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Old 05-15-2018, 3:07 PM
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Default 2.25 SWR install can't get it down

Hello, I have a 4 foot fiberglass antenna (from a farm store) spring mounted with a three way flange on the very top of my truck's ladder rack. I ran a 8 gauge wire from the aluminum ladder rack through the bed and directly to the frame of the truck. When I attached the wire to the frame I used a dremel tool to clean off all the paint first. I've tuned the antenna to the best of my ability and I still get a 2.25 SWR from 1 to 40. I've checked the cable and its free of shorts, the mount is grounded well almost no resistance.

Any ideas why the SWR is so high?
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Old 05-15-2018, 5:56 PM
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Adding a spring can affect rhe SWR. Pics are good and details help alot.
It would be helpful to know what the reading is on Ch1 and Ch 40. See this link and the area on antenna length and SWR,

https://www.rightchannelradios.com/b...g-instructions

Also are you using an internal meter or external?
This might get some feedback as opinions differ on the board but you have no real groundplane with that type of install, Thoughts anyone?
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Old 05-15-2018, 6:33 PM
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DC ground and RF grounds can be different things. Making sure the rack/mount is grounded is a good thing, but running a wire all the way down to the frame likely isn't helping.
Making sure the mount is grounded to the lumber rack is more important.

As Retro said, many of the antennas are designed to be used with a spring. The spring is part of the antenna and adds to the antenna length, which impacts the resonate frequency of the antenna. Without knowing the type of antenna you have, it's hard to tell what you need, though.

Adding the spring would also be a real good idea since that antenna is up high and will easily meet some tree branches.
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Old 05-15-2018, 6:45 PM
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I think the antenna doesn't have enough ground plane under it. As a test, take about 9ft of wire and attach to the ground side of the mount then string the wire horizontal away from the drivers side of the mount.

If the antenna will tune ok with the wire then its probably a ground plane issue. If not it would be good to let us know what the match is on ch 1 and ch 40. That can tell you if the antenna is too long or has been trimmed too much.
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Old 05-15-2018, 7:55 PM
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The SWR is a flat 2.25 from channel 1 to 40

I did quite a bit of trimming wire off the end to get to this point ( started with 3 at 40 and 2.5 at 1)

Thanks for the ground wire suggestion will try it in the morning. Think I'll also put a ground wire to the tool box directly below the antenna, can't hurt. If I can I'd really like to keep this mounting location.
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Old 05-16-2018, 2:55 PM
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Ok I've found out the antenna is a PowerComm 1000 watt 4ft whip.

Their catalog:
http://www.lyncoproducts.net/product...s-CB-Lynco.pdf

After I ran a 14 gauge wire from the ground side of the mount, the SWR readings went from 2.25 flat 1-40 to 2.5 @ 1 and 3.25 @ 40.

Then I tried moving the antenna to the center of the ladder rack with similiar swr readings. Then moved the antenna up to the ladder rack crossmember right over the cab center, but the swr readings stayed in the 3's.

I'm beginning to think I have a bum radio or cable. The swr meter is an external Astatic I got from flying J truck stop. I also ran power directly to the battery. I would never have guessed I'd have this much trouble installing a CB.
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Old 05-16-2018, 6:22 PM
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I think your radio is probably good; but the ladder rack is throwing the vswr all over the place. If you can place the antenna on a vehicle that has no obstruction as does the ladder rack, I think you may find the vswr lower when you run tests.
Hopefully you haven't cut too much off, "...quite a bit of trimming wire off the end...".
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Old 05-16-2018, 7:41 PM
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If the SWR is higher on ch 1 than ch40 then the antenna is too short.
See the article link I posted.
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Old 05-17-2018, 7:55 AM
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What if I was to install a sheet of aluminum from the front ladderrack crossbar to the middle and then put the antenna in the center of that do you think that would cure my ground plane issues and lower my swr to acceptable levels?
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Old 05-17-2018, 10:24 AM
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It would be worth the effort to see what the results would be. Larger the sheet of aluminum the better the outcome.
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Old 05-17-2018, 10:44 AM
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Quote:
Originally Posted by MikeMic View Post
What if I was to install a sheet of aluminum from the front ladderrack crossbar to the middle and then put the antenna in the center of that do you think that would cure my ground plane issues and lower my swr to acceptable levels?
You flattened the SWR from where you started which is good. 2.25 from 1 to 40 flat and I'm talking relative not the absolute number is interesting/odd. Maybe the absolute number is inaccurate, but the flatness across the band is good. The last time I saw that kind of flatness across a band of frequencies the manufacturer was selling a snake-oil antenna with a built-in dummy load.

