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high swr with co-phased 102" steel whips

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kamikaze701st

Member
Apr 21, 2009
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ok, im trying to figure out how to lower my swr in my mobile. i put in a brand new piece of 18 ft. coax, and i have mounted two 102" steel whips, one on each side of course, to the front of my pickup bed. i used a piece of 3/8" flat bar to run across the corner of the bed on each side. it makes for a very sturdy mount. anyways, the swr is a 2:1 bearfoot and drops to 1.5:1 when i turn my amplifier (300 watt Fat Boy). when i modulate with or without the amplifier on, the swr jumps almost off the meter. is this a grounding issue, or does anyone have any ideas about how to lower the swr on this particular setup? thanks.

"Kamikaze"
 

Can't really say much of anything unless we know how you are feeding those antennas.
There are a couple of things that can be said though. Using an SWR meter while speaking just isn't going to tell you much that's going to be useful since an SWR meter is typically calibrated for only a fairly narrow 'power' range. A change in power will mean that it isn't reading correctly, so any fluctuation doesn't mean anything. The only time a typical SWR meter's readings are accurate is when a constant power level is maintained. Which typically means any noise contributes to an incorrect reading. Which means turn the mic gain down/off. SWR doesn't change with the amount of power being used, it stays the same. What that meter can tell you does change with power and the meter's calibration. Don't worry about it.
Are you using a 'phasing harness' of some kind to feed those antennas?
- 'Doc
 
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i am just using an 18' co-phase setup i bought at the local pilot. i know its not the best stuff, but its whats in the budget. ok so putting the power aside, how can i lower the swr when its the same all the way across the board, around 2:1? i really would like to keep the dual 102" whips for the look. would fiberglass whips make a difference and how much power does the typical fiberglass 102" whip handle?
 
kamikaze701st said:
i am just using an 18' co-phase setup i bought at the local pilot. i know its not the best stuff, but its whats in the budget. ok so putting the power aside, how can i lower the swr when its the same all the way across the board, around 2:1? i really would like to keep the dual 102" whips for the look. would fiberglass whips make a difference and how much power does the typical fiberglass 102" whip handle?
Click to expand...


Your Pick up Bed may have rubber mounts between the bed and frame rails, you could start by sanding the paint where youre flatbar mounts to the bed, then use two pieces of grounding strap or thick gauge electrical wire.
where the cab meets the bed on the underside clean off a spot on each inside frame rail, screw them on with a sell taping metal screw of your choice, but make sure to cover the srew and wire ends with some sort of waterproof sealer so that it does not corrode with the weather, that should pretty much complete a decent ground, and if that does not help might want to use those 6 inch spring must people use with the 102 and if that still does not help your antennas may be spaced to close together?
 
I would be willing to bet that harness is the biggest problem. For it to work it has to be 75 ohm coax, and the total length will be something like 12 feet end to end, with a 'T' connector in the middle of it. RG-59 or RG-11, or the equivalent, NOT 50 ohm stuff.
That about grounding the bed to the frame is certainly a good idea. No idea about your truck, but mine has a couple of large ground cables like that from the factory. At least checking yours shouldn't be out of reason. Same for the cab, while you're under there, that wouldn't hurt either. Short grounding straps/cables are better than longer ones. Using 'star' washer is a very good idea, don't have to worry about removing paint/rust or whatever.
That ought to get you into the ball-park. May not get you to the right seat, but at least it'll be in the park.
Good luck.
- 'Doc
 
They usually have a 12' harness and an 18' harness at those truck stops. The 12' one is usually the right one. I'm with 'Doc on this and am willing to put money on the coax harness.
 
Possibillity that the 102" are about 6" short. Putting the 6" spring on each antenna might be considered.
Also: what impedance is the coax?
Aren't you supposed to use 75 ohm coax for co-phase setups?
And the distance between the whips is 1/4 wave?
Just checking...
 
