FEEDLINES AND AMPS

[John W. Brosnahan]

Eric, I couldn't agree more on the aesthetics of beautifully done open wire
line.  The Antenna book by Laport (RCA-1951 or so) is great.  It has wonderful 
pictures of big feeders and huge phased arrays with workers suspended on
ladders from the antenna elements. There is a great picture of a guy using  
the spreaders on an open wire feeder as a ladder.  And everybody is wearing 
fashionable hats from the era.
 
The ionospheric heater at Arecibo, located about 15 miles from the big
dish uses 32 (now probably 64) wire log periodics pointing vertically
that are fed with open wire feeders in the sub-sub-feeds and with simulated
coax (a number of parallel wires for the center conductor surrounded by
an even greater number of wires for the shield) in the sub-feeds.  The 
main feedlines are made of two diameters of irrigation tubing.  These
main lines were initially made of even larger simulated coax but the
birds kept getting into the cage and at 150KW on each line the bird
population went into decline simultaneously with increased smoke in
the vicinity.  It is possible to use wire construction to create an
unbalanced line that will match directly to 50 ohms but it requires a 
lot more wire.  Probably at least three for the center and six or more
for the "shield".   But you still have the ice problem and the precipitation
static problem.
 
The aesthetic issues I have to be concerned about are the ones associated
with open wire lines radiating out from the house in all directions and
the propensity for my wife and the deer to walk into them as well as being
generally annoyed by them.  To reduce the physical aspect of this problem 
would require using support poles at least 10 ft high (or even higher in 
order to get the concrete trucks in for the next tower(s), but this just
makes the aesthetics worse.
 
The radius rule (2.c.) for CQ reads.  "All TRANSMITTERS must be located
within a 500 meter diameter OR within the property limits of the station
licensee's address, whichever is greater.  All antennas must be physically
connected by wires to the transmitters and receivers."  
 
My AMPLIFIERS are probably just over 500 meters apart, although the exciters
(TRANSMITTERS?) are co-located within 10 meters of each other.  But 
everything IS within the property limits of my address.
 
The intent of this rule was to keep multi-multis from being "created" by
a number of different operators each using the same call while operating
from their home stations on separate bands.  They would have the
advantage of not having to solve the interaction problems that everyone
else (in the multi-multi class) faces.  My setup maintains all of the 
interactions  "problems" and adds even more technical "problems" with remote 
control of the amps.  (My 8031s really do just what the Alpha 87As do (except for the 
automated tuning) but add some additional diagnostic features.
 
NOTE:  I substitute the word "challenges" for the above "problems".
 
You can reduce the necessity for autotuning by making the amps single 
banders and by reducing the Q to 3 or 4. Of course you need Low Pass filters
to meet the purity regulations, but you need to exceed the regulations 
for harmonics in any case for a multi-multi station in order to 
reduce the interaction between bands.  You get the impedance transformation
required  (remember, the maximum ratio between plate load impedance and
the 50 ohm output impedance that can be matched is the square of the
Q of the pi-network) by using multi-step transformations.
 
The technical difficulties are just opportunities to show your
engineering capability the same way that the contest itself is just a
way to show your operating ability.  (If only my skills were a little
more balanced.  I would trade some technical capability for operating
ability.)
 
(If I can get all the good operators to work on technical issues maybe they
will be too tired to actually operate and I will have a chance!!)
 
73    John W0UN