Though the currents in both loops are reactive, connecting them in parallel causes the feedpoint, at the top, to be at nearly 50 Ohm, resistive.
The only problem remaining, is that the feedpoint, as with most antennas, is symmetrical. This means we shouldn't connect a coax cable (asymmetric) at this point. Though the antenna will be adapted, currents on the outside of the cable will offset the radiation pattern of the antenna.
Several solutions exist:
The 'twist' generates the desired hemispherical pattern, which permits us to hear from horizon to horizon.
|ON7EQ's QFH with balun||
A simple solution for eliminating the exterior cable currents, is
to wind the cable a number of times around the vertical support,
making the outside act as an RF coke (inductor).
The diameter should be as small as is practical, to reduce radiation as much as possible. The number of turns depends on the diameter, of course, but the inductance should be large enough (compared to 50 Ohm) to reduce the outside currents.
The windings should be as near to the top as possible, to reduce the length of the radiating parts.
Many questions arise about the infinite baluns...
The impedance at the top is the one we want at the bottom too. So we cannot tolerate impedance transformation through the balun. This means we have to use a cable with the same impedance - else there would be a transformation!
For UHF and SHF work, the most practical solution is probably to use coax cable for the actual antenna itself. For lower frequencies, we should use tubing, and feed a 50 Ohm cable through it.
Part of the infinite balun principle, the cable's braid should be connected to the antenna at the bottom center. In practice I have not been able to detect much difference.
|(c) John Coppens ON6JC/LW3HAZ||post|