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Author Topic: Unexpected effect of adding grounding strap  (Read 4136 times)
G7MRV
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« on: February 28, 2013, 04:29:27 AM »

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
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W9MMS
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« Reply #1 on: February 28, 2013, 08:46:21 AM »

How many did you say you've added?Huh?

You got to be kidding!
Please tell me this is a joke!

.... http://www.k0bg.com/bonding.html#where


UNREAL!!!


((((73)))) Milverton.
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G7MRV
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« Reply #2 on: February 28, 2013, 09:19:29 AM »

Milverton, try taking a day off and actually read what people write. Did I say that was ALL I was installing? No.

It states quite clearly that I have STARTED installing the straps, and am testing after each one.

If you cant be bothered to actually read what was put and if you cant provide any meaningful comment then I suggest you post on someone elses thread.

Any serious opinion out there please?

For anyone willing to actually help, I can offer the figures for the system with the 2nd strap now installed, this one bonding the bonnet (hood) - Lowest SWR at 14.296MHz 1.2:1, R=44 X=7, and X=0 now at 13.831MHz

The second strap now has the system showing the sort of effect I was expecting.

The straps for the doors, engine block and exhaust system will be added when I next have sufficient time free.

oh and W9MMS - help yourself to an ignore flag
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KB4QAA
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« Reply #3 on: February 28, 2013, 11:05:24 AM »

Martin,
This is the sort of thing we tried to warn you about when you first proposed this 'one at time then measure' method.

You are making minor changes then trying to derive 'significant' results.

Technique and materials are going to have more effect on the result than the single strap.

When you have 8-10 bonding straps installed, then we can look at measurements and discuss the results.

very best, Bill
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W9MMS
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« Reply #4 on: February 28, 2013, 11:59:46 AM »

Milverton, try taking a day off and actually read what people write. Did I say that was ALL I was installing? No.

It states quite clearly that I have STARTED installing the straps, and am testing after each one.

If you cant be bothered to actually read what was put and if you cant provide any meaningful comment then I suggest you post on someone elses thread.

Any serious opinion out there please?

For anyone willing to actually help, I can offer the figures for the system with the 2nd strap now installed, this one bonding the bonnet (hood) - Lowest SWR at 14.296MHz 1.2:1, R=44 X=7, and X=0 now at 13.831MHz

The second strap now has the system showing the sort of effect I was expecting.

The straps for the doors, engine block and exhaust system will be added when I next have sufficient time free.

oh and W9MMS - help yourself to an ignore flag

>>> Any serious opinion out there please? <<<

Bond the entire Vehicle then see the result! - can't spell it out any clearer now.

Do you get result after taking the first dosage of a prescription that states " Take twice daily for seven days? "
If you can't be bothered to follow the prescribed plan, Oh Well!

((((73)))) Milverton.

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G7MRV
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« Reply #5 on: February 28, 2013, 02:14:44 PM »

Do you get result after taking the first dosage of a prescription that states " Take twice daily for seven days? "
If you can't be bothered to follow the prescribed plan, Oh Well!

Actually, yes, you do, if you know how to monitor and analyse the body for the effects. (by the way, I have two trades - Broadcast engineer and biochemist)

ANY change has an effect, albeit often small. I am not looking for significant effects with minor changes, Im looking to understand the effect each change has and why. Theory says each bond should cause a lowering of the resonant point. So why did the first one raise it? Thats all I wish to understand. I accept that the variation may be within the limits of my measurement accuracy, in which case the observed change may be erronous. I dont accept that the entire process must be gone through before testing. Ive seen projects done that way, and ive taken them over and spent a long time finding the point at which a mistake was made that screwed the whole job.

The usual reason single task-single test isnt carried out is because people are too lazy to repeat measurements, or are under pressure to complete within a timeframe. I have no such restrictions and so can apply proper scientific method and measure at each change. Yes perhaps I wont see meaningful values until I have a lot of data. But I would rather have that data with a few erronous values than to get to the end and find something isnt right, then have to go back and locate the problem.
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G7DIE
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« Reply #6 on: February 28, 2013, 02:22:12 PM »

Martin,
This is the sort of thing we tried to warn you about when you first proposed this 'one at time then measure' method.

You are making minor changes then trying to derive 'significant' results.

Technique and materials are going to have more effect on the result than the single strap.

When you have 8-10 bonding straps installed, then we can look at measurements and discuss the results.

very best, Bill

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 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.
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AC4RD
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« Reply #7 on: February 28, 2013, 04:02:01 PM »

But, the readings I get now are - lowest SWR 1.4:1 @ 14.430MHz, R=42, X=14, and X=0 at 13.870Mhz.

Martin, this isn't an answer to your question--I'm not smart enough for that. ;-)  But I wanted to add one vague opinion, and there are plenty of people who do not agree with me on this.  But when I'm playing with my monoband HF mobile antennas, I work to get them nicely resonant at the desired frequency (X=0 at 18.150, for instance) and THEN worry about SWR.  I find that the antennas often have about 2:1 SWR when they're nicely resonant, and then I add a bit of a coil or a capacitor at the feedpoint to get the SWR down a bit more.  This seems to work for me, and it seems to produce good results.  But, as I said, lots of people don't agree with me on that.  Wink  And I'm going to stay away from the whole "bonding/testing" issue--though I've always found that with bonding, more is better.

73 and GL!    --ken
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AA4PB
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« Reply #8 on: February 28, 2013, 04:45:01 PM »

Ken, you are exactly right. When tuning an antenna, first make it resonant (X = 0). A fairly good mobile installation will have an impedance of 25 Ohms at resonance, which give an SWR of 2:1. Once it is resonant then you provide impedance matching at the base to make the impedance 50 Ohms which gives a 1:1 SWR. Once the antenna is resonant and providing an impedance of 25 Ohms then a 2:1 UNUN makes a nice, broad band impedance match.

