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   Home   Help Search  
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Author Topic: UNGROUNDED HF MOBILE ANTENNAS  (Read 4170 times)
K7FF
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« Reply #15 on: November 14, 2016, 06:19:20 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

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

But, what about a 1/4 wave antenna that is NOT grounded to the vehicle?

Are there any other types of the 'other side of the antenna'?
« Last Edit: November 14, 2016, 06:22:09 PM by K7FF » Logged
K7FF
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« Reply #16 on: November 15, 2016, 12:49:53 AM »

I have resurrected an HF counterpoise design for an HF portable antenna system that I created about fifteen years back, which I have decided to use with my 'K7FF Innovative HF Mobile/Portable Antenna System'.  

It has a VERY small footprint, is tuneable, and worked blindingly well on ten and twenty meters set up in my back yard in Fair Oaks.  It does not use radials, is isolated from earth ground and vehicle body, can be elevated, or mounted in or above a pickup truck bed.  The counterpoise can be elevated in relation to the bottom of the monopole.

I have all of the stuff I need to build several of the systems using this counterpoise arrangement.

Derek
K7FF
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W6ARY
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« Reply #17 on: November 15, 2016, 02:04:12 AM »

As electrons flow into an antenna, they have to come from somewhere.  Likewise, as they flow out, they need somewhere to go.
But, what about a 1/4 wave antenna that is NOT grounded to the vehicle?

Are there any other types of the 'other side of the antenna'?

Yes, there are a limitless variety of "electron reservoirs" you might use for a 1/4 wave antenna.

I already mentioned a second 1/4 wave antenna driven at a 180 degree phase relative to the first antenna.

You could also use ~1/4 wave length of feed line.  However, that would lead to the counter-charge oscillating on the outside surface of the coax shield, which could lead to problems if the feed line runs through the interior of the vehicle.

Any conductive body that could readily store a large amount of charge could be used as a counterpoise.

---Gary, W6ARY
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KH6AQ
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« Reply #18 on: November 15, 2016, 07:23:24 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.

Alan, yes there is some RF current in the ground around the mobile installation but how much? 

I'll put together a NEC simulation so we can get an idea of the magnitude of it.
« Last Edit: November 15, 2016, 07:41:39 AM by WX7G » Logged
KH6AQ
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« Reply #19 on: November 15, 2016, 07:50:52 AM »

Alan, yes there is some RF current in the ground around the mobile installation but how much?  I always assumed there was some and that it, along with the current intercepted by the vehicle body, travel vertically (along the axis of the monopole whip) and add to the system radiation.

I have a crude NEC simulation running at 3.9 MHz and it shows about 80% of the antenna current returning through earth and coupling to the vehicle body. So, in this simulation 20% of the current is intercepted by the vehicle body. This vehicle current travels vertically up the sides of the vehicle and leads to some radiation. The model and RLC model need some work but when they agree I think we'll have a better picture of what's going on. From there we can see if there's a way to reduce the RF current through the lossy earth.
« Last Edit: November 15, 2016, 08:02:47 AM by WX7G » Logged
K0BG
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« Reply #20 on: November 15, 2016, 10:25:56 AM »

Like radials, the body does radiate a bit of RF (that which isn't absorbed as heat). However, the radiation is at a very low angle, and ends up being absorbed by the surface in question. Thus is doesn't add any appreciable improvement in far-field signal strength.

By the way, this issue was well covered in QEX several years ago by Rudy Sevens, N6LF.
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K1DA
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« Reply #21 on: November 15, 2016, 01:12:11 PM »

THere's always a market for the 100 MPG carburetor. 
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KB2WIG
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« Reply #22 on: November 15, 2016, 02:55:20 PM »


Well, you don't need a 100 MPG carb if you use regular tap water, and the pill that the Illuminati and Big Oil have hidden.
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SOFAR
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« Reply #23 on: November 15, 2016, 03:00:13 PM »


Well, you don't need a 100 MPG carb if you use regular tap water, and the pill that the Illuminati and Big Oil have hidden.

I want to get my hands on the razor that never dulls. Hear that Schick has it locked away in a safe.
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K7FF
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« Reply #24 on: November 15, 2016, 05:04:58 PM »

Now that the trolling immature bozos have pirated this thread, its time to shut it down.

BUT, WHY DO THE RESPONSIBLE USERS ALLOW IT TO CONTINUE AND RUIN IT FOR THE REST OF US?
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KH6AQ
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« Reply #25 on: November 16, 2016, 04:52:42 AM »

The NEC model has been informative. I'd post some of it here if I could figure out how to.

Findings for the 3.9 MHz, 8' monopole model:

The monopole on a vehicle works with or without earth ground present. I think a good term is ground agnostic.

Without earth ground (the vehicle in free space) the displacement current from the monopole returns directly to the vehicle.

With earth ground present 20% of the displacement current from the monopole returns directly to the vehicle. 80% goes to earth ground and then to the vehicle body.

Things to investigate:

How much does the vehicle body add to the radiated field?  As pointed out by Alan there might be more attenuation of the radiation close to the ground.

Can the displacement current to earth ground be reduced?

I took a preliminary look at the last question and the answer is yes. By using a resonant radial and a feedline choke the earth current was reduced by half. But that trades lossy ground (10 ohms perhaps?) for a lossy radial loading coil (10 ohms perhaps?)and the radiation efficiency might be unchanged. I'll look into this some more.

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K0BG
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« Reply #26 on: November 16, 2016, 04:55:19 AM »

Rather than reinvent the wheel, David, look in the first two ARRL Antenna Compendiums.

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KH6AQ
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« Reply #27 on: November 16, 2016, 09:54:49 AM »

Alan, I'll check those references out.

I found this today by Roy Lewallan, an ARRL technical advisor.

"In a typical HF setup, the car is capacitively coupled to the ground, so the antenna is something sort of like a cross between a lopsided vertical dipole (with the whip being one side and the car the other) and a vertical with elevated radial system."

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M0GVZ
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« Reply #28 on: November 17, 2016, 11:28:21 AM »


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

But, what about a 1/4 wave antenna that is NOT grounded to the vehicle?


Your coax connecting the antenna to the radio, the radio body, the mic cable, the radio power cables and the vehicle wiring supply the electrons and also give them somewhere to go, aka common mode RFI.
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