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Re: Metal Crossboom was Re: clamps on M2 436CP30 antenna



Joe,

Thanks, I'm reading the stuff now. It looks like the first "practical" sweet
spot is 3/4 wave from the DE. That seems too close for balance. The next
spot is 1.5 wl from the DE and that might be about 34 inches...just rough
approximation. For those of you who are using wa5vjb's "trick", on 70 cm,
what spacing are you using from the antenna DE to the metal cross boom (in
inches).

I assume the listed distances are in free space wl, 984/f ?

Tnx, most interesting stuff.

One caveat, from the jpegs you sent, it appears one minute he's talking
about dl6wu linearly polarized yagis and the next he starts talking about
circular antennas, as if he has presented the data for them. Maybe that's an
artifact of  sending just the jpgs? I would be a little concerned about
generalizing data from linear yagis to CP crossed yagis, so can I assume
that the actual data presented in the graphs was from a CP yagi? He says on
the first page that all data are from DL6WU antenna, 70cm...I didn't realize
that WU designed any circular antennas. Maybe the WU design was the building
block for TWO crossed yagis for CP...if so, then it would all hang together.
His drawing of the mounting with associated graphs, only shows a linearly
polarized single yagi, not a crossed yagi. That is what is confusing me. The
first page shows a crossed yagi, the 2nd with all the measurements and data
show a linear yagi.

73
hasan schiers, NØAN
schiers@netins.net

----- Original Message -----
From: "Gary "Joe" Mayfield" <gary_mayfield@hotmail.com>
To: <amsat-bb@AMSAT.Org>
Sent: Wednesday, August 22, 2001 10:54 PM
Subject: Re: Metal Crossboom was Re: [amsat-bb] clamps on M2 436CP30 antenna


> Greetings Hasan,
>
>     No it is not a bad practice if done correctly.  The definitive paper
was
> written on the subject by Kent Britain, WA5VJB.  His email gave me
> permission to reproduce the article.  He also wanted to dispel a couple of
> other myths about CP antennas (that's what the helix stuff was).  John
> Becker, W0JAB has graciously agreed to put the paper on his web site.  He
> will post the address on the list when he is done.
>
>      Kent is a recognized authority in VHF/UHF/SHF antennas.  He backs his
> claims up with verifiable test data from an antenna range.  Kent's
> conclusions are "STRONGER, SIMPLER, CHEAPER and less LOSS.  Just avoid the
> pitfalls".  Kent doesn't sell expensive fiberglass masts so he has no
> motivation to encourage their use.
>
>      I will be willing to email 3 jpegs containing the article to anyone
> interested, but I ask that you be patient while John puts it on the web.
>
> 73,
> Joe
> ka0yos@amsat.org
>
> ----- Original Message -----
> From: "hasan schiers" <schiers@netins.net>
> To: "Gary "Joe" Mayfield" <gary_mayfield@hotmail.com>;
<amsat-bb@AMSAT.Org>
> Sent: Wednesday, August 22, 2001 5:37 PM
> Subject: Re: Metal Crossboom was Re: [amsat-bb] clamps on M2 436CP30
antenna
>
>
> > > It is a fairly common practice to run the coax from the feedpoint of
the
> > > antenna along the boom to the crossboom and then tape it to the
> crossboom
> > > following the crossboom back to the rotor.  This has the same effect
as
> a
> > > metal crossboom.
> >
> > Hi Joe,
> >
> > If I'm interpreting what you said here correctly, then it's bad
practice.
> > The other article you quoted was about helices and how to do mounting. I
> > don't think things got translated correctly.
> >
> > Yes, if you take a coax cable and run it along the boom and through the
> > array, then at right angles across the plane of the array, it is much
like
> > having a metal cross boom in the array. I have NEVER seen anyone
> > knowledgable do it that way, for the obvious reasons. It is for the very
> > same reason one doesn't mount a vertically polarized yagi on a metal
> > vertical mast. Now, yes, I have seen this done many times. It's a
terrible
> > idea. It was and is done a lot on 2m FM...only because they didn't know
> > better and were told "it doesn't really affect it 'that much'" It was
> being
> > done for convenience, not because it was the "right" way to do things.
> >
> > Have you noticed that all the drawings for the 70cm circular antennas
show
> > the coax coming off the BACK of the antenna....with a specification to
> take
> > the coax away from the back for a significant length to keep the coax
away
> > from the elements?
> >
> > Quoting from the M2 Manual:
> >
> > IN BOLD PRINT, no less:
> >
> > "Do not route the feedline to boom to mast plate as exiting the antenna
> here
> > will adversely affect the circular field."
> >
> > Further down it refines this requirement even more:
> >
> > "The 436CP30 is a circular polarized antenna and creates a field in all
> > planes or polarities. Performance DETERIORATES SIGNIFICANTLY if it is
> > mounted on a metal (conductive) mast or crossboom. A mast or crossboom
of
> > any NON-CONDUCTIVE material MUST BE USED."
> >
> > "Mount the 436CP30 so that elements tips are AT LEAST 12 inches FROM ANY
> > CONDUCTIVE MATERIAL (mast, tower, FEEDLINE)"
> >
> > I'm amazed that something that has been virtually gospel for 30 years is
> now
> > being dismissed because of an obscure reference to a symposium article.
> I'm
> > also not at all sure from the quoted sections that I saw, that the
> question
> > I asked was even being addressed. What I read in the post was virtually
> all
> > about the fact  helical antennas may not be as circular as assumed...in
> fact
> > it looks like they might be downright PICKY about the kinds of things
you
> > put in their near field, like dielectric supports, etc...
> >
> > It begs the question...if so many of these helices were so poorly
circular
> > due to various construction practices, we are all of a sudden to assume
> that
> > placing a CONDUCTIVE MAST or FEEDLINE right in the middle of the array
is
> > ducky? It makes no sense to me.
> >
> > I found Roy's remarks interesting...it sounded like he was saying if you
> > bisect the X formed by the elements with the boom (X layout instead of +
> > layout) and don't protrude too much of the mast into the X portion and
> then
> > route the coax along the boom and out the side of the X, that it "worked
> > great".
> >
> > If there were no other way to accomplish my antenna, I think that's a
fine
> > suggestion. To deliberately insert TWO FAT CONDUCTORS in the middle of a
> > circular antenna on purpose, when it isn't necessary, strikes me as very
> bad
> > advice.
> >
> > FAT CONDUCTOR #1 = Metal Cross Boom, partially inserted into the X of
the
> > array.
> > FAT CONDUCTOR #2 = LMR-400 Coax or larger (about 0.4 to 0.5 inch
diameter)
> >
> > It seems to me that there is a RIGHT way to do things and it's been
known
> > for many years. When all of a sudden someone comes up with "nah, it
really
> > doesn't matter"...I need more of an explanation than "it works great" or
a
> > series of comments on how poorly circular a bunch of helices are.
> >
> > The real question I asked remains:
> >
> > Is anyone seriously suggesting that placing two fat conductors in the
> middle
> > of a circularly polarized array will not have a deleterious effect on
that
> > antenna's performance?
> >
> > If that is what is being suggested, PLEASE explain, in detail, how this
is
> > possible.
> >
> > 73,
> >
> > hasan schiers, NØAN
> > schiers@netins.net
> >
> >
> >
> ----
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>

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