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For our fourth balloon flight, we wanted to use the VOR navigational
beacon to try and determine the position of our payload.
We needed a) good omnidirectional coverage, b) horizontal polarization,
and, of course c) extra lightweight construction.
Using a simulation program (nec2) I tried several combinations, like
crossed dipoles, fed at 90 phase difference, but most of these, without
an extra groundplane, did not give a nearly optimum radiation pattern.
The crossed dipole configuration had very poor performance at the
horizon, where sensitivity was important.
Two square loops (probably two circular loops give the same result)
were much better, and the slight phase difference between the lower
half and the upper part caused a very small preference downwards.
If the squares are then elongated vertically, things get even better.
The radiation pattern at 60 degree angle gave a 1.5 dB gain, while
at a slightly smaller angle still (50 degrees) the gain rose to 1.7 dB
or so, and the impedance declined to 33 Ohms - symmetrically. This
gave me the idea to use a 1:9 balun to 300 Ohms, and then do the
90 degree shift with a 300 twin lead (read lightweight) 1/4
section.
Paralleling the two feeds lower the impedance to 150 Ohm, which can
easily be obtained from the beacon.
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