Posted: 4/9/2012 6:03:12 PM EDT
I just picked up this beast a few weeks ago and am having trouble with the SWR's for my 10 meter. It's been a while since I messed with mobile radios so help me out if you can.
Since this will be an off-road vehicle, I opted for the 102" steel whip mounted to the left side of the rear bumper on a "gumdrop" mount so it doesn't get ripped off. (The 2 meter 1/4 wave whip on the roof has been seriously abused  ) Coax is Belden RG8X ran through the body, under the door sills, and out from under the dash to the radio with Amphenol PL-259's. Power out is about 25 to 30 watts.
The problem is the SWR is from 2.5:1 to 3:1 and swings backwards to about 1.8:1 with a little modulation on AM. The SWR's get better on the higher freq.s like the antenna is shorter than it actually is. Too much reflection off the back of the truck? Need a better antenna ground? (On my to-do list) Zombie plot to interrupt world-wide comms? Suggestions? |
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Is the SWR fluctuating while parked or moving? A whip that long is going to move around a lot and the SWR is going to change. The SWR on my screwdriver will change between the time I'm stopped and when I'm driving due to the wind pushing the whip back.
If your SWR is getting lower when you're modulating, it sounds like you're experiencing > 100% negative modulation which is resulting in a suppressed carrier and lower power output, thus giving a lower SWR... whoever tuned your CB 10 meter rig probably pushed it a little too hard.  See what it does into a dummy load.
If the amount of space between the whip and the body can vary depending on wind and driving, use a spacer (like a tennis ball) on the antenna to prevent it from getting closer (or worse, touching) the body. You also might try bending it into an NVIS configuration with a piece of string. |
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Your SWR does not/can not change with AM modulation, apparently your TX power is going down with your modulation.
Best solution is to get/borrow/beg/steal an antenna analyzer so you can really see what is going on with the antenna. Wondering what kind of "better ground" you are anticipating for your antenna. If the shield side of the coax isn't sufficiently bonded to the vehicle body at the antenna you will have problems, as there is insufficient capacitive coupling from the mount to the body at that low of a frequency. I have never grasped the fascination with the side fender ball mounts. |
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I just picked up this beast a few weeks ago and am having trouble with the SWR's for my 10 meter. It's been a while since I messed with mobile radios so help me out if you can. http://www.diamond-x.net/photos/i-GL5QmJr/0/XL/i-GL5QmJr-XL.jpg Since this will be an off-road vehicle, I opted for the 102" steel whip mounted to the left side of the rear bumper on a "gumdrop" mount so it doesn't get ripped off. (The 2 meter 1/4 wave whip on the roof has been seriously abused ) Coax is Belden RG8X ran through the body, under the door sills, and out from under the dash to the radio with Amphenol PL-259's. Power out is about 25 to 30 watts.
The problem is the SWR is from 2.5:1 to 3:1 and swings backwards to about 1.8:1 with a little modulation on AM. The SWR's get better on the higher freq.s like the antenna is shorter than it actually is. Too much reflection off the back of the truck? Need a better antenna ground? (On my to-do list) Zombie plot to interrupt world-wide comms? Suggestions? Sounds to me like the ant is indeed to long. Keep trimming a 1/2 in or so off the whip until the SWR is at its lowest point at the center of the band you plan on using. The whip may still be tuned to the 11m (CB) band. ETA: Don't pay any attention to the SWR changes with AM modulation....its no where near accurate. I would suggest you calibrate your SWR meter without any AM mod so you get an accurate reading....assuming, of course, you are using a meter that needs to be calibrated. |
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I had to trim my 102" for 10 meters as well. I also used a racket ball to keep the antenna from direct contact with metal.
Now, I use hamsticks, 102", and an LDG Z-11 Pro. Not ideal, but it works. Also did a lot of bonding on the vehicle. K0GB is a good resource. |
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Thanks for the replies so far.
To answer a couple of questions: I'm taking measurements while parked in an open area. The antenna is mounted through a hole in the bumper near the divers side end, but I think the whole truck could benefit from the addition of some bonding straps. The antenna is about 5 inches away from the body at the closest point near the base and you can see how the body lines move away from the antenna higher up in the picture above. Tennis ball installed as an anti-whack-a-body device. The radio may be "tuned up" a bit too much. I bought it a few years ago, made sure it worked, and it's sat on the bench since then. I'll have to check out some of these issues later in the week. I have to put together a presentation for a class in the morning. Thanks again! |
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Thanks for the replies so far. To answer a couple of questions: I'm taking measurements while parked in an open area. The antenna is mounted through a hole in the bumper near the divers side end, but I think the whole truck could benefit from the addition of some bonding straps. The antenna is about 5 inches away from the body at the closest point near the base and you can see how the body lines move away from the antenna higher up in the picture above. Tennis ball installed as an anti-whack-a-body device. The radio may be "tuned up" a bit too much. I bought it a few years ago, made sure it worked, and it's sat on the bench since then. I'll have to check out some of these issues later in the week. I have to put together a presentation for a class in the morning. Thanks again! No such thing....
