Topband: Vertical Tee (was Top Loading wires)

[Donald Chester]

 My 160m antenna is unique in that I have never run into anyone else with anything exactly like it. It consists of 127' of Rohn 25, insulated at the base, with 120 buried radials each 133'6" long. Probably overkill, but I got a good deal on a 16,000 ft spool of #12 bare soft-drawn back in 1974 when the economy had just tanked and the price of copper dipped precipitously. A few years later I constructed a homebrew radial plough that allowed me to  bury the whole roll in about 4 days.

The tower also holds up a 137' long doublet, attached at the top guy bracket at the 119' level. The legs of the doublet slope slightly downwards, at 18 degrees from horizontal, so that the ends are approximately 20' lower than  the mid point. The doublet is fed with a balanced open wire feedline, made of #10 wires spaced 2" apart; Zo calculates to 438 ohms. The open wire line runs down through the interior of the tower, with the two conductors spaced at the geometric centre of the triangle, exiting the tower near the base. There is no direct electrical connection between the doublet or feedline to the tower structure at any point. A knife switch at the base of the tower allows both conductors of the OWL  to be shorted directly to the tower for lightning  protection, and there is a lightning ball gap across the base insulator. Another knife switch in the ATU housing, about 8' from the tower, allows the OWL to be disconnected from the dipole matching network and float free.

I have never tried a thorough analysis of this system, which would appear to me to be very complex, but the antenna has been very effective ever since it was completed in 1983. This antenna  would probably be more accurately described as a vertical tee, rather than a simple quarter wave vertical, since the close proximity of the feed line along nearly the entire length of the tower appears to couple the doublet very closely  to the tower. The top-loading wires, formed by each leg of the doublet, are each 68' 6" long. If the top loading wires were bonded directly to the top of the tower in true vertical tee configuration, the maximum current point  should be near the mid-section of the tower, but I am uncertain as to  what actually happens with the present configuration.

I normally feed the base of the tower with both knife switches open and thus the OWL to the dipole floating. The matching network consists of a parallel tuned circuit with one end grounded and the tower fed via a tap on the coil. The  transmission line from the shack to the base of the tower is another 438-ohm OWL, coupled to the main coil through a 14-turn primary winding. Designed largely by trial-and-error, the ATU allows the transmission line back to the shack to be adjusted to a near-perfect match with no standing waves, anywhere in the band by simply adjusting the variable capacitor in the parallel tuned  circuit that comprises the secondary winding.

Here are the measurements.

Usual operation, with the OWL floating:
                                                 1800 kHz, Z = 142 ohms +  j286
                                                 1900 kHz, Z = 235 ohms + j378
                                                 2000 kHz, Z = 392 ohms + j477

Shorting the OWL to the base of the tower by closing the lightning switch, radically changes the measurements:
                                                  1800 kHz, Z = 66 ohms + j59
                                                  1900 kHz, Z = 112 ohms + j145
                                                   2000 kHz, Z = 162 ohms + j213

These measurements  were taken with the OWL grounded directly to earth via the radial  system:
                                                 1800 kHz, Z = 12 ohms + j52
                                                 1900 kHz, Z = 22 ohms + j62
                                                 2000 kHz, Z = 24 ohms + j69

I have never run any kind of test to compare distant signal strengths with the OWL floating, shorted to the tower, and grounded to the radials, perhaps something I should do.

Don k4kyv