>"Ben Dawson" <dawson_at_hatdaw.com> writes:
>> It is perfectly possible to use separate amplifiers for each element which
>> requires positive power flow, and dissipating resistive loads for the ones
>> which have negative power flow. In fact we just designed a system for an
>MF
>> array in which 15% of the power is consumed in the negative tower load.
>The
>> stability and bandwidth are better than for the case where the negative
>> power is put back into the system bus by a "re-entrant" circuit. (Similar
>> to putting the termination load power back into the system in a Rhombic
>> antenna. Negative elements have modest power flow in typical systems.)
>
>It would seem, though, that if you are operating in an environment where you
>are transmitter power limited (either by power available, or by regulatory
>fiat), one might want to avoid dissipating some of that power in a load. I
>suppose, in the regulatory case, it depends on where the "measurement plane"
>is for transmitter power. That is, if you dissipate power in a load
>internal to the physical transmitter (say, in a pad used for power trimming)
>is that any different than dissipating power in a load external to the
>transmitter.
>
>This brings up an interesting optimization constraint for a phased array.
>If the goal is to maximize the radiated power in a given direction, AND
>you're using an array of amplifiers, the optimum phasing might not be the
>one that would produce the highest "antenna gain" (as if you had a single
>feed). You'd want to maximize the power from each amplifier.
>
>Thanks to all for all the useful discussion.. even if we have strayed
>somewhat from NEC related issues....
>
>Jim Lux
Jim, I have lost track what the "NEC related issue" was.
NEC assumes that returned power (negative element impedance) is
recycled. Practically in a multiple fed antenna system the element's
Antenna System Tuning Unit (ASTU) by conjugate match does its best to
recycle all the returned power --- but not without some power loss,
For the crossed field antenna (CFA) the power surging back is almost
equal to the power surging out --- a terrible antenna system to tune
and match.
Finally we achieved this by using two Pi-networks (two elements to
feed) and baluns suitable placed to isolate transmitter ground from
antenna system "grounds" at the feed points. We used a Hitachi Model
6025 Digital Storage Oscilloscope. Current was sampled at the inputs
and outputs of the Pi-network tuners, using current transformers
(secondary windings isolated from tuner grounds). We could see
simultaneously the relative amplitudes of both currents (cylinder and
disc), and digitally read the time difference between the traces to
an accuracy of one nanosecond (about a degree in phase at the
modelling frequency 3757 kHz). The tuning problem was the achieve the
current ratio we wanted, we tuned for 1:1, since our purpose was to
validate NEC, the phase difference we wanted - 90 degrees, and the
combined input VSWR = 1:1. The art of tuning had to be learned.
The VLF transmitter station at Rhauderfene, GR uses eight umbrella
fed antenna elements --- no problem with negative impedances, since
all the elements were fed in phase. But for a particular directional
pattern, if one of the elements in a phased array exhibits a negative
impedance, separate transmitters make tuning easier --- you merely
tune the ASTU for each element to provide a conjugate match --- so
the transmitters see a VSWR = 1:1, it delivers the required power,
and the driving point impedance of each antenna element is what we
want (current phase and amplitude in accord with design --- phase
referenced to element in your design which for NEC was assigned 0
degrees) --- power is recycled if the input impedance is negative So
long as the element looks like it has a negative input impedance and
the circulating power is what we want, the array behaves as designed.
Jack Belrose
-- _____________________________________________ John S. (Jack) Belrose, PhD Cantab, VE2CV LIFE SENIOR MEMBER of the IEEE TA ARRL Fellow Radio Club of America Life Member AWA Life Member QCWA Athlone Fellow Radioscientist Emeritus Researcher Communications Research Centre Canada PO Box 11490 Stn. H OTTAWA ON K2H 8S2 CANADA TEL 613-998-2779 FAX 613-998-4077 e-mail <john.belrose_at_crc.ca> _____________________________________________ -- The NEC-List mailing list NEC-List_at_robomod.net http://www.robomod.net/mailman/listinfo/nec-listReceived on Wed Jan 12 2005 - 22:34:31 EST
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