4' Firestick II on Jeep TJ. NEED HELP!

[hacksaw]

Hi, I bought a 4' Firestick II and the proper mount and PL antenna mount for my 03 Jeep TJ Wrangler. I mounted the bracket on the tailgate, then grounded the bracket back to the body. (The tailgate on the Wranglers swings to the right, and needed a proper ground strap attached back to the accuall body.) This is the best place to mount a antenna on a Jeep in my opinion. I did not want to mount it off to the side. It is in the middle of the Jeep, hopefully using all the body as the ground plane. It is dirrectional out the front and rear. RX off the side is not great, Front and back is excellent. Now, to the problem. How do I lower SWR across the band of the Firestick II antenna? I am using the recomended 18' of coax. The SWR readings on the 11 meter band on channel 1 are 1.7:1, channel 20 1.4:1, and on channel 40 1.7:1 readings. This is where I have tuned the tip of the antenna for even SWR across that band. Is there any way to either shorten the coax, or lengthen the coax to bring the SWR readings down? Like 1.3:1 channel 1, 1.1:1 on channel 20, and 1.3:1 on channel 40. Thanks

Shawn

 

[Moleculo]

I have that same antenna on my TJ. I have it mounted slightly different, though. The length of the coax doesn't matter. If you want, you can shorten it up to just the length you need to get from your radio to the mount. Also, you should not try to lower the SWR by varying the length of coax. Just run however much you need and then make other adjustments.

That SWR measurement you're seeing is perfectly OK. Unless you're running a bunch of power (amplifier), I would leave it alone. There won't be any difference in signal strength by lowering the SWR from 1.7:1 to 1.3:1. Generally, as long as it's under 2:1, the radio is happy, too.

If you still want to try and lower the SWR, you're going to need to do some more bonding of the body and frame. Make sure that the ground strap you added on the tailgate is connected to the body metal by scraping away the paint on both connections. You probably will need to add some more ground strap, also. The way that the tailgate connects to the body just doesn't provide a good enough connection for RF. You should also bond the frame to the body and the hood to the body, as well. Run some ground strap from frame to body somewhere. (If you've installed a good gas tank skid that attaches the rear frame to the body then you can probably skip this step). The hood on these jeeps has a small piece of ground strap, but it needs to be upgraded also. Unless you've installed an aftermarket gas tank skid. Grounding the exhaust right at the rear is a good idea also.

Finally, take some pics of your mount and post them. There may be something there that can be improved, as well. You can see how much work this is going to be if you really want to do it...

 

[hacksaw]

Hey, thanks. Nice to see other Jeep owners on here. What I am using for a mount is a part of a mirror mount bracket. I used the aluminum L bracket part and attached it to the top of the spare tire holder, and it is basicaly flush with the top of the tailgate/tub. I used a crimp ring terminal, and #10 copper wire to attach a ground back to the body inside the tub where the cargo tie downs are in the back. I buffed the paint on the tub, and the under side of the hold down. The coax cable runs down the pass side of the tub and under the carpet. No spring is used, I used good quality mount, and PL connector for the antenna. Seem like I did good?

 

[Moleculo]

I used a crimp ring terminal, and #10 copper wire to attach a ground back to the body inside the tub where the cargo tie downs are in the back.

It sounds like you did OK, except for this part. The 10 guage copper wire isn't big enough for RF ground. You need to use some wide ground strap (like 2 or 3 inches) or something like that. Also, where the antenna mount is attached to the spare tire bracket, did you scrape away the powder coating to ground it right there?

 

[freecell]

"The length of the coax doesn't matter."

oh yes it does.
http://www.worldwidedx.com/installa...28331.html?highlight=resonant+line#post128331

 

[Moleculo]

"The length of the coax doesn't matter."

oh yes it does.

You can't say it matters as an abolute like that. You need a bunch of qualifiers to make that statement true, which is what your post in the other thread goes into a great bit of detail about.

Stating it like you did above adds confusion to the topic of this thread, IMO. Messing around with his coax length isn't going to improve the groundplane that his antenna is seeing, which is what he needs to improve to make the system work better.

