[TowerTalk] 75 ohm CATV ?? DIP or PRUNE ??

[K7GCO@aol.com]

TT:  Please delete previous posting of a few minutes ago.  I failed to do a 
spell check.  k7gco

 << In a message dated 11/14/00 8:46:25 PM Pacific Standard Time, 
kb7ww@chatusa.com writes:
  << 
   My 6 meter antenna is mounted at 75 feet on a Rohn 25 tower. The
   feed line from the antenna to just past the rotor is RG 213, then 
   down the tower 70 feet of 1/2 inch 50 ohm heliax, from the base of 
   the tower to the shack 110 feet of RG 214. 
   
   I would like to replace the 110 feet of RG 214 with CATV 3/4 inch.
   
   Now for the question, should I dip it to correct length or prune it
   for best VSWR at the shack.  Also why do you feel this way.  By the 
   way the VSWR is now about 1.2:1 at the shack.
   
   Thanks guys
    >>
  Question #1. What is the purpose of grid dipping it?  Both ends have to be 
shorted for grid dipping and it would grid dip some multiple of 1/2 waves at 
some frequency unless pruned to 50 MHz.  If you had a low SWR what are you 
trying to gain?  RULE OF THUMB: "Feedlines should be just long enough and no 
joints to reach the shack--no shorter--grid dipped or not!"  I have a magic 
length I use frequently cut initially that is a 1/2 wave or multiple 80-10M.  
I cut it for 3.562 MHz initially as a compromise frequency.  For .66 VP coax 
it's 91' 2" and it's handy if I decide to do phasing of verticals or beams 
initially or later.  I have a coax log and I know how long all my feedlines 
are and check their loss every year.  If I double the length it's a 1/2 wave 
at 1.7813 MHz.  If phasing verticals on 160M it doesn't matter if you operate 
at 1.825 if both are the same length for the initial matched lengths.  If I 
operated only on that band I might cut both for 1.825 MHz.  What are you 
trying to do or minimize?  You are not phasing another beam so it can be any 
length that just reaches the shack.  
  
  Question #2 Why prune for the best SWR?  In reality that doesn't really 
work unless the coax is very lossy or you have coaxes other than 50 ohms in 
the line and that's not good practice.  There are cases I found of improperly 
shielded bridges where RF spill over got into them, affected the SWR reading 
and it changed with different lengths of coax.  I've tested identical bridges 
that even gave different resonant frequencies.  What ever bridge you have 
obtain a duplicate and compare.  I've had identical bridges where one 
actually changed the apparent resonant frequency as I changed the feedline 
length and the other didn't.  
  
  You have to have a length longer than needed to prune it and that's very 
wasteful for really good coax.  It's true that coax loss will reduce the SWR 
but so what?  That's a dumb way to do it.  Rule of Thumb: Use the least coax.
  
  Suggestion: If you have 1.2:1 SWR in the shack on 6M you probably have 
1.5:1 or higher at the beam.  If you want to prune something--prune the SWR 
to 1:1---at the beam.  I'll guarantee you it will stay 1:1 all the way to the 
shack regardless of the length unless you have a bum bridge or coax which can 
occur.  I don't have my coax charts here and I presume RG 214 is 50 ohm coax 
also.  Forget the 75 ohm coax as you are introducing an impedance variable .
  
  I never have 2 or more different types of coax to an antenna as this means 
2 male connectors and a barrel or a male and female Type N for each joint.  
Avoid unnecessary joints.  Water often finds it's way into them.  On VHF&UHF 
I could have a long length of very low loss 50 ohm coax to the beam and a 
short length of coax for beam rotation.  The coax joint will be well taped.  
Or if I had a coax switch at the top of a tower (well covered) I'd run a low 
loss coax to the switch and then normal coax from the switch to various 
antennas.  
  
  In this day and age there is absolutely no excuse for having a beam with 
other than 1:1 SWR at the beam at the desired frequency and a matching system 
with none of the "DREADED RF SPILL OVER" on to the coax shield and tower.  If 
you have a beam with other than 1:1 SWR at the desired frequency, learn how 
to adjust it or--send it back.  If off frequency where say the SWR is 1.5:1, 
if you place your "Magic RF Hand" on the coax shield right at the feedpoint 
and the SWR changes on the MFJ analyzer, you have the "Dreaded RF Spill 
Over".  It affects F/B and picks up vertically polarized noise also.  I've 
even seen this happen even with 1:1 SWR on some bum matching systems.  1:1 is 
not necessarily pure.  Send the beam back and suggest a better design--it 
actually exists.  Worry about important stuff.  Forget pruning (which 
requires excessive coax), grid dipping and excessive coax splices.  Resonant 
lengths are OK but really necessary only with phasing lines feeding stacked 
beams or phased verticals.  If you have 1:1 at the beam and with even 
lossless coax of any length, you will still have 1:1 at the shack--with or 
without pruning.  Trust me.  
  
  I've talked to manufactures for years about clean matching systems.  Some 
listen, some don't.  Some know how to do it and some don't.
  
  I have a big roll of 75 ohm coax I'd like to use.  To justify it's use, it 
has to be properly used.  When I do I will have an antenna with a 75 ohm 
matching device and No RF Spill Over.  I will have an L network in the shack 
to match it to the 50 ohm rigs I have.  
  
  I modified the RF Wheatstone bridge in a MFJ Analyzer for 75 ohm coax.  
This is needed if you want to properly use 75 ohm coax.  SWR readings with a 
50 ohm bridge using 75 ohm coax will not be accurate.  
  
  75 ohm coax can be used for matching 50 to 100 ohm loads.  There are other 
tricks but are seldom worth the bother even if you knew how to do it.   K7GCO 
>>
  >>

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