SP3L's Antennas - 9-Band Spider Doublet

9-Band Spider Doublet

One antenna capable of working on many amateur HF bands and not requiring tedious trimming is something none of us can pass by indifferently. The first choice that comes to mind is probably the T2FD antenna and its derivatives. However, low SWR over a wide frequency range is achieved in their cases at a cost of significant RF power loss in the loading resistors. However, by adding a few more wires to a straight dipole, one can build an antenna covering all bands from 80 m through 6 m without any loading resistors (and power loss associated with them). Such antenna can be driven by any contemporary transceiver equipped with an ATU capable of matching loads with SWR up to 3:1.

It is reasonable to assume that if the SWR measured at the antenna feed point equals 4:1 or less, we have a good chance to get SWR<3:1 at the coax end connected to the transceiver. That's because the attenuation introduced by the coaxial cable reduces the SWR when we move from the antenna to the transceiver.

The antenna presented below, when fed via a 4:1 balun, features SWR<4:1 in its feed point for every band from 80 m through 6 m (50-52 MHz part), except for the 60 m band. On 60 m band, the antenna has SWR less than 5:1 so, there is still a chance that at least some transceivers would be able to match its output to the antenna-balun-feeder system also on this band.

The plots below show the antenna SWR - with EZNEC scaling of the Y axis and with linear scaling.

The above plot is the result of a simulation performed in free space. Proximity of ground influences antenna SWR. Find below the SWR plots for individual bands simulated in free space and in three heights over ground: 30, 20 and 15 meters.

As you can see, the antenna should be installed at least 20 m above the ground level. At lower heights, the SWR may become high, especially on 80 m band.

Quite like the other multiband antennas (e.g.: T2FD, OCFD, End-feds), its radiation pattern changes significantly when we change bands. See the figures below. The pictures showing the radiation patterns are presented in the same order as the SWR plots:

80 m 40 m

30 m 20 m

17 m 15 m

12 m 10 m

6 m

The multi-lobe radiation patterns on higher bands are due to quite large antenna length when compared with the wavelength on these bands.

When hung 20 m AGL or higher, the antenna covers at least 8 bands. It also covers the lower part of 6 m band (50-52 MHz). It can potentially cover also 60 m band but with higher SWR.

Construction of the 4:1 balun covering the range 3.5 through 52 MHz might be a little challenging. But if you choose material # 31 and the ring cores with 2.4" o.d. (FT240-31), it should be feasible. I would suggest to wind 9 turns. Figure below (taken from: https://vk6ysf.com/balun_guanella_current_1-4.htm) shows the schematics of the balun.

9 turns should ensure over 1 kohm common-mode impedance in the whole frequency range (3.5-52 MHz). Such value should be sufficient for the power levels up to 200 W. For higher power levels, additional common-mode chokes connected in series may be required. Of course, the coaxial feeder should be run perpendicularly to the doublet.

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