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Old 09-14-2013, 7:54 PM
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Default Home Brew Milair, Off Center Fed Dipole

Home Brew Milair, Off Center Fed Dipole (chunky but not fat)

Not sure if this will work for it's intended purpose but having nothing to loose it was worth a shot.
I could not find any on-line info for any OCFD construction using anything other then normal coax and
a few using 3/8" copper or aluminum tubing. I am aware of using 2" aluminum or copper for a Fat OCFD as per discussion in several threads mentioned by PRCGUY, and I plan on trying that very soon.

So this is constructed of stuff I had laying around in the garge from other antenna builds through
the years.

Material:
1" x 1" x 1/8" wall aluminum square tubing
1" x 1" solid plastic bar stock (non-conductive)

So I used two pieces of the tubing, the top section is 32 1/2" long, the bottom section is 30 1/2" long
with a combined overall length of 63 1/2", these were not cut to length, it's just how long they were
when I pulled them out from under the work bench, If I need to trim them I can. The gap between
them is 1/2"

I used the 1" x 1" solid plastic bar to hold the two pieces of tubing in-line with each other and to keep the 1/2" spacing, I counter sunk the screw heads into the plastic just to keep them out of the way. I also drilled and counter sunk two more holes into the plastic to attach the mast mounting bracket.

Nothing in the way of mounting is coming in contact with the dipole tubes.

I used a piece of 18awg commscope coax I had laying around, looks to be about 50' long, I attached the center conductor to the top section and the shield (braid) to the lower section using ring terminals and self taping stainless screws, The tape job is not very pretty and I plan on redoing it if this antenna performs as a good milair antenna, Also I did not have a balun on hand so I will add that if needed, again if this antenna works there will be some changes, But i wanted to see if it is going to work for milair, I don't need it for anything else I have all the other bands I monitor covered with the other antennas I have up.

The black electric tape holding the coax to the lower tubing will come off when I put this up in the air,
It was just used to keep the coax clear from drilling and getting everything together.

Plan on mounting tomorrow on a Plastic mast, so as to keep the aluminum clear from any interferance
of a steel mast.

Would love to hear comments or concerns you all may have, I am in a bad section because I am rural and
I am down in a valley, I need a Good milair antenna to help pull in the week signals and hopefully more
traffic in general.

I will be hooking this up to my GRE superamp to get the added increase in receive that I so desperatly need.

Will let you all know how it works out.

Your comments/concerns are appreciated.

Last edited by digitalanalog; 04-28-2014 at 7:57 PM..
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Old 09-14-2013, 8:06 PM
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The lengths of each leg in an OCFD are critical, both to the feed point impedance and the bandwidth. Did you use lengths specified by prcguy? I think he wrote about a milair version awhile ago here on RR. Very different than the normal 18 and 48 inch legs you use for all-band scanner antenna. But tough to search RR here on iPhone looking for correct milair band lengths for OCFD.
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Old 09-14-2013, 8:11 PM
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No, as i mentioned I did not cut the tubing, thats just how long they were when i pulled them out from under the work bench, I have no idea how long they should be because of the size of the tubing, and the fact they are square and not round, I am not sure if that matters.
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Old 09-14-2013, 8:13 PM
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the specs Prcguy mentioned were in relations to a 2" round tube, These are 1" square tube.
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Old 09-14-2013, 8:14 PM
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As well I should mention, I would really like to go with a Full Wave antenna. I need all the help I can receiving.
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Old 09-14-2013, 8:25 PM
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Whether round or square elements won't matter. But length is crucial to determining resonant frequencies of any antenna. You might get lucky, but easier to cut it to correct length before putting it up. Otherwise may be an unnecessarily bad performer on milair. Prcguy may chime in here... I notice current posts from him now on RR.
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Old 09-14-2013, 8:29 PM
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I agree, The proper length would make all the difference, But I looked and could not find anything to give me the information. everything is for Coax dipole, and that is not my desire, I don't think coax will help me, I need more then that, Like a FAT dipole and was hoping this would at least come close.
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Old 09-14-2013, 9:17 PM
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Give it a try! Heck, you put the work into it, I'd like to know how it performs for you.
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Old 09-15-2013, 5:32 AM
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For a simple half-wave dipole centered around 300 mhz milair with your tubing stock, just cut for 8.5 inches on each side.

The tap points you made for your coax should be a little closer to the end of the tubing.

What you don't want to do is tape the coax to the bottom element. You want it coming away from the vertical dipole horizontally for at least 16 inches before bringing it down a mast.

Last edited by hertzian; 09-15-2013 at 5:34 AM..
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Old 09-15-2013, 7:31 AM
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@ hertzian:

Yes as I mentioned in the OP, The tape holding the coax was used temp. just to keep the coax out of the way while I was building the antenna,it will not be this way when I put it up.
I was not aware of the coax being deployed horizontally before running down the mast, what is the purpose of this and why 16"?

