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Author Topic: UNGROUNDED HF MOBILE ANTENNAS  (Read 6424 times)
K7FF
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« on: November 13, 2016, 10:25:31 PM »

I am interested in the concept of effective HF mobile antennas that are not 'grounded' or otherwise electrically connected to the metallic vehicle body/frame etc.  As such, it would alleviate the need for extensive electrical bonding of vehicle body parts, frames, bumpers, etc.

Perhaps akin to HF vertical base antennas that don't require radial counterpoises, etc.

Are there any commercial antennas, or designs available for amateur home construction, that meet this description?

Derek
K7FF

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K0BG
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« Reply #1 on: November 14, 2016, 03:40:46 AM »

Whether you believe it or not, all monopole antennas, bar none, require some sort of ground plane under them. Call is a counterpoise (albeit an incorrect term here) if you wish, but that doesn't change the irrefutable truth. If there isn't one, the RF current flows back to its source over the path of least resistance (impedance in this case), and that is over the outside of the coax (or feed line) as common mode current. Yes, you can choke some of it off, you can use so-called counterpoise stubs, and believe the hyped advertisements to the contrary, but you're not going to change the laws of physics.
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KH6AQ
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« Reply #2 on: November 14, 2016, 05:51:55 AM »

I think the Stealth 9400 is such an antenna.

http://www.stealth.ae/plugins/custompages/detinf.php?id=372&id_categories=136

There have been other mobile loops and I recall an article many years ago in QST about a homebrewed one that was mounted horizontally. And one could mount two mobile antennas as dipoles such as the made by Hi-Q for the U.S. military. In either case (loop or dipole) there is auto body RF current; think of the image of the loop or dipole in the auto body.

I think the place to use a loop or dipole would be for close NVIS work. I've modeled loops and dipoles on cars and always came back to a monopole as the antenna to build. The dipole is close to ground and the "image antenna" in the earth, or auto body, reduces the radiation resistance, leading to low radiation efficiency. A vertically oriented loop looks like something to investigate further. The image doubles its loop area.

For a monopole, rather than think of the auto body as a flat, non-radiating hunk of metal I view it as a radiating counterpoise. RF current returning vertically along the body leads to the body radiating. It is similar to a ground plane with radials angled downward; they radiate. How much the auto body radiates and how much the monopole radiates can be found by integrating the "current-area" of each.

Two proofs for how ground independent a mobile monopole is are:

1) Driving from dry to wet earth, or from wet earth onto a dry street, does not change the resonant frequency or SWR by much. It roughly follows a NEC model.  

2) A NEC model of an auto body and monopole shows very little change in the resonant frequency as it is moved from a foot above ground to free space.





« Last Edit: November 14, 2016, 06:02:43 AM by WX7G » Logged
K7FF
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« Reply #3 on: November 14, 2016, 06:53:45 AM »

I think the Stealth 9400 is such an antenna.

http://www.stealth.ae/plugins/custompages/detinf.php?id=372&id_categories=136

There have been other mobile loops and I recall an article many years ago in QST about a homebrewed one that was mounted horizontally. And one could mount two mobile antennas as dipoles such as the made by Hi-Q for the U.S. military. In either case (loop or dipole) there is auto body RF current; think of the image of the loop or dipole in the auto body.

I think the place to use a loop or dipole would be for close NVIS work. I've modeled loops and dipoles on cars and always came back to a monopole as the antenna to build. The dipole is close to ground and the "image antenna" in the earth, or auto body, reduces the radiation resistance, leading to low radiation efficiency. A vertically oriented loop looks like something to investigate further. The image doubles its loop area.

For a monopole, rather than think of the auto body as a flat, non-radiating hunk of metal I view it as a radiating counterpoise. RF current returning vertically along the body leads to the body radiating. It is similar to a ground plane with radials angled downward; they radiate. How much the auto body radiates and how much the monopole radiates can be found by integrating the "current-area" of each.

Two proofs for how ground independent a mobile monopole is are:

1) Driving from dry to wet earth, or from wet earth onto a dry street, does not change the resonant frequency or SWR by much. It roughly follows a NEC model.  

2) A NEC model of an auto body and monopole shows very little change in the resonant frequency as it is moved from a foot above ground to free space.







Again, a nice and very informative dissertation!

What I take away from it is that in order for a vehicle HF monopole to be effective, it must have a significant metallic mass (ie. the vehicle body, frame, engine, etc.) under it to act as a 'counterpoise'.

So, what if the metallic mass wasn't actually metallic (and used a different form of non-metallic propulsion), but instead was carbon fiber?

