MilAir Test antenna

[mancow]

You need to solder those braids to the center conductors. Without that you are basically placing floating metal tubes around each antenna element. Although they aren't grounded to anything they are creating a shield.

 

[digitalanalog]

Prcguy & Mancow,

I didn't noticed the Shield issue until I uploaded the pics last night, then it stroke me something was wrong.

It's raining today so I will have to wait till it stops and take it down and add the shield to the centers.

No problem, I will Fix It...

 

[popnokick]

OCFD for milair

The builder of this antenna (RR user "NAVCAN") describes the milair performance as "AWESOME". It's an off-center fed dipole (OCFD) and very simple to make. In this RR post he includes lots of pics and SWR readings for the finished product:

http://forums.radioreference.com/sc...35-homebrew-off-center-fed-dipole-pt-2-a.html

 

[LtDoc]

How would this 'bundled' coax lengths antenna be any better than a simple tubular antenna element of the same length and diameter?
Those 'bundles' are a form of 'cage' antenna, whose benefit is less weight than a similar sized metal tube or conductor. Except at this frequency range, would there be any lessening of weight?
The only benefit of larger sized conductor element that I can think of is that they tend to have a wider usable frequency range than smaller diameter conductors. Would that off-set the difficulty/complexity of construction?
I like 'playing' with antennas too! Nothing wrong with that, and if one works, good!
- 'Doc

 

[prcguy]

Problem here is the OCFD was not designed for mil air, its not really "designed" for anything and its basically random lengths of metal with a TV balun that may or may not do anything useful.

If it works awesome then imagine how well a real antenna designed for that range might work.
prcguy

The builder of this antenna (RR user "NAVCAN") describes the milair performance as "AWESOME". It's an off-center fed dipole (OCFD) and very simple to make. In this RR post he includes lots of pics and SWR readings for the finished product:

http://forums.radioreference.com/sc...35-homebrew-off-center-fed-dipole-pt-2-a.html

 

[popnokick]

Wirelessly posted (Mozilla/5.0 (iPhone; CPU iPhone OS 5_1 like Mac OS X) AppleWebKit/534.46 (KHTML, like Gecko) Version/5.1 Mobile/9B176 Safari/7534.48.3)

No disagreement that an antenna specifically designed for milair is going to be best. However, I've got to believe that an OCFD is going to work better than the sheathed bundle of shielded, shorted center conductor coax cables pictured in this thread. And provide higher performance for less cost and effort. The pictures suggest what has been built is a vertical center-fed dipole with shielding over the radial elements. Maybe we'll all be surprised by the test results (?)

 

[mancow]

He said he was going to fix the shield issue. But, I would think that it would have been much easier to find a large diameter aluminum tube or perhaps use some of that tin roof flashing or a tin heating and air type tube as the elements. He could cut one side and reduce its diameter until it expands inside the PVC then pop rivet a connector on the end if he is using something that won't accept solder. It seems to me it would be much lighter, more robust and much simpler. That is, unless there is something inherently better about the caged design I'm not aware of.

 

[LtDoc]

The unconnected braid may as well not be there, it doesn't 'do' anything. One solution would be to connect that braid and center conductor (each section). Or, just do away with all but the outer sections, the inner ones aren't doing anything either (surface conduction at radio frequencies).
Bunch of alternatives, pick the one you like best...
- Doc

 

[ipfd320]

the way the pics look...he is using no shields at all...the feedline shield is soldered to 1/2 of the tube and the center conductor feedline is to the other 1/2 of the tube

Question??--would it be better for Reception if the tube was fixed in a horizontal than a vertical or possibly on a 45 degree angle since the higher radio signals would be comming out and downward from the aircraft

just a curious question not a suggestion.....let us know how it works....i wouldnt mind trying to build 1 myself

 

[prcguy]

As mentioned before, an aircraft at high altitude at a 45deg angle or 30deg angle up is still very close and you can pick that up with a paper clip for an antenna.

To receive aircraft at a great distance you need an antenna with low angle radiation above any surrounding obstacles because the aircraft eventually gets down low then below the horizon.

