Unusual SWR issue with a mobile

[Boll Weevil]

I drive a tractor-trailer and have been having trouble with achieving a low and stable SWR on a new tractor despite the fact that other drivers with new tractors are not having trouble.

After much experimenting and disappointment, I discovered that the only way to get an acceptable swr is to have weight on the fifth wheel of the tractor. It doesn't appear that just contact between the tractor and trailer will solve the problem but actual pressure on the suspension. I keyed up while hooking a trailer last night and watched it go down to around 1.5 on channel 20. Without a trailer I get over 2.0 and sometimes 2.5 or more. Any ideas?

A few details:

2014 Mack Pinnacle daycab tractor
Various 13'6" van type trailers.
Francis 4.5' antenna.
Coax is 18' clear mini-8 from the truck stop.
Mount is on grab handle above window behind the cab.

The above setup has worked very well for me in the past.

Thanks.

 

[9C1Driver]

Actually many new trucks aare a pain to get the SWR right, and the factory "all in one" coax sucks. Most common problem is bad grounding. Your going to have to rub the paint off at the mounting point and then make sure you add some ground strap to the frame.

 

[Lil'Yeshua]

CH Mack and high SWR
2013 Mack Pinnacle Test Drive Pt.1 - YouTube
2013 Mack Pinnacle Test Drive Pt.2 - YouTube

 

[222DBFL]

I will say this. My Nissan NV work van had bad grounds under the hood were all not properly grounded. Every one had paint under them. Had to sand down close to 20 grounds under the hood. Like said make sure all grounds are truly grounded. Good luck and hope you get it resolved

 

[loosecannon]

try this test.

grab the coax going to the antenna, and while checking the SWR (mic keyed) wiggle and move the coax around.

if the SWR moves while you are doing this, then you have a coax issue.

if you cant find anything physically wrong with the coax itself or the installation, then you probably have common mode currents riding on the outside of the coax.

you'll need to put 6+ inches worth of snap on ferrite beads on the coax right at the base of the antenna.

this isnt the only possibility, but mobile installs are notorious for having common mode currents on the coax.
LC

 

[M0GVZ]

Mount is on grab handle above window behind the cab.

You have no RF ground. Might as well be connected to a block of wood for all the good that mounting point is doing. The fact the SWR goes all over the place just confirms it.

Length of coax shouldn't matter.

 

[M0GVZ]

but mobile installs are notorious for having common mode currents on the coax.
LC

Only because people do everything they can to avoid drilling a hole. Mirror bracket and handle mounts like the OP is using are one of the worst you can use.

 

[Boll Weevil]

Thanks for the responses.

Loosecannon: I did in fact find fault with the original coax I was using by doing just that. It didn't solve the problem but is a good way to find what is probably the most common cause of coax failure. Unfortunately I was dealing with more than one issue.

222DBFL and 9C1Driver: I am convinced that my issue is a result of something that was done wrong during the manufacturing process simply because there are at least 15 more of these trucks at my terminal built between 2011 and 2014 and most do not have issues using an identical setup.

Here's what I did. I had a stainless steel, L shape mirror bracket that I had used on another tractor where I had limited antenna mounting options. One of the holes had to be bored slightly to accept the plastic spacer on the stud but it was otherwise good to go. I bolted it to the back of the cab using the very same bolt that attaches one end of the grab handle to the back of the cab. This bolt also secures part of the steel frame work that supports the roof fairing Without having to remove any paint I was able to get my SWR down to 1.3 on ch 1 and 1.2 on ch's 19 & 40.

Now being one who just can't let something go, I'm still curious as to how I was able to get a better ground using the same bolt. One theory is that the new mount is almost against the cab and is directly above a frame rail whereas the old location was about 6 inches from the back of the cab and directly over the driveshaft.

Is this plausible or am I grasping at straws here?

 

[Boll Weevil]

Only because people do everything they can to avoid drilling a hole. Mirror bracket and handle mounts like the OP is using are one of the worst you can use.

You replied while I was two-finger typing.

If it were my truck there would be a hole in the center of the roof and another through the fairing to feed the whip through, but many of the truck drivers having issues are in company owned equipment where such mods are not an option. It can be frustrating at times but we have to simply do the best we can with what we have. I would also like to use a better antenna I share the truck with a "city" driver who makes pick up's and deliveries all day, which can be tough on an antenna regardless of where it's mounted. If the truck was only used by me to run the interstates between company terminals I'd be able to experiment a bit more. I think my new mount and location might be the best I can hope for in this situation.

 

[loosecannon]

Boll Weevil,

if the majority of your antenna is below the roof line, then yes, you are going to have problems no matter what.
if that grab handle is within a few inches of the roof line, then you are probably ok.

i would suggest forgetting about the fact that others are not having the same problem when trying to solve yours.

it is probably making you disregard possibilities that you should not.

i believe, like M0GVZ, that you have chosen a location that has barely/ no connection to the vehicle frame.

whether or not the other trucks are like this doesnt really matter.

try this experiment.
get a piece of copper wire about 108" long, and attach one end to your antenna mount (the ground side, not the antenna side)
let the wire hang down freely. if it wants to make contact with the vehicle, then tie a string to it that will allow you to angle it out from the truck a bit just so its hanging in free air.

now check your SWR. if it has come down significantly, then you definitely have a ground issue.
no, this is not the greatest test for this, but it wont be too hard to try.

