About 30 years ago I built a 3el 75/80m vagi on a 70ft 45G boom. It was
installed on a 190ft 65G clone tower. Yes, it was heavy, over 1000 pounds.
All three elements were 150 ft long, highly tapered, insulated from the
boom and open in the center. With the open shorted with power relay the
elements were resonant near 3.8 MHz.
There were additional relays that switched in a small inductor to lower the
frequency for the reflector element and a capacitor to raise the frequency
for the director element. The two parasitic elements could be switched
from phone to cw, forward or reverse direction. The driven element had more
relays to switch coax stubs in/out to take care of the phone/cw swr changes.
It was a fun antenna to operate. The antenna stayed up for about 25 years
but got little use after I got bitten by the EME bug.
The first rotator was one of the large Telrex units. It lasted about a week
before a wind storm destroyed it. The second rotator was a large prop
pitch unit. It stayed together but I had to find and install an original disc
brake to keep it from windmilling.It is a bit unnerving to be at 190ft and
hear a prop pitch motor screaming as the antenna suddenly moves about 90
degrees!
73,
Gerald K5GW
In a message dated 3/31/2017 10:41:13 A.M. Central Daylight Time,
jimlux@earthlink.net writes:
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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