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Author Topic: Does the body of a mobile installation radiate RF power?  (Read 12577 times)

Posts: 3289

« Reply #15 on: December 18, 2012, 10:56:59 AM »

This it what I like about  these boards.
Everyone is an expert and everyone else does not know anything.
The rest hide behind what someone wrote in a book and use it a gospel for every situation.
I never said anything about a bottom feed in this case.
You are even condemning the K5OE design after hopefully seeing the polar response plot of his design and the use he put it to.
I don't know if I will get the same thing but it the attempt that counts.
I guess this is to be expected here  that ten people will interpet ten different ways of which at least nine will, be a condemnation.
I doesn't change anything.
Happy Holidays.

Not speaking of you in particular, but when someone doesn't know what they are doing they can rationalize any explanation.  The K5OE design is theoretical crap.

Posts: 3289

« Reply #16 on: December 18, 2012, 11:14:20 AM »

Here is a quick visual reference showing the vertical patterns of vertical antennas.

Lowest angle is from a 5/8L
Highest from a 3/4L
Moderately high 1/4L

Very simple, no need to re-invent the wheel.  For best performance, drill a hole in your car roof and mount them with an NMO.

Posts: 3289

« Reply #17 on: December 18, 2012, 11:34:39 AM »

(KM3F) I think your missing some points here.
The antenna has a broad vertical pattern from low angle of 10 to as high as 60 degrees.

The 3dB points of a 1/4L vertical are 9degrees-55 degrees.

-Congratulations.  By adding complexity, you have not achieved any better pattern than a simple one wire quarter wave vertical.

Posts: 2276

« Reply #18 on: December 18, 2012, 12:54:01 PM »

Does a carbody radiate?

Lets delve into the family of self contained (vertical) dipoles.

The vertical portion of the 1/2 wave vertical dipole contributes to it's maximum potential when the other half is the exact size and shape.The second half is closer to the ground than the top half by default.

The second half can be split into many tuned elevated parts and placed from horizontally to sloped.

A mobile carbody does not and cannot equal a properly designed physically straightened downward going counterpart half.

A better understanding is found when one views the circuit from the center feed point.

In a mobile application Center feed remains the same but the halves are dramatically different.

A Ground Plane antenna is a name for when a half that is vertical and the other half is divided into 4 or more parts and symmetrically distributed about the semi circle.strictly speaking radials held in the horizontal plane.
The 4 parts (of the whole second halve)are named radials and are oriented 90 degrees or in the horizontal plane hence ground plane) but their contribution to the verticals radiation is in securing a push platform but the field strength is concentrated in the vertical axis that by a quarter wave radiator but horizontal radiation is minimal as designed properly.

The perspective of all of the family of self contained verticals is from the feed point.

The ground plane mobile antenna system exchanges tuned elevated horizontal radials for a carbody.
A vertical conductor pushing against a carbody is fed at the bottom end usually a junction;, and the horizontal? carbody is a platform or plane that also completes the antenna.
It works but presents as a weird thicker shape hump compared to horizontal radials that are elevated and tuned, that hump reduces in size compared to lower and lower frequency applied to the circuit while the plane remains unchanged the vertical must be lengthened expand..coupling through to the earth surface where the carbody is usually sitting on rubber tires an insulated semi conductor. with a capacitance to earth below.
The carbody cannot just grow wires and radials are within our service however.

  Even when radials are horizontal such as a set of 4 tuned horizontal  and directly next to the feed point isolated radials fashioned symmetrically they cancel horizontal radiation therefore no field strength to measure in that axis but a remnant.

A better solution is in using a sloped second halve split into equal parts elevated ans isolated from ground.

So the sloped tuned elevated radials do NOT cancel all their radiation and is measurable out in the far field. In this caseThere IS contribution yet the sloping angle does not preclude the antenna from the family of self contained verticals but is a variant that can be utilised when installed properly above a mobile.

The carbody is a solid plane that works.

Tuned, elevated, sloped, isolated radials at an angle of apx 45 degrees is not the same but is a dipole.

PS the angle may be tilted upward and lends itself to some utility for hilltop mobileering where the isolated radials do not touch the ground.

73 Smiley


Posts: 817

« Reply #19 on: December 19, 2012, 10:05:57 PM »

w5wss, good explanation.
I just got done building the OCF design.
At first test the SWR was within 2.5 to 1 before any tuning was done because the elements were made with extra length resonating in the lower part of the 2m band..
After tuning and being placed on an arm off a low tower outside the shack, the SWR is less than 1.5 across the FM portion and down into the upper SSB area.
Testing suggest the SWR can be made flatter than this with a different feedline from what I have connected for now.
Test getting into distant repeaters is quite encouraging with one about 100 miles to the west.
To repeat the design; the vertical is 23  3/4 inches long, the radials are 8 inches long.
Coax feed is a PL259 at the junction of the two and below them with no matching of any kind used.
The antenna appears not to be less sensitive to it's surroundings for detuning.
It one wants to call it BS  or by any other description be my guest and knock yourself out.
It works as well as any other, so far and looks to be better until proven otherwise.
Later it will mount on a truck in the box away from the cab for trial testing over well known routes both going away from and to the repeaters with known signal performance using standard 1/4w, 5/8w and dual band antennas.
Good luck.

Posts: 2276

« Reply #20 on: December 20, 2012, 05:31:15 AM »

Right on! yeah I would suggest for the mobile support structure to be high enough Raised" enough so that the sloped radials clear the carbody roof top by about 6 inches. and that the pole is non conductive.

 Slope the 4 or more elevated radials and isolate them from ground.

The design produces what you are striving for and is an excellent choice for your endeavors. better than a mobile antenna centered on a large plane and electrically driving the plane with the feedline shield warning ONLY when you slope the 4 elevated,ground isolated radials at an angle of 45 degrees or so, downward and look to tune the antenna to a very close match using the slope angle in conjunction with radial length and diameter 

The construction details can be worked out.

Have fun!.....what 10 dollars to make?

Posts: 2276

« Reply #21 on: December 20, 2012, 05:50:06 AM »

Oh and the design I just described though is center fed and symmetrical not ocf.

Posts: 723

« Reply #22 on: December 20, 2012, 09:33:59 PM »

Easy answer, if the car body is metal, and current is flowing through it, then it is a part of the antenna system which is radiating.
The tough part tho, is the interaction of the car body aiding or detracting from the pattern?

N8CMQ   Jeff Retired...
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