SWR/Grounding Issue

[comoman]

I'm installing a 10 meter radio and Sirio 10 meter antenna in my 2012 Jeep Wrangler. I've run into an SWR issue and not sure how to deal with it. I'm using an MFJ- 259B antenna analyzer and can't get below 2.7 SWR at 50 ohms. The Jeep antenna mount is on the tailgate and doesn't have a decent ground so I installed a braided ground strap between the body and the tailgate. Something I find strange is that with the radio off I get 0 ohms through the ground wire from the radio to the vehicle chassis ground. When I turn the radio (Stryker ST 89MC 10 meter) on, the ohms reading on the ground is 0.16k to 0.20k ohms. The coax is new and check out ok.

Will someone please tell me what I'm doing wrong? I've got a suspicion the resistance on the ground wire is contributing to the SWR issue.

Any help will be appreciated.
KD5JNA

 

[mmckenna]

There was a very nearly identical post a year or so ago with this issue. Might need to do some searching to find it.

Here's what I recall:
Grounding was the issue.
Grounding one side of the tailgate was not enough, needed both sides grounded.
Striker on the latch side was part of the issue. It had a plastic cover/coating on it that was preventing a good connection. Removing the coating on the striker/pin helped resolve the issue.
You could try a temporary ground strap on the latch side of the tailgate to see if that helps.

Don't confuse DC ground with RF ground.

 

[comoman]

There was a very nearly identical post a year or so ago with this issue. Might need to do some searching to find it.

Here's what I recall:
Grounding was the issue.
Grounding one side of the tailgate was not enough, needed both sides grounded.
Striker on the latch side was part of the issue. It had a plastic cover/coating on it that was preventing a good connection. Removing the coating on the striker/pin helped resolve the issue.
You could try a temporary ground strap on the latch side of the tailgate to see if that helps.

Don't confuse DC ground with RF ground.

Thanks for the quick reply.

I do understand the difference between DC and RF ground.

The tailgate hinges from the passenger side. The braided ground strap I installed is on the hinge side of the tailgate and is grounded directly from chassis to the tailgate. That's so the tailgate has the same ground when opened or closed as the ohm meter indicates. The antenna mount is mounted on the bolts that fasten the spare tire carrier to the tailgate. When I check resistance between the antenna mount and the chassis ground I get 0 ohms. It's only when I turn the radio on that the 0.17k to 0.20k ohms indicates on the ohm meter.

Is this the thread you were referring to? http://forums.radioreference.com/cb...dd-swr-issue-jeep-wrangler-2.html#post2540969

 

[prcguy]

The antenna wants some sheet metal right under it and what you have is more of a DC ground than an antenna ground plane or counterpoise. You can sometimes lengthen an antenna and get a compromise between resonant frequency and impedance where an SWR meter shows an improvement but sometimes not.

If you have a mount like a 3/8-24 stud where you have access to the coax center and ground, try a 50pf ceramic disc cap across the center conductor and ground of the coax right at the base of the antenna. This will fool the antenna into thinking it has more capacitance from the whip to a ground plane and it should match better if not perfect.
prcguy

 

[KE5MC]

snip...

When I check resistance between the antenna mount and the chassis ground I get 0 ohms. It's only when I turn the radio on that the 0.17k to 0.20k ohms indicates on the ohm meter.

Is this the thread you were referring to? http://forums.radioreference.com/cb...dd-swr-issue-jeep-wrangler-2.html#post2540969

Something is not right with the radio's DC power sourcing or measuring technique. To me your highlighted comment indicates radio current (DC) thru the strap and the resulting voltage drop is changing the ohm meter reading. That is usually the case when working on the PCB and the radio is powered on or capacitors are charged if equipment is off and you get funky resistance reading or smoke the ohm meter. In this case I would expect the radio on or off would not have any impact on the ohm meters readings. I can't say exactly what is wrong, but it clearly is not as it should be.

P.S. What is the lowest SWR and what is the impedance? Don't expect the lowest to be at 50 ohms.

 

[SCPD]

".......can't get below 2.7 SWR at 50 ohms......."


