Anodized Alumium for Antenna Elements

Hi Tom,

The number of variables in the description of your (Mark's) method is rather considerable, so I will remark by the parts you offer:

So far, fine.

Commendable.

I might slyly point out how do you know the gain? It visits the age old logical knot offered: In a town of clean shaven men, there is a barber who shaves everybody who does not shave himself; who shaves the barber?

This method is called using a "transfer standard." As I offered, that requires an absolute knowledge somewhere, and you have identified it in this "known gain" yagi. However, the gain is actually immaterial until you begin making claims of absolute gain. That is, most of this correspondence is satisfied with relative gain comparisons as you point out:

Quite true, however, you having once acknowledged suspicions you then plunge back into the murky pool of absolutes:

Well, here we run counter to my experience with real life components. They varied by several times your 0.1dB, and this was often times for the same item tested repeatedly (I never measured any item less than five times and never five times repeatedly, in a row).

OK, the method is good and robust, but your sudden departure from expected results are on the scale of 5 to 6 times the range of your typical error.

If this is to be attributed to oxidation on the elements, that still seems suspect. The oxidation is not lossy, and certainly is not sufficiently thick enough to shift the resonance. Oxidation is one of the charms of aluminum, it is self sealing.

I would offer that if the elements oxidized, so did the connectors (or connections). Simple, repeated connector matings (like swapping in and out for the range test) were sufficient to break bad contacts and make the difference which was attributed to scrubbing the elements. In the normal course of my calibration of various items with connectors, I always inspected and cleaned them first. N connectors have erosion problems that will give rise to variations outside of

0.1dB - comes from those threads. The "standard gain" antenna should be suffering from this erosion by now, but you don't report it.

This raises suspicions for me - you have too much fulfillment of expectations which is truly extraordinary. I have made thousands of calibrations of isolators, pads, couplers, meters and so on that have shown a gaussian distribution of results for premium equipment. Your range experience shows very little variation - much too little when we are talking about being within 0.1dB.

Well I hope you shake the infection off. Further details are unlikely to resolve this corrosion as it is too much a matter of "you had to have been there" kind of thing.

73's Richard Clark, KB7QHC

Hi Tom,

Not one of their numbers against the characteristics. However, I am familiar with what you describe as the characteristics.

It is a tuned AC voltmeter, commonly used for SWR measurement in slotted lines connected to a the detector where the source is modulated at 1KHz. The meter is tuned to 1KHz and has a very high gain and selectivity. This allows it to employ a variable gain, by

10dB switch steps (and a variable knob to set zero, or the reference). The scale is read in combination with the attenuator (gain) switch and thus the scale offers considerable resolution, easily 0.1dB and better. It is probably an HP-415.

I've calibrated these too (Boonton, I think, also built them, but as Boonton was acquired by HP, it isn't a remarkable difference).

73's Richard Clark, KB7QHC

So for relative gain it's possible, in your opinion, to measure +- .1dB with this, if properly used?

tom K0TAR

Hi Tom,

Quite easily.

73's Richard Clark, KB7QHC

So he's using decent equipment. Whether it's used correctly is another matter. I'm betting he did a good job, given the results I've seen, and what I know of him.

But you are correct to be be skeptical on the results.

tom K0TAR

How close to the ocean are you? It sounds like you are right in the spray!

- Mike KB3EIA -

Hi Tom,

As I've offered, the test protocol is very precise, and the instrumentation (as far as has been discussed or inferred) is up to the resolution. However, many mistake what accuracy, precision, and resolution mean.

Resolution is the number of digits in your reading. It usually implies that you can read more digits than you report. So, to say you have measured a voltage to be 1.5V means that you have an instrument that can read in hundredths of volts.

Precision is the repetition of readings. High precision means your measurements all can be reported as 1.5V because they vary no more than 4 hundredths of a volt in readings around the reported value (or by more fancy regression techniques).

Accuracy is how far from actual your report is. It is enough to say that resolution and precision are not accuracy, but that they are necessary components of accuracy.

Insofar as the range goes, it remains to be seen if it has been calibrated in its own right. The test is not necessarily found in absolutes, but rather in its response to perturbations. In other words, inject a known variable and measure its ability to support a report that faithfully records the value of that variable as evidence of its robustness. You have to perturb the system with small changes as well as large changes to see if it is linear in its response. This is not easy and makes great demands upon not only the instrumentation, but the ingenuity of the tester. Then you repeat the tests from a different angle to see if it is symmetric. Then you test for background contributions - noise (actually this is probably best done first as it sets the boundaries of your low end and defines part of the dynamic range).

