Challenges of Long Boom Yagis

The very biggest of the big guns put up large yagis for all the HF bands and then stack them for additional gain. This is neither easy nor inexpensive. To do it you either have money to spare or an especially strong motivation to pursue a few extra decibels. There is no one right answer. For someone like me, aiming high but not crazy high, trade-offs are necessary.

The above diagram from W2PV's Yagi Antenna Design is one I've shown before on this blog. It should be obvious that achieving an additional 3 db of gain by increasing boom length alone is exceedingly difficult on HF. Consider that a 1λ boom on 20 meters is almost 21 meters long (70'). Even at half that length on 10 meters it is still a giant. Hence stacking.

A long boom on HF yagis is rarely longer than 48' (14.7 meters). There are longer boom yagis out there, and even commercial products, but as I said they're rare. Given the incremental performance they are, in the opinion of many, not worth it. Consider that a 48' boom with a 3" diameter has a projected wind area of 12 ft². Treating it as a cylinder the wind load when broadside to a 135 kph (85 mph) wind is 240 lb. On 20 meters and up a long boom yagi has its maximum wind load in this orientation.

Building a long boom yagi to survive these and higher winds is expensive. The antenna will be very heavy to lift. The rotator and tower must be rated to deal with the load. Required boom strength increases faster than linearly with boom length due to the bending stress from the added wind load, its length and its weight. I need to keep the weight reasonable enough to allow tramming the antennas up the tower so that I can avoid the expense and land access issues of a large crane.

Since the gain of a 20 meter yagi with a 40' (12 meters) boom is only ~1 db lower than its 48' long bigger sibling I have chosen this length for my planned yagis. On 15 meters I am going shorter still: 32' (10 meters). For the next year or two, for the duration of the solar cycle minimum I will not be building long boom yagis for 10 meters. Should I decide to build 3-element yagi on 40 meters it will also use a 40' boom.

All of these antennas have been modelled and compared to longer and shorter boom yagis. The one for 15 meters was described in this blog quite some time ago. The 20 meter yagi is similar to the 5-element 40' boom yagi described in the ARRL Antenna Book. The 40 meter yagi is of my own design, both 3-element and with a coupled resonator for broadband low SWR. The latter antennas I will likely describe in a future article once the madness of tower and antenna building season subsides.

Few hams use yagis with booms longer than 6 to 8 meters and even these look big close up. The longer ones don't look so big when they're high in the air. On the ground you can better appreciate their size. In the picture you can see the first two long booms I built earlier this summer. Each is 3" diameter. The short one is 32', for the 15 meter yagi. The longer one is 41', for the 20 meter yagi. The garage they're in front of is 26' (8 meters) wide.

Both yagis come from 3" tubing I acquired a few years ago, with a heavy wall pipe inserted into the centre of the 40' boom. I took the pipe to a machine shop where they used a metal lathe to turn down the ends to fit within the 3" tubes. They did a very neat job and it was inexpensive. I mated the boom sections with the tools in my own workshop. The completed booms were set aside while I focussed on other projects.

Neither of these booms is sufficiently robust to be mounted high and rotated. I base this on calculation with software tools. The data say they can do the job, though without a margin for ultimate safety that I consider sufficient.

Instead these two yagis will be side-mounted, fixed on Europe and stacked with electrically identical yagis rotated at the top of the tower. The booms for the top yagis will be stronger, using aluminum I have on hand plus heavy wall tubing I will purchase. I am awaiting quotes on the element tubing (0.058" for telescoping) since of the many aluminum suppliers within driving distance none stocks this wall size. I have already bought the aluminum bar from which I will fabricate element-to-boom clamps.

None of this is cheap! That said, it is more economical than buying commercial products and I learn a lot along the way. The real cost is that it takes time. I may not get as far along as I'd like before winter with this and other projects. But it's fun, and that's as it should be. For 40 meters I would like to experiment with element design by building one and mounting it for use as a dipole during the winter and spring. This will help me assess its survivability before proceeding with yagi construction.

Long boom HF yagis have one other irritating feature: tuning. Unlike most 3 and 4 element yagis the driven element on a longer yagi is far from the centre. You cannot reach the driven element from the tower to adjust matching. For my 5-element 20 meter yagi on a 40' boom the distance between feed point and boom centre is 13' (4 meters).

Most commercial yagis tell you exactly what measurements to use, but on a home brew antenna a substantial amount of adjustment may be necessary. My plan is to mount each yagi vertically on a suitable tower (one without any antennas that can interact with the yagi) for tuning.

When I am done I will have one tall tower (the present one) dedicated to 40 and 10 meters and one dedicated to 20 and 15 meters. This is common practice for many contesters. One or two of my tri-band yagis will be fixed or rotatable to fill coverage gaps, especially for higher elevation angles to the Caribbean, the US and other paths when needed. Full implementation will take me into 2019.

Speaking of which, look at what showed up last week.

This is the tower for 20 and 15 meters. If all goes well it will be planted in the ground by early October. With luck and hard work it will be raised before winter. Assembly of the yagis will be done concurrently. Unfortunately I only anticipate raising the two side mounted yagis this year, with the rotatable antennas delayed to 2019. The weather will soon turn against me.