Would the immutable laws of physics prevent the creation of an effective non-metallic mass counterpoise, and thereby 'fool' those very same laws of physics...?

Derek
K7FF

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AB3TH
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« Reply #4 on: November 14, 2016, 08:13:00 AM »


Would the immutable laws of physics prevent the creation of an effective non-metallic mass counterpoise, and thereby 'fool' those very same laws of physics...?


"Something" will act like a ground or counterpoise or the other half of a dipole, whatever you want to call it.  The cable, capacitively coupled car body,  car parts, etc.  You may not be able to model it.  You may not be able to match it.  The efficiency may be 0.001%.  It will radiate something.  Carbon actually has 10 or 20 times better conductivity than steel and better conductivity than copper.  Once you encapsulate carbon fibers in an epoxy matrix, not so good.  I have carbon fiber plate that measures at 300 Ohms/square.  1 ounce copper is 0.005 Ohms/square.  Galvanized steel sheet, around 1 Ohm/square.  How it behaves at RF, I don't know.  Steel is bad because of skin effect.  Carbon may improve at HF because of capacitance between fibers.  If there was enough demand from hams, maybe somebody would make a car with copper bodywork.
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K7FF
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« Reply #5 on: November 14, 2016, 08:32:56 AM »


Would the immutable laws of physics prevent the creation of an effective non-metallic mass counterpoise, and thereby 'fool' those very same laws of physics...?


"Something" will act like a ground or counterpoise or the other half of a dipole, whatever you want to call it.  The cable, capacitively coupled car body,  car parts, etc.  You may not be able to model it.  You may not be able to match it.  The efficiency may be 0.001%.  It will radiate something.  Carbon actually has 10 or 20 times better conductivity than steel and better conductivity than copper.  Once you encapsulate carbon fibers in an epoxy matrix, not so good.  I have carbon fiber plate that measures at 300 Ohms/square.  1 ounce copper is 0.005 Ohms/square.  Galvanized steel sheet, around 1 Ohm/square.  How it behaves at RF, I don't know.  Steel is bad because of skin effect.  Carbon may improve at HF because of capacitance between fibers.  If there was enough demand from hams, maybe somebody would make a car with copper bodywork.

As for the resistance of various conductors, the law of diminishing returns quickly comes into play.

AB3TH:

Thanks.

I assume that when you say 'steel is bad because of skin effect'. you are referencing stainless steel.

It seems odd, if that is the case, that stainless steel whips are universally used on mobile antennas, whether as whips or as resonator stingers.  So, relatively speaking, just how bad is it...?

As for vehicle copper body parts, plating with copper, or ?, should be sufficient.

Derek
K7FF
« Last Edit: November 14, 2016, 08:36:27 AM by K7FF » Logged
K0BG
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« Reply #6 on: November 14, 2016, 08:46:32 AM »

David, you've mentioned the "radiating counterpoise" before, and got shot down if you'll recall. You're forgetting that the body is capacitively coupled to the surface under it. It is similar to radials, in that RF does flow in them, but doesn't radiate in the same sense as the vertical element.
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K7FF
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« Reply #7 on: November 14, 2016, 09:41:50 AM »

WX7G:

Many years ago there was a QST article that detailed a series of tests to measure the RF level along the radials of, at closely spaced intervals, of an HF (160M) monopole (I think), done utilizing RF wattmeters.

It suggests to me that if there is RF present along the radials (with standing waves?), then it needs to be, according to the laws of physics, radiating.  Either heat or RF.

Derek
K7FF

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AB3TH
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« Reply #8 on: November 14, 2016, 11:06:20 AM »


I assume that when you say 'steel is bad because of skin effect'. you are referencing stainless steel.

It seems odd, if that is the case, that stainless steel whips are universally used on mobile antennas, whether as whips or as resonator stingers.  So, relatively speaking, just how bad is it...?

As for vehicle copper body parts, plating with copper, or ?, should be sufficient.

Derek
K7FF

Skin depth depends on frequency and the square root of permeability.  RF resistance depends on the conductivity, diameter and skin depth.  A magnetic material like plain steel has about 1/30 the skin depth and lower conductivity.  At RF steel has around 100 times the resistance of the same diameter copper wire.  That's why you use copper clad steel for antenna wire and not plain steel.  Galvanized steel is better but not as good as copper.  And, it has to be hot-dipped not electrogalvanized to get a thick enough layer to be somewhat useful. 

