On 2014-06-22 10:12 PM, Dan Hearn wrote:
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.
The fact that the feedline is not working into a matched load does *not*
create common mode current. Common mode on a feedline is created *only*
when the impedance to ground is different on each leg of the antenna.
If the impedance is the same - even if it does not match the feedline
impedance - the current into each leg of the antenna is the same, the
antenna is balanced and there can be no common mode current.
Mosley used to connect one side of the driven element to the boom with
a strap. Grounding one half of the driven element and connecting the
shield of the coax to the junction of the half-element and strap is
guaranteed to seriously unbalance the antenna, causing beam skew, feed-
line radiation, and common mode current. In addition, if the boom plus
end elements happened to be a multiple of a half wave, the 1/4 wave
each side of the feed point along the boom could easily cause all kinds
of strange behavior.
It is unbalance *not SWR* that causes common mode current to flow in
a feedline. In a properly balanced system, the currents on the center
conductor and inside the coax are equal and opposite regardless of
their level or their phase relationship to the voltage and no current
appears on the *outside* of the shield.
73,
... Joe, W4TV
On 2014-06-22 10:12 PM, Dan Hearn wrote:
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
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