Originally Posted by FrankJ
Well, I figured 300 Mhz for the center of the UHF Milair Band. Would that dipole be wide enough in frequency coverage to cover from 225 to 400 Mhz?
Well, the whole Mil air band band-width stretches from around 225Mhz - 400Mhz (at least here in Europe it does) – just short of a complete octave, so a plane/conventional single element dipole wouldn’t be my first choice (that’s my personal humble opinion only, nothing more than that).
A variation on dipole designs that I feel is going to give you better overall performance (over 1 octave* bandwidth) in a receive only antenna setup is going to be either the CAGE dipole, or a little heard of antenna on this forum known as a PRISMATIC POLYHEDRON dipole. This latter antenna is a very interesting design (no-where near as complicated to design and make at home as the name would imply) and offers huge potential.
Both the above antennas are easy to make (plans & dimensions are readily available on the net); they can be designed and built for whatever frequency/bandwidth combination you want. For 225Mhz – 400Mhz either type is neither too big to be impractical to make at home, nor to small to be awkward from a dimension tolerance perspective to construct.
Thin wall copper tubing between ½” – ¾” is great for the Pn** dipole, and a combination of similar diameter copper tubing “caged” with stainless steel multi-strand wire is ideal for the cage dipole construction. The very nature of these materials has the added advantage of ensuring they tolerate environmental exposure very well.
Of course these antennas, like cone designs (e.g. bi-cone, discone, inverted cone etc etc .... ) can be designed to cover bandwidths much wider than a single octave, but to optimize performance for reception, if you do decide to design/dimension one or the other, do so just for the frequencies & bandwidth you want to cover, and no more.
Get on to Google and research both antenna types – search Google images as well. Typing the antenna name/type in inverted commas will ensure only relevant articles & images are displayed, otherwise you could land up with 1000's of pages and images coming up which will just confuse matters and be a waste of time.
I’ve built both over the years, they are not difficult to construct - if you have any questions, feel free to ask.
* - an Octave in rf bandwidth terms is a doubling of preceeding bandwidth figure e.g. 200Mhz -400Mhz is 1 octave, 200Mhz - 800Mhz is 2 octaves, 200Mhz - 1600Mhz is 3 octaves
** Pn dipole & Prismatic Polyhedron dipole = same thing (Pn is an abreviation)