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Author Topic: lossy SS whips  (Read 4473 times)
WX7G
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« on: July 14, 2010, 01:10:03 PM »

I took measurements on a ground mounted screwdriver antenna to see how much loss the SS (Stainless Steel) whip contributes. This is something to think about the next time you're competing in a mobile antenna shoot-out.

Test setup:
Tarheel 200A-HP screwdriver (5' length) ground mounted
DX Engineering 102" SS whip cut to 72" or Hustler aluminun mast extension "whip" (72" x 1/2" aluminum)
64 radials 20' in length
3.5 MHz

Input impedance with SS whip: 21 ohms
Input impedance with Alum "whip": 18.5 ohms

Conclusions: The SS whip contributes 2.5 ohms of loss referenced to the antenna base. For this particular installation the SS whip dissipates 12% of the RF energy. An aluminum clad SS whip would be a nice item to have.
« Last Edit: July 14, 2010, 01:12:29 PM by DAVE CUTHBERT » Logged
K3GM
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« Reply #1 on: July 14, 2010, 02:53:00 PM »

It might be interesting to measure a Larsen Kulrod whip.  They are/use to be overplated with copper, then I think silver.  Aluminum cladding would last about a week in the corrosive highway spray up here in the winter.....so did the Kulrod.  ......Gold!
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K0BG
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« Reply #2 on: July 14, 2010, 04:07:39 PM »

Interesting outcome, but one I find hard to believe. The skin effect losses of stainless steel, and aluminum are virtually identical. In fact, the difference between regular old steel, and pure silver are so slight, even laboratory-grade instruments are hard pressed to measure the difference.

The other issue is the input impedance. Since the radiation resistance, the conductor resistance, ground losses, stray losses, and a few other variables all effect the input impedance, one cannot assume a change in input impedance is positive, or negative. Only a normalized, field strength measurement is valid for such comparisons.

As for the Kulrod...

Jim Larsen, K7GE (sk), was a friend of mine, and I once ask him a question centered around this very subject. His reply was, it is more of an advertising advantage, than a real one. And I agree.

Perhaps you should present the findings to Tom Rauch, W8JI. But be ready for his terse reply.
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KE3WD
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« Reply #3 on: July 14, 2010, 08:20:49 PM »

I also don't think it would be due to the difference in metals. 

However, your AL antenna certainly touts a much larger diameter than the SS whip and that translates to more AREA...
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K5LXP
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« Reply #4 on: July 15, 2010, 05:39:18 AM »

To augment KE3WD's comment, I think another useful comparison would be a piece of copper wire of similar diameter and length to the SS whip.

Mark K5LXP
Albuquerque, NM

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AA4PB
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« Reply #5 on: July 15, 2010, 05:51:52 AM »

I'm highly suspicious that the difference between 72" of aluminum vs. stainless amounts to 2.5 Ohms of additional loss. First off, you haven't accounted for the fact that the aluminum mast is of a much larger diameter than the stainless whip. That alone could account for a major difference in loss, even if it were made of the same material. I also expect that the dia difference caused you to retune the screwdriver which in turn changed the amount of loss in the loading coil.

The only way to do that test would be to have two idential whips (dia and length), one of stainless and one of aluminum. Then don't make any changes to the screwdriver coil setting after you change whips.
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WX7G
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« Reply #6 on: July 15, 2010, 04:32:03 PM »

The measured comparison is not SS vs aluminum per se but the thin SS whip vs the 1/2" alumimun tube.

Let's run the numbers. The SS whip is 17-7 stainless steel and has a relative permeability of 125. The DC resistivity is 8.3E-5 ohm-cm. One skin depth at 3.5 MHz is 0.8 mils. The RF resistivity at 3.5 MHz is 44 milliohms per square.

The aluminum DC resistivity is 2.7E-6 ohm-cm. One skin depth at 3.5 MHz is 1.7 mils. The RF resistivity at 3.5 MHz is 0.6 milliohms per square.

The SS has 60 times the RF resistivity compared to aluminum at 3.5 MHz.

The calculated RF resistance of the 0.5" aluminum tube is 0.03 ohms while the 0.125" SS whip is 7 ohms. The calculated difference in antenna base resistance is about 4 ohms. EZNEC says 4 ohms. I measure 2.5 ohms.

What accounts for the difference between calculated and measured loss? The two dominant error terms look to be measurement error and the actual SS whip permeability possibly being less than that published figure of 125.  

So what would be the simulated difference if both whips were 0.125" diameter? Still about 4 ohms.
« Last Edit: July 16, 2010, 06:05:37 AM by DAVE CUTHBERT » Logged
K0BG
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« Reply #7 on: July 15, 2010, 04:49:39 PM »

Even then, Dave, the difference is so slight in field strength measurements, it is all but nil. You might want to see what Tom, W8JI, says about this on his web site.
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WX7G
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« Reply #8 on: July 15, 2010, 07:21:55 PM »

My previous posts were not meant to be qualitative, but meant to be quantitative. We now know the RF resistance of the 17-7 SS whip stocked by DX engineering.

2 ohms for the 6' 17-7 stainless steel whip does not seem like much but it is equivalent to reducing the center loading coil Q from 300 to 200 for my center loaded 12' vertical.
« Last Edit: July 15, 2010, 07:41:17 PM by DAVE CUTHBERT » Logged
K0BG
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« Reply #9 on: July 16, 2010, 06:04:20 AM »

Dave, many field measurement test has been done, between SS whips, and even silver plated copper pipe. There is virtually no difference in signal strength.

And again, you cannot use the input impedance as a measure of a positive, or negative change. There are just too many other variables you can't (or didn't) measure.
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WX7G
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« Reply #10 on: July 16, 2010, 06:23:59 AM »

It's Simple
Let's look at this another way. With the aluminum mast the measured base impedance is 18.5 ohms. With the SS whip the measured base impedance is 21 ohms. The calculated radiation resistance is 1.4 ohms. Given 100 watts the antenna current at the base is 2.36 amps for aluminum and 2.18 amps for SS. The radiated power, being I squared R, where R is 1.4 ohms, is 7.8 watts for aluminum and 6.7 watts for SS. The difference is 0.7 dB. To go one step deeper the whip antenna distribution is altered by additional whip loss. The antenna radiation resistance becomes lower making the SS whip decrease the antenna gain by even more. This is a second order effect and can be ignored for now.

Indirect Measurements
Yes, base impedance can be used as an indirect measurement of antenna efficiency. Being a relative measurement (aluminum vs SS) some of the error terms in the error analysis do not change between these two experimental treatments. They "drop out" of the error analysis - they do not matter. Some other error terms are small and can be considered to be "second order" and can be ignored for now. An example of a second order term is the change in loading coil loss caused by the change in loading coil inductance between the SS and the aluminum whip. For my SS vs aluminum whip comparison the whip is the dominant term in an error analysis.

In the case of my 12' center loaded 3.5 MHz vertical the calculated difference in field strength, based on the measured 2.5 ohm base resistance measurement, is 0.7 dB. That can be easily measured using a VNA, measured with some difficultly with a spectrum analyzer, and quite possibly missed with crude mobile antenna shoot-out field strength measurement equipment. My indirectly measured GND loss resistance of 10 ohms is in the range of a mobile installation. Whether 0.7 dB is important is up to the user of the antenna. A 0.7 dB loss is the same as reducing transmitter power from 100 watts to 85 watts and it is equivalent (in my installation) to reducing the loading coil Q from 300 to 200. If one would not want to do either of these things they would not want to use a SS whip.

Antenna shootouts
One thing that I think is missed in antenna shoot-out measurements is quantifying measurement variability. To do this one takes repeated field strength measurements of the same same mobile installation. The vehicle can be driven away and re-parked for each measurement to see if this is a significant source of measurement variability. Another source of variability can be the person taking the measurement. It is interesting to plot measurements taken by one person and watch the data trend upward or downward and finally level out as that person gains skill in that measurement. One can also often see a difference between persons. Have person A take repeated measurements and have person B take repeated measurements. Plot the data and there is often a noticable difference. This type of error is not random but is systematic. Plotting the measurements can be revealing. One can "look through" the measurement variability. If there are enough measurements (30 or more) they can be plotted as a histogram. One should see a Gaussian distribution if the variability is random. If it is not there is something going on that should be looked into. If Gaussian, the standard deviation can be calculated. Remember to convert logarithmic units (dB) to linear units, perform the statistical calculation, then convert back to logarithmic units. We can then state the variability of our field strength measurement. Let's say we come up with 1 dB for one standard deviation. We can then say that with a single measurement statistically 68% of the time we are within 1 dB of what we would obtain if we took a large number of measurements and averaged them. For 2 standard deviations - this occurs 95% of the time - we are within about 2 dB. To reduce the variability of a measurement we can run repeated measurements and average them. The variability will be reduced by the square root of the number of measurements. Take 16 measurements and the variability is reduced by a factor of 4.

Another source of mobile antenna shoot-out measurement error is temperature. As the temperature changes during the day of measurements how much does the field strength measurement equipment change? One way to quickly quantify this is, at the end of the shoot-out, to take the first vehicle measured and remeasure it. All of this should be quantified and ironed out before the day of the shoot-out. For a valid engineering measurement I would budget one full day to this effort.

Let's say we find that our antenna shoot-out measurement has a one standard deviation variability of 1 dB. That is 1 dB of measurement "noise." The whip SS/aluminum 0.7 dB difference is below the measurement noise floor and we are likely to miss it. To reduce the measurement noise take repeated measurements like so: SS then Aluminum, SS then aluminum and plot the data. Take a look at the data and the true difference will be seen.

