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[TowerTalk] How to keep the RF off the outside of the coax

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Subject: [TowerTalk] How to keep the RF off the outside of the coax
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Date: Fri, 12 Nov 1999 15:54:47 EST
In a message dated 12.11.99 10:56:47 Pacific Standard Time, K7GCO writes:

   If you have coax cables running under the house with so much RF Spill Over 
on the shield that is causes RFI, you have feed systems that needs to be 
improved.  A coax shield can pick up induced RF if it runs horizontal to the 
beam off the ground.  Assuming there is minimum RF Spill Over and you still 
have problems, use the donut toroids on the coax before it goes into the 
house.  However they only work well at the high current area as they are RF 
Resistors.  You may have to vary where they are.  Coiling the coax to create 
a choke only works at the high voltage area and takes progressively more coax 
the lower the frequency and becomes expensive.  The coax coil is the least 
effective at the feedpoint (high current area) unless at high frequencies 
where a coil could be around 1/4 WL long.  When you have a way to measure RF 
Spill Over you can see all these affects.  
     Use the Palomar RF Current Meter Model PCM-1 to measure the relative RF 
values.  It clamps on the coax or other lines and has 3 ranges.  I check all 
my coaxes at various places and with the proper feed systems I have virtually 
no RF on the coaxes at the antenna or in the shack on all my coaxes.  I get 
high use of it.    
     You have to realize that existence of RF Spill Over first before you 
recognize the need to measure it and then eliminate it.  It's an almost best 
kept secret.  I got on to it back in 1939--with a burn.  All verticals have 
RF Spill Over except the AEA 2M Iso-Pole vertical with the double skirt.  The 
2nd 1/4 WL skirt is attached at a high voltage or impedance point and shorts 
the RF off the mast.  (A 1/4 WL radial or skirt is a Lo-Z (almost zero) 
connected to a high Z point).  Ohms law takes over.  
      When I adjust fed systems there are times when you can just make one 
antenna end longer than the other to eliminate or minimize RF Spill Over.  
Use the MFJ Analyzer, place your hand on the shield at the feedpoint (below 
the balun or whatever) or on the center of the DE that is T-Matched.  If the 
DE is unbalanced (not zero or minimum voltage), your Xc hand will change the 
SWR.  With a properly designed and adjusted feed system, your hand will not 
change the SWR when placed on the center of the DE (T-Matched) and properly 
balanced.  Gammas require unbalancing the DE physical lengths to compensate 
for the gamma arm--result is no RF Spill Over.
      I have a system where I feed the DE off center by running the coax down 
the center of the DE to the feedpoint where it's 50 ohms.  I fine tune the 
lengths and presto--no RF Spill Over there or in the shack.  I get great 
patterns that compare to Eznec and fewer noise problems.  I even have 
verticals that are fairly quite--only the antenna radiates and receives.
     In Radio Mag in 1939 Bailey had an article about RF Spill Over of a 
popular vertical of the day and reduced it.  This is the first ever article 
on this problem.  He summed it up briliantely by saying "Verticals are mostly 
50 ohm matching devices for the coax to the mast and/or coax shield."  The 
mast LW type radiation dominates and its high angle radiation overrides the 
on the horizon radiation of the antenna on top.  Reminton Rand had the 2nd 
article in a 1952 QST on RF Spill Over and how to eliminate it.  I've been 
pointing out whenever I can. 
      The sensitivity of the MFJ is such that the absorption of your hand to 
RF will change the load and SWR.  Higher powered SWR devices may not indicate 
the presents of your hand.  Another test is when using a short length of coax 
to the feedpoint is to short the ground of the MFJ case to the center of the 
DE, boom or mast.  If there is RF on the shield, this will alter it and 
change the SWR reading.  Remember RF Spill Over on transmit is mostly vertical
ly polarized RF Spill-Up on receive that is more susceptible to noise and 
lowers F/B ratios, decreases potential nulls and creates potential RFI.  I 
knew one high power ham with a fish tank with something electrical devices in 
it and his fish kept dying (RF Spill Over Cancer).  
      To illustrate how bad Gamma RF Spill over can get here is an example.  
A W6 ran 25 KW, his neighbor called him and told him his mast was on fire.  
He had his wife modulate the rig and watched.  The DE was mounted right near 
the mast.  His gamma match system had so much RF Spill Over, 1/4 WL down the 
mast the coax was arcing to the mast due to RF spill over.  This was the high 
voltage area of the RF Spill Over.  Even though the mast and the coax shield 
emerged at the same point and potential at the top, the differences in the 
mast and coax diameter altered the RF Still Over and caused enough RF voltage 
difference between them 1/4 WL down and it arced.  This arcing potential is 
there all the time with 1 KW but not enough to arc through the thin coax 
insulation.  This arcing and some other problems went away--when he installed 
a 200 ohm T-Match.  Barrow a 25 KW rig to test your system.  It shows up all 
kinds of deficiencies.  
      Learn how to control RF Spill Over and signals improve and you will 
hear more of the weak ones.  That's one reason why I like open wire line 
and/or balanced 100 ohm coax into a Johnson MB in the shack.  Create 100 ohm 
feedpoints for the latter.  Lengthen the DE and add series Xc.  High Rr 
feedpoints decreases feedpoint losses also.
 K7GCO   >>

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