New to Mil Air Comms

[ka3jjz]

Depends on the aircraft, I'm sure - some are equipped with HF (3-30 Mhz) radios, and use the HF-GCS (Global Communications System), otherwise known by its much older name, GHFS (Global HF System). I've heard aircraft making phone patches on this system numerous times. We have a wiki page on the subject

http://wiki.radioreference.com/index.php/USAF_High_Frequency_Global_Communications_System

73s Mike

 

[trainman111]

Mil-Air Antenna

Okay, I've done some research and I found some plans for a ground-plane antenna. Combining the plans from the two websites following this text, I would like to make a ground-plane tuned to 223mhz. with four radials and then your main vertical element. Would this be a good plan? I'm either going to put it up in the attic or up by the chimney (20-25 ft, 25-30 ft. respectively).

http://www.hamuniverse.com/2metergp.html
http://ku4ay.net/groundplane.html


Nick

 

[ka3jjz]

Quite possibly; you might want to retune the antenna for more of a midpoint - it's a large range of freqs after all - maybe around 300 Mhz or so. Getting it outside is preferable - you would not have issues with possible shielding from roof materials, siding and so on. The higher the better. Feed it with good quality coax and waterproof it well, and I don't see why it wouldn't work well as a starter antenna.

Not a bad choice at all. Have fun 73s Mike

 

[trainman111]

Well, it looks like ive got myself some construction ahead of me! Do you have a formula to calculate the lengths of the vertical element and the four radials? I have found so many different formulas I don't know which is right. Do you have a reliable one? Also, anyone know any plans to make a discone antenna? I looked at the Diamond D130J and thought, well maybe I can make this myself for under $95. I will do some searching on my own also, but if anyone's get some plans, please share!

Nick

 

[ka3jjz]

The formula on your hamuniverse link will work just fine. Remember, we're just listening here, not transmitting.

In fact, let me throw something else out here - you can make the ground plane dual band by cutting one vertical element for one set of freqs (say 140 mhz - which will put you right in the middle of the 138-144 milair band, where many air national guards hang out...) another vertical element for 300 mhz or so. Cut the radials 5% longer than the 140 mhz element and you should be good to go.

I'd be interested in where you saw other formulae for this.

As to a discone - I'd bet the ARRL antenna handbook has something on this. I would check the Scanner Antenna wiki links and see if something comes up....73s and GL Mike

 

[trainman111]

Thanks Mike...How much better is discone for mil-com monitoring versus a ground-plane. As for the formuals, i dont remember where I saw them...they were somewhere on the net when i was browsing...

Nick

 

[ka3jjz]

A discone is much more broad banded, and as such, is a better choice when trying cover a wide range of frequencies It has little or no gain because of this, and is a good choice for an antenna in an urban area, where a large amount of gain may overload your receiver.

However, don't let that stop you - a ground plane is a sound, simple choice - and as my old Marine DI used to say (he had retired from Parris Island, and was my first boss in the DP industry)...KISS 'Keep It Simple Stupid' (actually he had something more colorful for the second 's' but this is a family board....) It's much easier to build and won't cost an arm and a leg to get together.

As for the formula - interestingly, the much respected Antenna Elmer website has an online calculator- however, he doesn't lengthen the radials by 5% as the HamUniverse site suggests. I suspect the extra 5% is for pruning to get a good match - not really what we need to worry about here. Here's the URL for that online calculator...

http://www.qsl.net/w4sat/gndplane.htm

73s Mike

[edit] Since we're on the subject of broadbanded antennas, don't forget to look at the AC6V supersite - there are TONS of antenna links there, and I wouldn't doubt that he'd have something on discones. The link is in our Scanner Antennas wiki.

Something else that's quite broadbanded, and has attracted a good deal of attention in the past - the Off Center Fed Dipole. Again, from our Scanner antennas wiki...
http://wiki.radioreference.com/index.php/Homebrewed_Off-Center_Fed_Dipole

Several folks on the Antennas board have built this and had good success with it. The guy that put that article up is a member of this board.

Finally, to divert from antennas for a moment, what about frequencies...again, the Wiki comes to the rescue

http://wiki.radioreference.com/index.php/ARTCC
use the Washington DC page (at the bottom of this page) - the FAA ATA100 freqs are not the most accurate (as has been discussed here, on Larry Van Horn's Milcom blog, and other places) - though I'd take special note of the 'Do Not Publish' frequencies
(edit on that) I've been told by another source that those 'Do Not Publish' freqs are quite outdated - not very surprising - so stick with the data that appears after the ATA100 - and perhaps put the 'discretes' in a lower priority bank, since we're not very sure about how good they are...

http://wiki.radioreference.com/index.php/Potomac_Consolidated_Terminal_Radar_Approach_Control
which covers areas north of you, but with the number of flights coming and going, I have no doubt that you'll hear lots on these frequencies

http://wiki.radioreference.com/index.php/Andrews_Air_Force_Base,_MD
I'm not sure if you're in the flight path of aircraft leaving or going to Andrews, but if you are....'nuff said.

