Thanks for sharing be that. Just fascinating!
73
Mike
W4AAW
see www.remotecontesting.comOn Mar 31, 2017 12:14 PM, TexasRF--- via TowerTalk
<towertalk@contesting.com> wrote:
>
> 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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>
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