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Discussion Starter · #1 ·
will performance be affected at all by passing aluminum tubing directors, the reflector, and part of driven, right thru the Nibco PVC T fittings attached to the PVC boom, or do i need to compensate for electrical losses by increasing the elements lengths by a certain amount?


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Due to accuracy limitations, when modeling Yagi's in NEC, the metal Boom is usually MISSING.
When cutting METAL, the mutual impedance effect of a METAL Boom needs to be countered
by EXTENDING the length of the PASSIVE (not Driven) elements by a "Boom Correction".

The amount of Boom Correction depends on the Boom Diameter (or square equivalent)
and HOW the passive elements are attached to the Boom:
a) Thru the middle of the Boom, but insulated (plastic inserts in holes),
b) Thru the middle of the Boom and bonded (welded) to the Boom,
c) Mounted to the TOP of the Boom and bonded,
d) Mounted ABOVE the Boom and hence insulated.
and, or course, there is NO Boom Correction when using a non-metallic boom.....

Fol. article discusses how much Boom Correction is needed:
Note Boom Correction is MORE at UHF than at calculated VHF freqs.
Boom Correction is reduced the further away the elements are mounted
ABOVE the Boom, until there is ZERO correction when the elements are
mounted at a distance of half the boom diameter. A straight line
approximation could be defined between this ZERO point and the point
of maximum Boom Correction when the element is actually touching.
Hence if the elements are only slightly above the Boom, use the Max value.

Fol. article analyzed the frequency shift caused by various Boom to Element distances:

Additional articles by Dragoslav Dobricic, YU1AW, can be found here, incl.
Square Booms and various sensitivity investigations:

FYI: K7MEM's On-Line Yagi Calculator has two choices for metal boom attachment,
in addition to the non-metallic (i.e. NO Boom Correction) choice. Note that the
Active Driven Element is NOT assigned a Boom Correction, since it is assumed that
it is insulated and ABOVE/BELOW the Boom....hence ZERO or negligible correction.
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