On 12/27/2011 3:03 PM, Eddy Swynar wrote:
> On 2011-12-27, at 4:17 PM, N6FD wrote:
>> A lot of people (myself included) have been very effective with attic
>> antennas in wood structures.
>> But steel does often have lower loss than a wet tree.
> */Hi Eric,/*
> I'm going to play "devil's advocate" here, & try to put my "spin" onto
> what you've just stated above...! * : >)*
> *QUESTION #1*: you've stated earlier that trees are very lossy
> affairs, & because of that will dissipate RF fields radiated into
> them. If that's so, then why would your ".../attic antennas in wood
> structures/" be so effective...? Why wouldn't that environment have a
> lossy influence, too...?
> *QUESTION #2*: Why /not/build antennas in steel structures, if said
> steel has ".../lower loss than a wet tree"./..?
> Again, I hope you might see where I'm going with this. You can't have
> it work /one/way /one/time, & /not/the other, /another/time...at least
> not the way that I grasp this.
> Another question: why is it that in some antenna articles (*/QST/*,
> */CQ/*, etc.) the author muses aloud as to the possibility that some
> nearby /metal/structure may have had an adverse effect upon the final
> SWR / tuning of his antenna...? It's never, "/The nearby gooseberry
> grove may have affected my antenna/," but rather, "/The nearby
> aluminum-sided house / water tower / steel chicken coop / etc..../"
> that had a play in things...
> And I'll ask my question anew, because nobody has even come /close/to
> answering it, specifically, /your gut reaction/: in what environment
> would you rather have your new 160-meter 1/4-wave vertical in, (*A*)
> the middle of a forest of 120' tall trees, or, (*B*) the centre of a
> parking lot surrounded by ground-mounted 120' tall metal lamp posts
> placed about every 20', or so...?
> Think about it! Hi Hi
> */~73~ de Eddy VE3CUI - VE3XZ/*
My answer to question #1 is that the wood in a building is usually dry.
Dry lumber has much lower losses than wet lumber. Additionally, it only
has to go through 3/4" to 6" of wood to get out and I can orient my
dipole to be perpendicular to the rafters. In a forest, your average
tree is much thicker. Your mileage may vary depending on the roofing
material, but composition shingles don't add much loss. When the roof
is wet, losses are also higher. My experience was in Southern
California, so for 80% of the year, the roof was quite dry.
The answer to #2 is that steel is still a much poorer conductor than
aluminum or copper and isn't much used in antennas as they generally do
not need the structural strength that steel gives. Where it is needed,
something like copperweld is used where the steel is covered with the
copper. Since RF travels in the surface (skin effect) the wire behaves
like copper at RF.
I would take option "B" because I would expect more of my RF to radiate
out of the parking lot rather than warming up trees.
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