I don't think your high SWR is related to the DC connection between the tail gate and the Jeep chassis. You are asking a very shortened vertical to work against what essentially is a non-existent ground plane. Maybe, and I use that 'maybe' advisedly, if you could cancel the reactance at the base of the antenna you could reduce the SWR, but mechanically that arrangement would be difficult, unstable, and for a 4X4 vehicle, unacceptable. I have a Jeep Wangler too, and a 60" SS whip on a standard ball/spring mounted on the chassis. I use a internal- mounted antenna tuner directly behind it, inside the vehicle, connected by a 2" piece of copper wire-- achieving a 1.2:1 on 27 Mhz. I know, you may not want to hear the words "circle cutter" but unless you mount firmly to the whole frame, you will have mixed results at best. There are many antenna configurations all superior to bumper mounts, tail gate mounted arrangement. The ARRL Antenna handbook gives a great place to start the research, and I can't stress researching this topic like a scientist- especially the chapters on vertical antennas. Good Luck, Comoman
...............CF

 

[comoman]

Something is not right with the radio's DC power sourcing or measuring technique. To me your highlighted comment indicates radio current (DC) thru the strap and the resulting voltage drop is changing the ohm meter reading. That is usually the case when working on the PCB and the radio is powered on or capacitors are charged if equipment is off and you get funky resistance reading or smoke the ohm meter. In this case I would expect the radio on or off would not have any impact on the ohm meters readings. I can't say exactly what is wrong, but it clearly is not as it should be.

P.S. What is the lowest SWR and what is the impedance? Don't expect the lowest to be at 50 ohms.

I can get as low 1.7 SWR at 22.0 mHz with approximately 40 ohm reading.

 

[comoman]

".......can't get below 2.7 SWR at 50 ohms......."


I don't think your high SWR is related to the DC connection between the tail gate and the Jeep chassis. You are asking a very shortened vertical to work against what essentially is a non-existent ground plane. Maybe, and I use that 'maybe' advisedly, if you could cancel the reactance at the base of the antenna you could reduce the SWR, but mechanically that arrangement would be difficult, unstable, and for a 4X4 vehicle, unacceptable. I have a Jeep Wangler too, and a 60" SS whip on a standard ball/spring mounted on the chassis. I use a internal- mounted antenna tuner directly behind it, inside the vehicle, connected by a 2" piece of copper wire-- achieving a 1.2:1 on 27 Mhz. I know, you may not want to hear the words "circle cutter" but unless you mount firmly to the whole frame, you will have mixed results at best. There are many antenna configurations all superior to bumper mounts, tail gate mounted arrangement. The ARRL Antenna handbook gives a great place to start the research, and I can't stress researching this topic like a scientist- especially the chapters on vertical antennas. Good Luck, Comoman
...............CF

Don't know much about this stuff yet and any information is well appreciated.

I didn't know that not having a ground plane would have anything to do with what the SWR reading on an MFJ-259B analyzer would be.

 

[jwt873]

I get 0 ohms through the ground wire from the radio to the vehicle chassis ground. When I turn the radio (Stryker ST 89MC 10 meter) on, the ohms reading on the ground is 0.16k to 0.20k ohms.
Will someone please tell me what I'm doing wrong? I've got a suspicion the resistance on the ground wire is contributing to the SWR issue.
KD5JNA

There is no zero ohms. (unless you're using a superconductor ) Even heavy copper wire has a slight resistance. Perhaps .01 Ohms.

When the radio is on, there is current flowing between the vehicle chassis and the radio. If you use a voltmeter to measure across a resistor in a circuit you will see a voltage. As shown here --> https://cnx.org/resources/23b518766fc782980738a1595013ce7eebacedca/Figure_21_02_02a.jpg

In other words, you can't use an ohm meter to measure resistance in a circuit that's conducting current. The tiny resistance in your ground wire will cause a very slight voltage drop which is probably just enough to screw up your Ohmeter reading. I would trust the 'zero' ohm reading you get with the radio off.

I'm not familiar at all with the Sirio antenna. Does it have any adjustments? Can you adjust the length? Have you checked the SWR at frequencies outside of 10M to see if there is a point where the antenna is resonant?

 

[comoman]

There is no zero ohms. (unless you're using a superconductor ) Even heavy copper wire has a slight resistance. Perhaps .01 Ohms.