Bottom line... Does the setup communicate well with other radios. 2.25 is not great, but is not a deal killer, unless there is something amiss with the feed line or antenna. Maybe its as good as its going to get and more work will not improve the situation.

Good Luck,
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Old 05-17-2018, 4:37 PM
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sounds like it could be a coax issue as well, a weak solder in the connector or the base of the antenna,
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Old 05-17-2018, 8:15 PM
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Yeah, guys I was wondering if this el cheapo antenna was to blame. I picked up a 4 foot firestick with the screw top tuner, will try that out tomorrow. But today I pulled the coax out of the truck and went through the door and no change, I was thinking maybe the route had something effecting the signal? nope no change. Just on a whim I tested the vswr while driving down the road 55mph, and it had dropped into the 1.5 range on channel 19. Is that normal? Is vswr something that changes at speed? or maybe because the spring was cocked back it changed the situation? I'm tempted to use a old pizza pan with a hole in it, antenna sticking through, to test the ground plane theory. If that works I'll step up to a large piece of aluminum plate.

Just like working on cars, change one thing at a time until it responds...
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Old 05-17-2018, 8:30 PM
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Quote:
Originally Posted by MikeMic View Post
I tested the vswr while driving down the road 55mph, and it had dropped into the 1.5 range on channel 19. Is that normal? Is vswr something that changes at speed?
Shouldn't be. See more below...


Quote:
Originally Posted by MikeMic View Post
or maybe because the spring was cocked back it changed the situation?
Likely, changing the angle of the antenna to the ground plane would impact SWR. Or, the mount is faulty. Or the antenna is faulty.
I never liked those fiberglass antennas. Probably because I had cheap radio shack ones when I was younger. Seemed like they worked well for a while, then the antenna would flex enough to break the internal conductor, then the SWR would go to heck.

I've learned over the years to never assume a new part is perfect.


Quote:
Originally Posted by MikeMic View Post
I'm tempted to use a old pizza pan with a hole in it, antenna sticking through, to test the ground plane theory. If that works I'll step up to a large piece of aluminum plate.
Likely too small a ground plane to make enough difference. A ground plane doesn't need to be a solid sheet of metal. Your lumber rack should provide some manner of ground plane, even without the truck under it.

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Originally Posted by MikeMic View Post
Just like working on cars, change one thing at a time until it responds...
Just make sure you are changing the right stuff, or it gets expensive.
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Old 05-17-2018, 10:22 PM
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Quote:
Originally Posted by MikeMic View Post
Hello, I have a 4 foot fiberglass antenna (from a farm store) spring mounted with a three way flange on the very top of my truck's ladder rack. I ran a 8 gauge wire from the aluminum ladder rack through the bed and directly to the frame of the truck. When I attached the wire to the frame I used a dremel tool to clean off all the paint first. I've tuned the antenna to the best of my ability and I still get a 2.25 SWR from 1 to 40. I've checked the cable and its free of shorts, the mount is grounded well almost no resistance.

Any ideas why the SWR is so high?
Your situation sounds a little like mine. My antenna configuration was a little backwards. I had an insulator in the wrong spot, on my mount bracket.
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Old 05-19-2018, 7:23 AM
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I moved the antenna to front crossbar right over cab, here is a pic of the connection.

I also pulled the cable out of the route I had it in and ran it through the door as a trouble shooting option, but no improvement.

So with this setup I'm getting SWR of 1.7 at chan. 1 and 2.3 at chan. 40, this might be as good as it gets...

If I understand theory correctly (which I probably don't) if I used a smaller antenna would that mean I need less ground plane? and possibly work better in this installation?
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Old 05-19-2018, 8:02 AM
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If I looked it up correctly, That model on the website is not a groundplane less antenna so therefore a groundplane is required. It looks like you are making progress.
Google Procomm.inc to get to their page.

As a rule when adjusting antennas as described in the aforementioned link I posted. Be aware that adjusting/cutting is done in small increments as its easy to remove but near impossible to put back.
How much have you adjusted it so far?

