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Grounding increases SWRs?

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Cruiseomatic

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Just grounded my Imax with 6ft. of copper rod with copper wire and my SWRs increased by .5. How or why did this happen? Since when did grounding increase anything? I know that 1.5 and below is "normal" but I'm the kind that strive for lowest possible.
 

prcguy

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Some antennas like the Imax, A99 and others with no ground radials do not decouple the antenna from the coax very well and anything you add to the ground side of the antenna can change the tuning. The antenna is reacting to a new ground counterpoise and not the actual earth ground.

I'll bet if you leave the ground wire attached to the antenna and disconnect from the ground rod it will still show a different match than without the ground wire. In this case there will be some length of ground wire that will have little effect on the match or you could check into one of the ground radial kits for these antennas which could fix the problem.
prcguy
 

Cruiseomatic

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Tried removing the rod and no change. Then removed the ground wire and no change. Wind started blowing and the SWRs reflected that. Now that I'm really thinking about it, I bet one of the two coax lengths on the pole is either bad or has some loss is it and when the wind flexes the mount, its flexing the coax. Other day when the SWRs were low and no wind, I was making some DX contacts and now, none.
 

Ironfist

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The braid strap will not effect your SWR. It will help with a lightning strike if your antenna don't split. I recommend 3 or 4 "1" foot circles in your coax to spin off lightning as well. You more than likely have a loss in the connector or the coax itself somewhere. Start testing from the nearest point available on your setup. Right before your inline meter and ending at the antenna. Pull the antenna down and pull the coax off of it and get the multi-meter out and check the resistance at the tip of the coax. It should read around 50 ohms. Make sure to keep the other end of the coax hooked up to your radio no matter which connect point you are testing.. 1.5 is not bad but a flat SWR is easily achievable with an IMAX 2000. Make sure that you have a good inline meter as well.
 

jonny290

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First of all, SWR stands for Standing Wave Ratio. It is singular. Only one ratio.

The braid strap will not effect your SWR. It will help with a lightning strike if your antenna don't split. I recommend 3 or 4 "1" foot circles in your coax to spin off lightning as well.
Lighting doesn't 'spin' off coils of coax, though visualizing this is highly amusing.

Those loops are decoupling loops meant to keep RF off the feedline.

Pull the antenna down and pull the coax off of it and get the multi-meter out and check the resistance at the tip of the coax. It should read around 50 ohms.
Er, no. RF impedance has little to do with DC resistance, especially in a coil-matched 5/8 wave (assuming imax 2000 in the OP). If an Imax 2000 is working right it will read as a DC short to a multimeter.
 
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Ironfist

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First of all, SWR stands for Standing Wave Ratio. It is singular. Only one ratio.



Lighting doesn't 'spin' off coils of coax, though visualizing this is highly amusing.

Those loops are decoupling loops meant to keep RF off the feedline.



What I meant.

Er, no. RF impedance has little to do with DC resistance, especially in a coil-matched 5/8 wave (assuming imax 2000 in the OP). If an Imax 2000 is working right it will read as a DC short to a multimeter.

Er yea, if the coax is still hooked to the radio then the meter will read the Ohms from the radio to the coax which should be 50 Ohms for an CB. The radio has to be on when testing. If the coax has a break in it or there is a bad connection or a short it will read much higher or lower or nothing. It's actually a great way to check ones setup if the SWR is off.
 

LtDoc

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"I was making some DX contacts and now, none."
That has nothing to do with the SWR of the antenna system. You'll have to talk to 'Momma Nature' about 'her' propagation, that's where that would come in.
In general, a '0.5' change in SWR isn't a biggy. Changes like that can be from quite a few things happening, that wind being only one of them.
Using a ground line on an antenna is not a bad idea. But that's a safety ground in the case of an 'A99' or an Imax antenna, has very little to nothing to do with RF grounding. Also, if that antenna is mounted on a metal pole/mast/tower, why not use that pole/mast/tower as the 'ground line'? It's already there, use it. And then you get into the subject of safety grounding and it's not 'simple' at all.
Have fun.
- 'Doc
 

Cruiseomatic

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"SWRs" as in, It changes on multiple freqs. and as the DX contacts are, Wasnt just propagation, guys in a city 10 miles away that used to hear me well couldn't anymore. I did eventually find the problem. Had high loss in a stretch of old coax. Replaced it and everything is good for now. And you're right Doc, I am now working on both safety AND RF grounding.
 

