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Cushcraft Junior Boomer Modifications
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REWORK OF THE CUSHCRAFT JUNIOR BOOMER:
Why Tri-Reflectors Don't Work: Amateurs are always searching for a method to beat the system. The NBS tri-reflector played perfectly into the amateur's hands. It seemed to give something for nothing, 0.75 dB gain without a longer boom. Let's look at it from a practical approach. The gain of any antenna is obtained by redirecting some of the energy radiated from the it into a specific direction. The theoretical isotropic radiator has no gain (or a power gain of 1) since it radiates equally well in all directions. Even the lowly dipole has gain (2.14 dBi) since its E plane has some directivity.
Let's now take a given Yagi that has a 20 dB front-to-back ratio, that is the strength of the rear lobe is 1/100 of the main lobe. Assume that I have just created the perfect quadrilateral reflector which is guaranteed to reduce the strength of the rear lobe to 0, that's an infinite front-to-back ratio without affecting any other part of the Yagi's pattern, except the main lobe. For this to happen whatever energy that was in the rear lobe would have to go into the main lobe and hence increase the Yagi's gain. Well, how much would be gained by redirecting energy that was already only 1% as strong as the main lobe.
To simplify the problem we will also assume that the beamwidth of the rear lobe is exactly the same as the main lobe (usually it will be narrower). When we calculate out the difference we find that the gain increase will be only 0.04 dB, that's only 4 hundredths of a dB. In essence we can put a good faraday shield behind a good yagi and not see any significant change in its gain.
On the other hand, the tri-reflector significantly lowers the impedance of the driven element. This adverse effect gives us the clue to what is happening when we add multiple reflector arrangements. The feed point impedance change means that the new reflectors are having inter-element mutual impedance effects. Stated another way, the additional reflectors are causing a tuning change in the directors of the Yagi. This is why the gain peak frequency dropped with the tri-reflector. This also means that if you have a Yagi that does not have its director string tuned correctly it may be possible to add some sort of multiple reflector arrangementwhich will increase its gain. Conversely, if the directors are properly optimized, it will be impossible to add any sort of reflector (dual, trigon, plane, or tandem) which will give significant gain increase. A properly tuned long Yagi with a single reflector will always win any gain verses wind load battle.
On the Cushcraft 214B, Tom Williams, WA1MBA, reported that moving the original middle reflector (40 3/16-inches long) out to 16 3/8-inches spacing from the driven element and moving out the shorting straps on the driven element to 4 1/2-inches gave good results. The computer analysis gives the natural driven element impedance for a stock 214B as 11 ohms and 20 ohms whena single reflector is used.
Fred Stefanik, N1PDM, provided information on his Cushcraft 214B rework. The trigonal reflector was removed and a new boom mount point drilled at 16 3/8-inches from the driven element.
Fred checked the remaining elements and found them to all short for an antenna designed for 144.0 to 144.3 MHz, but not by a uniform amount. His best guess is that the 214B was designed to operate around 145.250 MHz.
Directors #4 through #9 were shortened as follows:
- Director #4: remove 3/32-inches from each end.
- Director #5: remove 1/8-inches from each end.
- Director #6: remove 1/8-inches from each end.
- Director #7: remove 1/8-inches from each end.
- Director #8: remove 1/8-inches from each end.
- Director #9: remove 3/32-inches from each end.
Fred also found that the balun was one inch too short. A new balun which measured 25-inches from tip-to-tip was made from RG-213A/U. To obtain the best match, the shorting straps on the T-match were set for 10 1/16-inches inside dimension. This corresponds to a 4 1/2-inch measurement from the center of the connector center pin.
The resulting NBS/Cushcraft/N1PDM 2 meter 12 element yagi has a very clean pattern with one clean main lobe and a couple of small rear lobes. The front to back ratio is 20 to 25 db.
Last Update: February 15, 2002
