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eHam Forums => Mobile Ham => Topic started by: G7MRV on February 28, 2013, 04:29:27 AM



Title: Unexpected effect of adding grounding strap
Post by: G7MRV 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


Title: RE: Unexpected effect of adding grounding strap
Post by: W9MMS on February 28, 2013, 08:46:21 AM
How many did you say you've added?????

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

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


UNREAL!!!


((((73)))) Milverton.


Title: RE: Unexpected effect of adding grounding strap
Post by: G7MRV 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


Title: RE: Unexpected effect of adding grounding strap
Post by: KB4QAA 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


Title: RE: Unexpected effect of adding grounding strap
Post by: W9MMS 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.



Title: RE: Unexpected effect of adding grounding strap
Post by: G7MRV 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.


Title: RE: Unexpected effect of adding grounding strap
Post by: G7DIE 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 ;D

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.


Title: RE: Unexpected effect of adding grounding strap
Post by: AC4RD 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.  ;)  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


Title: RE: Unexpected effect of adding grounding strap
Post by: AA4PB 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.


Title: RE: Unexpected effect of adding grounding strap
Post by: W9MMS 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



























Title: RE: Unexpected effect of adding grounding strap
Post by: G7DIE 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 ???

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 ;)


Title: RE: Unexpected effect of adding grounding strap
Post by: K5LXP 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


Title: RE: Unexpected effect of adding grounding strap
Post by: KB4QAA 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 ;D

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.  ;)


Title: RE: Unexpected effect of adding grounding strap
Post by: KB4QAA 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.


Title: RE: Unexpected effect of adding grounding strap
Post by: M6GOM 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



Title: RE: Unexpected effect of adding grounding strap
Post by: G7MRV 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 ;D

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.  ;)

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



Title: RE: Unexpected effect of adding grounding strap
Post by: G7MRV 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.


Title: RE: Unexpected effect of adding grounding strap
Post by: G7DIE 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.  ;)
[/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 ;)


Title: RE: Unexpected effect of adding grounding strap
Post by: M6GOM 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.


Title: RE: Unexpected effect of adding grounding strap
Post by: G7MRV 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.


Title: RE: Unexpected effect of adding grounding strap
Post by: WN2C 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


Title: RE: Unexpected effect of adding grounding strap
Post by: AA4HA 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.