MilAir Test antenna

Status
Not open for further replies.

digitalanalog

Active Member
Joined
May 18, 2007
Messages
562
Location
United States of America
Put this together today for testing.
It's only up about 18 feet, no where near high enough but
I ran out of day light so I just put it up real quick.

Tested so far with amp it seems to be working fine
I have to use an amp on all my milair stuff or I receive NOTHING.

Ugly looking right now,I will raise it and see if it's going to work as well as I hope it will
then I will make it look better, then pictures will be posted.

But so far so good.

Here is a simple CAD drawing for an idea of what it is and how I made it.
 
Last edited:

popnokick

Member
Premium Subscriber
Joined
Mar 21, 2004
Messages
2,177
Location
Northeast PA
If I understand the CAD drawing you sent, you've connected the center conductors of all the pieces of coax together, essentially creating one antenna element that is 36" long. This is not going to offer any gain or improved performance on milair over what you'd get from a 36" telescoping or wire antenna.

If you have 36" of vertical space to hang an antenna, you'd be much better off using the coax cable lengths to build a vertical colinear antenna that is tuned for milair. Colinear antennas offer very high gain.

Check out these two links. The first one is for 70cm UHF, but includes the formulas you'll need to determine the lengths of the coax sections based upon the milair freqs you want to cover.

Build A 9dB, 70cm Collinear Antenna

This article is also for UHF amateur band, but has useful info on the colinear antenna and how it works.

Coaxial Colinear For 432 MHz

There are also other articles if you search. One way to support all of the interconnected pieces of coax in a colinear is to hang it inside a piece of PVC pipe.
 

majoco

Member
Joined
Dec 25, 2008
Messages
3,792
Location
New Zealand
What's with the unscreened 'center conductor only' feedline? Perhaps if you'd joined the RG8 coax in the center of the two 12" legs with the screen to one leg and the center to the other to make a dipole you'd get better results.
 

ipfd320

Member
Banned
Joined
Jul 30, 2008
Messages
753
Location
W.Babylon N.Y. 11704
is there anyway you can post a picture of you creation instead of the cad drawing?

is the final result being sleeved in a tube of some sort?
 

popnokick

Member
Premium Subscriber
Joined
Mar 21, 2004
Messages
2,177
Location
Northeast PA
More on milair colinear

In my original reply I wrote "...if you have 36" of space to hang an antenna...". That was incorrect, and you're going to need more than 3 feet of vertical height for a milair colinear. At 432 mHz (much shorter wavelength than milair) an 8 element/section colinear is about 7 feet long. A colinear cut for milair frequencies is going to be longer than that. However, you may want to try a 4 element/section colinear at first, for two reasons:
1) While lower gain than an 8 element, it will take up less vertical height
2) Using fewer elements/sections increases the beamwidth, something that will probably be advantageous for milair use since your received signals are coming from above rather than the horizon.
 

prcguy

Member
Joined
Jun 30, 2006
Messages
11,000
Location
So Cal - Richardson, TX - Tewksbury, MA
Due to the nearly 2:1 frequency range of UHF milair its not practical to build a collinear gain type antenna for this band due to element to element spacing problems. A classic collinear project using offset 1/2 wavelength sections of coax might have 20MHz of useable BW at best at 300MHz.

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


In my original reply I wrote "...if you have 36" of space to hang an antenna...". That was incorrect, and you're going to need more than 3 feet of vertical height for a milair colinear. At 432 mHz (much shorter wavelength than milair) an 8 element/section colinear is about 7 feet long. A colinear cut for milair frequencies is going to be longer than that. However, you may want to try a 4 element/section colinear at first, for two reasons:
1) While lower gain than an 8 element, it will take up less vertical height
2) Using fewer elements/sections increases the beamwidth, something that will probably be advantageous for milair use since your received signals are coming from above rather than the horizon.
 

