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Author Topic: Mobile HF woes  (Read 6227 times)
KJ1H
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« on: October 16, 2012, 03:34:29 PM »

So here's the scoop.

Rig: Alinco DX-70 with a new EDX-2 tuner
Antenna: Shark Mini FMs for 10, 15, 17, 20, and 40 meters
Mount: "puck" mount through trunk lid
Car: 2003 Ford Crown Victoria Police Interceptor

I've been having a heck of a time getting anywhere with this. I seem to be battling multiple issues here. I know I've got an Extra license and should arguably know this stuff, but I can't seem to figure out a pattern and am at wit's end, so I'm calling out for help. Smiley

I got this car in February, and tried to run 10 meters with a Radio Shack HTX-10 (which I had success with in my last car) and a Shark Mini FM for 10 meters.  I haven't been able to make a single contact.  A couple of months ago I picked up the DX-70, and bought additional Shark Mini FMs for the other bands. I still haven't been able to make any contacts.  I know the bands have been variable, but I should at least be able to talk to someone.

With an SWR meter, I've been able to tune all but the 17 meter antenna fairly well for the band. (17 is such a narrow band that I've had trouble finding a starting point to begin tuning it either shorter or longer.) Except it seems that just when I get the SWR reasonable, it suddenly starts pinning the meter again. After retuning it, it changes again. This may indicate a bad connection somewhere.

I picked up a new EDX-2 tuner to try and reign in my issues a little bit. I figured that if I could at least get the SWR close, the tuner could handle the rest of the job, and effectively widen the usable bandwidth available through these antennas. (I'm tuning them for the voice portion of the band, but I'd like to be able to park and play CW as well.) I mounted the tuner in the trunk, and have a short coax run to the bottom of the antenna mount.  I made sure to ground the tuner as well, and used a bolt in the seatback that appears to have been put there by the police department for that very purpose.  Yet I was still unable to make any contacts.  So I checked the coax, and all the components, and it turned out that my original antenna mount was shorting out somewhat - not zero resistance, but not infinite, either. So I just replaced it with a Breedlove mount, which appears to be a far superior design. At this point, my shiny new tuner stopped working. It powers on and off with the rig, but it won't tune, and it doesn't connect the antenna to the radio. I receive no signals when the tuner's hooked up, but with a bypass coax connecting the rig directly to the antenna mount, I can receive just fine. Other than powering up the tuner, the rig acts like it isn't even there.  I've already gotten in touch with GRE America (Alinco support in the U.S.) about that.

So I installed the bypass coax semi-permanently and tried to get the antennas retuned with the Breedlove mount. I'm back to the same problem of highly variable SWR. I get it tuned close, and then it all changes suddenly. If I do my initial tuning at low power, get a decent match, and then switch to high power, the SWR skyrockets, even though the only thing I changed was the power level.

My grounds appear to be good, though I'll be improving them anyway.  I scraped away some paint on the underside of the trunk lid so that part of the antenna mount could touch bare metal and get a good ground.  The car already has a couple of ground straps between the trunk lid and the body for the police radio antenna it had in its past life, but I plan to replace those anyway just to make sure I have a really good connection.  I've been using the quick releases on the antennas, but I've removed them from the mount and my 20 meter antenna for now, and will focus my troubleshooting efforts on that band because I should always be able to talk to someone there. And the radio really is transmitting according to my SWR/power meter - 10 watts on low power, 100 on high.

I just want to get on HF in the car. I can't believe it's been this hard to do it. Lots of time and money spent and I have no contacts to show for it. I'm about ready to give up on the mobile HF idea completely. Please talk me down from the edge of that cliff. Smiley  Any suggestions?  What have I missed here?  Because clearly I've missed something. Please go easy on me. Smiley
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73 - Justin
WX7G
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« Reply #1 on: October 16, 2012, 06:26:37 PM »

The VSWR changing sharply indicates a bad connection; possibly the coax. You can shotgun this problem by replacing the coax and connector(s). And maybe the puck mount. Make sure the mount is grounded to the car body and not just the trunk.

Some will tell you the Shark antennas are no good. They look like Hamsticks and as such the 20-10 meter Sharks can work quite well.

No being able to make contacts when the VSWR is good indicates there might be a problem with the DX-70.
« Last Edit: October 16, 2012, 06:28:53 PM by WX7G » Logged
AC4RD
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« Reply #2 on: October 17, 2012, 04:14:41 AM »

Agree with WX7G--try using a new run of coaxial cable and see if that helps.  For years, I used various Hamsticks on a trunk-lid mount, and any time I suddenly had big changes in SWR, it meant the connections were faulty again.  Taking the mount off, cleaning everything, and re-tightening everything always solved that problem.   And I have never had a decent working mobile HF setup that relied just on the antenna mount for bonding to the car body.  I *always* run a separate piece of braid, as short as I can get it, from the antenna mount to a bolt on the car's body.  Many people don't do this, but I've had far fewer problems and better signals since I started doing this.

I don't think the tuner is going to help you until you work out the underlying problem(s)--I'd suggest taking it out until you have the basic setup working well.  And again, I agree with WX7G that bad connections/coax are the very first thing to suspect in this situation.

I tried one of those "Shark mini" antennas for 20 meters, a few years back.  I was surprised to find that it worked reasonably well, considering how physically short it was.  It was a couple of S-units down from a regular Hamstick, but that's better than I was really expecting.  Wink  I recall it was a bear to tune for resonance; if you have an antenna analyzer or can get a nearby ham to come over with his analyzer, it'll probably help a good bit.