 

[trainman111]

Thanks for all the great info Mike! You def. helped a great deal! About the calculations...what is the wavelength that that calculator uses? 0.78' doesn't seem like the right size compared to other lengths I've seen for the Mil-Air band. If he's using 1/4 wavelength per se, but you want to make it 1/2, do you just multiply your calculations by two? Again thank you for all the great info!

Nick

 

[ka3jjz]

He's using the old 234/f(mhz) quarter wavelength formula - it states that right on the page. The result is being presented exactly the same way as if you did it by hand, or using Windows calculator. Try it, you should get exactly the same result.

73s and GL...Mike

 

[trainman111]

"He's using the old 234/f(mhz) quarter wavelength formula - it states that right on the page"

-Duh...how could I not see that? I don't know, anyway, what's a good diameter to use to make the radials and the vertical element? By browsing the net, I found a couple different sizes people have used. I know that if i make the diameter larger, the antenna becomes more broadband. That's what I want right??

Nick

 

[ka3jjz]

Whatever you can do to make the antenna more broadbanded is going to work in your favor, particularly when working milair like this. I've read - maybe it was on the Off Center Fed Dipole article - that the thicker the diameter of the pipe, the more broadbanded it becomes. I can't say I have any experience with that - I don't - but if that's what you have handy, by all means use it. As an aside, the thicker elements would in all likelihood stand up better to the weather. 73s Mike

 

[trainman111]

Thanks Mike. I've been looking on the net and I found a ground plane antenna tuned to the VHF aviation band. It's a pretty good size antenna. Now, if I wanted to make a UHF antenna, thats gonna be a really good size antenna! Should I stick with my first plan or buy this one? How would it work with UHF Mil-Air Comms? It's a $28.95 buy-it-now deal on E-Bay.

LINK:
http://cgi.ebay.com/Base-Scanner-An...4QQihZ015QQcategoryZ14956QQrdZ1QQcmdZViewItem

Nick

 

[ka3jjz]

It looks to be a bit too long for milair (altho it might work passably in the 136-144 mhz range); in fact, from the logo, it's almost certain to be;

http://www.centerfireantenna.com/scan.html

Do yourself a favor - make one. You'll learn more that way. 73s Mike

 

[trainman111]

Do yourself a favor - make one. You'll learn more that way. 73s Mike

Thats what I thougt too ;->. Does 22" or so sound like the approx right length for the radials and element? (I have more precise measurements but I wanted to run it by you to make sure I got the right measurements. I don't remember what wavelength the calculator was at.)

Nick

 

[ka3jjz]

Go to message 27 in this thread and do the math using the calculator and Windows calculator. The lengths don't have to be spot-on but in the ballpark will do. Remember, we're not transmitting, just receiving. And don't neglect a good length of coax - cheap RG58 or 59 will definitely ruin your day. 73s and happy building Mike

 

[trainman111]

Hey Mike, what would you recommend for a good coax? Right now I'm using a belden type (can't think of the exact name of the top of my head) Also, if I wanna make the wavelength 1/2 and not 1/4, do I just double the measurements that the calculator gives me?

Nick

 

[prcguy]

For about the same price you can get an ok looking Discone that will cover VHF air, UHF mil air and lots more. See Ebay item 290017660270. I have seen Workman or Tram brand versions for as low as $19.95 or $16.95 in qty of 10.
prcguy

Thanks Mike. I've been looking on the net and I found a ground plane antenna tuned to the VHF aviation band. It's a pretty good size antenna. Now, if I wanted to make a UHF antenna, thats gonna be a really good size antenna! Should I stick with my first plan or buy this one? How would it work with UHF Mil-Air Comms? It's a $28.95 buy-it-now deal on E-Bay.

LINK:
http://cgi.ebay.com/Base-Scanner-An...4QQihZ015QQcategoryZ14956QQrdZ1QQcmdZViewItem

Nick

 

[ka3jjz]

Hey Mike, what would you recommend for a good coax? Right now I'm using a belden type (can't think of the exact name of the top of my head) Also, if I wanna make the wavelength 1/2 and not 1/4, do I just double the measurements that the calculator gives me?

Nick

This has gone too far off topic - so I'm making this the last answer here - post to the antennas forum if you have further questions .

Now as to the coax - please read the WIKI. We have a couple of good links on coax and the loss you incur with each type. If you're not going too long, you can probably get away with RG6. Don't forget to waterproof your connections with a good sealant and taping.

Don't change the figures the calculator gives you . A ground plane is based on a 1/4 wave design. I'd suggest once again, reading the wiki and start learning about antennas in general. The AC6V site has got numerous links - and I'd almost be willing to bet that some of them are basic enough for even a newcomer to get.

As to that discone for USD19.95 - I'd be concerned about just how well they are made. Sometimes cheap discones will fall apart in rough weather - the RS one has had problems with this in the past, for example.

That's it for now. C U in the antennas forum 73s Mike