There are two basic kinds of stainless - magnetic and non-magnetic.  Magnetic stainless steels can't be used for antennas.  Non-magnetic stainless steels have a permeability slightly greater than one and an RF resistance of maybe 20 times copper.  Mobile antennas use non-magnetic steel for structural and corrosion reasons.  It isn't great stuff. 

The steel body of the car is magnetic.  By itself, it would be a very poor ground plane.  Luckily, most of it is galvanized.  But, that layer is not very thick.  So, RF resistance gets progressively worse below maybe 10 MHz.  Yet another reason why mobile antennas are more complicated than they look.
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W1VT
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« Reply #9 on: November 14, 2016, 12:26:18 PM »

I remember someone in California showing me his Saturn station wagon that had a a small transmitting loop suspended inside it!  Instead of sheet metal, Saturns had polycarbonate body panels.

http://www.orionmicro.com/ant/ddrr/ddrr1.htm
The second type is a double ring system isolated from the ground with the same two vertical posts connecting the opened extremities of the rings. One is solid aluminium, the other one contains a Vacuum capacitor. A third aluminium post cut in the middle is used to feed the antenna and can be moved to adjust the SWR. The coax enter in the middle of the right post, then up then down to the "feed point" where the shield is connected to the top part and the center of the coax to the bottom part of the post.
A ham actually built this for 75 meters!


Zack W1VT
« Last Edit: November 14, 2016, 12:28:56 PM by W1VT » Logged
K7FF
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Posts: 156




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« Reply #10 on: November 14, 2016, 01:29:58 PM »

I remember someone in California showing me his Saturn station wagon that had a a small transmitting loop suspended inside it!  Instead of sheet metal, Saturns had polycarbonate body panels.

http://www.orionmicro.com/ant/ddrr/ddrr1.htm
The second type is a double ring system isolated from the ground with the same two vertical posts connecting the opened extremities of the rings. One is solid aluminium, the other one contains a Vacuum capacitor. A third aluminium post cut in the middle is used to feed the antenna and can be moved to adjust the SWR. The coax enter in the middle of the right post, then up then down to the "feed point" where the shield is connected to the top part and the center of the coax to the bottom part of the post.
A ham actually built this for 75 meters!


Zack W1VT

Zack:

VERY interesting!

On what kind of vehicle was it mounted and where on the vehicle.  And, how well did it work out?

I am having difficulty visualizing the arrangement.  Perhaps, you could take a photo of your memory and send it to me!

In any case, this may give lie to some of the previous 'expert' information promulgated herein...

Derek
K7FF
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W1VT
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« Reply #11 on: November 14, 2016, 01:34:39 PM »

The web site I referenced has a diagram of the antenna.
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K7FF
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« Reply #12 on: November 14, 2016, 02:34:02 PM »

The web site I referenced has a diagram of the antenna.

I read the article on the DDRR antenna, and remember the articles well from way back then.

Because of its size, perhaps a railroad flatcar might be better to haul it around!

Thanks,

Derek
K7FF
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W6ARY
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« Reply #13 on: November 14, 2016, 04:30:45 PM »

I am interested in the concept of effective HF mobile antennas that are not 'grounded' or otherwise electrically connected to the metallic vehicle body/frame etc.  As such, it would alleviate the need for extensive electrical bonding of vehicle body parts, frames, bumpers, etc.

Derek,

As electrons flow into an antenna, they have to come from somewhere.  Likewise, as they flow out, they need somewhere to go.

In the case of a dipole, the electrons come from the other half of the dipole.  The two halves trade electrons back and forth.  When one side has a positive charge, the other has a negative charge and vice-versa.

In the case of a 1/4 or 5/8 monopole with radials, the radials trade electrons back and forth with the monopole.

In the case of a small magnetic loop, the capacitor donates electrons from one plate while accepting electrons onto its other plate.

In the case of a 1/4 wave antenna grounded to a vehicle, the vehicle body and frame supply the electrons.

You can also use an array of two identical 1/4 wave antennas, fed 180 degrees out of phase with each other.  In this case, one antenna will supply the electrons when the other needs them and vice-versa.  (Together, they act as a 1/2 wave dipole.)

By considering ways to source and sink electrons, you can come up with many more options than a traditional "ground".

---Gary, W6ARY
« Last Edit: November 14, 2016, 04:53:55 PM by W6ARY » Logged
K0BG
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« Reply #14 on: November 14, 2016, 05:32:52 PM »

And just to add to Gary's well-worded reply, attempting to use a dipole mobile just doesn't work very well, because you can't mount one high enough, except possibly for 10 meters.
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