or I should have bought a Dream Beam-18 in the first place
Originally published August 17, 2010
Ever wondered what would happen if you combined a 40M rotary dipole with a fixed-position reflector or director element? Me too. So I tried it.
The SteppIR 3-element yagi with 40M/30M dipole is a great antenna, but it doesn’t deliver gain on 40M. That’s fine for domestic work from VE7, but for DX — particularly working Europe — I need at least a bit of gain to make the difficult path over the North Pole.
I love verticals and half-squares, but no matter where I put them they interact with my 80M vertical array — there is never any free lunch. Something is always traded for something else, and so I have not been able to fix my Europe 40M problem with those great options.
Studying the layout of my back yard, where the SteppIR is on a small 40-foot-tall crankup tower, I realized that nearby trees could serve as supports for a passive element on 40M.
While this is not a novel concept, it was rather exciting to realize I could create a passive element that could be switched from being a reflector (aiming the array at Europe) or a director (aiming at W6).
And that’s how the hybrid SteppIR 40M yagi came into being.
The model case
To see what it would look like, I created a model of the SteppIR 40M trombone element in MMANA GAL (optimized for 7.050 Mhz), and added a 40M wire spaced about 33 feet away.
My SteppIR 40M dipole MMANA GAL .maa model
dipole at 27′ | dipole at 47′
The layout of the trees means the wire element is off the yagi’s center axis, shifted quite a bit to the west of the SteppIR — the east end of the wire element is about even with the east tip of the SteppIR’s 40M element.
The alignment of trees also means the wire element is broadside to 30°, which is an ideal configuration for me — when the dipole is aimed at Europe, it is in perfect alignment with the wire element.
A key point to mention is the “boom length” of the hybrid antenna. The wire element is about 33 feet (10m) away from the 40M dipole element (giving this 40M 2-element yagi a 33-foot boom length). None of this is even close to ideal, but seeing as this is an experiment, I figured “who cares?”
In the spring of 2010, I build a 69.9-foot-long (21.5m) wire element made of 14-gauge aluminum fence wire, which I have found hard to work with but very strong for wire antennas. Once the element was cut to length, I cut it apart 6 feet (1.8m) from one end and inserted a simple SPDT relay.
The “solo” 40M dipole at just 27′ in height exhibits about 6.52 dBi (over an isotropic radiator). By adding a passive reflector or director adds about 2.1 dB of gain — pretty good for being almost totally free (for the cost of a $6 relay and 70′ of wire.
27′ high — unaided SteppIR 40M dipole (6.52 dBi at 90°)
The unaided 40M dipole at just 27′ in height exhibits about 6.52 dBi (over an isotropic radiator).
27′ high — with wire director, aiming South (8.62 dBi, 47°)
Adding a passive reflector or director adds about 2.1 dB of gain and cuts in half the takeoff angle of maximum radiation — pretty good for being almost totally free (for the cost of a $6 relay and 70′ of wire).
27′ high — with wire reflector, aiming North (8.63 dBi, 43°
Now, let’s take a look at what happens when we crank up the tower, so the SteppIR dipole has about the same height as the new wire element. (I say “about” because I can only estimate the wire’s actual altitude, comparing it with the SteppIR’s height).
47′ high — unaided SteppIR 40M dipole (5.39 dBi at 43°)
The gain falls a bit when going up 20′ in height, but the high-angle stuff is reduced — the takeoff angle of maximum gain is cut in half from “straight up” to 43°.
47′ high — with wire director, aiming South (8.3 dBi at 36°)
The gain is up by 3 dB, or a doubling of power, plus the takeoff angle falls to 36°. That’s not great by DXing standards, but we’re still only 47′ in the air. A half-wavelength on 40M is 70′ up.
47′ high — with wire reflector, aiming North (8.96 dBi at 38°)
The gain shoots up to almost 9 dBi. Front-to-back is about 10 dB.
What about other directions?
If you’re interested enough to read this far, you’re probably thinking, “Yeah, but what happens when you turn the SteppIR?”
That’s a great question — and one of the first things I wondered about. This is where the MMANA model helped me decide to go ahead. Bottom line: as the SteppIR dipole is rotated until it is 90° to the wire element, the effects of the wire element diminish and almost disappear. Plus, there are some interesting areas of coverage as you turn.
Note: my modelling is not very good. Because I don’t have the skill to “turn” the Steppir dipole (which is made up of many segments) I turned the wire element instead. Doing this, I approximated distances, so you will see some variations in the plots. That said, the basic patterns should serve to illustrate where the antenna points depending on the spatial relationship of the dipole driver and the passive element.
|Steppir 90° to wire reflector
||Steppir 90° to wire director
|There is a slight gain advantage to the east, but the typical low-height “peanut without a waist” is the basic pattern.|
A quarter turn to the right….
|Steppir 45° to wire reflector||Steppir 45° to wire director|
A quarter turn to the left…
|Steppir -45° to wire reflector||Steppir -45° to wire director|
And in full “yagi mode” with 33′ spacing, parallel to the wire…
|Steppir 0° to wire reflector||Steppir 0° to wire director|
Performance on the air
I love the SteppIR dipole even when it is only 27′ in the air (remember, my 27’/47′ tower looks out over sharply sloping hillsides, so it behaves like something a bit taller). But a dipole at less than 70′ isn’t going to get me over the North Pole very often. Adding a wire element to the SteppIR dipole makes a difference.
Studying the predicted patterns, I expected the hybrid yagi to behave no better or worse than the basic 40M dipole when listening to most of North America. Indeed, Midwest and East Coast stations are no stronger or weaker with the antenna in reflector or director mode. Turning the dipole their way brings them up, and switching the hybrid element has no effect on them.
The hybrid element comes into play with W6 and Europe. The S-meter does jump on California and deep South American stations, when going from reflector to director mode. And European stations are an S-unit or two better with the antenna aimed at them in reflector mode, as they should be.
The gain is not always pronounced, depending on signal origin and incoming angles, but when this contraption is the right antenna for the job, your ears know it.
This part-fixed, part-rotatable antenna arrangement isn’t going to be totally predictable. If you absolutely, positively need to know where your signal is going, put up a fully rotatable 2-element yagi.
Only time on task — using the thing, and comparing real-world experience with the predicted patterns — will provide a good sense of performance around the compass. I certainly don’t have that good sense yet, but I will provide updates and examples here as I figure it out and play with things.
One of the areas I’d like to experiment with is adjusting the SteppIR dipole length to see if that offsets in any way the unchangeable too-long 33′ boom length of the hybrid yagi.
If over time this thing doesn’t work better than other antennas I’m using, I’ll say so.
Thanks for reading, and all the best on 40M!