When the radio is on, there is current flowing between the vehicle chassis and the radio. If you use a voltmeter to measure across a resistor in a circuit you will see a voltage. As shown here --> https://cnx.org/resources/23b518766fc782980738a1595013ce7eebacedca/Figure_21_02_02a.jpg

In other words, you can't use an ohm meter to measure resistance in a circuit that's conducting current. The tiny resistance in your ground wire will cause a very slight voltage drop which is probably just enough to screw up your Ohmeter reading. I would trust the 'zero' ohm reading you get with the radio off.

I'm not familiar at all with the Sirio antenna. Does it have any adjustments? Can you adjust the length? Have you checked the SWR at frequencies outside of 10M to see if there is a point where the antenna is resonant?

Using my MFJ-259B analyzer I can get 1.7 SWR with approximately 40 ohm reading at 22.000mHz. There is an adjustment on the antenna. I have adjusted the stainless steel element from one extreme to the other several times. I've adjusted the antenna with as little as 1/8" to 1/4" movements with each adjustment.

Sirio Performer 5000 3 8 10M CB Mobile Trucker Antenna for 3 8 24 Mount | eBay

I use the same antenna on a car trunk mount and it work fine. The only difference that I'm aware of is the car has a ground plane where the Jeep Wrangler does not.

 

[jwt873]

It looks like a good antenna... I can't think of much else to do. How high above the vehicle body is the coil? If it's close to metal, there could be some sort of interaction.

I have a Hustler antenna system on one of my cars where you screw on different coil & whip assemblies for different frequencies.

It's bumper mounted on the left rear corner of the car. But, the mast is 54 inches long which puts the 10 meter coil out in the clear about 2 feet over the trunk line and another 2 feet over the roof line. It's pretty broad and works great on 10M and 11M. With an antenna tuner, it performs just fine on 12 Meters.

 

[K5MPH]

I had a friend years ago do this very same thing not with the same antenna never the less the tail gate was a no go very bad ground you need to move the antenna some where else .......

 

[prcguy]

I think you've answered your own question which is what several others have pointed out. Now you just have to find a more appropriate location to mount it.
prcguy

I use the same antenna on a car trunk mount and it work fine. The only difference that I'm aware of is the car has a ground plane where the Jeep Wrangler does not.

 

[NDRADIONUT]

You could just fool your radio with a matcher like this....

Maco Model 91 Base Mobile 26 28 MHz Antenna Matcher w 3' Belden Coax Cable | eBay

 

[SCPD]

Coroman, I have (I think I have- if I have figured out the photo posting here) post'd some pictures of my whip antenna arrangement on my Jeep. I use it for CB primarily, but with that MFJ tuner it will load anything from 7 to 30 Mhz.... SWR can be adjusted 1.2:1 across all those frequencies.
If you look at the RF path that occurs when you mount the antenna base on the tail gate you can see it is very convoluted, and very ineffective at best. With that arrangement, a short antenna- at 28 Mhz --you will have all sorts of reactance, multipoint returns, metallic interactions with the roof- the list goes on and on...
But, as I said, you must not fear the "Circle Cutter"...(laughs) I hope this helped a bit

.........CF

http://forums.radioreference.com/at...s/images/attach/jpg.gif337&stc=1&d=1457024870

 

[comoman]

Coroman, I have (I think I have- if I have figured out the photo posting here) post'd some pictures of my whip antenna arrangement on my Jeep. I use it for CB primarily, but with that MFJ tuner it will load anything from 7 to 30 Mhz.... SWR can be adjusted 1.2:1 across all those frequencies.
If you look at the RF path that occurs when you mount the antenna base on the tail gate you can see it is very convoluted, and very ineffective at best. With that arrangement, a short antenna- at 28 Mhz --you will have all sorts of reactance, multipoint returns, metallic interactions with the roof- the list goes on and on...
But, as I said, you must not fear the "Circle Cutter"...(laughs) I hope this helped a bit

.........CF

http://forums.radioreference.com/at...s/images/attach/jpg.gif337&stc=1&d=1457024870

I'm sure your setup works great and thanks for the input but I have always had a disliked for having to drill holes. If I had a beat up old truck or something that wouldn't be an issue but I'm going to try working this out with the antenna mounted in the traditional location by the spare tire.