LtDoc

Member
Joined
Dec 4, 2006
Messages
2,145
Location
Oklahoma
Two things I can think of to make hearing more milair stations is to increase the antennas height, and to use an antenna with a fairly high angle or radiation.
That first 'thing' is common for any antenna especially VHF/UHF stuff. The second 'thing', the shape of the antenna's radiation pattern can get more 'tricky' and is certainly not 'absolute' by any means.
A typical 1/4 wave antenna has a sort of round, or 'ball' shaped radiation pattern with no gain at all (in fact, it has a negative gain when compared to a standard 1/2 wave antenna, which is the standard for gain). Stacking elements on top of each other (vertical array) does produce gain but at the expense of that 'ball' shaped pattern, the pattern get's sort of 'squashed', more a round 'oval' or donut shape. Not a bad shape for things not very high, but the radiation/reception from over head stuff is reduced. [That's how 'gain' is produced with any antenna, radiation/reception is moved from where it would normally be to some place else where it might be of more benefit. That's where the stations you wanna hear are, rather than over head, for instance.]
So, using a 1/4 wave antenna (groundplane type?) may do more for over head stations than those off to the sides. So how about getting some gain out of them? That's possible, but don't expect a lot of it. Then again, since it's VHF/UHF and 'line of sight', you shouldn't need a lot of gain anyway.
I don't have a good antenna suggestion for you. This is just to remind you about radiation pattern shapes (which are the same for reception pattern shapes). Some times, lots of gain (but in the wrong direction(s) ain't so good.
Have fun.
- 'Doc
 

prcguy

Member
Joined
Jun 30, 2006
Messages
11,000
Location
So Cal - Richardson, TX - Tewksbury, MA
A high angle of radiation is not needed, you want low angle. For an aircraft at say 35,000ft at a 45 deg angle up its less than 10mi away line of sight. You can probably receive that with a bare PL-259 connector center pin and no antenna. At a 30deg angle up it would only be 13.25mi away.

When an aircraft at altitude is very far away it gets further down near the horizon with more potential obstructions and you need a low angle antenna to work best.
prcguy


Two things I can think of to make hearing more milair stations is to increase the antennas height, and to use an antenna with a fairly high angle or radiation.
That first 'thing' is common for any antenna especially VHF/UHF stuff. The second 'thing', the shape of the antenna's radiation pattern can get more 'tricky' and is certainly not 'absolute' by any means.
A typical 1/4 wave antenna has a sort of round, or 'ball' shaped radiation pattern with no gain at all (in fact, it has a negative gain when compared to a standard 1/2 wave antenna, which is the standard for gain). Stacking elements on top of each other (vertical array) does produce gain but at the expense of that 'ball' shaped pattern, the pattern get's sort of 'squashed', more a round 'oval' or donut shape. Not a bad shape for things not very high, but the radiation/reception from over head stuff is reduced. [That's how 'gain' is produced with any antenna, radiation/reception is moved from where it would normally be to some place else where it might be of more benefit. That's where the stations you wanna hear are, rather than over head, for instance.]
So, using a 1/4 wave antenna (groundplane type?) may do more for over head stations than those off to the sides. So how about getting some gain out of them? That's possible, but don't expect a lot of it. Then again, since it's VHF/UHF and 'line of sight', you shouldn't need a lot of gain anyway.
I don't have a good antenna suggestion for you. This is just to remind you about radiation pattern shapes (which are the same for reception pattern shapes). Some times, lots of gain (but in the wrong direction(s) ain't so good.
Have fun.
- 'Doc
 

digitalanalog

Active Member
Joined
May 18, 2007
Messages
562
Location
United States of America
It's Done

After receiving the PROPER Guidance, I decided to redo this project again and this time for keeps.
As the pics will show, I took my time and considered all things possible and the end results are not mounted 35' above my house, No Milair received at the time of this posting, I will comment on it's performance after listening for a few day's.

This one probably the hardest built I have done so far, But it turned out very good and I am pleased, Now just waiting on some Air Traffic.

Here are the build pics.
 
Last edited:

prcguy

Member
Joined
Jun 30, 2006
Messages
11,000
Location
So Cal - Richardson, TX - Tewksbury, MA
Now that's starting to look like an actual antenna. Are the shields of the bundled coax connected to the center conductors? Basically the bundles of coax are just a replacement for fat metal tubes and if the center conductors are used but not the shields it might cause problems.

Instead of mounting the antenna from the center using a "T" you could use another PVC coupling at the bottom instead of the pipe cap, then maybe another section of PVC pipe a few feet long so you can attach to your mast with hose clamps and that would put the whole antenna above the mast and in the clear.

Otherwise looks interesting and it would be fun to check it with an antenna analyzer to see how it tunes across the mil air band.
prcguy
 
Last edited:
Status
Not open for further replies.
Top