As long as the antenna is resonant as the desired freq, you have a sufficient "ground plane", and no close objects, the amount of RF power will not effect the SWR. |
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The radio may be "tuned up" a bit too much. I bought it a few years ago, made sure it worked, and it's sat on the bench since then. No such thing....
As long as the antenna is resonant as the desired freq, you have a sufficient "ground plane", and no close objects, the amount of RF power will not effect the SWR. When I read "tuned up" in quotes, it kind of makes me think of a "10 meter" radio, in quotes. Are we talking about an illegally converted "10 meter" radio, that was "tuned up" for use in a band other than the 10 meter amateur band? |
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The radio may be "tuned up" a bit too much. I bought it a few years ago, made sure it worked, and it's sat on the bench since then. No such thing....
As long as the antenna is resonant as the desired freq, you have a sufficient "ground plane", and no close objects, the amount of RF power will not effect the SWR. When I read "tuned up" in quotes, it kind of makes me think of a "10 meter" radio, in quotes. Are we talking about an illegally converted "10 meter" radio, that was "tuned up" for use in a band other than the 10 meter amateur band? Could be...I don't know. I was speaking with respect to RF input to an antenna.....what radio it is shouldn't really matter IMO. 
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Thanks for the replies so far. To answer a couple of questions: I'm taking measurements while parked in an open area. The antenna is mounted through a hole in the bumper near the divers side end, but I think the whole truck could benefit from the addition of some bonding straps. The antenna is about 5 inches away from the body at the closest point near the base and you can see how the body lines move away from the antenna higher up in the picture above. Tennis ball installed as an anti-whack-a-body device. The radio may be "tuned up" a bit too much. I bought it a few years ago, made sure it worked, and it's sat on the bench since then. I'll have to check out some of these issues later in the week. I have to put together a presentation for a class in the morning. Thanks again! No such thing....
As long as the antenna is resonant as the desired freq, you have a sufficient "ground plane", and no close objects, the amount of RF power will not effect the SWR. We're almost certainly dealing with a modified CB here. Since the SWR is going down when a modulated signal is present, I'm betting we're dealing with a four quadrant multiplier-based AM CB. With class AB finals, AM can use a two quadrant or four quadrant multiplier in the modulator. With a two quadrant multiplier, negative modulation causes severe distortion but with a four quadrant multiplier, it results in carrier suppression, moving to double sideband. The suppressed carrier is reducing the output power, hence the lower SWR. By "tuned" I mean Bubba adjusting slugs until maximum output it shown. |
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Thanks for the replies so far. To answer a couple of questions: I'm taking measurements while parked in an open area. The antenna is mounted through a hole in the bumper near the divers side end, but I think the whole truck could benefit from the addition of some bonding straps. The antenna is about 5 inches away from the body at the closest point near the base and you can see how the body lines move away from the antenna higher up in the picture above. Tennis ball installed as an anti-whack-a-body device. The radio may be "tuned up" a bit too much. I bought it a few years ago, made sure it worked, and it's sat on the bench since then. I'll have to check out some of these issues later in the week. I have to put together a presentation for a class in the morning. Thanks again! No such thing....
As long as the antenna is resonant as the desired freq, you have a sufficient "ground plane", and no close objects, the amount of RF power will not effect the SWR. We're almost certainly dealing with a modified CB here. Since the SWR is going down when a modulated signal is present, I'm betting we're dealing with a four quadrant multiplier-based AM CB. With class AB finals, AM can use a two quadrant or four quadrant multiplier in the modulator. With a two quadrant multiplier, negative modulation causes severe distortion but with a four quadrant multiplier, it results in carrier suppression, moving to double sideband. The suppressed carrier is reducing the output power, hence the lower SWR. By "tuned" I mean Bubba adjusting slugs until maximum output it shown. I'm fairly certain this thread is getting way more complicated than it has to be... Speargun, how are you "checking" the SWR? Are you using an external SWR bridge (meter) or does your radio have it's own SWR meter function built in? -SV |
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I'm fairly certain this thread is getting way more complicated than it has to be... If Phurba and I are correct, I'm fairly certain this thread will be heading towards a lock.
But my guess is problems with the transmitter, and/or modifications done to it, as Phurba explained in better detail. |
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Sorry guys but I cant get my head around your thought process...
