A couple of closing comments on the multi-amp discussion. I don't believe
the contest rules specify a maximu of 1500 watts output, only that the
participant will be limited by the rules of his licensing authority. I
don't know the power limit in EA8, but I'll bet that it may be legal in
a number of countries to run more than 1500 watts.
The comments by Steve, K0SF may well be true, but running multiple sources
for large phased arrays is not as difficult as he seems to suggest, at least
not at HF and VHF. The issues of mutuals and driving point impedances and
all of the other factors are there whether you use multiple sources or not.
Typical ham phased arrays have long been poor on guaranteeing proper
CURRENT distribution. The work by W7EL and others as described in the
most recent edition of the ARRL Antenna Handbook show some ways of
forcing currents to be equal in the elements of arrays
with different driving point impedances. (The four stack of 8 element
28MHz Yagis that I presented at Dayton last year forced currents to be
equal by using the property of the 1/4 wavelength transmission line that
forces equal voltages at the paralleled ends to have equal currents at the
termination ends regardless of the termination impedance. I know this
array works properly by using 10M EME as a calibration source and getting
returns that are as predicted for the path loss. I now have a Four by
7-element array on 15 meters and I am getting EME echoes here also. ALL
WITH LEGAL POWER!!!!!!!!!!!!!)
With proper design of a multi Yagi phased array at HF, the use of multiple
amplifiers presents no major technical problems. But even if the phases
of the amplifiers are 180 degrees apart you won't get a "bulbous pattern".
You'll get a pattern with the main lobe somewhat elevated and the higher
angle sidelobes (H-plane) will increase somewhat. This feature was used
in my 10M array to steer the lobe vertically by inverting the phase of
either the 3rd Yagi or the 4th Yagi or both the 3rd and 4th Yagis.
Gain is reduced by 1dB or so (it goes into the sidelobes) but the array had
greater than 20dBi gain (with ground reflections) and dropping down to 18 or
19 dBi but at a better elevation angle was a worthwhile mode of operation.
This technique has long been used in the U/L/BIP/BOP switching in 2 stacks.
(And is now used in my 15M array where Upper is the two upper 7s and Lower
is the two lower 7s.)
In summary Feed your antennas with equal lengths of coax that are odd
multiples of 1/4 wave in length and tune your amps for equal output
VOLTAGES of the same phase and you are in business. (Even if things aren't
perfect that power will go somewhere. In the case of contesting it doesn't
matter as much where it goes as it does in an FM/CW radar.)
HOT TOPIC FOR THOSE THAT READ THE ENTIRE MESSAGE!!!
But this discussion does raise an interesting point. I had long wondered
if maybe I should mount my amplifiers at the feedpoint of my antennas
(or at least at the top of the tower) to reduce feedline loss. But the
FCC doesn't state where the power should be measured. What if I stick
my watt meter at the top of the tower?
This topic is expressed in a very interesting but short section
in Dave Leeson's book on Yagi design.(Sec 10.9 page 10-18).
Dave said he has had informal discussions with several FCC staff
members and "I think the answers I got mean that the power is
measured at the antenna, as it is, for example, at AM broadcast
stations. It appears that if the rules are silent on a subject, the FCC
does not quarrel with the licensee's reasonable interpretation of them."
Now remember that this statement is printed in a book published by the
ARRL and could be considered as approval by the ARRL of this
interpretation.
One thing I have been impressed with is with the various operators
of W0UN over the last two years (W0UA, N2IC, K0GU) and their interest
in fair play. They have all been religious in keeping the Alpha at
1500 watts maximum. But with big arrays you have long feedlines and
relatively higher losses, maybe an Alpha 87A isn't big enough to overcome
the feedline loss in such a situation. But I also seem to recall
something about the FCC refusing to type accept a 2 hole 8877 Alpha 77S
a number of years ago, demonstrating that there is some limit in what
they will accept.
Comments anyone? John W0UN
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