One other bit of advice, and lots of people think this is bunk but it works very well for me:  tune your mobile antenna for RESONANCE first, then worry about the SWR.  That is, use an analyzer to get your antenna resonant where you want it in the band, and then use a coil or cap to trim the SWR.  It works for me!

GL HTH!  --ken
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KJ1H
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« Reply #3 on: October 17, 2012, 04:48:41 AM »

Thanks for the advice. I've already tried 3 different runs of coax - the original, the one that came with the EDX-2, and an entirely different one, which I'm using now. The mount is new, the antennas are new, the rig is new - everything but the car, really!

But I know grounding is key, so I'll replace the existing ground straps, and add one to the antenna mount itself. I'd like to get my hands on an analyzer. It would be much easier than essentially guessing at a starting point like I'm doing now(and haven't managed to do for 17 meters). And you're right, I have issues beyond what the tuner can fix, or else I'd already be making contacts. The tuner's not working anyway so I'm working without it for now.
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73 - Justin
KB4QAA
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« Reply #4 on: October 17, 2012, 05:12:01 AM »

While the variable SWR may result from coax or connection faults, my immediate impression is that RF is getting back into the radio.

I recommend adding a toroid choke on the coax as close as possible to the tuner.  Use as many turns as you can get on.  If the radio has a remote head add chokes at both ends of the extension cable.  Likewise if you are using an external speaker.

-Ohm out the  "grounding bolt" to a nearby chassis point to confirm it is a good ground
-Ohm out your coax cable.  No sense buying an entirely new cable without checking

I don't see any fuses in the tuner, nor anything resettable.  I recommend doing a reset of the RT.  

bill

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K0BG
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« Reply #5 on: October 17, 2012, 05:46:20 AM »

The first thing you need to do is go to my web site, and read the Auto-Couplers article. Aside from the fact the EDX may be defective, the feed from the EDX to the antenna must be as short as possible, and not coax!
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N6AJR
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« Reply #6 on: October 17, 2012, 11:23:35 AM »

Also another problem I have run into  specificall on trunk mounts, is the trunk lid "floats".  I had a a problem with variable swr and picket fencing when moving. What was happening was the trunk lid sits on rubber all the way around and only makes contact through the 2 hinges and the latch.  when running down the road, the trunk lid floats up and down a bit on the hinges pins and latch.. I fixed the problem by running a ground braid from the bolt on the trunk lid side of the hinge to a bolt on the chassis side of the hinge.  this gave it a decent chassis ground and got rid of the picket fencing and the changing swr when in motion.
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KCJ9091
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« Reply #7 on: October 17, 2012, 04:33:48 PM »

The trunk lid is bonded from the factory on the CVPI.
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M6GOM
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« Reply #8 on: October 19, 2012, 07:15:20 AM »

The trunk lid is bonded from the factory on the CVPI.

How? Wire makes a good electrical bond but a poor RF one. RF flows on the surface of a conductor so you want as much surface area as possible. Flattened RG213 braid is a start.

Here is how I do mine.

http://i300.photobucket.com/albums/nn9/computershack/Amateur%20stuff/Icom%207000%20install/4Uu89.jpg
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KJ1H
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« Reply #9 on: October 19, 2012, 07:48:32 AM »

My car, having been originally built for police use, came from the factory with two ground straps connecting the trunk lid to the body. However, I'll be replacing these anyway, plus adding another ground from the antenna base directly to the body, as was suggested. I'll also re-recheck for any loose connections at this time.

Failing that, a local ham offered his help and his antenna analyzer, which would be quite helpful if this is something more than a grounding issue. As for the tuner, still no help from GRE. I may have to carve out some time to try and get a live human on the phone. I'm not going to try to use it in the car at this point, but I would like to get it working again. It's new and under warranty, after all.

Thanks to all for the tips!
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73 - Justin
KCJ9091
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« Reply #10 on: October 19, 2012, 04:50:00 PM »

The trunk lid is bonded from the factory on the CVPI.

How? Wire makes a good electrical bond but a poor RF one. RF flows on the surface of a conductor so you want as much surface area as possible. Flattened RG213 braid is a start.

Here is how I do mine.

http://i300.photobucket.com/albums/nn9/computershack/Amateur%20stuff/Icom%207000%20install/4Uu89.jpg

Last one I did an install in was a 2009.  It had a 1" flat braid next to each hinge.
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K0BG
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« Reply #11 on: October 19, 2012, 05:15:30 PM »

Quote
The trunk lid is bonded from the factory on the CVPI.

Maybe! The up-fit package for the Intercepter DOES NOT include any trunk lid bonding. However, if you have purchased the Intercepter used (the only way you can buy one unless you're a government agency), anything is possible. To be sure, the trunk lid is grounded, but only for DC.
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KCJ9091
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« Reply #12 on: October 19, 2012, 08:01:29 PM »

It had 76 miles on it fresh from GSA Fleet operations when I first put my butt in the seat.  Unless Fleet ops put the straps in, which is doubtful, they were there from the factory as they act only as the broker doing no work on the vehicles other than the paperwork.
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WB2DSH
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« Reply #13 on: November 26, 2012, 08:06:47 PM »

If you'd like to avoid the frustrations of mobile operation, please look at my post about beta users for a remote base system I'm working on for mobile via cellphone.  If you think you might be doing a lot of mobile work, I could give you one to try out. 
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