半波長ダイポールアンテナ	498
半波ダイポールアンテナ	0

Re Hendrik's latest reference, it really confirms that 半波 is most likely an abbreviation of 半波長.
I'll close this one off, and propose an [abbr] entry for 半波.

After submitting my previous comment I found another reference, close in topic to the original reference, that highlights the second way of parsing 半波長:
https://ja.fmuser.net/content/?6795.html

LOL. I did not pay attention to the fact that 半波長 can be parsed 半+波長 in addition to 半波+長 (the latter is more obvious to me, since the use of 半波 in 半波長 is just one of several possible contexts in electrics and electronics, and 半波 by itself is "a thing" for us electronerds). A less ambiguous reference for my original submission would have been https://www.rohm.co.jp/electronics-basics/ac-dc-converters/acdc_what2 or https://kotobank.jp/word/半波電位-1578986. My suggestion is therefore to leave this entry as 半波, since we do have 波長 in the database already (on the other hand, I would not object to you adding both 半波 and 半波長 to the data base).

It's often used prenominally but I don't think it's adj-no. The 〜の n-gram terms aren't adjectival.
GG5 tags it as a physics term.

@@ -12,2 +12 @@
-<pos>&adj-no;</pos>
-<field>&electr;</field>
+<field>&physics;</field>

GG5, 理化学英和辞典

Note that the quoted text had 半波長 which is what is in the references. It's 半+波長.

@@ -5 +5 @@
-<keb>半波</keb>
+<keb>半波長</keb>
@@ -8 +8 @@
-<reb>はんぱ</reb>
+<reb>はんはちょう</reb>
@@ -12,2 +12,3 @@
-<gloss>HW (half wave)</gloss>
-<gloss>half wave (HW)</gloss>
+<pos>&adj-no;</pos>
+<field>&electr;</field>
+<gloss>half wavelength</gloss>