From a pure antenna system standpoint.....it does not matter if you are using a modified CB on 10m or a FT1000MP...at all (even if he is, most likely he will be on ssb anyway no?). A radio is a radio is a radio. It outputs RF into an antenna system. If the antenna system (coax, ant, ground plan) is tuned to the desired frequency, then all is well... With that said...If you are concerned with what the PA of said radio see's as a match, then yes, you need to pay close attention to output caps and coils in the radio. -SV |
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Sorry guys but I cant get my head around your thought process... From a pure antenna system standpoint.....it does not matter if you are using a modified CB on 10m or a FT1000MP...at all (even if he is, most likely he will be on ssb anyway no?). A radio is a radio is a radio. It outputs RF into an antenna system. If the antenna system (coax, ant, ground plan) is tuned to the desired frequency, then all is well... With that said...If you are concerned with what the PA of said radio see's as a match, then yes, you need to pay close attention to output caps and coils in the radio. -SV That's what I'm getting at. The radio has been monkeyed with and needs to be connected to a dummy load to see what the SWR is into a known good 50 ohm load. If it was a stock radio or using a tuner I'd say the problem is with the antenna, but at this point I wouldn't recommend chopping off pieces of the antenna until the radio has been ruled out. |
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Sorry guys but I cant get my head around your thought process... From a pure antenna system standpoint.....it does not matter if you are using a modified CB on 10m or a FT1000MP...at all (even if he is, most likely he will be on ssb anyway no?). A radio is a radio is a radio. It outputs RF into an antenna system. If the antenna system (coax, ant, ground plan) is tuned to the desired frequency, then all is well... With that said...If you are concerned with what the PA of said radio see's as a match, then yes, you need to pay close attention to output caps and coils in the radio. -SV That's what I'm getting at. The radio has been monkeyed with and needs to be connected to a dummy load to see what the SWR is into a known good 50 ohm load. If it was a stock radio or using a tuner I'd say the problem is with the antenna, but at this point I wouldn't recommend chopping off pieces of the antenna until the radio has been ruled out. I would agree if he is depending on an internal SWR meter. Otherwise he can set the SWR with an external meter or an analyzer...and have the radio's output stage checked whenever he gets around to it. The sooner the better. Glad we made it to the same page
-SV |
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Sorry guys but I cant get my head around your thought process... I think they are thinking along the lines of the OP using an SWR meter that is built into the radio, and possibly one that simply measures return power and calculates SWR based on an assumed forward power. If that is what it is, then increasing the forward power would result in artificially high SWR readings. Or if it has a forward power adjustment/calibration, that said control may just be improperly set when trying to take the SWR reading. CB/10m radios with onboard SWR aren't very intuitive as to how you should use that meter, if you're not experienced at operating forward calibration type SWR meters. The OP's description of indicated SWR decreasing with AM modulation let to a little sidetrack of what would cause output power to decrease with AM modulation, and also indicates that the OP is using some kind of SWR meter which doesn't directly read SWR: but rather reads SWR from a reverse power reading based on either a preset forward power or an adjusted forward power reading. |
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-snip- If your SWR is getting lower when you're modulating, it sounds like you're experiencing > 100% negative modulation which is resulting in a suppressed carrier and lower power output, thus giving a lower SWR... whoever tuned your CB 10 meter rig probably pushed it a little too hard. See what it does into a dummy load.
-snip This is the question I was referring to when I mentioned "tuned up." I don't know the history of the radio so it may be that it has been tuned by bubba since it's pretty much a simple "1 screw" adjustment once you get under the cover. I used a Dosy meter to measure the SWR's, but I'll put the Bird on it this weekend to double check. Either way, the initial (dead key) SWR was high; I just remember seeing the backwards swing issue years ago when I installed CB's and couldn't remember what we would do to fix it. ....The more I think about it, the more I want to say it was a bad "ground" (shield side of the antenna mount not bonding to the body well) issue. 
Just to clear the air.... I'm not a CBer. I tried it in college, inhaled, gagged, and gave it up on the spot. 
I considered putting a CB in the truck, but it would be pretty much a worthless endeavor. The only way anyone would ever hear you around here is if you had a 400 watt 'modulator' driving your '6 pill' amp into your monkey antenna powered by 6 alternators and 3 Caterpillar tractor batteries. 
Seriously though, the only people around here you'll find on the CB is dog hunters, during hunting season only (but only on the low channels), and drunk old farts cussing each other and playing music 24/7. I hooked a CB up a while back in my house with the idea of talking to a buddy about 10 miles away every now & then, but any time we got on the radio, the local drunks would follow us around the 40 channels playing music, roger beeps, and misc noise toys to the point that conversation was impossible. Besides, I never could understand the "mouth full O' marbles" CB talk very well anyway.
If I ever need short range, car to car comms, I can always use some of the many bubble pack radios the kids have. They would probably have better range than a CB without all the crap. Since I already have 2 meter/440 covered and 10 meters is fairly popular around here, I opted to install a 10 meter rig. Now back to your regularly scheduled program. 
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