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Author Topic: Screwdriver Problems - No 80m Zero Reactance?  (Read 3576 times)
K5END
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« Reply #15 on: August 27, 2009, 11:21:05 AM »

what frequency gives you r = 38, x = 0 ?

and what happens if, at the (30, 0) point if you vary the analyzer frequency by 10, 20, 30 & 40 KHz in either direction?

is this a ham stick?
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K5END
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« Reply #16 on: August 27, 2009, 12:57:32 PM »

i forgot to ask my main question.

what kind of screwdriver is this?
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K1KRV
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« Reply #17 on: August 27, 2009, 07:51:58 PM »

Allen, thanks for your remarks on stray capacitance.  My coil is clear of any sheet-metal and the 'hand near the coil test' doesn't alter anything.

Concerning 'ground losses' K0BG wrote:

"Since the ground losses, and the radiation losses are in series, and cannot be separated (quantitatively measured), one factor can increase, while another decreases. As a result, comparing just the input impedance as a measure of efficiency without considering the other factors, will yield mixed results."

I do 'get' this.  And this thread has helped me understand it more thoroughly!  I realize it's the 'nature of the beast', but not being able to 'separate and measure' these factors independently is still frustrating!

In the "Bonding" article K0BG wrote:

"As with the number of radials, the more you have, the better. However, as the number of radials increases, the less effect each one has. This is true of bonding too, but I have to say, I've yet to find the limit."

My bonding installation seems pretty respectable: pickup bed to frame (4 corners), pickup bed to cab (2), exhaust (front & back), hood to firewall (2 corners), engine to frame (1).  I'll add more, but I may be reaching a point of diminishing returns.  

What do you think of the following?

Taking 25ohms as the target, I seem to be 'shy' 10ohms -- (Freq: 14200 R=38 X=0)  That's 'close' to the 12.5ohm difference between 'Q' 50 and 'Q' 300 that you discuss in the "Antenna Efficiency" article -- (coil loss 15ohms vs. coil loss 2.5ohms)  

Is it reasonable to think that my bonding system is pretty close and now it's time to consider that antenna 'Q' may be the reason for high impedence?

[BTW: Thanks for the suggestion on 'Electro-Motion' EM20 braid.  I really like that product!]

K5END, thanks for asking!

Freq: 14100 R=42 X=0
Freq: 14200 R=38 X=0
Freq: 14250 R=38 X=0
Freq: 14300 R=39 X=8

This 'screwdriver' is an Alpine.  The coil is formed around a grey material that looks like PVC, so the Q may be lower than some.

There is no 'fingerstock' in this antenna. The coil is exposed through several 'cut-outs' in the top of the mast tube.  A stainless-steel spring wraps around the tube (like a rubber band) and seats in these cut-outs. The spring contacts the coil through the 'cut-outs', connecting it electrically to the mast.

At the bottom, the mast is 'sliced' to create an integral matching coil. These 'cuts' are 1/16th wide and 'coils' about the same.  Formed from the tube itself, the 'coils' are flat and 'wrap' around a section of PVC pipe inserted in the tube.  There's an aluminum sleeve inside the PVC tube that slides over a 1/2" x 4" stud that's bolted to the mount.

Thanks for all the help!  I'm learning a lot!

73,

Kurt
K1KRV
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K1KRV
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« Reply #18 on: August 27, 2009, 08:11:43 PM »

Allen,

Correction,

I'm actually 'shy' 13ohms -- R=38 vs R=25 'target'.  

That's almost 12.5ohms exactly!  The difference between 'Q' 50 and 'Q' 300 -- (coil loss 15ohms vs. coil loss 2.5ohms) that you outline "Antenna Efficiency".

Is this just a coincidence?  Or is it telling us that my 11 bonding straps are probably doing the job and the high impedance is likely to be the result of low antenna 'Q'?

Thanks again!

73,

Kurt
K1KRV
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K5END
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« Reply #19 on: August 27, 2009, 10:03:26 PM »

Kurt,

Your numbers look pretty good on 20 meters...almost too good.

Without evaluating the system personally, I'd have to take a shot in the dark, a stab at it, a hail mary play...or any other cliche' for the word "guess" and say it looks like you might have a lossy antenna, or coax, or connector, or any of a number of other things.

But it looks like something is not so fresh in Denmark. It could be the analyzer, even.

Do you have a good power meter, like a Bird 43, or even a Daiwa SWR meter? It would be useful to measure the actual power going to the antenna. That might tell us something.

I would not worry about hitting a "target" 25 ohms. I expect Alan would agree 25 is not a "goal." It's more of a typical result, in his experience. I do concur that 25 shows up a lot with good verical antennas. But so does 50, and 75. There are too many variables affecting the impedance to write law on this.

For example a good vertical with four perfectly tuned, elevated radials can be made to raise its impedance by angling the radials downward a bit. All this does for you is help MATCH the impedance to the radio and transmission.

