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Author Topic: Very high SWR?????  (Read 2578 times)
AE5KH
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Posts: 29




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« on: February 18, 2013, 03:02:49 PM »

I am having trouble with my setup but I am not sure where to go next. I am running a mobil station with a IC7000 and a Larsen Special 2m/70cm antenna in the antenna 2 port VHF/UHF. I connected an inline SWR meter and I am getting a very high SWR reading, around a 5/6 SWR @ 147.270Mhz. I checked both the jumper cable and the main feed cable with a ohm meter and I did not get any reading when I crossed from the pin to the shield on either of the connectors and then I checked the continuity through the cable on both the pin and shield and that was fine. I checked the continuity through cable by crossing one connector and checking it at another and it was fine. I even disconnected the mount from the truck and tested the SWR and got the same number. I have calibrated my meter several times to check and I keep getting the same numbers. I have not connected or setup the HF side yet so I don't know what the SWR are for the internal meter on HF. I keyed up a repeater that is about 30 miles away and got good audio on a quick test. I thought if I was mismatched or high SWR audio would suffer if I could key the repeater at all. Not sure what else to do. I know the antenna and mount are good because I removed them from my last truck and mounted it in the same place on this truck.

Not sure what else to do? My next option I guess would be to get a new SWR meter?
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WB2WIK
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« Reply #1 on: February 18, 2013, 03:13:28 PM »

How's it mounted?

Also, it always pays to test SWR at a number of frequencies, not just one.

What is it at 144.1?  145.0?  146.0?  147.0?  147.9?  This would help reveal if there's a serious problem or it's just mistuned for the new environment and the antenna needs tuning.  (Normally, they can be tuned slightly by adjusting the length of the radiator whip.)

Could also be a bad NMO coil at the base -- they can be damaged, and they do go bad sometimes (usually due to mechanical shock).
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AA4PB
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« Reply #2 on: February 18, 2013, 03:26:45 PM »

Normally high SWR won't affect the audio on the FM transmitter. The transmitter will normally reduce its power output in order to protect the finals from the high SWR (over 2:1) and that will reduce your range, but it usually doesn't make any noticeable affect in the audio.

30 Miles seems like a pretty good range for an antenna with such a high SWR but that will be affected by your location and the repeater height so it's hard to tell.
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WB6BYU
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« Reply #3 on: February 18, 2013, 03:32:14 PM »

Particularly check that the mount is well grounded to the sheet metal
skin of the truck RIGHT AT the mount.  Grounding the mount though
the coax shield back to the rig will look OK for a DC meter, but won't
work properly.
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AE5KH
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« Reply #4 on: February 18, 2013, 05:26:43 PM »

I checked SWR at all frequencies listed above and the numbers are the same (5/6). The mount is grounded to the stake pocket that the mount is inserted in. The mount bolts in the pocket and grounds directly to the shield side of the PL259 connector.
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KB4QAA
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« Reply #5 on: February 19, 2013, 05:35:31 AM »

Tossing out ideas and questions....

-Is the meter a VHF/UHF meter?
-If it is an Power/SWR meter, does the power reading agree with the radio power output setting?
-Confirm you are using one of these modes: FM, AM, CW to take your measurements and not SSB (no carrier)

-How good is the 'ground' connection between the mount bracket and the wall pocket?  Have you Ohmed out between the coax shield and some point nearby on the bed wall?

-Have you put some sort of compound on the mounting bracket bolt to exclude moisture and maintain low resistance?  e.g.  GB Oxigard, Penetrox, Noalox, Vaseline?

-Consider adding bonding strap(s) between the bracket and the bed wall.

-Is the antenna length adjustable?  Have you tried adjusting it?  (although the flat SWR is very suspicious, that something else is the culprit)

-Can you borrow an antenna analyzer or bridge from someone?  Note, ensure that it has fresh, fresh batteries, as the MFJ devices tend to give flakey readings when battery voltage drops much at all.

bill
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KE3WD
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« Reply #6 on: February 19, 2013, 05:46:10 AM »

... checked both the jumper cable and the main feed cable with a ohm meter and I did not get any reading when I crossed from the pin to the shield on either of the connectors and then I checked the continuity through the cable on both the pin and shield and that was fine. I checked the continuity through cable by crossing one connector and checking it at another and it was fine....

Was the Antenna connected to the other end of the coax when you found no DC continuity from center pin to coax shield at the radio connection end? 


