Hi folks, I'm in the process of building myself a screwdriver antenna for my truck. Being the vain sort, I'd like it to end up being black as that matches nicely with the other bitties sticking out of the vehicle.
Q.- Does anodyzing negatively impact aluminum's ability to efficiently radiate? I seem to recall reading somewheres that anodyzing leaves a non-conductive surface, but on the other hand aluminum oxide ain't supposed to be that great a conductor either & that whats on the surface of any piece of aluminum thats seen air for more than a few minutes.
The radiating surface don't care. Just make sure all mechanical contacts which need to conduct penetrate the anodizing. Like where the coax connects, or where the coil is connected.
Anodizing the aluminum is kind of over kill as I have measured new antennas and old antennas with 30 plus years of exposure to the elements. No difference measured.
What did throw me a curve was buying Anodized Surplus Aluminum Tubing in Burbank Ca, and making a vertical for the VHF Low Band. Ok no problem getting it together and should have tuned in a few minutes.
But no it was responding to a much higher frequency than the 31 Mc I wanted it to tune to. After checking and double checking everything and replacing the coax and antenna analyzer it still measured a way high frequency SWR null.
Now faced with the strong reality that this thing was gonna kick my a$$, after a hole day of going nutzoid. I got my old Simspon 260 VOM and measured continuity from all the elements. I found that one of the vertical elements had been anodized on the inside causing a no connection condition.
Cutting off a metal wire brush and putting the shank in Mr. 1/2 inch chuck, drill motor, and the 100 foot cord (I have given up on battery operated drill motors) the inside of the tubing was now nice and shinny bright aluminum.
The antenna tuned up as designed. So keep your eyes open when using anodized aluminum tubing. And I think this is why some antenna manufactures do not anodize their aluminum tubing. Plus the added manufacturing costs.
Jay > The radiating surface don't care. Just make sure all mechanical contacts
That might be ture in the Mojave but come up here on the norhern California coast - on the ocean - and you 30+ aluminum antenna will be noting more that a memory.
Interesting, as I have measured antennas with only 2 years exposure that had .5 dB measured difference. A 432 MHz antenna scrubbed with a ScotchBrite showed a .6 dB gain increase.
So B as in B, S as in S, as we say here in Minnesota.
Well, you'd have to ask Mark Thorsen, WB0TEM, what equipment was used, but the range is checked several times against the reference antenna during each band we run, and is generally within .1 dB between checks. Except that one day in KS. Boy was it hot.
I wouldn't bet absolute values are on the mark, but an antenna measured against itself was reproducable. So I do believe the antenna improved due to having the oxide scrubbed off.
But the point of accuracy, even reproducible accuracy, requires a very absolute source to compare against. Sometimes that absolute is quite simple to achieve, but now you have upped the ante to 0.1dB. This implies a measurement accuracy of at least three times better; which, in turn, means you have access to a standard that can discern 0.8%.
To say you test the antenna "against itself" does not really say much when it comes to power and gain. That is no benchmark. The presumption here is that you have an external source of power that is constant. This then raises the same question. Over a span of time, what guarantees this degree of accuracy? By what method is it confirmed? That source's "absolute" power level needn't be an issue, but there is no way to escape casting that "absolute" requirement into another standard to confirm the fact of its stability.
Based on some of the other responses you received it may not be worth it but there is another type of coating for aluminum called alodyne which protects the same way as anodizing but is electrically conductive.
Jim Miller wrote: there is another type of coating for aluminum called alodyne
Alodine (R) coatings are not conductive. You can specify a "type 3" coating that is thin enough that fasteners will usually punch through the coating layer.
Another corrosion-inhibiting coating for aluminum is iridite. There are conductive and non-conductive versions, something I learned the hard way long ago.
I was the engineer responsible for transferring the design of the then new Phoenix Missile IMPATT diode transmitter from the development lab to the production floor.
The transmitter had three stages: a single diode driven by a phase-locked Gunn oscillator fed a three diode cavity that drove a 16 diode cavity.
The development hardware used aluminum cavities that were comprised of two pieces, with third copper piece that mounted the sixteen diodes. So there was one aluminum-to-aluminum and one aluminum-to-copper interface in each sandwich. Since this was a product for the U.S. military, "passivation" was required for all aluminum parts.
I won't go into the considerable amount of detective work that it took to decide that despite being "conductive" Alodine and its ilk are not suitable coatings for rf components.
Gold is your friend, if of course, it's thicker than a few skin depths, which is another long story. [g]
Ok. I'll give the details I remember, but be advised I didn't implement Mark's system, I just assisted in using it.
The range has a source yagi for each band, that yagi has a low power AM modulated signal on it (as I remeber, might be wrong on this). The other end has a reference yagi off to the side from the test position. A yagi with "known gain" is run against the reference yagi, and the relative signal strength is measured. That gives us a known gain point on the meter. A yagi is then placed in the test position and the relative gain or loss is recorded. The "known gain" yagi can be put back into the test position at intervals to check the calibration. Obviously the absolute values may be suspect, but relative measurements work well. The results also agree very well with YO predictions, with a yagi in the 18.4 dBd predicted range being low by .3 as measured, which is roughly what he expected to happen. Most more normal gain, 14 to 15 dBd for 432, were within .1 of predicted. Bands tested on this range were 144, 220, and 432.
Multiple prototype 2M and 70cm EME antennas that my partner and I built, stored safe from corrosion, tested the same +- .1dB with a several year gap between the tests.
What equipment he uses for the ratio measurement, and precisely how it is done, I don't know. I will attempt to contact him and find out if I can get this damn sinus infection under control in the next few days.
PolyTech Forum website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.