 

[freecell]

"You can't say it matters as an abolute like that."

yes i can.

the vswr is something other than 1:1.
standing waves are present on the line.
the line is resonant.

the qualifiers are in the thread linked to.

the ONLY condition in which the length of the line is a non-factor is when the match is 1:1 and there are no standing waves on the line, when the line is non-resonant, when the impedance measured AT ANY PLACE in the line is 50 ohms and BOTH VOLTAGE AND CURRENT ARE IDENTICAL AT EVERY LOCATION ALONG THE LINE. Z (impedance) is EQUAL TO E (voltage) DIVIDED BY I (current).

"Messing around with his coax length isn't going to improve the groundplane that his antenna is seeing, which is what he needs to improve to make the system work better."

yes it will. forget about the ground plane, you'll never have enough of it on a jeep. the problem is the lowered input impedance at the feedpoint which is in the range of 29 - 38 ohms (1.7:1, 1.3:1) and the real problem resulting from that is reduced power output from the transmitter which can be corrected by adjusting the length of the line to provide a 1:1 match from the transmitter looking into the line. the values of impedance available on a line with standing waves under these conditions are present at regular intervals along the line and if locations representing impedance values of roughly 38 - 43 ohms are selected and the line is terminated at these points the conditions for match are satisfied.

sq rt 29X50 = 38.0
sq rt 38X50 = 43.5

 

[Moleculo]

forget about the ground plane, you'll never have enough of it on a jeep

You're telling me that it's not worthwhile to change from a ground plane that is comprised of a 2'x3' tailgate to one that is made up of the entire vehicle and frame? I beg to differ. My experience with that same vehicle installing vhf/uhf and 3-30mhz antennas on it tells me otherwise., and so does a lot of other published information about running HF systems on less than optimally sized vehicles for HF. Sure, it's never sufficient, but you might as well maximize what you have to work with. It's also a lot faster and easier to run a bunch of ground strap than it is to tinker around with cutting and resoldering $3 PL-259s until you find the magic length of coax. He can get his SWR lower than that on that vehicle with that antenna by using the entire vehicle for counterpoise. I know because I did it already. If he does that and still can't lower the SWR and decides he wants to get it lower, then you have to start looking at other options, like messing around with the coax or using a transmatch of some kind.

 

[W5LZ]

Instead of trying to make the feed line do something it was never intended to do (impedance transformation), why not do that transformation at the point were it will do the most good, the feed point of the antenna? That allows the feed line to do exactly what it is intended to do and not something it was not intended to do. It 'corrects' a problem where the problem exists, not where it doesn't exist, and by doing that it increases the over all efficiency of the antenna system. Losses? Less than what would be lost in that 'coax length' silliness (uh, unless you're benefiting from that 'silliness' in some way?). There are times when using a 'Q' section does make things 'easier'. It very, very seldom ever makes things 'better'. Those 'Q' sections are not the 'cure all' they are made out to be.
I don't think that a comment is required about the tailgate/whole body groundplane thingy. Seems pretty self evident to me.
- 'Doc

 

[freecell]

"You're telling me that it's not worthwhile to change from a ground plane that is comprised of a 2'x3' tailgate to one that is made up of the entire vehicle and frame?"

yeah, you're wasting your time. you don't have any image plane for an entire 180 degrees off the back of the jeep. that's the cause of the lower than 50 ohm (29-38) input impedance. neither the tailgate or the frame change that.

"tinker around with cutting and resoldering $3 PL-259s until you find the magic length of coax."

not necessary. once the source and load impedances have been determined the impedance values as well as their locations along the line are not only predictable and simple to calculate but the values repeat themselves at regular intervals. the impedance of the mismatched load establishes the range of impedances available along the line. the larger the amount of mismatch the wider the range of impedances available.

 

[Moleculo]

yeah, you're wasting your time. you don't have any image plane for an entire 180 degrees off the back of the jeep. that's the cause of the lower than 50 ohm (29-38) input impedance. neither the tailgate or the frame change that.

So then I shouldn't be able to get my Hi-Q 3-30mhz antenna working there either, right? How do i cut the coax to make it work at that entire range of frequencies?

you don't have any image plane for an entire 180 degrees off the back of the jeep. that's the cause of the lower than 50 ohm (29-38) input impedance.

I'm not disagreeing with you about that. I didn't say it was optimal groundplane, but you have limited choices with a Jeep and with most mid sized vehicles. Don't forget, part of the point is to try and minimize the ground losses that are going to occur in a mobile setup. I disagree with you about it being a waste of time. What you're saying is exactly the opposite of published, accepted, and tested mobile HF installation practice. The vast majority of HF installations for lower frequencies are off of the rear of the vehicles, and a lot of them are on mid-sized vehicles. The only way you get it to work 1/2 way decently is to bond as much as you can.

