Thursday, December 27, 2012
The DOUBLE TURNSTILE ANTENNA
I designed these originally for contest use. The first one was for 4m. It was so successful that we used them for all bands from 6m to 70 cm. The 70 cm antenna is actually two of these stacked, making it a quad turnstile not to be confused with a turnstile quad, which uses two quad elements in an octahedral arrangement.( I even used the 70 cm one on 23 cm, but it no longer has an omnidirectional pattern at the higher frequency!) I have also built one for 10m.
Design philosophy. As we were entering the restricted section, we were only allowed one antenna per band. We were always short handed, so needed to have an antenna that was easy to erect and simple to use. The obvious antenna was an omni, because it did away with the rotator.( Less to carry, less to wire up, less to go wrong, less power required therefore smaller generator could be used and less fuel etc etc.. All in al less weight to carry up the hil! But it needed to be horizontally polarized and have gain. The 4m antenna turned out to have more gain (> 8dBi) than the average 4m beam that contesters were using at the time ( 4 el Yagi) and it had a smaller footprint on the ground. The main lobe was also at a very low angle ( about 6 deg above the horizon) and there were no minor lobes below this. So ground noise pick-up was also virtually eliminated. But the main advantage was that it stopped the operators twiddling the rotators we previously had and wasting time because they would insist on peaking up an already fully readable signal because " your not very strong old man". Another advantage was that being omni every other station heard you all the time wherever theyt were ( propagation conditions permitting) which keeps the operating frequency clear. ( More effective if running QRO rather than the (RSGB) QRP limit of 25 W that we ran. In fact we only had 10 W on 4m! It didn't stop us working Cyprus one year on 4m) Time to erect one of these antennas on site is about 10 - 15 mins for two people and slightly more than double that for a single person. In practice we actually used 3 or 4 people for safety and put them up to the regulation maximum height of 10 m. Anymore just got in the way. I regularly put these up on my own albeit at lower heights when out portable.
The design: All the elements are driven via a phasing/impedance matching harbness. This makes construction easy as all the elements are the same length. There are four half-wave dipoles altogether in the double turnstile and eight half-wave dipoles in the quad (UHF) turnstile. Cut them to size using a tape measure and pipe cutter or hacksaw. The insulators to support the elements are crucial. They need to be able to support the antenna in the correct orientation ie in a cross and attach & insulate them from the vertical boom. Most of the phasing harness is also enclosed in the boom. This is for aesthetic reasons and electrical reasons - no radiation/pick-up on the harness. ( The UHF version is different in that I was unable to cram the harness inside the pole without compromising the antenna with thin lossy co-ax.)
The original design used a rather mechanically and electrically weak joint in the co-ax, which was buried in the middle of the supporting pole (boom). The design has since been replaced by the one shown, which only has connections at the elements and below the support pole, where they are accessible. The insulators were originally commercial crossed dipole centres (originally manufactured for Band I TV antennas), with the centres machined out on a lathe, to take the support pole.These are still available, but are relatively expensive. For the larger antennas on the lower frequencies, I used some 40 mm plastic pipe clamps which were originally part of the packing for some hand rails. Extra support was added in the form of hardwood strips about 40 mm wide by 5 mm thick and about 300 mm long. The elements were cable-tied to these. Metal clamps and plastic insulating strips could be used, but I would avoid Perspex. A suitable source of plastic are polypropylene/polyethylene kitchen chopping boards sold in hardware stores etc.
The coaxial harness introduces the correct 90 deg phasing between the crossed dipoles in each pair and keeps top and bottom pairs in phase. It is important that the elements are connected as shown ( ie don't interchange braid and centre conductor). I normally leave the antenna with stubs connected ( 10, 6 & 4m) to the phasing harness; the commercial centres uysed in the 2m version can be left wired and just the elements removed using the wing nuts. The 70 cm version is so small that this is easily transported on a roof rack without any disassembly. There are two sections of 75 ohm coax ( ordinary low loss TV coax can be used here) which act as quarter wave transformers. The velocity factor of the cable needs to be taken into account when measuring these. The effect of these is to give a better match to the 50 ohm coaxial feeder line, but it isn't perfect. Do not attempt to fiddle with these lengths or the lengths of the elements to get a better match as this will destroy the radiation pattern. It is pretty close to circular and the take-off angle just above the horizon to take advantage of normal tropospheric range enhancement due to refraction. The higher lobes ( see radiation pattern diagrams ) are useful if there is any Es or F1/F2 propagation (mainly 10 & 6m), as they can enhance single & chordal hop propagation.
Lastly the 10 m version is included here. Not really VHF, but close! A beam to give you 8-10 dBi of gain is serious engineering and expense. The double turnstile I made for this band cost me about £30 in new aluminium in 2001 and has about the same erp as a 400 W linear to a dipole, when driven by Joe Averages 100 W Tx! Plus you get all that gain on receive. Each element is 2.5 m long; cable lengths are: 1 lambda = 6.66m; 1/2 lambda = 3.33 m, 1/4 lambda = 1.66 m. The top of this antenna should be guyed with non-conductive guys (nylon or polypropylene).
All these antennas should be modelled against real ground to get the true picture. As free space antennas they look pretty poor. Best results will be obtained if you can mount them on top of a conductive cone-shaped hill, which is what we had at Burton Dasset contest site. This ultimately led to the omni flyswatter for 23 cms and above, though it also owed a lot to an airfield lighting design I was involved with in the 1980's.
at 1:51 PM