Design
Designed as a vertically mounted dipole and reflector two element.
Two units mounted at right angles and fed 90 degrees out of phase using a quarter wave of coax.
Design optimised on polar diagram and gain.
Construction
The version pictured is constructed from 8mm microbore central heating pipe and 40mm waste pipe easily
obtainable from DIY shops. A vertical dipole with elements made from two strands of 12swg copper wire
twisted together is mounted on the end of the horizontal section of pipe.
Construction Notes
These notes are brief as there is nothing especially taxing with the construction...
It is advisable to fit a short length of plastic rod between the dipole halves - if necessary
bulk with insulating tape to make a good tight fit
Secure the dipoles in place with zip ties either side of the pipe.
An inspection cover at the top of the tube allows the connections to be made easily using solder tags
and self tapping screws then popping on the cover makes it waterproof.
Secure the reflectors in place with either self tapping screws or zip ties.
A "dirty balun" should be formed by taking the coax out from the inside of the tube,
wrapping it around 5 times and then taking it back inside. The necessary holes should be marked
and drilled before connecting the coax.
Polarisation
To achieve the necessary right hand circular polarization one dipole of the pair is
fed through a quarter wave of coax from the feed point of the other. This diagram should
make plain where the inners need to go. Getting this wrong will result in left hand circular
polarisation and an antenna that doesn't work.
Dimensions
Dipole: 99cm
Reflector: 106cm
Spacing: 39cm
1/4 wave delay line - I used 75ohm TV Coax and using the MFJ analyser the length worked
out to 39.5cm. EZNec plots
Performance
I have stopped using the vertical dipole as a secondary antenna in the diversity mode supported by my
receiver. Although at low elevations it provided a higher signal level than the turnstile it seems that
much of that signal was noise as the image quality in the extreme north and south were better without it.
This second picture of the antenna mounting will go a long way towards explaining why there is plenty
of noise evident on the top of the images - that chimney is due north of the antenna. An alternate mounting
position is desireable but currently not possible. I will be trying my hand at constructing a quadrifilar
helix or QHA next to see how that performs. I will probably be trying the tall design which has higher gain
at low elevations and using the turnstile as a diversity antenna as that performs well at higher elevations.
Update
I have now built a QHA and taken the opportunity to incorporate the turnstile on the same pole and relocate
it. The new position alters the angle of the chimney away from due north to west of north. Results so far operating
the turnstile with the QHA are looking better than with the turnstile and vertical in the old location.
See the QHA page for a picture.