The maximum power transfer theorem states that maximum power is delivered to the load when the generator and load impedances are equal. In any other case, excess power is transformed to heat in the coax...or in the finals of the transmitter! The energy has to go somewhere, and the law is, the law. You can't break that one! If you can, let me know so I can rent a tux to attend the ceremony where you get the biggest Nobel prize of all time.

To clarify accuracy, power and energy are not the same thing, as the previous paragraph might appear to imply. It was written sloppily but I am too tired to rewrite it now. Energy = power X time.

One comment. The impedance is not a function of Q.

Efficiency, among other things, can be a partial function of Q, however.

Q is a characterization of the system, and is, in part, a result of the impedance.

A comment on bonding. Yes. Do it, and emphasize conducting current toward a plane low on the vehicle and parallel to the ground. You are making a better capacitor.

Even though at HF frequencies a typical car with all of its holes will "appear" to the wave as a solid metal brick (a good example of this is the wire grid dish reflectors,) that does not change the fact that the current still needs a good path to Mama Earth and the capacitance is still a function of the "plate" you have made parallel to the road.

Gotta run. Good luck.

Larry K5END
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K0BG
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« Reply #20 on: August 28, 2009, 06:10:22 AM »

We're mixing apples and oranges.

Bonding, using braided strapping to interconnect the various bolted on pieces of the chassis and/or frame, is an important undertaking. What the bottom line here is, you're attempting to make the body of the vehicle present a low impedance to the flow of RF. For example, if you mount an antenna atop a trunk lid, and measure the input impedance, and then bond the lid across the hinges, you can easily measure the difference in the input impedance.

While I make reference to radials (I probably should revisit that), bonding and radial systems only have one thing in common. And that's a effort to reduce ground losses. In an HF mobile installation, they are always too high! As I have said, proper bonding is a worthy endeavor. It should be noted, however, that a bunch of ground straps from the antenna's mounting hardware to ground, is not a substitute for proper bonding, or proper mounting of the antenna itself.

If putting your hand near the coil has little or no effect on the input impedance, then something is amiss. That might be (?) because the coil is too close to sheet metal. It could also mean the coil Q is very low.

The Q of any coil is dependent on a lot of factors, and some of them aren't evident. And all of them are very difficult to measure, if in fact they can be measured at all! The more important ones are these: Any metal mass in close proximity to the coil effect the Q. The mast, the whip, the body of the vehicle, large supporting end caps (Hustler high power coils are a good example); The dielectric properties of the coil form, weather covers, wire size, turns spacing, and even the coating (if any) on the wire; Length to diameter (form factor) is also a biggie, especially when the coil has a large amount of inductance (like 80 meter loading coils for example).

All of the factors are reflected in the input impedance. The lossier the coil (low Q), the higher the input impedance will be, all else being equal. It is this last part (all else being equal) where the rub comes.

As I alluded to previously, some changes might very well reduce some specific portion of the input impedance, yet increase another, giving one a false sense the change was positive. I'll give you a very good example; the cap hat.

You often see cap hats mounted directly atop the coil housing. As stated above, this reduces the Q of the coil, which increases the input impedance of the antenna. The general consensus is that cap hats increase the radiation resistance, but this is only true if they are facilitated correctly. So what really happened in the example, the coil's resistive losses increased (the radiation resistance dropped slightly), but the readout from his antenna analyzer told him what he want to see, not what actually happened. As I said, apples to oranges.

Alan, KØBG
www.k0bg.com
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K5END
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Posts: 1309




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« Reply #21 on: August 28, 2009, 06:21:57 PM »

"We're mixing apples and oranges. "

I'm not.
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K5END
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« Reply #22 on: August 28, 2009, 07:03:27 PM »

Kurt,

Read this page as if it were from the Bible.

http://www.w8ji.com/loading_inductors.htm

Alan, you might want to read it too.

This guy explains better than I did how these misconceptions arise. It's a very well-written explanation.

Based on what you've said, I think your antenna needs professional help, whatever the make and model is. You're not likely to solve the problem on an Internet thread.

LK
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K1KRV
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« Reply #23 on: August 28, 2009, 07:46:42 PM »

Larry,
 
I love a good 'hail mary' play!  Your comments are insightful and clever as well!  :-)  

I'm not 100% sure why you say my numbers are 'too good' but I do have a Daiwa meter and hope to try some tests over the weekend.
 
I'm not "worried" about 25ohms, but simply thinking of it as a benchmark to flesh out the 'odours in Denmark' ;-)

Alan,

Thanks for your fine comments!  

At your website, the difference between radials and bonding is crystal clear.  I copied one sentence here, for reference, so it reads out of context.  
 
