Remote Amps

[John W. Brosnahan]

RE: K3NA's comments on remote amps versus transmission line tradeoffs.
 
Eric, is absolutely correct that remote amplifiers provide an enormous
number of engineering challenges, but life is full of tradeoffs and
decisions.  While I don't think I would actually bolt my Alpha to the
boom of my antenna, having a number of relatively low power solid state
amps distributed throughtout the system could reduce some of the technical
problems compared to remoting a single big tube amp.
 
My one experience with remoting a large number of amplifiers, involved
building 74 50KW pulse transmitters using a pulse rated 8877 (3CPX1500A7)
in the final.  64 of these were used at a time (the other were spares)
and each one driving 192 dipoles (4x48 coaxial collinears).  These were
"gutless wonders" to keep the cost down and resembled good home brew
practices (I built these in the basement of my home as well as the
12,288 dipole array).  Each transmitter required .25 volt drive and 
resided in its own little outhouse.  The HV (8KV and 4KV) was distributed
from four common power supplies via RG-8!!  (Don't grab the wrong coax.)
These were built in 1978/79 and ran for many years just north of 
Fairbanks, AK and then the equipment was divided up and pieces are now 
operating at a number of sites throughout the Pacific.
 
So the technical problems are not really that severe.  But it does take
extra engineering (read money) but if your technical requirements (ie.
you live in the black hole of zero land) it may be a valid choice, but
clearly not for everyone.  And probably not for me in the sense that
I won't stick a number of amps out in the weather.  But I do have four
towers that are 2000 ft from the house with a separate building for
amplifiers and I will remote the amplifiers for these antennas.  The
rig will drive 2000 ft of coax on transmit taking the 3-10dB loss 
(depending on band) with a solid state brick to bring the remaining
watts up to the 100 or so required to drive the amps.  The amps are
operated and monitored with a built-in 8031 microprocessor over
twisted pair and a PTT line will probably be used although I am
looking at using PIN diode T/R switching.  Receive preamps will
establish the S/N ahead of the 2000 ft run back to the shack.  Fortunately
many of the black boxes required to make this work have been developed
in the course of my work.  Now you know why I have used "guest ops" the
last 2 years and why I am only on 1 band at a time so far, the technical
work is quite extensive.  In addition, I don't have "big bucks" to do this
so everything takes time to scrounge the materials.
 
My point is, the technical challenges can be as rewarding as the
operating.  It just depends on where you get your kicks.  But don't
get discouraged because of the difficulty, get challenged!!  You'll
learn a lot (about things that work AND THINGS THAT DON'T).
 
Transmission line improvements are clearly a more attractive option for
many.  I also (as Eric does) especially like the 4-wire transmission line.  
It has much better balance to ground than the two wire version, so the 
potential of picking up noise and signals from the transmission line are 
greatly reduced.  At amateur power levels you can use relatively close spacing
and if you pick 2 inches on a side with #10 wire you end up close
enough to 200 ohms to use a 4-1 balun at each end of the run to match
to 50 ohm coax.  I have a 1500 ft run to my 4-square on 160 (1/2" hardline)
and lose enough power (even at 160) to consider 4-wire line seriously.
Ice can be a problem although it is relatively rare in Colorado, and
the potential from precipitation static (and wind static in dry climates
like Colorado) could be a problem.  Aesthetics and maintenence (compared
to coax) are probably the biggest negative factors.
 
My apologies to anyone more interested in operating techniques than
system design for contesting for these long notes.  I'll be travelling
for a week or so, you'll get a break!!  I just didn't want anyone to
get too discouraged by the technical problems.  Remote amps have been 
done on low budgets. 
 
73  John  W0UN  and 
 
(Eric, K3NA, I have a couple of my programs ported over to the
PC for mast design for wind load with up to a 5 stack of antennas and the
element strength program that includes ice loading and predicts the sag
of the tips.  Also have the shorted stub transmission line matching
program pretty well ported over.  I'll send you a copy in a couple  of
weeks to beta site, then make them and a number of others available as 
shareware for next to nothing.  When they are cleaned up with some
instructions, I'll let everyone on cq-contest know, in case they
are interested, hence this "broadcast".)