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Author Topic: lossy SS whips  (Read 3955 times)
WX7G
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« Reply #15 on: July 25, 2010, 10:37:46 AM »

Alan, I encourage you to repeat my tests and to add field strength measurements.
« Last Edit: July 25, 2010, 12:13:05 PM by DAVE CUTHBERT » Logged
K0BG
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« Reply #16 on: July 25, 2010, 11:16:58 AM »

Hummm. Sixty four, twenty foot radials, with a ground loss of 4 ohms at 25 MHz? That doesn't make sense, unless you're doing this with NEC or EZNEC, as they don't do ground loss calculations to any degree of accuracy.

I've done this experiment about a dozen times over the years, and I get the same measurements, no matter the material used. While I might believe there is a slight difference, it is within the accuracy fuzz of any common measurement methodology available to amateurs. You certainly can't do it with an MFJ-259B, and even using a miniVNA Pro, the difference is so slight, as to be nonexistent.



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WX7G
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« Reply #17 on: July 25, 2010, 11:38:53 AM »

Alan, think about it. The antenna has 64 radials 0.6 wavelengths long. This is a very good counterpoise approaching what AM BC stations use to obtain a 1 ohm ground. The 4 ohm ground loss is found by measurement. The measured 40 ohm resistance input for a 90 degree vertical having a radiation resistance of 36 ohms means the ground loss resistance is 4 ohms.

Now whether or not the ground loss is accurate the 10 ohm steel whip loss measurement is valid. The only change in the measured system between the two treatments is the antenna material. 10 ohms measured difference agrees with 10 ohms calculated agrees with 11 ohms simulated. This is the sign of a valid experiment.

Now it's your turn to repeat these measurements and report your findings here.
« Last Edit: July 25, 2010, 12:15:07 PM by DAVE CUTHBERT » Logged
K0BG
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« Reply #18 on: July 25, 2010, 02:26:29 PM »

Okay, tell me how you measured the ground loss? Since there isn't an accepted methodology, the science is not exact.

Yes, you can calculate it, but before you give me an answer, perhaps you should present this problem to Roy Lewallen at w7el@eznec.com.
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WX7G
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« Reply #19 on: July 25, 2010, 03:52:21 PM »

Measured base impedance = radiation resistance + ground loss. Some algebra and we have ground loss = base impedance - radiation resistance. Base impedance is 40 ohms; radiation resistance is 36 ohms.

40-36 ohms = 4 ohms

The N6LF paper Experimental Determination of Ground System Performance for HF Verticals Part 3, Table 3 shows a measured base impedance of 39.7 ohms for 64 radials, 1/4 wavelength long. Also see Experimental Determination of Ground System Performance for HF Verticals Part 3 Figure 1 and Figure 3. In Part 6, Table 5, column C4 the radial system is composed of 32, 33' radials. This system is the most similar to my system of 64, 20' radials. At 28 MHz for a 1/4 wavelength vertical N6LF measured a base impedance of 43.8 ohms. Part 5, Figure 4 shows the change in base impedance going from 32 to 64 radials as being 2 ohms.

Using this last reference we can bracket the expected ground loss. It cannot be less than 0 ohms. And based on the N6LF paper Part 6, Table 5, column 5, along with Part 5, Figure 4 we would not expect it to exceed 42 ohms. From this we can say that the predicted ground loss must be between 0 and 6 ohms. I measured 4 ohms.

Based on the information cited my measured base impedance of 40 ohms at 28 MHz for a 1/4 wavelength vertical over 64, 20' radials is believeable.  

N6LF points out that the standard counterpoise for a BC station is 120 radials, 0.4 wavelengths long. My counterpoise at 28 MHz consists of 64 radials, 0.6 wavelengths long putting my counterpoise in the arena of very good counterpoises.  

I have used the 'measured minus the radiation resistance method' for years to obtain ground loss resistance. If I remember correctly, one or more of my AntenneX articles use this method and were reviewed by L.B. Cebik. The N6LF paper Experimental Determination of Ground System Performance for HF Verticals Part 3 Figure 1 uses this method. The J. Sevick paper An Experiment Investigation of The Short Ground-Mounted Vertical Antenna in the H.F. Range also uses this method to determine ground loss. See also the K0BG paper A Simple Ground Loss Explanation part of which is quoted here:

EZNEC will give you the calculated input impedance, and an antenna analyzer will give you the actual input impedance. Simple subtraction will give you the approximate, overall loss. For example, if the input impedance of your antenna measures 45 ohms, and the EZNEC calculated input was 25 ohms, the difference (20 ohms) is additional loss most-likely caused by a poor Image Plane, and to a lessor degree, coupling losses.

In a recent QST technical note, Dr. Jack Belrose, VE2CV, suggested this very method to calculate the efficiency of a mobile antenna system. In other words, use EZNEC to calculate the input impedance. Compare that with the measured input impedance taken from an antenna analyzer, and then increase the EZNEC ground loss figure until both measurements agree. Ah Ha! You have your efficiency, or at least a very close approximation.


« Last Edit: July 26, 2010, 01:30:55 PM by DAVE CUTHBERT » Logged
WX7G
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« Reply #20 on: July 25, 2010, 05:21:13 PM »

Alan, we have wandered from the point of this thread and that is the loss of a 17-7 stainless steel whip.

For such a gross difference of 10 ohms even an SWR meter can be used with a loss of accuracy. It will measure 1.0:1 for the 17-7 stainless and 1.2:1 for the aluminum.

The loss is measureable and predictable. Now that this loss can be quantified - we know the loss, and know how to use it in antenna calculations and simulations - the mobile antenna designer has one more parameter under his control.

I wonder if your zero loss measurements are the result of antennas in which it makes very little difference? For example, my Tarheel 200A HP screwdriver with a 9' whip measures only a 1 ohm base resistance difference going from aluminum to 17-7 stainless steel. The 7 MHz measurements indicate that either the stainless steel permeability is lower at 7 MHz or that I have a measurement error or both. I'm giving this some thought.

It is the "pure" installation on 28 MHz you suggested where the loss really stands out. I'll repeat this measurement with two whips that are resonant around 40 MHz. That is a 6' aluminum and a 6' stainless steel whip. For instrumentation I have SWR meters, an MFJ-259 and an AIM-4170. I can do transfer calibrations (which I haven't yet) to resistors then to a calibrated ohm meter to obtain NIST traceable measurements.
« Last Edit: July 27, 2010, 05:40:46 AM by DAVE CUTHBERT » Logged
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