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Author Topic: Hamstick, Hi-Q or Tarheel 40 or 75  (Read 17743 times)
WX7G
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« Reply #30 on: December 06, 2012, 04:28:39 PM »

W5WSS, what difference (in dB) did you measurement between the two antennas?
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W5WSS
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« Reply #31 on: December 07, 2012, 04:01:33 AM »

Hello Dave, The test site is a 20 square acre grassy Pasteur that happens to be a hill top in Oklahoma. I held the range constant as I measured the Field strength using an industry brand Field strength meter calibrator and  reliable.

A meter that I used repetitively to stage antenna sites for clients.

The measurements were plotted to form a 360 degree circle limited to 6ft above flat topography every three degrees!

The methodology was carefully followed and readings were recorded to reflect a series of 360 cross sections forming a circle down range and attenuation was observed as normal. The technique was repeated in succession three times.

The methodology was held constant as the two antenna systems were compared.

The first system was an Aluminum tube tapered using two smaller sections nested forming a 20m quarter wave radon and mounted on an insulator made of Teflon centrally located above the carbody and driven by the center conductor.

The counterpoise was bonded via 6 1/4-20 bolts starwashers and nuts, through an Aluminum mounting plate and the carbody rooftop low resistance bonding (Stripped of paint leaving bare metal outer and inner) throughout was assured and driven by the coaxial shielding.This methodology was intended to exceed normal installation practices.

The mobile quarter wave system was tuned matched and considered optimal and Field strength measurements as previously described were painstakingly conducted.

Asymmetry nor symmetry was assumed.

The results were consistent.

The second system can be seen here at Eham see my picture at my callsign.

The feedline was transfer ed to normalise the test controls.
The base height was selected andheld atop a non conductive pole. the feed point isolated electrically speaking.
The equipment was isolated and powered with a yellow top battery NOT connected to the vehicle.
Radials 4 #14 copper wire and insulated were selected and isolated from ground using non conductive anchors.
Slope angle was set at 45 degrees.
radial placement was set at 0,90,180,360. at feedpoint location and transferred down to anchors where transfer ed means again 0 90 180 and 360.

The quantitative findings are debateable and can be argued.I will not post them here.

The upward bump in strengths measured were seen downrange and attributable to the radials slope angle as a contributor vs the carbody.

Symmetry vs asymmetry and mutual coupling relative to Earth conductivity was held as a constant.

The second system reaches to exceed my expectations with a simple exchange between sloped elevated isolated symmetrical radials and the relative carbody used for the common structure.

The carbody can be expressed as a raised hump of semi conductive material underneath the antenna system number 2..but not for number 1 antenna system.

The difference can be rechecked and was using antenna modeling.

73









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W5WSS
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« Reply #32 on: December 07, 2012, 04:09:31 AM »

Fwiw my extrapolated dbd findings were 2-3dbd favoring the radial exchange everything else held constant.
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WX7G
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« Reply #33 on: December 07, 2012, 04:53:55 AM »

Wow, that is an excellent experiment with lots of work involved. At what distance did you take FS measurements?

Using EZNEC and the model of my vehicle with your antenna setup the difference in E-field at 200' in the Y direction and 6' above ground is 2.1 dB.

Next I removed the vehicle and replaced it with a vertical wire having the same length as the vehicle height. The difference in E-field at the same measurement point is 1.5 dB.



« Last Edit: December 07, 2012, 05:53:00 AM by WX7G » Logged
W5WSS
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« Reply #34 on: December 07, 2012, 07:30:16 AM »

I neglected to mention that in order to add the radials @45 degrees the base height was raised so there is a difference (unavoidable) with height so Try looking with the base height at 13ft. as opposed to the mobile system being 5ft
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W5WSS
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« Reply #35 on: December 07, 2012, 07:33:05 AM »

Your are modeling an inverted quadrant center fed L different than my antenna experiments.
But sounds interesting.
I built the antenna that you are modeling and found that one can rotate the horizontal leg and effect changes in FS at point b
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W5WSS
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« Reply #36 on: December 07, 2012, 09:11:24 AM »

I had to review the notes for the distances from the two 20m antennas being analysed. following from closest to furthest 33ft 1/2 wave 66ft 1 wave and 700ft10 wavelengths respectively. done in three complete radius's measurements taken every 10 degrees or so.
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WX7G
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« Reply #37 on: December 07, 2012, 10:22:04 AM »

For the mobile system I placed a 17' wire on the top edge of the 5.75' tall vehicle.

For the modified system I placed a 17' wire with the base at 13.75' and four 17' radials sloping downward at 45 deg.

