[TowerTalk] 80 Meter beam

StellarCAT rxdesign at ssvecnet.com
Fri Mar 31 17:26:46 EDT 2017


"It is a bit unnerving to be at 190ft and hear a prop  pitch motor screaming 
as the antenna suddenly moves about 90
degrees!"

a day in the park Gerald ...

Gary
K9RX

[says a guy that can't make it past about 40' :<)



-----Original Message----- 
From: TexasRF at aol.com
Sent: Friday, March 31, 2017 12:14 PM
To: jimlux at earthlink.net ; towertalk at contesting.com
Subject: Re: [TowerTalk] 80 Meter beam

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.

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