So, that a mobile antenna shoot-out measurement did not show a difference between two whips doesn't mean there is no difference. Without knowing the measurement noise we can't say with certainty what the difference is.

Reality
I have quantified by indirect measurement and by mathematical analysis the loss difference between a SS whip and an aluminum whip in my antenna installation. The SS whip version has 0.7 dB more loss. My measurement and conclusion meet the criteria for a valid engineering measurement.

Whether the 0.7 dB of additional loss (of the SS whip) matters is up to the user. Quantitative vs qualitative.

Non-RF Indirect Measurement
Indirect measurements are used all the time in industry. As an example, say we want to know the die temperature of a power MOSFET at the end of a 1 kW, 30 us pulse. We can't stick a thermocouple onto the die and take this measurement. But we can still accurately measure the die temperature. Here's how: We pulse the MOSFET with the 1 kW, 30 us pulse and then immediately (like 1 us later) measure Rds ON or the forward voltage drop of the intrinsic diode. We then heat the die until we obtain the same Rds ON or diode voltage drop. We can measure the temperature of the fixture with a thermocouple.

Likewise with some antenna perameters, such as changes in gain for changes in loading coil loss, antenna material, etc. they can be measured indirectly. For a valid engineering measurement we need to be able to quantify what we measure in simulation or calculation. If measurement and simulation/calculation agree we might have a good measurement. If measurement and simulation/calculation do not agree we either have a bad measurement, a bad simulation, or both.


« Last Edit: July 16, 2010, 09:07:42 AM by DAVE CUTHBERT » Logged
K0BG
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« Reply #11 on: July 16, 2010, 10:56:44 AM »

Dave, I suggest you spend some time on Tom Rauch's site: (http://www.w8ji.com)

From other posts, I understand that your background is in broadcasting, and you are an electrical engineer. That's all good, I'm sure. While it appears that you can use the base input impedance to compare some change you've made in the antenna as proof the change was positive, alas you cannot.

For example, a base loaded antenna will have a slightly lower input impedance than a center loaded one (a couple of percent at best). Yet, the center loaded antenna will outperform the base loaded one, because the radiation resistance is higher (about double), because the current node it higher in the antenna.

Top loading (via a cap hat) will raise the radiation resistance even higher, yet the input impedance won't change significantly. In fact, most of the time it'll decrease slightly. As you alluded to, the only way is to measure the field strength, in a normalized scenario, and that is beyond most amateurs' capabilities.
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WX7G
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« Reply #12 on: July 18, 2010, 09:58:27 AM »

Alan, you are correct that I'm an electrical engineer. One area of my expertise is antenna design (with an emphasis on physically small antennas). I have written over 30 antenna articles, have had several antennas in production, and hold an antenna patent. So, antenna design and analysis is not new to me.
« Last Edit: July 18, 2010, 10:35:10 AM by DAVE CUTHBERT » Logged
K0BG
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« Reply #13 on: July 18, 2010, 10:43:14 AM »

Well, mine is in civil engineering, with a background in communications, primarily with the phone factories.

Let me add this. I, like you, used to assume that a change in the input impedance could also indicate a change in performance. I suspect it still can, if the change is huge. However, after several lengthy conversations with W8JI, as well as several articles by N6LF, I now realize you really can't rely on input impedance measurements.

Bob Lewis, AA4PB, in his previous post, outlined the accepted way to see changes in the input impedance when comparing antenna makeup A vs. B. The problem is, we can't assume that some specific part of the input impedance remained the same, while we played with another. They are, after all, in series with one another.

And as Bob pointed out, we might get closer to a positive change, if the two configurations were identical, but made up of dissimilar metals. However, the actual field strength measurements between one conductor material, and another, is typically within the accuracy fuzz of the measuring device in question. This is certainly the case with the MFJ-259B.

I have a new VNA coming from Germany (it's the only one I could afford; the HP stuff is out of sight!). It is going to be interesting comparing the readouts with what I already have on record for the various assortment of HF mobile antennas I own.
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WX7G
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« Reply #14 on: July 24, 2010, 06:16:23 PM »

Alan, today I measured the base-referred loss of a 106" 17-7 stainless steel whip. This was compared to an aluminum mast. No loading coils, just the whips placed over sixty four 20' radials. The ground loss resistance is 4 ohms. At resonance (around 25 MHz) the aluminum whip input R is 40 ohm. The stainless steel whip is 50 ohms.

The stainless steel whip base-referred loss is 10 ohms.

A NEC simulation reports a base-referred loss of 11 ohms for the 0.18" 17-7 whip. This is using the published resistivity of 8.3E-7 ohm-m and a relative permeability of 120.

« Last Edit: July 24, 2010, 07:25:00 PM by DAVE CUTHBERT » Logged
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