On the subject of testing a bonding installation, allow me a 'last shot' to distill this in my mind:

K0BG wrote:

"The lossier the coil (low Q), the higher the input impedance will be, all else being equal. It is this last part (all else being equal) where the rub comes. "

So, IF stray capacitances, cap hats and other variables are not a factor. And IF the bonding system is 'as good as it gets' -- the 13ohm difference between my installation and a common 25ohm benchmark (R=38 vs R=25) MAY be attributable to the 12.5ohm difference between 'Q' 50 and 'Q' 300.
 
Am I understanding that correctly?

And finally, one hypothetical question:

If you removed a 'screwdriver' antenna from it's mount and replaced it with a 1/4w vertical wire -- would that isolate the bonding sytem for testing purposes?  That is, wouldn't a plain wire, tuned for resonance, eliminate the variables inherent in the electrically shortened 'screwdriver' such as 'Q'?

I'm not suggesting this as a procedure, but wondering about it theoretically.  If you mounted a resonant quarter wave wire on a parked vehicle and then added bonding until the impedence dropped as far as it will go -- wouldn't that, more or less, equate to the best bonding acheivable?

You and Larry have given me a lot to work with!  I really do appreciate it!  If the rain stops, I hope to get out this weekend an apply what's been discussed in the thread.  I'll report back with what I discover.

Thanks again,

73,

Kurt
K1KRV
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K1KRV
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« Reply #24 on: August 28, 2009, 08:04:03 PM »

Hi Larry,

I think we posted simultaneously!  Thanks for the link!  I have read through W8JI's superb website and plan to study it further.  

I don't expect to precisely 'solve' my specific antenna issues on an internet thread.  But this thread has helped me think through the problem!  I consider it a great learning opportunity and appreciate your contribution.
 
I installed the mobile to re-ignite my excitement for amateur radio and it's certainly done that!  This discussion has helped 'blow the cobwebs' off my brain and gotten me motivated.  I'm even planning to get back up in the trees and fix all those niggling little shortcomings in my home installation.
 
Thanks very much for your part in that!

73,

Kurt
K1KRV
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K0BG
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« Reply #25 on: August 29, 2009, 10:13:04 AM »

In all due respect Larry, you need to read it again too!

The whole point here is this. The input impedance of an HF mobile antenna comprises three main ingredients: The ground losses, typically the largest fact; Coil resistive losses, which can actually be higher than the ground losses if the Q is under 50 or so (frequency dependent), and the radiation resistance, which is mostly depending on the effective length of the antenna. There are stray capacitive losses, conductor resistive losses, and even skin effect losses, but typically they're small compared to the big three.

If you decrease the ground losses by one ohm, but add one ohm of radiation resistance, the net result can't be measured. The example I used earlier, is valid. If you mount a cap too close to the coil, you will indeed see an increase in the input impedance. If you didn't know better, you'd think this was an increase in the Rr, when in reality it is caused by an increase in coil losses.

Based on this, as Tom points out, you can't tell what you've done by just relying on the input impedance.

Alan, KØBG
www.k0bg.com
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K5END
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« Reply #26 on: August 29, 2009, 06:57:24 PM »

Alan,

You're just repeating yourself with general statements, most of which are very basic stuff.

You haven't said anything specific about the problem.

Try again.

Here is an idea.

I will write down a well-known physical law that any sophomore engineering or physics student with mediocre grades would understand and recognize immediately.

If you can explain the importance or even the meaning of this law—without looking it up or consulting others—then you will prove that you know something about this topic. Conversely, if you can’t, then, well...

Vector notation is not an option here in plain text format, so I'll write it verbally. Here it is.

“The 'scalar' or ‘dot’ product of the "Del" operator and the B vector equals ZERO.”

<Jeopardy theme playing>

Here is your chance, Alan. Amaze us. No cheating.
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K0BG
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« Reply #27 on: August 30, 2009, 05:51:14 AM »

Larry, rather than attack me personally, why don\'t you explain yourself when you don\'t believe what I say is correct? Perhaps we both can learn from the experience.

Alan, KØBG
www.k0bg.com
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K5END
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« Reply #28 on: August 30, 2009, 07:29:04 AM »

Well, here is the answer.

This is one of Maxwell\'s equations. The "readers digest" version of it is there are no magnetic monopoles. You can\'t have a magnetic South without a North, and vice versa.

If you cut a magnetic dipole in half, you just get two more dipoles.

As the same is not true for electric charge, that is, you can have a positive or negative charge only within a region of space, this has some implications on the nature of magnetism.

Alan, it\'s not a personal attack, and it\'s not that I think what you are saying is not true.

I just ask that you say something specific instead of rambling the same old "cut and paste" mantras.

As far as what statments I disagree with, it\'s not my job to clean up your web site or to teach you electromagnetic physics.

Be more careful with the advice you give. Stick to what you know.

You do know a lot, clearly. so stick to that. Antenna science is not your field--pardon the pun.
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K5END
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« Reply #29 on: August 30, 2009, 07:36:18 AM »

PS. That was the differential form of the law. There is an integral version as well, as is the case for all of Maxwell\'s equations.
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