Dipole for UHF milair band

[FrankJ]

I'm experimenting with making a dipole and I want to cut it for the center of the UHF milair band (300 Mhz).

What lengths should I cut each element for?

Does it matter if it's a 1/4 or 1/2 wave?


Frank

[ka3jjz]

Each leg is a 1/4 wave long (actually if memory serves you would need to knock off 2% or so for ground effects if it's too close to the ground...).

The problem you're going to run into is that the further you go from the desired frequency, the response would become more problematic, depending on how high you have it off the ground. Without modeling it's hard to know precisely how it would react.

I assume you're going to mount this vertically...if you're going this route, why not just make the Off Center Fed Dipole we have the plans for in our wiki? As I understand it, the response is rather high-angle, which would be pretty good for hearing planes in the distance. It's not necessarily cut for any frequency, but from all that I've heard about it, it's a pretty decent performer for aircraft comms. Easy to keep hidden, too...

Here's the link

Homebrewed Off-Center Fed Dipole - The RadioReference Wiki

Mike

[FrankJ]

Yes, I want to mount it vertically just like the OCF dipole. I have used this antenna before with good results, but just thought I would get better results on UHF milair with both elements cut to 300 mhz.

I know there is an antenna length calculator, but again, not sure if I should cut the lengths for a 1/4 or 1/2 wave?


Frank

[prcguy]

If you make the dipole elements very fat like around 2" diameter as found on some military dipoles it can cover the entire band with a reasonable match. Unfortunately its hard to calculate the length on something that fat for the UHF band and its probably easier to cut and tune with an antenna analyzer.

On the high angle antenna for airplane reception, aircraft at a distance are low angle and when they are high angle to you they are usually close enough to hear with a paperclip for an antenna.

If an aircraft is at 35,000ft altitude and a 45degree look angle, its only about 10mi away. At a 30 degree look angle its only about 12mi away. When it gets down to about 5deg its maybe 80mi away and so on plus you start getting obstructions where a little antenna height helps.

Good aircraft reception = a good low angle antenna.
prcguy

[benbenrf]

Originally Posted by FrankJ

I'm experimenting with making a dipole and I want to cut it for the center of the UHF milair band (300 Mhz).

What lengths should I cut each element for?

Does it matter if it's a 1/4 or 1/2 wave?


Frank

Frank - is it just +/-300Mhz frequency that interests you? What about the rest of UHF Mil air band?

[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?

[hertzian]

For casual monitoring, that would be ok if using tubing. Each side is 1/4 wave:

234/f mhz = length in feet. 234/300 = 0.78 feet. 0.78 * 12 = 9.36 inches each side.

Use large tubing, 1 to 2 inches for bandwidth. Wire is much narrower in bandwidth. Run your feedline away from the dipole horizontally for a few feet and then down. Otherwise, the coax couples with the mast and skews the nice pattern, typically upwards.

In reality, even with tubing it is hard to cover that wide a frequency range really well especially at the ends of the band. If you have a lot of activity either higher or lower than 300 mhz, I'd recalculate and cut for that as the center. But 9.25 inches each side should get you in the ballpark.

[benbenrf]

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?

Frank

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)