UA9BA 80m Spitfire Antennas
I had overwhelming feedback on my article about 160 m spitfire installation at UA2FW's for the CQ WW CW 160m DX CONTEST 2021 ( https://dxnews.com/ua9ba-spitfire-160m/ ). So, I got hooked on the spitfires myself and continued evaluating the antenna's behavior under different conditions. In this article I describe few construction ideas for 80 meter spitfires and provide the MMANA files for your own evaluation. I hope the material will help you to design on your own spitfire antennas.
Common input data for evaluated spitfires
When thinking on how to simplify the construction of the antenna I bounced into a rather innovative idea of a simple method of controlling the resonance lengths of parasitic elements. In a classic configuration the director/reflector lengths of parasitic elements are controlled by switches along the lower part of parasitic elements like on Fig.1. The switches control overall lengths of the parasites
The bad part of such method is that if a sub-band switching is required, then one has to install as many additional supports for the switch boxes as there are so many numbers of required sub-bands. In my evaluations I use angle capacitors for controlling the resonant length of parasites. Thus, only one support for the switch box at the angle of a parasite irrespective of number of required sub-bands is needed. Illustration on how it works is on Fig.2
All described antennas have a center element 25 meters high. I tried other heights of center element and 25 meters is about the best compromise between pattern and input impedance. Higher center elements do raise the input impedance which is good for the efficiency of the antenna, but the F/B ratio degrades. With lower center element we get higher F/B, but input impedance becomes too low. So, let the center element be 25 meters high in all antennas here. The center element construction starts with 80 mm OD Al tubing and ends with 60 mm OD Al tubing.
All antennas have lower parts of parasites 2.5 meters above ground and the angles of parasites are 18.5 meters away from the center element.
All calculations were done for average ground - dielectric constant of 5, ground conductivity of 5 mS/ m without radials.
Necessary and sufficient condition for comparing the performances of different spitfires is VSWR not higher than 1.5:1 within evaluated sub-band (either 100 kHz or 150 kHz wide) under "matched condition" in MMANA options.
Classic spitfires for 80 meter band. Does OD of parasites matter? Does construction of center element matter?
I had evaluated two cases - 3 mm OD wire parasites and 9 mm OD wire parasites with the same center element 25 meters high. The MMANA files are #1 and #2 respectively at the end of the article.
Table 1 of antenna performance parameters for 3 mm OD single wire parasites.
Table 2 of antenna performance parameters for 9 mm OD single wire parasites.
It is easy to see that thicker parasites improve gain and F/B numbers. So, the thicker the parasites the better performance of the antenna.
How does the construction of the center element of the antenna influence its performance? We'll take a 9 mm OD parasites and make our center element 2 dimensional one -1.5 m wide and 25 meters talllike in Fig.5. The MMANA file for it is #3.
Table 3 of antenna performance parameters for 9 mm OD single wire parasites and 2-D center elementas shown on Fig.3.
As one can see - the gain numbers became higher, F/B remained generally the same and a real part of the impedance got slightly higher if compared to the best previous performer in Table 2.
So, from now on in further evaluations I will use for the center element the construction shown on Fig.3.
Now. Can we still improve the performance of spitfire antenna? What about a 2 dimensional approach to the parasites? I use a 1.5 meter wide parasitic elements like shown on Fig. 4. Those elements have a bigger equivalent diameter and we've seen in preceding chapter positive effect of a bigger diameter wires on antenna's performance. MMANA file #4
Table 4 of antenna performance parameters for 2 dimensional parasites and 2dimensionalcenter elements shown on Fig.4.
There is obvious improvement in gain and overall F/B performance of the antenna as can be seen from the table 4 figures compared against those of tables 1, 2 and 3.
The tables 5 and 6 show the numbers for two other sub-bands 3.6-3.7 MHz and 3.7-3.8 Mhz respectively.
I show two examples of 4-directional spitfires made around 3-dimensional center element of 24 and 25 meters high. The parasites are 2-dimensional 1 meter wide elements made of 3 mm OD copper wire. There are shorts on parasites. These shorts come along together with plastic spreaders. The plastic spreaders alone have some dielectric constant in the range of 2.5 to 4 or even higher values (wet wood spreaders for instance). Those spreaders lower the resonance frequency of the parasites and the performance of the antenna in practice differs a lot from virtual model thatdoesn't take into account the impact of spreaders. To ensure a good match between theory and practice I just shorten those dielectric spreaders with copper wire shorts. By the way, it could be a thin wall aluminum tubing instead of plastic spreaders and copper wire shorts.
Table 7 Performance parameters for 4-directional spitfire shown on Fig.5 with 24 meter high center element. MMANA file #5.
Table 8 Performance parameters for 4-directional spitfire shown on Fig.5 with 25 meter high center element. MMANA file #6.
As one can see the input impedance raised to 41 ohms from 32.6 when we went from 24 meters to 25 meters of height for the center element. That's very good news, indeed! The cost was marginal - 0.1 dB of gain, which will be easily reimbursed by higher efficiency of the antenna. A slight decrease in overall performance in rear direction is acceptable in my opinion. You may see it differently, though. It's up to you!
2 spitfires in a collinear configuration.
We can further enhance the performance of spitfires by putting two of them side by side at a distance slightly higher than 0.5 wavelength (47 meters in our case) and feeding them in phase. See Fig.6 for the layout of the collinear antenna and Fig. 7 for the pattern on 3.550 MHz of this antenna fed in phase. MMANA file #7.
Actually, one can feed the two in many different ways. It was covered in my article on UA2FW's 160m collinear array.
I wish you success in building your own spitfire antennas.
73! Willy (Vladimir) Umanets UA9BA
Karataban of Chelyabinsk oblast 11 June 2021