I used a RF Signal Frequency Wavelength Calculator found at csgnetwork.com to run so numbers.
I admit I am not sure if these dimensions are for Full length(overall) or for each element of an antenna.
So here is what that particular calculator showed.

using 300.00MHz for calculations:
1/4 wave = 9.84 inches or 0.82 feet
1/2 wave = 19.68 inches or 1.64 feet
5/8 wave = 24.59 inches or 2.05 feet
3/4 wave = 29.52 inches or 2.46 feet
full wave = 39.36 inches or 3.28 feet

So with this information (however accurate it is), with the two elements I am using (32" and 30")
I should fall into the 3/4 wave section IF the wavelength is for Each section and Not for Overall.

So, I admit I am not an antenna designer, but I am more then willing to try anything that might help
plus it cost just about nothing to try other then time....make some that did work and made some that failed....not going to stop me from trying, Just can't seem to justify some of the way over priced antennas.

This is going to be put up today, problem is there is so little milair traffic in my area, plus it's the weekend, so it may take some time before I hear anything to be able to tell if it's working or not.
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Old 09-15-2013, 7:38 AM
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That looks interesting.
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Old 09-15-2013, 11:59 AM
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What hertzian wrote (8.5 inches each side) is for a 1/2 wave milair dipole.... and is going to give the best results for the antenna pictured (along with ensuring the coax leaves the antenna perpendicular to the elements for at least 16 inches).
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Old 09-15-2013, 12:11 PM
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Understood 8 1/2" for 1/2 wave, still don't understand why the coax has to be perpendicular, not seen any other mention of this in reference to any other antenna.

As well, I don't want a 1/2 wave antenna, I want a Full wave.
Am i correct a Full Wave Antenna will be a better receiving antenna?

Remember, i am Rural as well as down in a valley.
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Old 09-15-2013, 12:22 PM
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To cover the entire UHF mil air band I would make a 1/2 wave dipole and not a modified off center version and use fat elements in the 1.5 to 2" diameter range. You can make it a coaxial dipole where the coax exits the bottom of the lower element but you want the coax to be centered as it exits the element and a doughnut made of Styrofoam pushed up into the bottom element can work well for this.

Spacing between the dipole elements would have to be determined by experiment and a common mode choke on the coax near where it exits the lower element would help with tuning. The elements will also be shorter than calculated due to the fat diameter.

Making one of these has been on my list for awhile and if successful I'll post the info. Otherwise I already have a bunch of UHF mil air band antennas and would only be doing this for the curiosity.
prcguy
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Old 09-15-2013, 12:50 PM
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Quote:
Originally Posted by digitalanalog View Post
@ hertzian:

This is going to be put up today, problem is there is so little milair traffic in my area, plus it's the weekend, so it may take some time before I hear anything to be able to tell if it's working or not.
Looks like a fun project. You might be able to see if there are any remote UHF transmitters near you. The ATC controller side is usually simulcast with so you may be able to pick up the ground side even if there is no milair traffic. Just a thought in case you didn't think of it.

Good luck!
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Old 09-15-2013, 5:17 PM
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O.k., So I shortened the tubes to 9" each side, I also ran the coax up the center of the bottom tube and used foam rubber to center the coax.

It's up about 35' , so now it's just wait time to see if it is going to work any better then the ST2 antenna's I was using.

It is run into a GRE super amp them split to two scanner.

A few pics as well.

Last edited by digitalanalog; 04-28-2014 at 7:57 PM..
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Old 09-15-2013, 5:20 PM
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I will add, It's picking up a Bunch of garbage on 265.370 - 265.650, can't really tell what it is, it sounds like some type of TV broadcast but way out of tune, I tried tuning it in with the BC-895xlt at 5khz steps, but it will not tune in fully to be able to under stand exactly what it is, So i locked all the garbage channels out.

Never had this problem using the ST2 antennas, so it is picking up Something better just not sure what it is.
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Old 09-15-2013, 8:30 PM
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You could be getting the satellite downlink pirates that steal access on military satellites. If so, you've made an antenna that may also work very well on milair. If not, you may need to add a filter to reject whatever you're getting in that range. Either way, stay tuned and let us know how it does. Based on other posts here on RR regarding using dipoles for aircraft reception, you should have a good antenna there.
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Old 09-15-2013, 8:56 PM
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Quote:
Originally Posted by digitalanalog View Post
Never had this problem using the ST2 antennas, so it is picking up Something better just not sure what it is.
You're hearing SatCom signals.
I can pick it up in NY also.

I didn't bother switching antennas around. This is what that part of the band looks like with an ST2 antenna and 30 db gain preamp mounted at the antenna.

Rich

Edit: By the way, if you can hear that one, try tuning to 256.600 there's 'currently' a much stronger SatCom (pirate) signal there.



Last edited by rbm; 09-15-2013 at 9:02 PM..
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Old 09-15-2013, 9:56 PM
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Quote:
Originally Posted by popnokick View Post
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You could be getting the satellite downlink pirates that steal access on military satellites. If so, you've made an antenna that may also work very well on milair. If not, you may need to add a filter to reject whatever you're getting in that range. Either way, stay tuned and let us know how it does. Based on other posts here on RR regarding using dipoles for aircraft reception, you should have a good antenna there.
I made it for milair, that's the only purpose for it, if it does not work for milair it's coming down.
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