On 3/31/17 7:42 AM, Kelly Taylor wrote:
Here’s a thought: if one can use a fan dipole to make an inverted vee
cover both 80 and 75, how about a fan reflector?
It would be a bit tricky.. in a Yagi you rely on the resonances of the
elements to couple energy between them with the correct magnitude and
phase so that it "forms a beam" in the right direction.
If the frequencies of interest are far enough apart, you can make an
"interlaced" Yagi (e.g. the Force 12 multiband units).
But for 80 and 75, they're fairly close... the reflector for 75 might
interact with the driven and reflector for 80, etc.
Not to say it can't be done... that's what optimizers and NEC are for -
when you have multiple interacting resonances, you just let the computer
grind away and figure out how best to get there.
As I think about it, here's a case where shortened and loaded elements
(which have narrower bandwidths) might work better - the element BW is
smaller, so less chance of interaction.
The other approach would be to do an all driven array - two wideband
elements (e.g. fans or cages) and feed them with two different phases..
for wires you're probably not rotating it, so separating the two
antennas by 1/4 wavelength (60-70 ft) and driving them 90 degrees out of
phase would work fairly well.
The feed could either be done by analyzing the impedances and picking
just the right lengths of coax (a'la Christman feed commonly used for 2m
and 440), or with some sort of hybrid coupler (as used in 4-squares) and
3/4 wavelength lines.
(or, of course... two amplifiers, two adjustable matching networks, and
low level phase/amplitude adjustment feeding the PAs)
Or, one could make a switchable loading at the center - 1 wire for each
element, and they're either both 80 or both 75.
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