[TowerTalk] BigIR question

[Jim Thomson]

Date: Thu, 7 Dec 2017 17:53:19 -0000
From: "David Robbins" <k1ttt at verizon.net>
To: "'Grant Saviers'" <grants2 at pacbell.net>, "'turnbull'"
<turnbull at net1.ie>, "'Keith Dutson'" <kdutson at sbcglobal.net>,
<john at kk9a.com>, <towertalk at contesting.com>
Subject: Re: [TowerTalk] BigIR question

<Or do like I did after burning up several of the M2 Yagi baluns (really just
<small ferrite binocular chokes)... replace them with coax air core chokes.


<Air core is better than cracked ferrites and melted coax in those little pvc
<cans!!  And can be done quickly with materials at hand without having to
<worry about getting enough ferrite mass to handle whatever burned up those
<supposed 3kw rated 'baluns' 

<David Robbins K1TTT

##   M2 has NO clue on how to build a balun, still dont... just junk.   Binoc  format
is very difficult to get to work correctly, esp on the lower bands.  The
only advantage to the binoc method is  you can wind in  half turn increments,
and thats it.   The C between turns in the binoc method is very high. 

##  The ferrite in the  M2   CM.. balun burns up simply cause they used the wrong material,
wrong coax...and not enough RS.   You dont require a huge stack of cores to handle a lot of 
power either. 

###  http://myantennas.com/wp/product/cmc-230-5k/#prettyPhoto    This uses only 
four of the 2.4 inch OD, type 31 cores, and also wound with RG-400.    RS is from
8000 ohms to  11,300 ohms, depending on band.   It covers  160-10m. 
And no, you wont blow it up with 5 kw.    With sky high values of RS,  CM current is 
down to nothing.  Cores dont heat up, its that simple.
If you want balanced connectors on the yagi side, he can do that too.  If you want to
duplicate the design, its depicted in the pix,  just staggered assemblies,  with more turns
on the 1st assy  vs the 2nd assy. 

##  If  you want to see how effective your coax air core chokes are.... measure the CM
current using a MFJ- 854 RF ammeter.   Covers  6 ranges, from 3A...down to 1 ma.  The
1st 3 ranges are passive, the last 3 are active.   The MFJ-854 is calibrated by  1st feeding exactly 
50 watts into a dummy load.   50 W  = 1 Amp of RF.    Tweak calibration pot till you read 1A. 
Coax shield and center conductor  has to be separated, where they are stuffed into the
dummy load, so the MFJ-854 can then be clamped around the center conductor only...
or the pigtail braid. 

## Use the same  power output  to conduct all tests.  And measure in the same place on
the coax for each test.   800 watts into 50 ohms = 4A of current, etc.  Ideally  you want no
current on the braid of the coax.   The MFJ-854 has limitations though, it will  only clamp
around coax as big as 213-U,  so in some cases,  you might have to temp insert a short  jumper
made from 213-U.    You will quickly find out how ineffective the air core chokes actually are. 
You will also see how ineffective a lot of the ferrite chokes are.   In some cases, quite a few,
they  will actually increase the CM  current. 

##  If the yagi is truly balanced, and with 1.5 kw measured at the feedpoint, the voltage the CM
sees is only 137.5 volts.   With  8000 ohms of RS,  that equates to only  2.37 watts  of dissipation
in the cores..which is dick.   4 x cores will easily handle that, and them some. 

##  depending on your local environment, an RS of 8000 ohms, can in some cases, reduce the
ambient RX noise by a bunch.   Other benefits are reduced RFI, and no degradation of ant
pattern.   On a side note, the MFJ-854   can also be used to measure current on any cable
in the shack, including rotor cables, PC cabling, power cabling, radials on verts, wiring  in 
your neigbours home, even the wifes clothes line.   You will discover RF current in places
you would not have imagined.  It’s a good tool. 

Jim   VE7RF