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[TowerTalk] Re: Coax and tuner losses

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Subject: [TowerTalk] Re: Coax and tuner losses
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Date: Tue, 16 Nov 1999 17:54:16 EST
  I would suggest that hams measure the loss of your coax (per 100" at 29 
MHz) and you will get some surprises.  For RG8 .66 VF the loss should be no 
more than 1 dB/100' at 30 MHz.  Measure the VF also.  Remember this loss is 
only at 1:1 SWR.
     There are tuners on the market with even more than 30% loss with certain 
loads.  You have to understand that many mfg. rely on the ignorance of the 
customer not to notice certain deficiencies as too many customers look at 
price only.  After a transmission I check the inductors of my tuners with 
full power and they are stone cold.  I can even run 600W SSB on 10M into the 
275W AM rating Johnson MB on 10m with just perceptible heat and no arcing.  
If you can't feel any heat at a KW (after you transmit) you can assume 
negligible loss.  Remember the heat loss is distributed over the full length 
so the loss has to be real high before you notice it.  With sharp bends there 
can be center conductor shift if over heated.
      So few hams ever see anything properly designed they spend too much 
time disputing performance levels others enjoy all the time with techniques 
from the 30's and for reasons best know to them--never try and duplicate it.  
I suggest that until they do--stop disputing things they have no knowledge of 
and when many are telling them they are wrong.  Don't they ever get tired of 
being wrong?
      To show you how far the mfg.'s go in compromising design for the so 
called new hams here is a prime example.  Linears are set to what is called 
class B for SSB to get linear amplifier of the SSB signal applied to the 
input.  The linears run about 60% efficiency.  When you are not talking the 
idling current of one tube wastes about 30-40W.  2 tube linears have been 
known to heat up a shack in the summer.  In the old days Power Amplifiers 
were run Class C on CW.  The bias was higher and with the key up there was 
zero idling current--zero heat loss and waste--inaddition to more power 
out--about 15%.  This is the amount lost in 100' of RG8 coax at 29 MHz just 
to put it in perspective.  The CW Power Amplifiers of today are still biased 
to class B on CW requiring you to waste more input power to get the legal 
limit out.  They didn't want you to have to throw a switch from class B to 
class C for CW.  In a KW amp that convenience cost you about 150W of wasted 
power on transmit when you are on CW.  You pay for operator convenience.  
      However there has been a design break through of the highest order in 
the MFJ AL-80B linear.  During voice pauses (or when the power output is 
lower than 100 mw), or long listening periods, the idling bias is higher and 
cuts the idle current to zero.  The tube runs much much cooler and saves you 
money and extends tube life.  
      That circuit is available from MFJ to those who want to use it in their 
amps.  It works the same way on a class B amp on CW. I'd just as soon it not 
be used on CW and just increase the bias like the old days which increases 
the output and reduces wasted heat 2 ways.
    The AL80B also has the ALC circuit in the grid circuit where it should 
be.  Others have it in the output circuit after the amp has got up a head of 
steam figuratively speaking and the ALC has to then wipe the sweat off the 
tube's brow.  This ALC circuit in the grid acts as a clean compressor to the 
exciter SSB drive in a very effective way.  They also have done something to 
increase the efficiency of the amp (I might add an increased bias switch for 
CW myself as I like perfection).  I'm so impressed with this fresh design 
improvement in linear amplifiers I'm going to sell my 30L1 and get one as 
soon as I get back to SD.  This amp really talks on SSB and for a great 
price.  I'm told by the mfg. that I was the only one to object to the class B 
operation on CW.
     There was an article in CQ years ago called the Class C Linear.  It 
operated at Class C efficiency and was still linear.  I made it.  I sent this 
to 2 mfg. and suggested they use it and they didn't.  QST had a lower power 
amp article that operated at 85% efficiency--Class Z I think it was.  No big 
heat sinks were needed.
      This all illustrates that hams live in a sea of inefficiencies.  
Disputing tuner and open wire efficiencies is like all those who holler about 
increased ATM charges and not hollering about the excess big taxes they pay 
and vans that get 6 MPG.  I could mention some other inefficiencies in ham 
radio but I'll save that to spoil another day.
 Long live high efficiency open wire line and linear-CW amp designs.
  It seems we often get fooled by such things.  I remember acquiring a 
  bunch of old but apparently good RG8 coax.  Before I ran it "up the 
  tower" I decided to test it at 29 MHz.  I put a dummy load on one end, 
  and my Bird 43 and transceiver on the other end.  I sent 100W down 
  the line (I couldn't measure any Watts coming back).  Then I moved 
  the Bird to the dummy load end, and keyed up.  Only 85 watts were 
  making it down there! "That's a fifteen percent loss!  This stuff is 
  junk!"  I then checked the manufacturer's specifications... the coax 
  was in spec. for NEW cable!
  Fifteen percent = about 0.7 dB loss for however many feet I had.  
  Gee, 0.7 dB doesn't sound like as much as 15%!
  (Note! this was for about 80' length and is about correct.k7gco)
  George T. Daughters, K6GT
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