A Stub Matched Lazy H for 17 M

Transcription

1 A Stub Matched Lazy H for 17 M Introduction The author has experimented with various configurations of the classic Lazy H antenna and a version optimised for operation on the 17 M band is shown in Figure 1. The antenna conductors are 14 SWG seven strand copper, and all transmission lines and the stub are constructed from 450 ohm window line. 29 ft. 50 ft. agl 450 ohm transposed 20 ft. 25 ft. 25 ft. agl 14 ft. 2" all transmission lines 450 "window" 450 ohm TL to rig approx. 160 ft. 3 ft. 4" s/c stub Figure 1 Lazy H Dimensions Note that the stub method of matching described here is inherently narrowband, and prevents the antenna being used effectively on other bands, particularly 7 MHz, 14 MHz and 21 MHz, where it can still produce useful radiation patterns. If this is a concern, then it is preferable to just accept the slight loss of gain and trickier tuning with the unmatched configuration. If the transmission line run is relatively short, say less than 50 ft., then this should not be a problem in practice, but with longer runs the losses can be quite significant if the SWR on the 450 ohm line is already high, and the matching can be very susceptible to water or wet snow on the line. In my installation the transmission line was required to be around 175 ft. in length so I decided to install a stub at the antenna to match the antenna impedance to the 450 ohm transmission line impedance over the 17 M band. Antenna Pattern and SWR Plots Figure 2 shows the elevation radiation pattern of the Lazy H over medium ground. Maximum gain is 12.3 dbi at an elevation angle of 18 degrees. At 5 degrees the gain is still a respectable 5.4 dbi. The patterns are independent of the stub match since the model assumes zero transmission line and mismatch loss.

2 Figure 2 Elevation Radiation Pattern The corresponding azimuth pattern at 18 degrees take off angle is shown in Figure 3. Figure 3 Azimuth Radiation Pattern The azimuth pattern maxima are at right angles to the plane of the antenna, or NE / SW as installed at the author s QTH in eastern Ontario. Figure 4 shows the antenna SWR into 450 ohms, without stub matching and plotted from 15 to 20 MHz.

3 Figure 4 Unmatched 15 to 20 MHz VSWR At MHz the SWR is 6.4:1, with Z 0 = j734. Variation over the band is insignificant. The antenna therefore presents a fairly high inductive reactance loading, but with a reasonably well-matched resistive component. Line Losses and SWR The SWR measured by a 450 ohm balanced bridge at the transmitter end of the line, before matching and in dry weather, was 6.5:1 at MHz, or close to the model value. A repeat check made when the feeder was wet due to moderate rain gave a measured SWR of around 4. The rain also required retuning of the Z match tuner usually used to match the antenna feed line to the 50 ohm unbalanced transceiver RF port. The loss due to 175 ft. of 450 ohm transmission line between the tuner and the centre of the lower element of the antenna was estimated at 0.12 db / 100 ft. from the chart (Fig. 19.4) in the ARRL Handbook (1999). Applying the formulae shown below gives the actual loss (around 0.5 db) for 175 ft. of line operating at 6.5:1 SWR. This may not appear to be a particularly significant loss; however the calculation assumes dry transmission line under ideal conditions. In wet weather the line losses almost certainly increase, and there is the additional inconvenience of having to reset the antenna tuner.

4 Matched line loss: SWR at load: ML :=.12 ML 10 a := 10 SWR := 6.5 db per 100 ft. from ARRL HB pp.19.5 ρ := SWR 1 SWR + 2 Mismatched line loss: Measured line length: MML := 10 log MML = L := 175 MML = a 2 ρ 2 a 1 ρ ft. db ( ) 2 MML := L MML 100 db per 100ft. Matching Stub Calculations The first step is to calculate the position of the first current maximum on the transmission line, working from the antenna. In the absence of a suitable current probe, the best method is to work in from the outer end of the antenna elements to find the position of the first current maximum on the transmission line. At this point the impedance on the line consists of a real resistive component equal to the characteristic impedance of the line in parallel with a reactive component. In this case the length of one of the lower elements (29 ft.) is 0.53 λ, where λ = ft.. Since the element length is of the order of ½ λ, the next accessible current maximum occurs at ¾ λ from the outer end of the element, or at L = = 0.22 λ along the transmission line from its junction with that element. Taking the velocity factor of the line into account, L is given by: L = x 0.22 x 0.95 = ft. In the case of the upper set of elements, also 0.53λ in length, there is an additional 20 ft. of phasing line to take into account. Ideally the length of this line should be 0.5λ or ft, accounting for the 0.95 velocity factor, in which case the current maximum due to the upper elements would coincide with the position of the maximum due to the lower elements. However a shorter phasing line length of 20 ft. is used in the antenna described here because it allows tension to be maintained in the phasing line and, according to the model, the shorter phasing line actually improves the gain by a few 10ths of a db (attempts to use the ideal ft. length with 25 ft. vertical support halyard spacing resulted in excessive thrashing under high wind conditions and broken phasing line connections). The current maximum for the upper elements thus occurs at 5.75 ft. closer to the transmitter than that for the lower elements. A compromise stub position of /2 = 14.2 ft. (14 ft. 2 ) is therefore indicated. The next step is to find the required length of a short circuit stub to connect at the current maximum to in order to compensate for the antenna s reactive impedance. The procedure is to draw a circle on the Smith chart as shown in Figure 5, corresponding to the measured 6.5:1 SWR and centred on R/Z 0 = 1. This circle crosses the unit resistance circle at a point corresponding to an inductive reactance of j2.3. Follow the j2.3 curve to the edge of the chart and note the reading on the

5 wavelength scale. Then calculate the total length in wavelengths from this point to the infinite susceptance (short circuit) point on the right of the horizontal axis. In this example the j2.3 curve intersects the 0.185λ point on the outer scale, and the difference between this value and 0.25λ, corresponding to infinite susceptance, is 0.065λ, or x 0.95 = 3.34 ft (3 ft. 4 ). Figure 5 Smith Chart A short circuit stub was made from 450 ohm line to the above length and attached in parallel with the transmission line at the previously calculated position of the current maximum (14 ft. 2 ). The measured SWR improved to 1.2:1 at the center of the 18 MHz band, with negligible variation over the band. If necessary the SWR can be optimized by adjusting the stub dimensions after observing the frequency at which minimum SWR is obtained. For example if this frequency is higher than the desired operating frequency by X%, then the stub spacing from the antenna feed point and stub length can be increased by 0.95X%. In practice it appears that stub position is the more critical dimension so try this first. The stub match was found to considerably reduce environmental detuning effects. An additional benefit is that the stub provides a DC short circuit across an otherwise open circuit antenna thereby reducing the chances of static build up and allowing for straightforward ohm-meter transmission line continuity checks. Nick Shepherd VE3OWV