BUILD a TURNSTILE 75M ANTENNA
by Tom, K1JJ
Brentina's (W1IA) successful 100' high loop project inspired me to do some
antenna thinking here for "local, omni-directional coverage. His loop
appears to have filled in the pattern nulls seen by most dipoles off the
ends. To keep up with the Tinas and PT Cruisers of the whirl, I'm working on
a new 75M antenna project today for "local" coverage...
I'm also motivated cuz one tree support snapped on my existing 50' high
local dipole and I'm tired of working guys off the ends of this dipole and
seeing 10-20db nulls into certain areas. Whenever someone from Florida
breaks in I think, oh darn, he's off the side of the antenna - PW
time....SO.... here's the solution : a Turnstile -
Take two standard coax fed dipoles and mount them at the same height and at
right angles, feedpoints next to each other but not connected - They look
like a "+" plus sign looking down from above. Feed one 90 degrees out of
phase from the other - IE, feed the second one with an additional 42' length
of coax to produce a 90 degree phase delay.
The result is an almost perfect 360 degree HORIZONTAL omni-directional
pattern within 1 db! The vertical take-off angle is the same as any dipole
based upon height above ground.
The compromise is that a standard single dipole has about 3.1 db gain over
the turnstile in the BROADSIDE direction, BUT, the turnstile does not have
the 10-20 db nulls off the ends and corners like a flesh-shushi single
dipole has - just whirl-wide circular coverage.
So, I plan to mount the feedpoints at about 100' and the ends at about 80'.
It will be interesting to operate knowing that the signal is now virtually
equal in all directions and still has a reasonable take-off angle for
farther away contacts.
Will be PT Cruising and kaleidoscoping for reports over the next few days.
Your turnstiles is a phenomenal success . The difference between your
weakest and strongest antenna tonight was on the order of 24DB. The
turnstiles was the winner at my receiver (75A-3) with a pretty good linear
The best of luck with it and it was fun working you (and the rest of dem
Joe Cro N3IBX
Steve - WB3HUZ wrote:
How are you producing the phase shift between the two dipoles? Coax section?
Can you switch it?
Yo Big Country,
I have it all hardwired as a turnstile with no switching at the moment. A
coax section is used for the 90 degree shift.
Here's how the two dipoles of the turnstile mounted at right angles, 72'
high are fed:
From the shack I run 75 ohm hardline up the tower to 72' high and into a 2:1
step down torroidal balun. The balun's output is now 37 ohms. The balun
output connects directly to the first ~75 ohm dipole. At this same point I
have a coiled run of RG/11U 75 ohm coax, 42' long (90 degree phase shift)
that then goes to the second ~75 dipole. Since both dipoles are about 75
ohms each and electrically in parallel, they combine to make a perfect ~37
matched load for the balun and a 50% power split.
I checked the phasing line length with the MFJ analyzer to make it 1/4 wave
at 3860 as well as the element lengths. The dipoles are essentially flat and
at perfect right angles. All of the above is important for symmetry, for if
not, the pattern will not be a nice circular omni, but rather a distorted
one. The swr seems rather broad across the band compared to a standard
dipole for some reason and is around 1.2 :1 on 3860 for a 75 ohm match.
Yesterday I was testing it on receive and found Irb in NJ to be +30 on the
turnstile and only S9 on the high quad loops to Eur. These kinds of tests
tell me it really is a great high angle, local antenna. Station tests from
other areas like Canada and VA seem to show it is indeed omni when compared
to other known antennas here.
Highly recommended antenna for local, general coverage.