Antenna dimensions for other bands

The results from modeling for other bands are given on Table 1. MMANA and NEC2 give similar results. This antenna is predominantly horizontally polarized and it needs height to have good gain and take-off angle. The design height is chosen to be 18 m for 7 MHz. For 7 and 14 MHz bands, where the relative bandwidth is higher, there are two versions – for the center of the band and for CW portion. The center frequency is chosen at the point where the SWR curve begins to rise quickly. This is a compromise between SWR and directivity. Look at Fig. 15 to Fig.19 in the main article. The rise in SWR is a very good sign where the antenna is tuned. (Fig.1)

Band [MHz]	Wire Length [m]	C [pF]	Wire diam [mm]	Height [m]	R [Ω]	X [Ω]	Max Gain dBi	Max Gain @deg	SWR50	R [Ω]	X [Ω]	Max Gain dBi
MMANA										NEC2 Eps=13, Sigma = 5
7.1	10.84	470	1.2	18	37	-8	7.64	35	1.46	36	1	8.05
7.05	10.94	470	1.2	18	33	-4.5	7.67	35	1,54	33	1	8.43
10.12	7.65	300	1.2	12	32	0.7	7.59	36	1.54	34	3	8.4
14.15	5.47	235	1.2	12	36	3	9	27	1.37	37	5	9.77
14.04	5.52	235	1.2	12	34	4	9.2	27	1.47	35	5	9.79
18.12	4.27	200	1.2	12	40	-11	9.56	17.5	1.38	38	-5	10.26
21.2	3.66	150	1.2	12	37	-8	9.56	17.5	1.42	41	-2	10.11
24.95	3.12	120	1.2	12	34	2	9.78	14.8	1.47	37	5	10.34
28.3	2.76	100	1.2	12	37	5	10.28	13	1.37	46	8	10.4

Table.1

Fig.1 SWR for 7 MHz CW

When directions are switched there might be slight difference in SWR due to imperfect symmetry of the real antenna. We have mounted this antenna permanently on the roof of a house and measured SWR at 14.1 MHz was between 1.3 and 1.7 in different directions.

It is interesting to test this antenna for 80 m band. If a 30 – 40 m tall mast for higher band yagi is available and also large land property, adding 4 sloped wires is not a big problem. The antenna bandwidth will be acceptable for CW or SSB (DX part).

Horizontal placement

This antenna can be mounted horizontally between e.g. tall trees as a cross. The benefits are lower take-off angle and increased bandwidth (Fig.21 in the main article). Modeling gives shorter elements. We have calculated a horizontal beam for 40m band at 15 m height for the CW part. The length of element is 10.78 m with 470 pF capacitor, 1.2 mm bare wire diameter. For antenna centered at 7.1 MHz the length must be 10.64 m.

Balun for 50 Ω feeder

When the antenna is used with 50 ohms feeder there must be a current 1:1 balun placed between the antenna and the cable (Fig.2). We used a homemade balun which consists of two tube ferrite cores used widely in the industry for common mode baluns in various cables. These cores are cheap and exhibit very good results. The cores we used are 17.5 x 28.5 x 9.5 mm, type RH-17.5X28.5X9.5 (B8) from http://www.coremaster.com.tw/, material is Ni-Zn something like 43 material, 7 turns thin (2mm) Teflon coaxial cable, L = 225 uH. This balun was tested at 350 w RF power at 14 MHz – the core increased its temperature very slightly. Similar cores are produced by many manufacturers and can be found in the list of many electronic parts suppliers. Choose cores for 1 to 30 MHz range. This balun has impedance > 4 KΩ from 3.5 to 18 MHz – the peak is almost 10 KΩ at 7 MHz

Fig.2

Printed circuit board

Here an example of PCB layout. The PCB is single layer, only on several points there must be wire jumpers. This board was used in our antenna. The control cable connector is for RJ11 plug. Direction switching with computer

Fig.3 PC Board for coaxial cable

Fig.4 PC Board for transformer and symmetric feeder.

Instead of using mechanical switches, a simple hardware can be implemented to use the DTR and RTS terminals of the serial COM port of a PC. The description is given in http://active-antenna.eu/tech-docs/pc-control-active-antenna-amp.pdf. There is a small program http://active-antenna.eu/software/aaacontroller-1.1-bin.zip which can control these two lines.

More pictures

Here we show some additional pictures for the field-day style antenna which are self explaining. Hot glue and cable ties are widely used to secure the cables.