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Author Topic: Ford E150 Install Followup  (Read 1372 times)
KC0TLN
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Posts: 31




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« on: July 07, 2005, 09:13:31 AM »

Greetings.  This is a followup to my earlier thread, regarding the installation of my Icom 706mkIIg in my Ford E150 Conversion Van.

http://www.eham.net/forums/MobileHam/6878

After following much good advice from here and from Alan's (K0BG) site, I have an excellent mobile install with only one remaining problem.  The SGC-239 autocoupler matches all bands on my nine foot whip except for 15m and 17m.  For some reason, probably internal resonance in the vehicle(?) it will not match on those bands.  For the last few weeks I have been on a campaign of bonding:

1) The antenna coupler is bonded to the base of the antenna, the van body at the antenna, and the frame underneath the right rear.

2) All doors are bonded to the body.

3) The tailpipe is bonded to the frame at the right rear corner.

4) The frame is bonded to the body at the four rubber grommets on the four corners of the vehicle.

5) The radio itself is grounded to the body directly beneath the mounting on the center console.

I have run a test with a stiff piece of wire, three feet long, added to the end of the whip. This changes the match on all bands, but still the same 17m/15m instability remains.  My current thinking is that this test shows that I am still in pursuit of a bonding/grounding issue, rather than a fundamental change in the antenna.

I guess that my next bonding effort might be underneath the center of the body, since that I haven't crawled under there with straps yet. <smirk> Possibly cross bonding of the frame elements under the mid section will get this bug. I welcome any further ideas for things to bond... I still believe that this is an internal resonance issue, but I'll be glad to entertain ideas about other test methods similar to the wire-on-the-antenna test. I continue to appreciate the help!

--Ron
KC0TLN
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AA4PB
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Posts: 15065




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« Reply #1 on: July 07, 2005, 01:29:05 PM »

How about capacitance between the whip and the vehicle body? Is the whip mounted such that it runs close to and parallel to the body for some distance or is it in the clear?

What about capacitance between the feed wire and the body (ground). It is not coax I hope. How long is it and is it spaced away from the metal?

What kind of mount and insulator are you using at the base of the antenna?
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Bob  AA4PB
Garrisonville, VA
KC0TLN
Member

Posts: 31




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« Reply #2 on: July 07, 2005, 01:40:14 PM »

Sorry, I should have summarized all this from that previous thread, though it is there in verbose form:

Ford E-150 Converstion van;
Icom 706MkIIg
SGC-239 autocoupler
Hustler stainless steel ball mount with 9' whip.

Autocoupler and whip are mounted on right rear corner about 6" below the roofline; all but 6" of antenna is above van roofline (14'6" overall height).  Wire from autocoupler RF out to antenna is 5" of sparkplug wire.  Autocoupler is grounded to body at base of antenna with 5" of copper flashing, all the rest of the bonding straps noted above are tinned 1/2" braid.

Antenna mount and insulator are plain jane Hustler SS.

Radio is mounted on the center console of the van, with 10ga leads direct to battery (short run, 5') fused on both positive and negative.

Appreciate any thoughts you might have.

--Ron
KC0TLN
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K0BG
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Posts: 10248


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« Reply #3 on: July 08, 2005, 01:54:18 PM »

There are a couple of folks who don't agree with me on this, but it is vehicle resonance that causes the problem with 17 and 15 meters. I don't have the 15 meter problem with mine, but 17 continues to be a problem. I went so far as to take the cover off of the coupler in order to see which relays were closed. I sent the info to SGC, but I haven't heard from them. The ground under the vehicle has a great effect on the problem. For example, if I turn to 17 (it takes about 15 seconds to make the match), and then pull out of my drive way, the coupler will try to rematch as soon as I transmit. If I set it up in the street, everything is okay until I switch to 17 again. It always resets and never remembers the settings like it does on every other band. Incidentally, Terry Ditmer at SGC is aware of this problem. Whether they change the programming to correct it is another question.

Alan, K0BG
www.k0bg.com
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WB6BYU
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Posts: 18518




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« Reply #4 on: July 08, 2005, 02:56:31 PM »

If this truely is a ground resonance problem, then it must
be because the vehicle body is half wave resonant and
presenting a high impedance.  A quick check would be to
attach a horizontal quarter wave radial and see how much
of a difference that makes.  Perhaps park in the driveway,
connect the radial to the ground lug on the tuner, and
run it out from the car with some string on the end to
a fencepost or tree.  If this radial allows the antenna
to tune then you probably have a ground resonance issue.

What to do about it?

1) Run an insulated radial under the car from the corner
by the antenna to the opposite corner.

2) Do the same thing, but connect the radial to the frame
on the diagonally opposite corner of the vehicle and run
it back towards the antenna.  This should change the
car to something close to 3/4 wave.  Maybe.

3) Mount the antenna in the middle of the vehicle.
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WA5UHK
Member

Posts: 131




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« Reply #5 on: July 08, 2005, 05:03:29 PM »

K0GB's comment on 17mtr is ironic.  Its the only band I have RFI on.  I've bonded everything and I still have interferrence with my anti-lock brakes with that band.  I'm still working it with ferrite etc.  I'll let you know when I fix this...I know I can fix it...
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KC0TLN
Member

Posts: 31




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« Reply #6 on: July 10, 2005, 06:44:56 PM »

WB6BYU, I had two thoughts based on your comments...

First:  I'm not sure that the theory is correct, if the vehicle is providing a halfwave resonance, it would have a high impedance, true, but the autocoupler would either match it or not.  The symptom I see, is that the autocoupler with find a match for a moment, then the SWR will spike up and the autocoupler begins hunting again.  This is not the behavior if I try to tune up on a wild mismatch -- If I try to match 160m, it simply continues to hunt.  This suggests that there is something in the vehicle that is presenting a _variable_ impedance.  I think.

Second thought was, why not try it, ham radio is all about experimentation.  So I cut a quarter wave piece of wire, connected it to the ground lug, and tied it out horizontal with a piece of string.  Unfortunately, this gave the same result, I get momentary matches before returning to hunt.  I had this same behavior when I extended the whip with a piece of wire.

Still stumped.
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N2XNB
Member

Posts: 3




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« Reply #7 on: July 14, 2005, 06:20:17 PM »

this may sound dumb but what meter are you using for the 17 and 15 meter checks? if you are using the internal meter of your icom please do a web search as there is grounding that has to be done in your icom 706 for those two bands as it will show high swr without them!! i found the mod i think on mods.dk website. check it out might help you.....n2xnb just the lowly tech....shaun
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WB0NRE
Member

Posts: 3




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« Reply #8 on: July 16, 2005, 09:04:07 PM »

This probably won't affect the match problem, but is the spark plug wire copper or the resistive wire sold for really high-voltage ignitions?  The resistive kind may match, but you won't have much signal out (or in for that matter).  
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KC0TLN
Member

Posts: 31




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« Reply #9 on: July 17, 2005, 08:59:52 PM »

The meter where I see the SWR is the meter in the IC706mkIIg, _BUT_, the SGC autocoupler has its own internal measurment system where it picks up the SWR and feeds that into its logic to determine if the match is good enough.  The symptom is that the SGC autocoupler will find a match for short periods of time, but then it reads high SWR and goes back to searching for a better match.  I am able to see the SWR moving around on the internal meter of the radio, but that meter isn't controlling anything.

The spark plug wire is "old fashioned" high voltage conductor.  The local ham radio shop has it in their wire supply for this application, they will give you a chunk of it if you ask, when you buy an autocoupler.

Thanks for the questions.
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