The original Bird product line started at 25 MHz and went up from there. Each letter type covered a frequency range of just over two-to-one from lowest to highest. Type "A" 25-60 MHz. Type "B" 50-125 MHz, type "C" 100-250 MHz, type "D" 200 to 500, I think. Type "E" covers 400 to 900, I think.
The product line has expanded since the 1950s, and custom slugs for broadcast and industrial customers show up on fleabay from time to time. Don't know when the "H" slugs first appeared for sale.
But the tricky part of making a wattmeter's pickup element accurate is the frequency compensation. A 2-to-1 range is not so tough.
But the "H" elements cover a range of 15-to-1, 2 to 30 MHz.
Much trickier. As a result, much more expensive.
Bottom line for 27 MHz is that the difference is price. The "A" element, or equivalent from Coaxial Dynamics or Dielectric Communications will tend to cost half the price of a "H" slug of the same power rating.
At 27 MHz you're 10 percent below the top of its 30 MHz upper spec. You're about 10 percent above the lower limit for the "A" slug.
If you really don't care to go below 25 MHz, the "A" slug will save you money.
And that's the biggest difference that I see.
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