Reception to 200mi is not difficult and from an 800ft hill I have communicated with an aircraft at 34,000ft to a distance of 389mi on VHF as measured by GPS. This was with a 1/4 wave whip on the roof of a vehicle and if I was using a high angle antenna the range would have been much less.
prcguy

Question??--would it be better for Reception if the tube was fixed in a horizontal than a vertical or possibly on a 45 degree angle since the higher radio signals would be comming out and downward from the aircraft

just a curious question not a suggestion.....let us know how it works....i wouldnt mind trying to build 1 myself

 

[nanZor]

A single fat dipole is probably the most sensible project antenna for this band.
prcguy

I couldn't agree more. And that makes a perfect reference antenna to see if further changes improve or degrade the signal.

The first thing I see off the bat is that the element lengths are too long for milair, assuming 300 mhz as the center of the band.

For a dipole to help the op (I know you know this..)
468 / f Mhz = length in feet, so 468/300 = 1.56 feet (18.72 inches) total. Divide this in half for each side of the dipole and you come up with 9.36 inches per side.

With the last "in the air" photo, I'd run the RG8 transmission line through the cross-brace to each side of the vertical elements inside the tube with something simple, like #10 wire.

But here the objective is a wider bandwidth for the large spectrum of milair, so a larger element is desired. One GREAT option would be to use 9-inch copper tubing (each side) jammed into the plastic T section itself. Use end-caps to keep water from going down inside. Keep the leads from the RG8 transmission line to the tubing short.

The reason to use 9 inches or even perhaps 8.75 inches (not super critical) instead of the formula figure is that the formula assumes a small(ish) wire instead of tubing. When using fat tubing, the resonance goes lower, so you have to purposely shorten the antenna to compensate to raise the resonant frequency back higher up. This also applies when using wires inside pvc - it too lowers the resonant frequency, so you purposely shorten the elements a little bit if you put wire inside a pvc covering.

Instead of tubing, you could also build a "cage" of wires around the existing pvc as a support. 4 to 8 wires could simulate the tubing - any more and you might as well use tubing! Here again, we are placing wires right next to pvc, which tends to lower the resonant frequency, so maybe 8.5 to 9 inches in length on each side is more appropriate than the calculated formula.

If the bundled RG8 is used as a substitute for fat tubing, or a cage of wires, then make sure the braid is actually the antenna by shorting the center conductor of the individual bundle elements to their own shields, and cut to about 9 inches each side, BUT you MUST weatherproof the coax bundle - moisture WILL wick itself into the shield, causing corrosion and generally detune all your hard work in a short matter of time. Don't worry about any references to velocity factor, since we are using the shield as the elements, where only a small VF could be calculated for purists - but for this antenna, don't sweat it. Personally, I'd stick to a single fat wire(s), short tubing, or a "cage" of wires, rather than use RG8 as the antenna element itself. Use the RG8 just for the transmission line back to the shack.

Either way, I commend the DIY spirit going on here!

 

[digitalanalog]

Thanks to all for your comments and suggestions.
I appreciate them all.

I have been testing with the antenna "AS IS" and the results are this.
Civilian Air Bands = Fair at Best
MilAir Band 225-400MHz = VERY POOR TO NOT AT ALL(wow and this is what it was built for)
150-160MHz = Fair
400-500MHz = Fair

I tested it only on the bands I normally scan.

And I must say "I am NOT Impressed"

So I will re-Do the entire antenna taking some of the suggestions above and
see what happens, I will repost new pics of the new design (not sure yet how I will be doing it),

Again, Thanks to all for your comments and suggestions, I know there are better and probably easier way's to achieve what I want, But Buying an antenna is the easy way out (for me anyway) and who know's maybe I will come up with something that works Better then I hope for .....

Hoping to get on the roof today to take it down (maybe) and start redesigning AGAIN.. lol

 

[nanZor]

Again, Thanks to all for your comments and suggestions, I know there are better and probably easier way's to achieve what I want, But Buying an antenna is the easy way out (for me anyway) and who know's maybe I will come up with something that works Better then I hope for .....

You're doing fine! Since the simple dipole is a common standard against which others are judged, I'd start out by building this simple reference antenna, take your measurements and notes, and then build up from there for comparison, ie going wideband, multiband, etc. (hint - for multiband, think "fan dipole" - vertically of course) 468/f mhz = total length in feet which you cut in half for each side.

Building the simple dipole first (with either wire or tubing) might cut down on burning through a lot of material at the outset. Try just using 9 inches of material (wire or tubing) on the center conductor and shield of the coax leading to the shack, and see how milair does first. Then you can get more complex but now you'll have something to compare it to, other than just needing an amp.