clips to find maybe a better ground...no difference.

the antenna is mounted near the front of the gamiviti rack - could the signal be reflecting that badly off the front of the rack.

John at HRO said these little antennas like a DC ground. (direct chassis??)

Also...is it dumb to drive to Moab with a 5' tall ham antenna on my roof?
I just mounted a 48" Comet SBB25 on my 40 and Dave put his meter on it and I had consistent (high) 2.5 SWR also. We tried grounding it and nothing. He said it's just the nature of the beast with these Japanese antennas. We also tried putting 2 other borrowed antennas on (long and stubby) and the SWR was still high.

I would just run your 5'. Not like you're going through trees or anything on the way to Moab. Just remember it's there when you pull in the fast food drive through.
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Old 04-30-2011, 07:48 AM
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I run my big antenna everywhere. ESPECIALLY on the Hwy.
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Old 04-30-2011, 07:59 AM
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Depending on the length of the long antenna, it may not need a ground plane. If it's a 1/2λ (or other ground plane independent design) and your stubby is 1/4λ, then with an imperfect ground plane that would explain why your SWR readings are like they are.

I'm with Marco on it though, I always run my 5/8λ whip on the interstate, and I run my 1/4λ dual band around town.
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Old 04-30-2011, 08:26 AM
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Default sweet! I'm running the giant antenna.

good to know. I'll figure something else out for a small antenna, someday. and bring it with me. I'm 11' tall with the antenna on the roof.

is swr of 3.0-3.2 too high to use on a trail run even with limited use?
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Old 04-30-2011, 03:15 PM
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Originally Posted by CardinalFJ60 View Post
good to know. I'll figure something else out for a small antenna, someday. and bring it with me. I'm 11' tall with the antenna on the roof.

is swr of 3.0-3.2 too high to use on a trail run even with limited use?
Eh, If it were me I wouldn't run it at 3.0-3.2. You will probably do some damage to the finals in the radio. I'm not sure how much it takes to cook the finals, but I wouldn't be in a hurry to find out

For academic purposes, assuming 50w output, at 3.2 vswr:
You'll lose 27.4% of your transmit power, and 10.7w will be reflected back to the finals in the radio.

FWIW, if you're running an NMO mount, I'd recommend the Larsen stuff. The whips are stainless, and are replaceable without replacing the coil. If you run it into something, you can just swap the whip. They make several different ones. The 5/8λ you do have to cut to tune, but if you know someone with an SWR meter, or better yet an antenna analyzer, then you're good to go. I'm not sure how tall I am over all with the 5/8λ, but it's up there pretty high. I've run it into all sorts of things and it's still working like a champ.
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Old 04-30-2011, 06:04 PM
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Martin's Comet measured 2.5:1 across a very wide band. We tried a ground wire from the base of the antenna to the frame with no affect. He has his antenna mounted on the right fender, which strikes me as a pretty decent place. We tried Nakman's Diamond, measured the same. I was using my MFJ-269 analyzer, which is maybe not lab quality but works fine enough.

He was using a pre-terminated SO-239 mount, so I wasn't too concerned that he had a bad PL-259. It measured DC fine.

Are the connectors and terminations good and clean? How and at what power are you measuring SWR? Is your truck in the open, away from buildings and metal (importantly metal)?

Very low SWR is generally a good thing to shoot for, but that is not the only criteria for antenna performance. Sometimes things are not what they seem. None-the-less, here I think it's telling you something is not happy.

First, the SBB-1 is a 1/4λ on 2m and the SG-7900 is a 7/8λ on 2m. The SBB-1 very much relies on a ground plane to operate, while the SG-7900 is probably less dependent. This would be my main guess as to why you have a high SWR, a bad RF return. RF is a physical thing and it wants a complete circuit, just like a headlight in your truck or an outlet in your house. So the antenna itself is 1/2 of the required parts, the RF that excites free space electrons needs a way to get back home to the ground of the radio, which is why a ground plane or antenna ground are critical.

A photo would help!

I'm picturing that you have a mount that is a small tab with the NMO or SO-239, where the antenna screws on. The tab is connected to the roof rack, but both are painted and there is not real good conductivity. Further, the rack itself is painted and electrically isolated from the body of the truck.

My guess is scraping some paint from the mount and a length of 1/2" braid running to a /good/ ground (e.g the frame) will fix your SWR. You could ground the strap (NOT WIRE, use proper braid) to the body of the truck and the body to the frame in a couple of spots, if you want.

It's also conceivable that the roof rack could be a parasitic radiator that's causing a reflection. SWR is a measure of the ratio of power sent forward out of the radio against power reflected back. But it's nearly impossible to tell if the reflection is due to a bad connector, kinked coax, bad coax-to-antenna connection or power reflecting from nearby objects. So you have to work through and eliminate them. Since your reference antenna has a low SWR, you can probably safely assume the radio through mount connections are OK, so it's an interaction of the antennas themselves.

Could also just mean the antenna needs to be tuned. Do there happen to be a couple of Allen screws holding the whip on? If so, try loosening them and making the antenna longer or shorter and see if the SWR changes.
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Old 04-30-2011, 09:32 PM
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If you know someone with a 1/2λ antenna, you could always try that and see if it's any better.
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Old 04-30-2011, 09:56 PM
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Quote:
Originally Posted by xodeuce View Post
If you know someone with a 1/2λ antenna, you could always try that and see if it's any better.
The SBB-1 is 1/2 wave on 70cm. If we know the SWR on that band it might be useful, too. Can't see a way to tune it from the photos on the web. Sorta looks like a HT rubber duckie with an NMO mount. :-)
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Old 05-01-2011, 08:25 PM
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Quote:
Originally Posted by DaveInDenver View Post
The SBB-1 is 1/2 wave on 70cm. If we know the SWR on that band it might be useful, too. Can't see a way to tune it from the photos on the web. Sorta looks like a HT rubber duckie with an NMO mount. :-)


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