Spring 2007 Issue

ANTENNAS

Vivaldi, Cell Phone Booster,
and Discone Antennas

By Kent Britain, WA5VJB

Photo A. PCB broadband Vivaldi antennas, 10–25 GHz and 20–30 GHz.

This time we start off with Vivaldi antennas, and a 10–30 GHz version is shown in photo A. The rhombic and V-beam antennas go back to the earliest days of radio. If you place two long-wire antennas in a V formation, then you have gain in the direction of the V.

In figure 1 you can see a typical V-beam. The longer and narrower you construct the V, the more gain you will have and the wider the bandwidth of the antenna. Bandwidths of 10 to 1 are possible. On HF this would be a beam antenna that would work very well on all frequencies between 3 and 30 MHz. The minor technicality that would keep this antenna out of most back yards is that it’s 700 feet long and 400 feet wide.

To help reduce side lobes and for the antenna to have a smoother impedance transformation or SWR over this wide a range of frequencies, the V-beam wires are slowly tapered apart along an exponential curve. The wire forms of the antenna also should have the diameter of the wire increase as you progress toward the ends. This can quickly become impractical on HF, where the wire would need to be many feet in diameter. Wire cages can be used instead of 10-foot diameter copper wire, but Vivaldis are really not HF-type antennas.

However, at about 1 GHz, exponential antennas of the Vivaldi type start to become much more practical.One of the first uses was in horn antennas such as the ones shown in photos B and C. The exponential ridges will greatly increase the usable frequency range of a horn. A simple WR-90 horn antenna is typically rated at 8–12 GHz. The addition of exponential ridges results in a horn of the size similar to the ones shown in photo B, and they now have a usable bandwidth of 10 to 25 GHz. The much larger ridges shown in photo C give the horn a 2 to 18 GHz bandwidth.

You can think of the PC-board versions of a Vivaldi antenna as a ridged horn without the horn.

Vivaldis make great broad-band antennas with SWRs less than 1.5:1 over most of their bandwidth. Normally the coax would be soldered across the gap 1/4 wave from a back short. However, using the circle as a back short greatly increases the low SWR bandwidth over a 1/4-wave back short. Also, while the SWR starts to rise below 10 GHz, the antennas still have forward gain down to 3 GHz.

Exponential antennas of many types are becoming quite popular these days in the microwave range, and even 2.4–5.8 GHz WiFi service is taking advantage of their wide bandwidth.

Cell Phone Booster

Many of us have seen the advertisements for these stick-on antennas (photo D) in late-night TV commercials. Oh, the claims they make for them: works in sealed elevators, works underground, increases range, also cures gout and arthritis! However, I am most interested in how an antenna can reduce static on a digital signal.

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