[TowerTalk] Comments by K9YC

[Gary Schafer]

I have an old Mosley TA33 and yes they did recommend grounding one side of
the driven element to the boom with a strap. If I remember right they also
recommended a choke made of several (8 or 10) turns of coax.

I found that grounding one side of the driven element made the beam have a
nice wide SWR bandwidth. It actually had two dips, one higher in the band
and one low or below the band of interest. 

This was about 30 years ago. I called the factory and they said that they
never heard of anything like that. I couldn't figure out why it wouldn't be
like that as the driven element was severely unbalanced!
I removed the ground strap and the antenna performed as expected with a much
narrower single SWR curve.
I guess that was their method of achieving a "wide bandwidth" antenna.

73
Gary  K4FMX


> -----Original Message-----
> From: TowerTalk [mailto:towertalk-bounces at contesting.com] On Behalf Of
> Joe Subich, W4TV
> Sent: Sunday, June 22, 2014 9:37 PM
> To: towertalk at contesting.com
> Subject: Re: [TowerTalk] Comments by K9YC
> 
> 
> 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.
> > _________________________________________________________________
> > 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.
> >>
> > ----------------------------------------------------------------------
> ---------------------------------------
> > 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.
> > ----------------------------------------------------------------------
> ---------------------------------------
> > 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."
> > ----------------------------------------------------------------------
> ------------------------------------
> > This is true but I fail to see what it has to do with my note.
> > ----------------------------------------------------------------------
> -------------------------------------
> >
> >> 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.
> >>
> > ----------------------------------------------------------------------
> --------------------------------------
> >
> >
> > 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.
> > ----------------------------------------------------------------------
> -------------------------------------
> > 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.
> > ----------------------------------------------------------------------
> ------------------------------------
> >
> >
> > 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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