Loss resistance can also increase the impedance and lower the SWR but at the expense of turning power to heat instead of radiating it. Often as you reduce grounding losses by improving bonding or adding radials you find that the SWR actually increases as you reduce the loss.
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W9MMS
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« Reply #9 on: March 01, 2013, 03:22:20 AM »

Ken, you are exactly right. When tuning an antenna, first make it resonant (X = 0). A fairly good mobile installation will have an impedance of 25 Ohms at resonance, which give an SWR of 2:1. Once it is resonant then you provide impedance matching at the base to make the impedance 50 Ohms which gives a 1:1 SWR. Once the antenna is resonant and providing an impedance of 25 Ohms then a 2:1 UNUN makes a nice, broad band impedance match.

Loss resistance can also increase the impedance and lower the SWR but at the expense of turning power to heat instead of radiating it. Often as you reduce grounding losses by improving bonding or adding radials you find that the SWR actually increases as you reduce the loss.


Let's visit a previous post by Martin on the very same subject of "Bonding"

http://www.eham.net/ehamforum/smf/index.php/topic,88237.0.html

Now let's put one plus one and see what is the  total.
What is Martin trying to achieve here?
Based on his Statements, one has to ask if he is trying to tune his Vehicle to match his Antenna.
Mark (K5LXP) has highlighted the folly of such a thought process, but it seems to fall on deaf ears.
It is obvious, that he is being overly analytical.
Or, is it that Martin is trying to Re Invent the wheel?

((((73)))) Milverton

























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G7DIE
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« Reply #10 on: March 01, 2013, 04:46:46 AM »

Let's visit a previous post by Martin on the very same subject of "Bonding"

http://www.eham.net/ehamforum/smf/index.php/topic,88237.0.html

Now let's put one plus one and see what is the  total.
What is Martin trying to achieve here?
Based on his Statements, one has to ask if he is trying to tune his Vehicle to match his Antenna.
Mark (K5LXP) has highlighted the folly of such a thought process, but it seems to fall on deaf ears.
It is obvious, that he is being overly analytical.
Or, is it that Martin is trying to Re Invent the wheel?

((((73)))) Milverton


I don't know if you have some issues with the G7MRV, but I think that may be clouding your judgement, after reading the previous thread, and this thread, I see he is being true to what he set out to do, analyse the changes made at each step, I don't know why that would be considered over analytical Huh

I know, having read numerous forums posts, numerous websites and numerous literature, that bonding my car panels is a good thing, it's what I should do for HF mobile operations, but rather than glibly follow the many links to Alan Applegate's website and do things parrot fashion, I'd want to know the what, why, and how for my own installation, and maybe pass my experience on whilst doing it, I assume G7MRV is doing the same.

It is after all a technical hobby, self study and technical investigations being some of the reasons for getting a licence in the first place Wink
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K5LXP
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« Reply #11 on: March 01, 2013, 06:32:58 AM »

So everything seems to have shifted up, rather than down? Not what I was expecting. 
Can anyone offer an explanation for this?

One that I might offer is you have some common mode/third wire that may be affecting the overall response.  That, and/or what you've now coupled together isn't the homogenous conductive plane you imagine it to be but rather an RF-circuitous structure.  Two panels may be directly adjacent to each other but may in fact be held together with an insulating adhesive or weather gasket, and the conductive path between them can be feet away.  Plastics are used extensively in modern vehicles and you can't even trust there's a DC path anywhere, much less an effective RF path.

This is why when you bond a vehicle you typically "shotgun" it, and ensure there won't be any of these less than direct paths.  Common mode is a problem you'd address through direct bonding of the antenna mount and sometimes by choking the feedline with ferrites.   The science experiment aspect of this effort will likely prove the decades-established premise that a vehicle chassis is an RF-discontinuous structure which is remedied by the liberal application of bonding straps.


Mark K5LXP
Albuquerque, NM
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KB4QAA
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« Reply #12 on: March 01, 2013, 08:03:24 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
« Last Edit: March 01, 2013, 08:05:28 AM by KB4QAA » Logged
KB4QAA
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« Reply #13 on: March 01, 2013, 08:18:53 AM »

Martin,
You will probably not receive an answer to what results you observed. 

Common sense and experience tell me that you made a minor change to the antenna system, that had little effect. 

Why exactly?  Who knows.  Maybe your strap is too narrow, or you placed it too far inboard/outboard, or you didn't get a good connection, or maybe it was damp.  Get yourself an EE PhD and a laboratory.

Keep adding bonding straps and you will eventually see some trends.  Very little information can be derived from one data point.   

Conversely, if the one data point doesn't meet your expectations, perhaps your expectations/model is wrong! 

b.
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M6GOM
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« Reply #14 on: March 01, 2013, 08:32:46 AM »



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

First of all, ignore anything other than what you get when X=0. That is the resonant point of the antenna.

I think your homebrew antenna may have a lot to do with it. If you can find a way to use a whip in place of it, even one not resonant on any amateur band, you will probably see what you expected.

Also are you using serrated washers to cut through the paint down to the bare metal? Without them you're peeing in the wind.

The fact you are seeing change proves something is happening though but as others have said, keep on keeping on. You should notice a significant change when you do the bonnet.

Here is how I did the tailgate on my Mondeo. The strap is RG213 like yours with a serrated washer under the solder tabs to cut through the paint. I have not done any sanding back of the paint to metal. The strap is about 3" long.

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

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