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AE5KH
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« Reply #7 on: February 19, 2013, 11:26:00 AM »

The meter is a VHF/UHF SWR Watt meter.
When I check it @ 148.900 it does not agree, it is about 20 watts off but at 144.100 it is correct to what it is supposed to be on FM.
I checked the continuity from the shield side of the connector at the radio to the bed of the truck and got a good reading. Both the radio was disconnected and the antenna was removed from the mount.
I am using a analog SWR meter.

All testing on the cables were done with both ends disconnected.

I also tried a spare dual band antenna I have and I got similar readings except at the low end of the band I got a 3 and on the high end I got a 5.

I removed a redid my connectors just to be sure.

I believe the antenna is adjustable because it does have allen screws at the base. Not sure how much to move it though??

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KB4QAA
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« Reply #8 on: February 19, 2013, 12:01:18 PM »

Quote
I also tried a spare dual band antenna I have and I got similar readings except at the low end of the band I got a 3 and on the high end I got a 5.
That's better, seeing some variation, though again high.

Given all that, there is obviously a problem with the antenna mounting and grounding.

-Try moving the antenna to a different pocket.  You may want to experiment with some different locations and configurations temporarily.

good luck, bill
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AE5KH
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« Reply #9 on: February 19, 2013, 12:59:38 PM »

I just removed the mount and filed down the edges and reinstalled the mount and I realized something when I tested it again. I got it wrong in my above post.

When I check the original antenna @ 148.900 it does agree to what it is supposed to be on FM, showing 50 watts output but with a high SWR @ 5 but it is about 20 watts off at 144.100 and only a 3 SWR.

This does not make sense to me, I thought the output would be reduced by the radio if you had a high SWR.(as stated above from AA4PB) My meter is showing opposite from this.
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WB6BYU
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« Reply #10 on: February 19, 2013, 01:09:20 PM »

Does the SWR change if you unscrew the antenna from the mount?

How far is the antenna from the back of the cab?

What happens if you check the cable+mount with an ohmmeter, with
the antenna removed?

How long is your coax, and of what type?


Oh, and I would only check SWR at low power - I use the lowest output
setting sufficient to get an indication on the meter, especially when I'm
having problems, because it provides additional insurance against damaging
the rig.
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AE5KH
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Posts: 29




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« Reply #11 on: February 19, 2013, 01:43:17 PM »

I will do all tests in future under low power. thank, did not think about that.

1. I adjusted the antenna all the way up and got: 144.100 2 @ 20 watts adjusted antenna all the way down and got: 144.100 3 @ 30 watts.
                                                                   148.900 4 @ 50 watts                                                             148.900 5 @ 40 watts

2. With antenna removed from the mount I get a 10+ SWR @ 5 watts With antenna installed @ 5 watts I get a 3 SWR all tested at 144.100

3. The antenna is about 1 ft away from the cab of the truck. Which is the same as it was from my previous truck. No issues same antenna different radio. I will try and remove the mount from the truck and set it further away from the cab and see what I get.

4. I tested the cable with the antenna removed and it is the same. I don't get a reading unless I cross the pin and shield on either the radio side or the antenna side. I get a continuity reading if I cross the pin and shield and test at the NMO or I check from the thread and pin of the NMO connector and cross the connector at radio.
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AE5KH
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« Reply #12 on: February 19, 2013, 02:06:13 PM »

I moved the antenna down the bed a little ways but I don't have a ground at all so I got 10+ SWR.

I tested @ 440.100 and I got a 10 SWR @ 5 watts

I am really confused. I can try and run another ground wire from one of the screws to metal under the stake pocket or to the frame of the truck.
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WB6BYU
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« Reply #13 on: February 19, 2013, 02:25:41 PM »

I hope you are testing at 147.900 MHz, as 148.900 is out of the ham band.
Generally you can just check it at 146 or 146.5, though sometimes it is useful
to check it across the band to see which way it needs to be adjusted.

Having the metal cab 12" away will certainly affect the feedpoint impedance.


Remember that any conductor more than an inch or two long is NOT going
to provide a good RF ground on 2m and 440.  And shorter is better, especially
on 440.  Your best bet is to scrape off the paint and make sure the mounting
bracket is connected directly to the sheet metal of the bed as well as you can.


From your SWR curve, it may be that you need to shorten the antenna a little,
since SWR appears to be better at the low end of the band on 2m.  Not sure
how that affects the 440 resonance, however.
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AE5KH
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« Reply #14 on: February 19, 2013, 02:59:49 PM »

Yes you are correct the frequency I have in the radio is 147.900 my brain is not functioning trying so many things to get this fixed.


I have all ready removed the mount and filed the edge where the mount comes in contact with the paint. It did not help.

The antenna is set to the shortest I can get it.
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