Are you using the kind of SWR meter that has to be set to full scale, then switched to "REF" to read SWR? If so, I completely agree with 'Doc's first post. Check SWR with the mike gain completely down, and don't talk into the mike while you're doing it. Then remove the meter from the line. SWR isn't changing, but the amount of reflected power is changing. The RATIO between the amount of reflected power and the amount of forward power remains constant. You're seeing the reflected power, and that's not "SWR".
 
kamikaze701st said:
i am just using an 18' co-phase setup i bought at the local pilot. i know its not the best stuff, but its whats in the budget. ok so putting the power aside, how can i lower the swr when its the same all the way across the board, around 2:1? i really would like to keep the dual 102" whips for the look. would fiberglass whips make a difference and how much power does the typical fiberglass 102" whip handle?
Click to expand...

First issue is they are way too close together, second, they will be very directional, and unless that is what you need, a single whip in the center of the truck will be much better.
 
kamikaze701st said:
ok, im trying to figure out how to lower my swr in my mobile. i put in a brand new piece of 18 ft. coax, and i have mounted two 102" steel whips, one on each side of course, to the front of my pickup bed. i used a piece of 3/8" flat bar to run across the corner of the bed on each side. it makes for a very sturdy mount. anyways, the swr is a 2:1 bearfoot and drops to 1.5:1 when i turn my amplifier (300 watt Fat Boy). when i modulate with or without the amplifier on, the swr jumps almost off the meter. is this a grounding issue, or does anyone have any ideas about how to lower the swr on this particular setup? thanks.

"Kamikaze"
Click to expand...

701, the ground at the two locations you plan to install the dual antennas is important as already noted. Check each antenna, individually, at both locations first and see if they work well and with similar results. You could be spinning your wheels if the locations are not working to start with, and the ground your antennas see has much to do with good results.

You might get truck stop, one size fits-all cophase harnesses to work and then maybe not. Even if they work, it is my opinion that they probably aren't working well and if you see a good match---probably most of the match is coming from, reactance that worthless heat and ground losses.

Tuning is required in these harness else the little efficiency you can gain is lost. This is because proper phasing is involved and that does not happen as a lark with some random length of coax that will reach to the points you want. The harness that I have constructed work well and they end up being very short, like 6' - 7 ' in total span using 102" whips only. The harness can be extended if needed, but that is another issue.

I have tried other 11 meter mobile antennas sold, but non worked as well as the 102" whips and that was without any springs. As a note though, I did mostly work SSB in the freeband area above 11 meters.
 
thanks for the info. my truck is a 99 superduty crew cab, so the 12 foot harness is out of the question. i also checked for ground straps, and there are like six or seven of them spread out from the hood to the bed. the co-phase harness is rg-59, i made sure and checked that when i bought it. i am going to take the antennas and mounts off and redo them (making sure i have a good ground). still have another question though...how much would 2 fiberglass 102" whips handle (wattage)?
 
First issue is they are way too close together, second, they will be very directional,
Click to expand...

He didn't say how far apart they are. If it's just as wide as a typical truck bed, they are too close. However, making them too close together makes them far less directional than they would be otherwise. The reason to phase antennas is to make them directional in some manner. What determines that direction is a function of how far apart the antennas are spaced and the phasing angle degree. Phasing angle degree has to do with the timing difference between when the signal reaches one antenna vs. the other. The way you change and control this is by varying the length of coax between the two antennas.

Most people that run a cophase setup with CBs want a directional signal to the front and back of the vehicle. The way you accomplish this is to get the antenna 1/4 wavelength apart (about 8 feet) from each other and feed each antenna with 75 ohm coax that is in lengths of odd 1/4 wave multiples. You also have to take the velocity factor of the coax into consideration to figure this out. Your 75 ohm coax lengths can be any combination of 1/4, 3/4, 5/4, etc. wave lengths. Any variation from that will ruin your signal pattern. You can run 50ohm coax out to a certain convenient point, T off of it and then run the odd 1/4 wave multiples of 75 ohm. As you can see, this isn't the simplest installation to do.

If you can't get the antennas far apart to begin with, there isn't a huge reason to go through all this trouble. Just install one antenna, run some 50ohm coax to it and be done.
 
What is the jumper length between the radio and the amp? 3 OR 6 ft is preferred -if I'm not mistaken. I make mine exactly 3 ft 1/4 inch from tip to tip, or 6 feet 1/2 inch for the longer one. I'm not sure if it is that critical, but I seen to have no problem when running either length.
Just 2 more cents...
 

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