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Author Topic: Unexpected effect of adding grounding strap  (Read 4369 times)
G7MRV
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« Reply #15 on: March 01, 2013, 11:04:03 AM »


Through experimentation comes understanding, this is exactly how I'd approach the task, one at a time and measure, learn what is THE difference that made a difference, maybe I'll get I round to it one day Grin

I work in aircraft maintenance, Airbus and Boeing produce step by step manuals on how to diagnose and rectify faults on their products, I know that if I ordered all the spare parts that could potentially fix a problem, fitted them all, and the problem disappears, it could lead me to think this is the way to do it, but I'd never know which one item fixed the defect, I wouldn't learn anything from it.
I too have worked on airplanes.  You wouldn't start to replace a major panel on the airplane, install one rivet and take it flying to see how much the structural strength improved and expect to obtain meaningful data.  Wink

Although, strangely, the major manufacturers, when develioping a new aircraft, and indeed Empire Test Pilots School, DO exactly that. I wonder why?

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G7MRV
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« Reply #16 on: March 01, 2013, 11:10:25 AM »

Thanks to G7DIE, K5LXP and M6GOM for some good info.

IM curious as to whether the physical makeup of the tailgate is having an effect. Intruiging.

GOM - these first two are direct to bare metal with sheet metal screws, serated washers, and then reprimed and painted. Some, those on the doors, will go via the hinge bolts as in your case. Which reminds me I need to order some more washers.

Yes I think the compromise antenna might have a sway on the results. As im using the analyser (and as, through my employment, I have legal access to many non-ham frequencies for test purposes) i'll put a straight whip on when I can next work on it, and test at the resonant point of that. Although I probably wont be able to go quite so low in frequency with it.

Its likely to be at least two weeks before I am able to do further work.
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G7DIE
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« Reply #17 on: March 01, 2013, 01:27:04 PM »

I too have worked on airplanes.  You wouldn't start to replace a major panel on the airplane, install one rivet and take it flying to see how much the structural strength improved and expect to obtain meaningful data.  Wink
[/quote

If you've been involved in any structural repair or design work you'll know there is a standard rivet pitch, advisory circular AC43.13-1B section 4-57 for example, there will be a standard rivet pitch based on skin thickness and rivet diameter, however dependant on where you make the repair you may only need to use a percentage of a standard repair, say intermediate frames where only 60% of the number of rivets may be required. There's a reason we know this, that's because clever people have carried out stress analysis in order to minimise weight and still maintain structural integrity.

Now because amateur radio is a technical endeavour, where experimentation and self training is encouraged, why not learn something along the way rather than just do it because other people say so. G7MRV has already found a negative result by grounding his tailgate/trunk only, I'd say that's a result, it may have had a negative result, but it's a positive outcome, he knows, not just because someone told him, that only grounding his trunk will be a bad thing, I doubt there's many people that grace the various ham forums could have told him that Wink
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M6GOM
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« Reply #18 on: March 02, 2013, 06:45:45 AM »

Thanks to G7DIE, K5LXP and M6GOM for some good info.

IM curious as to whether the physical makeup of the tailgate is having an effect.

When I had the tailgate up on my Mondeo I couldn't get a match below 2.5:1 so it does have an affect to a certain point although I'd have thought that almost closed would be OK.

Looks like you're installing them OK so nothing to suggest in that respect although you really don't need to bare the metal. The idea of using the serrated washers is that it cuts through the zinc coating to the metal to make the contact then the zinc seals around the washer and keeps the anti-corrosion in place.

I will say though that I feel the performance on mine using a K400 on the hatch is dire compared to the previous install on my MK3 Mondeo where it was mounted by a 3/8 mount via hole drilled in the roof method. R is 27 Ohms when X=0 so that is about as good as it gets and I've made it to VK on 20m with 100w (I'm a full license holder but can't be bothered to change my call on here) so it works but it just seems harder to make contacts. The bottom half of the hatchback on mine is actually plastic with a metal frame which I only found out when trying to put on a magnetic GB sticker last summer.

Keep us informed.
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G7MRV
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« Reply #19 on: March 02, 2013, 11:14:48 AM »

It will be at least a week before I can next work on it.

Even with these compromise antennas (20m and 17m) which are adjusted for a max 1.5:1 SWR, meaning the resonant point is a bit lower than ideal, have done pretty well, with such places as Brazil, Argentina, Martinique, Kuwait, Australia, Indonesia all logged mobile during my commute to and from work.
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WN2C
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« Reply #20 on: March 02, 2013, 04:25:54 PM »

Ok, so ive finally started adding the bonding to my car. As I had previously decided, Im doing one strap at a time and testing after each. Although even if id decided to not test until the end, i'd still only have got one done, as its freezing cold and I cant feel my fingers any more!

Ive so far then installed one strap. This bonds the boot/trunk door to the main bodywork. Car is a Ford Fiesta 5-door 2007 model. Strap is a 6inch long section of braid from RG-213 coax. Antenna in use is a homebrew base-center loaded (ie loading coil not quite at the base, but not quite at the center either!) whip on a through panel SO-239 mount rear-center of roof panel. RG-58 coax to radio is around 4ft long.

Before adding the strap, measurements from my MFJ-259 were - lowest SWR 1.3:1 @ 14.318MHz, R=44, X=11, with X=0 being at 13.757MHz.

Now, on testing after fitting the bonding strap, I expected the resonant point to drop. But, the readings I get now are - lowest SWR 1.4:1 @ 14.430MHz, R=42, X=14, and X=0 at 13.870Mhz.

So everything seems to have shifted up, rather than down? Not what I was expecting.

Can anyone offer an explanation for this?

The figures might not be perfectly accurate, as they involved me closing the boot door as much as possible whilst wedging myself into the boot with the analyser!


Martin G7MRV

You said you had to turn yourself into a pretzel getting into the trunk.  What about capacitance from the body...may that have changed your readings?  They actually don't seem that far apart. 

I too am in the process of doing some bong of my truck and am anxious to see what your results are.

Rick  wn2c
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AA4HA
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« Reply #21 on: March 02, 2013, 07:00:14 PM »

You may see some results that are counter-intuitive with bonding. Sometimes body panels are not welded to the reinforcing structure of the door and are only crimped and tack welded around the corners and glued in the middle portions of the body panel. This may be done by the manufacturer to minimize the amount of surface refinishing prior to painting, cost savings or any other reason. Where you bond to may be that reinforcement structure so there may be mostly a capacitive connection to the body panel except at the edges (think of the entire thing like that, unwelded areas with LC values).

This is getting crazy-analytical if you try to wrap your brain around it. Also think of a flat conductive surface bonded in one spot as having "ripples" of standing waves across what you might think of as a ground plane. Again, this requires you to think RF and not DC and to be able to grasp in your minds-eye how complex the answer may be to what is a fairly simple question (BTW, these are the things that keep me awake at night. No wonder why I suffer from insomnia).

If you ever tried to model a complex structure with a wire model in NEC (like an aircraft or a missile) and predict how a patch antenna should work then you get an inkling of what sort of fairly minor appearing variations add up in unexpected ways. That is why programs like NEC may have the ability to add hundreds or thousands of wire segments to capture the detail of structures like a car body. Outside of what an automotive manufacturer does for their own integrated antenna systems you will not see them taking a second glance at what a radio amateur may mount on the roof or the middle of the bonnet.
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Ms. Tisha Hayes, AA4HA
Lookout Mountain, Alabama
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