[TowerTalk] 80 Meter beam

[W4AAW Mike Lonneke]

Thanks for sharing be that.  Just fascinating!

73
Mike
W4AAW

see www.remotecontesting.comOn Mar 31, 2017 12:14 PM, TexasRF--- via TowerTalk <towertalk at 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 at 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.
>
> _______________________________________________
>
> _______________________________________________
> TowerTalk  mailing  list
> TowerTalk at contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
>
> _______________________________________________
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk at contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk