The start of the project
Sergey UA2FW called me on phone one evening in late August 2020 and invited me to the group of fine operators for the CQWW CW 160m 2021 CONTEST. He also asked if I was willing to propose and design a transmitting antenna that would be as good or better than a regular 4SQ and also help out with layout of a transmitting and receiving sites on the available property. I agreed on both invitations and started to work on the antenna design.
The choice in favor of a spitfire
I won’t describe in details how Sergey and I came to the decision of building a totally NEW TX/RX 160m site at UA2FW’s for the CQWW 2021 contest, neglecting already existent and fully available site with a full size 4SQ with many beverages around but defeated with inevitable man-made noise and not a very good terrain profiles in main directions. The good thing about this project was availability of lots of land (some 80 acres or so), so land wasn’t a limiting factor whatsoever. In this article, though, I will rather concentrate on describing the design of transmitting antennas.
Making a choice on the type of the transmitting antenna was a rather interesting process. The limiting factors were these: from the practical stand point – we only had 5 months to design and build an effective TX antenna on 160m band – it had to be a vertically polarized antenna, the antenna must feature the ability of simultaneous transmitting in at least two directions of the operator’s choice with gain exceeding that of a regular 4SQ, the azimuth pattern must cover all major target areas such as North America, Caribbean, South America, Japan and South-East Asia with Australia and New Zealand behind, Africa with Middle East and the deep Pacific Ocean.
The “must” for any two direction simultaneous transmission was the hardest feature to satisfy. Once I realized that, I came to the crucial decision – the transmitting antenna must consist of two separate antennas. The easiest and a straightforward solution to the dilemma was a pair of 4SQs. But what about gain that must exceed that of a 4SQ?
I had once built a two element vertical array with one half of wavelength separation between the elements. It was in mid 90s when we were active as RZ9AZA. We were either feeding the elements in phase or out of phase (180 deg. phase shift) and the antenna was a real killer on Top Band. Remembering that experience, knowing of the VY2ZM’s approach at his magic QTH on Prince Edward Island I at one point thought of two spitfires horizontally stacked at 0.5 wavelength. I immediately got to the computer and quickly modeled the antenna. Man! Did I love the numbers! 8.5+ dBi of Gain versus 6.2+ of a regular 4SQ with reasonable front-to-back ratio exceeding 25 dB at design frequency segment. So, instantly I knew I found the solution to the task.
The spitfire antenna
In this chapter I will explain why the parasitic elements – director and reflector – were made of folded elements. Let’s see what a regular spitfire looks like and how implementation of folded parasitic elements improves its performance characteristics.
In the example down below I will first deal with typical spitfire antenna made of driven element 49.266 meters high 200 mm OD (just as it is in the erected antenna) with parasitic director and reflector both made of single wires 4 mm OD. See it on the picture down below.
I optimized the antenna within the frequency range of 1810 kHz to 1900 kHz. The optimization goals were – input impedance 75 Ohms, equal Front-to-back numbers at the band edges (1.810-1.900 MHz). The parameters of interest to be analyzed are – 1.5:1 VSWR Bandwidth, Gain curve, F/B curve. See those down below on three Figures. (The name of MMANA file for this one is ” SPITFIRE 160 REGULAR PARASITES.maa”)
Well, those are pretty neat curves and numbers for such simple antenna, aren’t they? But let’s see what it looks like with parasites made of folded elements with wire of 2 mm OD. (The name of MMANA file for this one is ” SPITFIRE 160 WIDEBAND PARASITES.maa”).
One can see a noticeable difference in favor of the latter model – the 1.5:1 VSWR bandwidth has risen to almost 100 kHz, Gain curve shows a 0.1 dB increase and the F/B is also higher at the edges of the frequency range by more than 3 dB and so within the band. So, that was the reason why I had chosen this configuration for the parasites.
Stacking two spitfires side by side
The stacking distance was an easy case. I knew from the books that for two stacked elements fed in phase the compromise between clean main lobe pattern with minor lobes under 13 dB and highest gain was laying within 0.65 -0.75 wavelength of spacing. And indeed! The MMANA modeling shows a 9.3 dB of gain with -3dB beam width of 41 degrees at spacing of 0.75 wavelength. On the other hand, for instance, the USA from Palm Beach FL. to Seattle WA. is seen from Kaliningrad within segment of azimuth angles of 41 degrees. And we know when working the USA on Top Band every 0.1 dB counts. So, I just made a decision that the spacing will be 85 meters. Not 95, not exact half wavelength of 82+ meters, just 85. The beam width with that spacing is 55 degrees with gain figure still showing a high number of 8.6+ dB. See the pattern on picture below.
Actually, this pattern is very similar to that of a 5 element 20 m yagi on a short 12.5 meter long boom.
And yet, if pointed right in the middle of the USA segment the states of Florida and Washington would receive 1.7 dB less power from the antenna than the Mid West states and that was a no-no situation for me. I just thought of HF band operations with highly directive antennas where an op must rotate the antenna like crazy for the whole period of a contest. So, why not use same technique on Top Band, but with phased two element array that can be electronically swiped over a limited range of angles. I liked that idea and we implemented it in the antenna. See the patterns down below. As one can see the main lobe gain is only 0.15 dB down from the “in phase” position and is tilted by 13.5 degrees, making a -3 dB point appear at 41 degrees, thus making overall beam width of the array a respectable value of 82 degrees. Now the Florida and Washington stations will only be short of 0.3 dB of signal strength of UA2FW compared to Mid West stations.
Practical design of two collinear spitfires
I must explain the chosen height of the center driven element of 49.266 meters. This height ensures higher values of active part of the input impedance of the antenna, thus making it more efficient minding the ground losses, yet presenting a rather small positive reactance at the base which is easy to compensate with practical value capacitors of reasonable reactive power handling ability (we did the job with the ceramic ones, so no expensive nicely looking attractions for thefts vaccum ones were needed). The MMANA file for the array that was installed is named ” 2 x 3el gp160 for cqww 160 2021 @ ua2fw’s FINAL.maa”. You can extract all the dimensions from the file. I will show down below the three pictures like in the text above, so one can get a better idea of the antenna’s features.
Author – Vladimir Willy Umanets, UA9BA.