It looks like your antenna is too long as the SWR is higher on Ch40 than Ch1. This Is opposite to earlier so the change in position has influenced its SWR....
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Old 05-19-2018, 8:16 AM
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Quote:
Originally Posted by MikeMic View Post

snip...

If I understand theory correctly (which I probably don't) if I used a smaller antenna would that mean I need less ground plane? and possibly work better in this installation?
Antenna can be "designed" to be shorter physically, but electrically it still must be of a specific length related to frequency. In your antenna that spiral underneath the white heat-shrink is the reason the antenna is shorter. That spiral is very specific in diameter, spacing and length. Ground planes are related to frequency like the antenna, but you have some and its enough or need more. No designing a ground plane to look bigger than it is.

Shorter antennas are less effective radiators, but survive longer as they get smacked around less.

Good Luck,
Mike
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Old 05-19-2018, 12:37 PM
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Quote:
Originally Posted by MikeMic View Post
I moved the antenna to front crossbar right over cab, here is a pic of the connection.
That's a better location. For omni-directional performance (equal radiation in all directions) you want equal ground plane around the antenna. Placing it off to one side will make the radiation pattern lopsided.

Quote:
Originally Posted by MikeMic View Post
I also pulled the cable out of the route I had it in and ran it through the door as a trouble shooting option, but no improvement.
Won't make a difference with coaxial cable. Route it the safest/shortest route. Avoid doors and windows as pinching the cable will introduce a whole host of new issues you don't need.

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Originally Posted by MikeMic View Post
So with this setup I'm getting SWR of 1.7 at chan. 1 and 2.3 at chan. 40, this might be as good as it gets...
I agree with the others, the SWR you are seeing indicates the antenna is too long. Try removing the spring and see if it comes down to more acceptable levels.

You should be able to do better. When I've run CB's, I use Larsen NMO-27 antennas on permanent NMO mounts on the top of the truck cab. Never had an issue getting SWR down very low. Here's what it looks like on an antenna analyzer.
Just looking at CB channels 1-40. You want to shoot for lowest SWR on channel 19 (center frequency of the CB band, and that should give you equal SWR numbers on channel 1 and 40:


Zooming out and looking at 26-28MHz, you can see the low SWR dip a bit more clearly:


With your SWR being lower on channel 1 than it is on channel 40, it's indicating that the resonance of the antenna is still too low. Shortening the antenna will fix that. How much you need to shorten it is the thing you'll need to figure out. Removing the spring will take a bunch of length off quickly. If SWR improves, then that'll tell you something. Ideally, you want to do the adjustments in much shorter increments, 1/8 to 1/4 inch at a time, no more.

Quote:
Originally Posted by MikeMic View Post
If I understand theory correctly (which I probably don't) if I used a smaller antenna would that mean I need less ground plane? and possibly work better in this installation?
Not quite….
The radiating element (vertical part of the antenna) is frequency dependent. You can coil it around a fiberglass rod and make it "look" shorter, but it's still the same amount of wire. Trying to jam 108 inches of wire (1/4 wavelength at 27MHz) into shorter packages just reduces efficiency. Ideally you want the full 108" whip antenna, but that's a bit excessive on top of a full size truck. The wound whips like you are using are a good compromise if you keep it reasonable. A 4 foot fiberglass antenna should still work well. Making it smaller might look better, but it isn't going to help performance at all. You might get lucky and find a shorter antenna that has better SWR, but low SWR doesn't equal good performance. I can put a 3 inch long 50 ohm terminator on there and you'd get perfect SWR, but your transmit range would be on the order of feet, not miles.
And, the ground plane stays consistent, no matter what antenna you use. Ideally a "perfect" ground plane for CB would be 108" in all directions, or 18 feet across. You won't be able to legally drive down the road with an 18 foot piece of metal sticking out both sides of your truck. You are limited to the size of your vehicle in this case. Making the ground plane smaller isn't going to help anything. Actually, you have a pretty good ground plane as it is. I'd not mess with that part of your install. Focus on getting your antenna tuned correctly.

I think you are on the right track, just take small steps, and be careful about doing anything you can't undo.
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Old 05-19-2018, 3:15 PM
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Default Check the spring

Bend that antenna over a bit while looking through the coils of the spring. You should be able to see a braided cable in there going end-to-end. This keeps the spring from looking like an inductor.
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