M0GVZ, i dont disagree with your statement about common mode currents, except to say that unless you mount another identical vehicle on top of yours, upside down, and isolated from the bottom vehicle; then you are going to have some common mode currents due to the counterpoise not being a mirror image of the radiating element.
LC

 

[M0GVZ]

M0GVZ, i dont disagree with your statement about common mode currents, except to say that unless you mount another identical vehicle on top of yours, upside down, and isolated from the bottom vehicle; then you are going to have some common mode currents due to the counterpoise not being a mirror image of the radiating element.
LC

If you use a fixed mount mounted correctly in the middle of the roof and do all the bonding then the RF flows over the surface of the vehicle instead of the surface of the coax.

The counterpoise doesn't need to be a mirror image of the radiating element, it merely needs to provide enough electrons to draw on to balance those being fed to the radiating element. In a mobile installation that's the body of the car and when that's not sufficient, the surface underneath the car that provides them through capacitive coupling of the vehicle body to the surface below it. That's why we do bonding, to maximise the surface area the RF "sees" and to improve the capacitive coupling to the ground below the vehicle. In a VHF/UHF installation the body of the vehicle will be able to provide enough electrons alone which is why you very rarely get common mode issues on a VHF/UHF install with them mostly occurring when using glass mounts or those stupid 1" magmounts.

You maybe think a mirror image of the radiating element is what you need because of dipoles - because each leg is the same length, one leg provides enough electrons to balance the other. The reality is that it doesn't need to be a mirror, just an adequate supply of electrons.

I'm not going to go any further than that because we start getting involved in electric fields, charge and some serious physics stuff which I'm quite happy with as I'm doing a BEng EE but would probably bore most people to death.

 

[Mudfoot]

 

[hotrod]

Thanks for the responses.

Loosecannon: I did in fact find fault with the original coax I was using by doing just that. It didn't solve the problem but is a good way to find what is probably the most common cause of coax failure. Unfortunately I was dealing with more than one issue.

222DBFL and 9C1Driver: I am convinced that my issue is a result of something that was done wrong during the manufacturing process simply because there are at least 15 more of these trucks at my terminal built between 2011 and 2014 and most do not have issues using an identical setup.

Here's what I did. I had a stainless steel, L shape mirror bracket that I had used on another tractor where I had limited antenna mounting options. One of the holes had to be bored slightly to accept the plastic spacer on the stud but it was otherwise good to go. I bolted it to the back of the cab using the very same bolt that attaches one end of the grab handle to the back of the cab. This bolt also secures part of the steel frame work that supports the roof fairing Without having to remove any paint I was able to get my SWR down to 1.3 on ch 1 and 1.2 on ch's 19 & 40.

Now being one who just can't let something go, I'm still curious as to how I was able to get a better ground using the same bolt. One theory is that the new mount is almost against the cab and is directly above a frame rail whereas the old location was about 6 inches from the back of the cab and directly over the driveshaft.

Is this plausible or am I grasping at straws here?

ANY swr below 1.5 across 40 channelsis very good. imho when you went to the stainless steel L shape bracket i think that helped ya out. [sounds like u change your mount]
orignally i was thinking your antenna wasnt above your roofline enough that can
be an issue too. sounds like everything isgood now enjoy !!!

 

[GnG8d]

I'm not going to go any further than that because we start getting involved in electric fields, charge and some serious physics stuff which I'm quite happy with as I'm doing a BEng EE but would probably bore most people to death.

You won't bore anyone, let's hear it!

 

[loosecannon]

I am not an EE. far from it. therefore i will just post a link to the man i put my trust in.
Common Mode Currents

the car thing was meant to be a joke, and when i said "mirror image" i was referring to RF current. (yes, the groundplane can be bigger, as long as equal RF currents can flow)
probably not the best analogy/ joke to use. LOL

from what ive read, its still a good idea to decouple the coax regardless of whether or not the vehicle has been bonded or not.

my Chevy S10 pickup is a perfect example. the entire truck is bonded with straps, including the exhaust pipe.
wilson 5000 hard mounted to the center of the roof.
radio is a mildy tuned 21GTL (not a splatterbox by any means)
with a texas star DX400 amp.

using the amp was impossible without causing airbag, and wheel sensor codes to be generated. freq didnt matter, and anything more than about 40 watts would cause the error codes.

i bought about 18" worth of mix 31 ferrite beads, slipped them over the LMR240 coax right at the base of the antenna, and voila! no more codes generated even at power levels of over 500 watts (had a 667 in there for a short while)

Like i said, i dont disagree with any of your statments. its obvious that you are in the know with RF.
i commented the way i did because you used the term "only" in this sentence:
"Only because people do everything they can to avoid drilling a hole."

RFI is a prissy b!tch and she wont always calm down just because you tell her she's breaking the rules.

LC