I recently sent the following to towertalk reflector. It appears that K9YC
does not understand it.
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On 6/21/2014 3:23 PM, Dan Hearn wrote:
> It is interesting to note that the Mosley claimed gain is about 2db
> greater than other tri band beams. They use dbd reference instead of dbi
> while I think their claims would be about right if they used dbi reference.
>
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Maybe I should have used a few more words to explain. In the fine Yagi
tests described by K7LXC and N0AX in their book on tri band Yagis. each
companies reference for their gain ratings is given. Mosley gave dbd as
their reference. That implies that they have about 2db greater gain than
other companies who use dbi. People who do antenna modeling get their gain
answers referred to dbi, an isotropic model. If this is confusing, You can
go to our clubs web page, www.sdxa.org and look under Articals to find a
clear discussion of dbi,dbd, and db gains.
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K9YC comment.
"Exactly the opposite. The peak gain of a dipole is 2.2 dBi. So a gain
specification of 4 dBd is equivalent to 6.2 dBi."
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This is true but I fail to see what it has to do with my note.
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> My second comment
> I suspect if you used a good line choke at the coax feed point and subtract
> 2.1db from their claimed gain you would have a typical triband beam.
>
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I don't agree with that logic or that statement. A simple common mode choke
simply reduces common mode current, which mostly prevents RF received on
the feedline from filling in the nulls in the pattern.
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Here I do not believe Jim understands the problem. The choke will do what
he says ON RECEIVING but that is not the problem we are trying to solve
with understanding the lousy test results for the Mosley tri bander. They
connected the coax directly to the driven element as Mosley said. The coax
saw many places on the 3 bands where the feed point impedance was not 52
ohms. That will result in spill over of current from the inside of the
shield to the outside at the attachment point and ultimately radiation
which screws up the beam pattern and sends rf into your shack. That is why
you need a good line choke at the coax attachment point.
I have a lot of respect for Jim and his work on line chokes and other
things. I have built and measured many of his designs with excellent
results. I do not understand why he went on such a rampage over my post.
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In his book, "Array of Light," N6BT shows optimized designs of 2-element,
3-element, and 4-element MONOBAND Yagis. He put these IDEAL gain figures in
a table with the CLAIMED gains of a dozen or so competing products. In
nearly all cases, the advertised gains were 2-3 dB better than the best
monoband beam. In other words, they claimed impossible gains. After the
first edition of "Array of Light" was published, nearly all of those
manufacturers revised their gain claimed downward by several dB.
73, Jim K9YC
--
Dan Hearn
N5AR
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