For the wire in place of the vehicle the vehicle was removed and a 5' vertical wire placed in its spot.
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W5WSS
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« Reply #38 on: December 07, 2012, 10:33:22 AM »

OK fb sounds good now look out at 10 lambda@6ft height and look to see a bump up in signal strength that is indicating a difference that seems to preclude surface wave attenuationinvolved with the closer readings measured when closer to the antenna.

Another words a reinforcement suggesting :sloped radials that are contributing to real radiation increase strength.out in the far field vertically.

I did not sound the area for more and less strength as I was doing allot of walking.
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WX7G
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« Reply #39 on: December 07, 2012, 11:51:01 AM »

OK I will simulate at 10 wavelengths.

The 45 deg sloping radials have 0.7X the current-area that the vertical radiator does and will contribute 0.7/(1.0+0.7)(100%) = 41% of the far-field E-field strength.

A 6' tall vehicle has as much as 0.25X the current-area that the vertical radiator does and will contribute as much as 0.25/(1.0+0.25)(100%) = 20% of the far-field E-field strength. The reason I say "as much as" is because some of the displacement current from the radiator to the vehicle terminates on the horizontal top of the vehicle thereby reducing the current along the vertical length of the vehicle. Because of this the radiation from a vehicle will be less than a vertical wire of equal vertical length. This can be quantified using a NEC simulation.

The current along the 17' radiator is close to a sinusoid and the current area is 0.7 x 17' x 1 amp = 12 A-ft. The current along the 6' vehicle is triangular and the current area is 0.5 x 6' x 1 amp = 3 amp-ft.



« Last Edit: December 07, 2012, 11:59:46 AM by WX7G » Logged
W5WSS
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« Reply #40 on: December 07, 2012, 01:44:49 PM »

OK Dave, yeah and this common mode as I see it from my perspective as I evaluate your modeling compares quite similarly to what I saw.

The average mobile system suffers from asymmetry to a more or lessor degree as a function of off central positioning of the radon etc. as a consequence common mode develops to the degree that is relative to the size of the vehicle vs frequency applied.and position of the antenna relative to the vehicle etc.

What you have demonstrated in your modeling exercise confirms this revelation to those whom assume any positioning on an HF mobile works properly.

Not the case and further investigation suggests that yes asymmetry increases loss and skews (undesirable) the desired omni directional pattern and field strength that would but does not fully develop.

The problem is accentuated as we move lower in frequency such as 40m and down where the vehicle becomes by %x smaller and smaller.
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WX7G
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« Reply #41 on: December 07, 2012, 04:54:22 PM »

I will revisit this in my modeling. So far, modeling has convinced me that asymmetrical placement of a mobile antenna results in greater current-area and higher radiation resistance compared to symmetrical placement of the antenna.
« Last Edit: December 07, 2012, 05:00:24 PM by WX7G » Logged
WX7G
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Posts: 5908




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« Reply #42 on: December 08, 2012, 06:10:49 AM »

Robert,
I played with my antenna model again and moved the antenna from the edge to the center of the roof. The antenna is a 7.5' base loaded whip at 7.0 MHz. For this simulation the loading coil loss is set to zero.

The feedpoint impedance is 3.74 ohms for both cases. The Average Gain over medium ground is -8.24 dB for the center of the roof and -8.52 dB for the edge of the roof. The Average Gain test accounts for all losses including those in the far-far field. Because of this it cannot be used to assign a radiation efficiency number but it can be used to compare the relative radiation efficiency of two antennas. The result is that at the center of the roof the antenna has 0.28 dB more total radiated power.

The azimuth pattern is skewed by the edge of the roof mounting. The front-to-back ratio, if I may use that term, is 2 dB.

Dave

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W5WSS
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« Reply #43 on: December 08, 2012, 01:12:13 PM »

Hello Dave, Your contribution to our quest to better understand RF radiation is appreciated.

I had a end fed 1/2 wave vertical positioned on a Dakota truck located inside the truck bed (right rear corner) the antenna uses a matching base coil tapped and is an excellent candidate for which placement about the vehicle can be done.

I experimented with 4 fat Aluminium tubes serving as radials but did NOT drive them with the shielding. I simply set them in positionaround the semi circle of the antenna symmetrically and observed mutual coupling,while these tubes originated directly at the base feed point they were not tightly connected via a dc connection but rather loosely coupled around the antenna mounting bracket.The tubes were 1-1/4" diameter and 4ft in length.
An upward slope angle of 45 degrees yes upward produced increased field strength.
The tubes held their position and were shorter and quite feasible for your 40m endeavor.





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W5WSS
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« Reply #44 on: December 08, 2012, 02:46:08 PM »

Again neglected to mention that the base height was 12ft above Earth surface and the antenna was upon a non conductive pole.
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