 

[comoman]

Well I did find out part of my issue. The Sirio 5000 Trucker antenna was designed for a mirror mount with a conventional ground on a truck. My Jeep Wrangler has a fiberglass top without a ground plane and I had to run a 6' braided cable from the chassis ground through the tail gate to the base of the antenna so the antenna would have a ground and it was playing tricks with the Ohms in the coax. The best ohm reading I could get was around 35 or 40. Talking to the radio technical that does all of my radio repair work, he told me to hook up up a piece of 50' coax to the system and it would get the ohms back to a 50 ohm reading before trying to adjust the antenna for better SWRs and it actually worked. I now have a perfect 50 ohm system with 1.4 SWR. The antenna is more resonant at 26.105 MHz and I can adjust the antenna a little to get it more resonant at the frequency I run.

 

[SCPD]

I'm glad to hear that you are happy with the results.

What troubles me is it was achieved with using 50 feet of coax cable. While a 50 foot coil, if it is RG-58 is not that big or awkward to deal with, it does pose a loss of about 1.3-5 Db at that frequency. Worse, while it may seem to be improving the SWR, it is actually working as a 50 Ohm dummy load against an unknown impedance-- the longer the line, the better the match!
I often use a 500 foot coil of RG8 cable, un-terminated (or I can short it out, it doesn't matter), as a dummy load for 2 Ghz and above-- Feeding into this long cable- with no antenna, no dummy load on the other end ---Viola !, a perfect 1:1 SWR. I think your improved readings are reflecting something similar, though on a smaller scale (shorter cable, longer wavelength.) It's become an interesting project No?...

 

[comoman]

I'm glad to hear that you are happy with the results.

What troubles me is it was achieved with using 50 feet of coax cable. While a 50 foot coil, if it is RG-58 is not that big or awkward to deal with, it does pose a loss of about 1.3-5 Db at that frequency. Worse, while it may seem to be improving the SWR, it is actually working as a 50 Ohm dummy load against an unknown impedance-- the longer the line, the better the match!
I often use a 500 foot coil of RG8 cable, un-terminated (or I can short it out, it doesn't matter), as a dummy load for 2 Ghz and above-- Feeding into this long cable- with no antenna, no dummy load on the other end ---Viola !, a perfect 1:1 SWR. I think your improved readings are reflecting something similar, though on a smaller scale (shorter cable, longer wavelength.) It's become an interesting project No?...

The coax is Belden RG-8/U. I understand the 50' length of coax I used acts as a balun. That's about the limit of my knowledge on the subject. I guess as long as it's working it really doesn't matter but if there's a way to create a 50 ohm balun to replace the 50' of coax I would be all for it. It's a bit difficult to find a place in a Jeep to hide 50' of coax.

 

[SCPD]

Well,.... unfortunately that coax is not acting as any sort of matching network- it is not a Balun. Coax cable is a radio wave guide (The "RG" in RG8) and in this configuration, nothing else (though in other situations it can be, but beyond the scope here.)

Why you see an improvement in your SWR is because of the loss in that cable. SWR measures both the forward and reflected RF to and from the antenna. For instance; If you have, say, an SWR of 2:1 with a short length of cable on Frequency-X, the coax having say 0.2 Db loss, and you increase the same type of coax to a longer run, now with a loss of 5 Db's and see the SWR drop to 1.5:1 (or something like that, I just made up the numbers)-- it isn't a coupling improvement to the antenna you are noting, but the loss in the return path in the cable. SWR measured -as seen at the transmitter- is looking at both the forward and reflected powers-- in this case, you will see 100% forward, but a decreasing amount as the line length increases. That is because the reflect wave is getting burned up in the return path. It looks nice on the meters at the transmitter, but is a fools paradise. As I said, make the cable long enuff and you will have a perfect match. This may not be possible, but if you can measure the SWR at the base of the antenna you will seem some alarming numbers. Treat the coax cable as a wave guide and nothing else ( and be aware of the velocity factor and how this effects the RF length of the path, you want to avoid certain fractions of wavelengths-- another topic.)
Again, Good Luck-- hope this cast some more light on this tricky subject.

..........................CF