Long boom 4 element yagi stack based on Cushcraft elements
By K9MU
As you probably know, 50 Mhz is my go-to band. It was the first band I was QRV on as a new ham back in 2001 at the peak of solar cycle #23. I had a 10w IC-551 and a dipole up on my parent's roof. I remember hearing Z22JE CQ'ing on SSB with no responses and I couldn't get his attention. I did work plenty of other good stuff though! That started my quest for 6m DXCC and grid chasing. I finally reached DXCC in the summer of 2020 thanks to this 4 element yagi stack!

I use a M2 6M7JHV at about 55 feet. It is nestled about 5 feet above a large HF tribander and then I have an assortment of UHF/SHF yagis above it. My 4 element yagi stack is mounted directly on the legs of my Rohn 45G. The yagis are at about 28 and 40 feet AGL. I use an Elecraft K3 and a NESDR USB dongle with a preamp for receive comparison between the stack and the 6M7JHV. The stack almost always out performs the 7 element when they are pointed in the same direction no matter what receiver I use. For our summer time multi-hop Es season, I point the stack to Europe and I have worked many European stations on FT8 that the 7 element never picked up. I'll add, the 6M7JHV probably is not optimal with all the other yagis in its capture area.

I highly recommend building this small yagi stack for more 50 Mhz antenna options! I had a couple old Cushcraft 50 Mhz yagis laying around and an assortment of old aluminum tubing I used to make the booms longer. Each yagi has a 50 degree 3 dB beamwidth. For contesting, I split the yagis to cover nearly 100 degrees of azimuth (ME down to FL for me!). I used the DOS based Yagi YO modeling program to optimize Cushcraft’s element length and spacing.

Cushcraft Long Boom (15' 3") 4 element yagi
# Element Position Total Element Length Length - 3/4" tube (fixed) Length - 5/8" tube (each end)
Reflector 0 57.33 24 34.22
DE 58.192 54.137 24 31.013
Director 1 118.874 52.630 24 29.5
Director 2 182.703 51.977 24 28.842
All lengths are in inches

2 yagi stacking distance/info (4 over 4 stack)
10.7 dBi for one yagi with a 50 degree 3 dB beamwidth
1/2 wavelength apart is 117.8" for 12.05 dBi gain
5/8 wavelength apart is 141.4" for 12.42 dBi gain

No fancy power divider needed! You'll need two pieces of 3/4 wavelength of 75 ohm coax to each yagi from a T-connector mounted midpoint between the yagis. The end result is a 50 ohm match for your feedline to the shack.

I used RG-11/U for my 75 ohm coax. My brand had a velocity factor (or velocity of propagation) of 78% (0.78). Coax velocity factors can been found on the coax datasheets. You can find your coax length by: 8856 multipled by your velocity factor and then dividing that by your frequency. In my case: 8856 x .78 divided by 50.1 Mhz equaled 137 14/16" of coax length to each yagi from the T-connector.

Element Position is the element placement, with respect to the reflector (0.00). The reflector is placed about 1-1.5 inches from the end of boom.

Columns 3,4,5 are the element lengths. The A50-5 antenna elements are tapered, with a 3/4" tubing in the center and 5/8" element tips. To attain the proper element lengths, slide the 5/8" element ends in or out of the fixed length 3/4" tubing.

Extending the boom The original boom length is 12 feet for the Cushcraft A50-5S. The above element spacing requires a boom length of 15 feet 3 inches. The original boom is constructed using 3 sections of 4' aluminum tubing. One of these sections needs to be replaced by an 8' or two 6' sections. DX Engineering sells 6' sections of 1.25" OD tubing... get two! (Ex. The original center section is 50" of 1 3/8" tubing, .058 wall thickness. Replace it with an 8' length of the same diameter tubing.)



Yagis split to cover more azimuth


Azimuth plot of yagi stack