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Author Topic: Yet another FT-857D/ATAS-120A grounding issue  (Read 1555 times)
KE7NPE
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Posts: 2




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« on: June 23, 2007, 09:21:20 AM »

Hi folks,
  I did both forum and google searches without finding anything pertaining to my exact issue.

I just finished installing my new 857D/ATAS-120A combination in my pickup.  The antenna is mounted on top-center of my fiberglass camper shell (yeah, I can hear the groans already).  I used a comet 3d5m through-the-roof mount, with two 5" diameter stainless plates.  I 'sandwiched' the fiberglass between the plates, then solidly attached a 10 ga. wire to the inner plate and ran it to the frame of the truck.  DC grounding is rock-solid.  I can hit the local 2 meter repater with no problem, but can't tune anything on the HF bands, not even manually.  SWR is through the roof.  I'm fairly certain it's a ground plane issue and am considering including a 4' x 5' sheet of thin sheet metal in the 'sandwich' between the lower plate and the roof of the camper shell.  Sort of a new roof layer.  Would this work?  If so, would the type of metal make a difference?  I was thinking of the thin stuff the heating and A/C folks use to make ducts out of.  If it would work, what would be the best way to ground it to the frame of the truck?  Grounding runs would be 2 or 3 feet to the truck body, and further to the frame.  If someone has knowledge of this sort of install and needs to see photos of what I'm doing, I can take some.  Thanks for your help.

Bruce
KE7NPE
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KE3WD
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Posts: 5694




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« Reply #1 on: June 23, 2007, 09:50:37 AM »

If you can find a piece of copper window screen, maybe at an older hardware store that still stocks same, it can be applied to the underside of a fiberglass camper shell.  I used 3M adhesive plus a few strategically placed pop rivets with washers on the screen side and a bit of sealant on the outside and it worked very well.  Bond the screen to vehicle ground, of course.

Friend of mine removed his campertop and painted the underside with conductive paint -- can be found at www.stewmac.com as paint used for shielding guitars, that's where he got his, likely other sources.  Then he shot a second coat of auto paint the same color as the original inside of the camper shell to protect the flat black conductive paint from abrasion and make the shell look good in there.  Left a small area of the conductive paint in the clear for attaching a grounding braid over to the truck frame.

Personally I like the copper window screening better, especially if you intend to run QRO mobile.  

Vertical antenna needs that ground plane right underneath it, these methods can do that for you.  

Likely your VHF performance will get a lot better, too, especially for longer distances.  


73,
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W3LK
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Posts: 5644




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« Reply #2 on: June 23, 2007, 12:35:35 PM »

(1) At best, the ATAS series antennas are lousy HF antennas, even when properly installed, as your is not.

(2) The issue is NOT DC grounding; that has virtually nothing to do with HF performance. What DOES is a proper RF ground plane UNDER the antenna. Essentially you have NO ground plane for the antenna to operate against on HF and therefore your SWR is off the scale

At the very least, you would have to put the previously mentioned screen over the entire underside of the fiberglass and even then the performance would be marginal, if that.

Go to www.k0bg.com and read Alan's section on installing HF antennas. It's the best place to start.

Almost anything can be made to work on 2m; HF is a whole different ball game entirely.

73,

Lon - W3LK
Baltimore, Maryland - soon to be Naugatuck, Connecticut
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KE7NPE
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Posts: 2




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« Reply #3 on: June 23, 2007, 03:10:36 PM »

OK, I tried a little experiment - I went down and bought a couple of rolls of aluminum foil and rolled them out on the roof of my camper shell, poking a small hole for the antenna mount, and screwed the antenna down on top of it.  I spread it out nice and flat and weighted the edges down, then went to the cab, dialed up a 40 meter freq. and hit 'tune.'   Wham!  Instant results.  The 120A tuned a nice, low SWR immediately on every band I tried it on.  I forgot today was field day - Listened in on lots of QSOs, but couldn't get out.  Most likely due to being parked in my driveway (right under power lines) and the crappy nature of the tinfoil setup.  I'm on the right track now, though.  I'll put a layer of sheetmetal inside the topper and I should be rockin'.  I have no illusions about it being the most efficient setup out there, but at least I'll be able to tune and listen and hopefully get out fairly well.

Thanks!

Bruce
KE7NPE
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W3LK
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Posts: 5644




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« Reply #4 on: June 23, 2007, 05:36:44 PM »

I'm happy your experment worked!

73,

Lon - W3LK
Baltimore, Maryland - soon to be Naugatuck, Connecticut
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