Read k0bg: "Bonding, sometimes referred to as strapping, is one of the three most important aspects of mobile radio. (...)
The best mobile antenna money can buy, isn't any better than the ground plane it is mounted over. Maximizing the available ground plane is what bonding is all about. On the HF bands, our vehicles act more like a capacitance to ground, rather than a ground plane. The bonding doors and trunk lids has a lessor effect on noise, but does aid in maximizing the ground plane If you doubt this reasoning, here's a little experiment you can try.
Install your antenna first and use an MFJ 259B or similar antenna analyzer to measure the on-resonance input impedance of your antenna. Then follow the suggestions below and once you're done, measure it again. The resonant point will drop slightly and the input impedance will drop perhaps as much as 25 percent. This occurs because bonding lessens the ground losses which are reflected in the input impedance. The better the quality of the antenna, the more noticeable the change will be."

once the source and load impedances have been determined the impedance values as well as their locations along the line are not only predictable and simple to calculate

The average guy that wants to install a CB in his jeep/truck to go off-roading with his buddies is not going take the time to research and learn how to do that. And he shouldn't be expected to, either. Even if he wants to learn how to do that, he might end up at the clown forum and get told how to do it wrong.

 

[W5LZ]

"yeah, you're wasting your time. you don't have any image plane for an entire 180 degrees off the back of the jeep. that's the cause of the lower than 50 ohm (29-38) input impedance. neither the tailgate or the frame change that."

Does asymmetry of this 'image plane' cause this lower than 50 ohms impedance? No. In fact, that 29 - 38 ohms impedance is very typical/normal. About the only thing that asymmetry will affect is the resulting radiation pattern, which will tend toward the front of the vehicle. That's 'tend', not shift completely, and in most cases that tendency is not all that noticeable anyway. Considering the mounting location, or the lack of other mounting locations that are agreeable to the user, that 'directional' tendency is something that he will just have to live with. Everybody puts up with it to some extent. NOT a big deal at all.
- 'Doc

 

[hookedon6]

i gotta disagree with some things said in this thread. some things mentioned will affect the efficiency but not the VSWR.
the ONLY reason the VSWR changes when the coax length is altered is because the coax is radiating.

 

[freecell]

"input impedance will drop perhaps as much as 25 percent"

how is that an improvement? the input impedance is already between 29 and 38 ohms due to a lack of image plane and you want to lower it even more?

"the ONLY reason the VSWR changes when the coax length is altered is because the coax is radiating."

another myth that the gullible and naive use to support "accepted practice". High VSWR does NOT cause feed line radiation.

most radiation from co-axial cable is caused by terminating this unbalanced feed line with a balanced load. the remainder of the radiation is due to other problems such as: dis-continuities in the outer conductor (braid corrosion is a major factor), improperly installed connectors and signal pickup caused by routing the feed line too close to, and parallel to the antenna.

"The average guy that wants to install a CB in his jeep/truck to go off-roading with his buddies is not going take the time to research and learn how to do that. And he shouldn't be expected to, either."

well he must not be the average guy because he's here asking questions. the fact that he is indicates that he's taking some time to research and learn something. let's not start lying to him or repeating misinformation just to prevent confusion. it's obvious there's already plenty of that going on.

 

[W5LZ]

" "input impedance will drop perhaps as much as 25 percent"

how is that an improvement? the input impedance is already between 29 and 38 ohms due to a lack of image plane and you want to lower it even more? "

Two things here. The first is that the outlined procedure increases the 'groundplane affect', which means an increase in effectiveness/efficiency. The other thing is that the input impedance is never going to be the ideal 50 ohms, and matching whatever the input impedance happens to be is the part of the goal in tuning an antenna. The other part of that goal is making the thing resonant (no reactances, or as little as possible).
Part of this deals with where the antenna is mounted, which basically comes down to what's the most practical for any given situation. Sometimes the 'best' place to mount an antenna just isn't very practical, for whatever reason. So, you have to make the best of whatever that 'non-best' location is. And, as explained by K0BG, making the most of that 'capacitive effect' of a vehicle's body at HF is a pretty good thing to do.

"let's not start lying to him or repeating misinformation just to prevent confusion. it's obvious there's already plenty of that going on."

I agree completely! Why don't you join us in that endeavor?
- 'Doc