You do all the above, and then some, pool the results and describe your limits of error. Test results that are reported without knowing the limits of error are not very informative. Hence, when I hear that readings are repeatable to 0.1dB for UHF and I hear nothing of the range of error (I must presume that it is no greater than 0.033dB); then I am more than skeptical because 1% accuracy in power determination is the extreme of very tightly controlled laboratory conditions.

That there are repeated measurements in the field to this level of precision is suspect because there is very little instrumentation AND combinations of many pieces of gear that come close. It takes only two pieces of 1% gear to create a situation that is at best 1.4% accurate and you are already crossing the 0.1dB threshold. For those trying to balance the ledger, a 1% accurate determination requires a method that is at least 3 times more accurate. The usual aggregation of error arrives through RSS (root sum square); some may like to gild their prospects and compute RMS (root mean square) and if they are lucky, this is not far off. Given enough results, luck washes out to sea and RSS dominates. Given enough results that conform to RMS, then you find you have qualified your methods and instrumentation to superlative standards.

73's Richard Clark, KB7QHC

About a half mile. I live in the northern California "Mendocino" coast. We have a lot of rain too and that combination is a killer for aluminum exposed to the elements.

Danny

HP416A.

tom K0TAR

Hi Tom,

By description and application, probably, but I need a picture or manual to be able to confirm. I've calibrated and used so much different gear that the numbers blur.

73's Richard Clark, KB7QHC

The (inappropriately named) Pacific coast where Danny lives has fair-sized waves almost constantly, particularly in the winter. When they break along the shore, a very fine mist of salt water droplets is created, and those drift for a long distance. In the winter, the prevailing wind direction is from the west, so the salt water mist is blown farther yet. The result is that the air itself contains a suspension of salt water. Aluminum corrodes fairly quickly, and good sized bare copper wire turns into a blue powder in a year or less. Where I live, in the Willamette valley of Oregon which is about 70 miles inland, it rains pretty constantly from about October through June -- not an extraordinary amount, but everything outside stays wet for the whole winter because of the lack of direct sunshine and the frequent rain. But aluminum lasts forever and so does copper, which only gets a thin, dark oxide coating. It's the salt water suspension that's the killer on the coast.

Roy Lewallen, W7EL

Dan Richards> >

Yea, but those launch angles to the west.

Danny, K6MHE

GAD!

I had forgotten that white elephant. Thanx Wes.

Tom,

It is pretty much in the same class of expanded range, 1KHz tuned AC Voltmeters. As long as you reference and return to a Cardinal point on the scale, accuracy you describe can be supported.

In fact, this class of instrumentation is probably the best leverage to building a very good RF lab. You can spend more, you could even find equipment that does most of the grunt work for you, but it is still a long shot that you will obtain more accuracy.

Accuracy is steadfastly bound to method and this style of instrumentation reveals an example of method thought out and polished with the best of engineering thought.

73's Richard Clark, KB7QHC

Richard Clark wrote: ...

Errrk?? From Boonton's web site:

"In July 2000 we became a member of a larger family as we were acquired by Wireless Telecom Group, Inc. (doing business as Noise Com). Being a wholly owned subsidiary of Wireless Telecom Group, Inc. has enabled us to further our product development and customer service initiatives."

See

Since you're wanting to make it black to match other accouterments on your vehicle, consider that most, if not all, black-dyed anodization will fade in the sunlight, and it doesn't take all that long. In addition, depending on the aluminum alloy, the "black" may not be all that black to begin with. Why not instead give the antenna a coat of automotive enamel or lacquer? You can either mask off areas that need to have electrical contact, or scrape them after painting. Perhaps you can paint after the antenna is assembled. That way, you can get a color that really matches other items.

Cheers, Tom

They left out part of their history.

H-P *did* acquire Boonton at one time. I have both a black crackle Boonton 250 RX meter and an H-P gray HP 250 RX meter out in my storage building.

Once long ago I was driving down highway 101 in my VW squareback, operating mobile CW late at night on 40 meters. Rig was homebrew, about

8-9 watts output (10 watts input). Antenna was a CB mobile whip on a bumper mount, base loaded with a coil wound on a powdered iron toroid core, Q about 200 - 250. Worked JA, solid copy. Yea, that salt water does wonders for a vertical.

Roy Lewallen, W7EL

Dan Richards> >

Thanks Tom (& everybody else). The problem is that I'm planing on having an 8' two section whip on the top that will power extend & retract through the loading coil into the bottom mast.

Why?

Call it a combination of lazyness & a prepondarence towards low ceilings in parking garages. Also (bonus), with the coil retracted I'll be able to vary the height of the whip for the upper bands.

So what I was really looking (hoping) for is a black coating that maintains electrical contact between the inner & outer whip sections as one slides in & out of the other.

Don't look like I'm gonna get it :(.

Maybe it would be easier just to build a test range insted :).

H.