AC9BX

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When making DC measurements on feed lines and antennas you should not expect to see nominal impedances. That 50 ohms is how RF behaves in a reactive circuit. It will not matter if your radio is on or not. Depending on how the output circuit is configured you could read anything from a dead short to high resistance. A coil of wire at RF is a reactive component. At DC it's just a wire. A basic antenna without a DC ground will always measure as open, not 50 ohms or whatever the nominal impedance should be.

The first thing that comes to mind is wind and water. SWR changing with wind should be so small you could barely see the needle move. If you're seeing it shift around I suspect something is loose, shorting, opening, or wet. A 5/8 wave CB antenna is 24 feet long. That puts a lot of stress on the mount in the wind, so much that it can disrupt the quality of the ground connection. Could the whole thing be shifting around a bit? I would start there. Then I would disconnect the cable at both ends. You should measure an open circuit from the center conductor to shield at either end. If not you've found a potential problem. It's potential because a short or resistance to DC may not be of concern at CB frequencies. However, if it's not open it's generally an indication there is a problem with your cable or a connector. Then create a short at one end. You should measure nearly a dead short at the other. There will be some small resistance over the length of the cable and back, a few ohms.

I'm not familiar with this antenna's construction but the features list says DC ground. So measuring it will likely show a DC short and tell you nothing about its tuning or function. This can make such troubleshooting difficult.

The features list says it offers SWR tuning, well, tune it. If you're getting 1.5:1 over most of the band I would suggest it's okay but could be better. Is it higher at one end of the band than the other? That's a sign of needing to be tuned.

I love marketing like this, this thing says it handles 5000 Watts. HA!, for 2 milliseconds maybe.

A coil of several turns of coax can help prevent lightning damage by imposing a very high impedance to lightning making the cable an unattractive conductor. It may be very effective in keeping small static discharges out of the radio. It is wise to install a grounding block at the point of this coil to insure there is an alternate path for the current to take. Electricity, if pushed hard enough, that is with enough voltage, will go through absolutely anything, wood, glass, ceramic, fiberglass, you, anything. It might destroy the material on the way but it's going through.
 

JayMojave

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Hello All:

When coax has a lot of loss you will see a increase in the SWR Bandwidth, as less RF Energy gets to the antenna and less is returned indicating a wider than normal SWR Bandwidth. Something to keep an eye on.

These antennas I hope are really not rated for any 5 KW rating, as they will last the noted 2 Mil sec, but also will be the "Big Fuse in the Sky" See
New Page 1

Jay in the Mojave
 

SCPD

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check cable

sounds like a connection or cable problem antenna is fine. 90% of the time the problem is in the cabling to a antenna.

P.S. check the cable or connections.
 

Cruiseomatic

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Found A problem. Bad 30 ft. stretch of RG-8/U coax. Measured wave ratios at the antenna and they're flat. In the house and now they're at 1.5 on 40 and 1.6-7 on 1. Replacing it tomorrow.
 

Cruiseomatic

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Did an experiment and added a 6ft. jumper before the meter in line with the old coax stretch still on. Wave ratios went FLAT. WTH? I just spent $60 on 50 ft. of new coax and it looks like I don't need it or do I?
 
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LtDoc

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Make that antenna 50 ohms with no reactance and it won't make any difference what length of feed line you use. Of course, why should anyone believe that!? And the flip side of that, if that antenna isn't 50 ohms with no reactance, the feed line length can always be adjusted to make that meter read whatever you want.
- 'Doc
 

Cruiseomatic

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Not sure I understand Doc, I have read that if you change feedline length and it affects wave ratios that you have a common mode current but I don't have any of the symptoms. I can shake, rattle, roll, grab, etc... that cable and no change. Remove the jumper and previous ratios come back, add it and they're flat.
 
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