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Can I convert my 900MHz Grid to a 750MHz Antenna?

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Wild Bill
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Joined: 06 Dec 2007
Posts: 36
Location: Erin, NY

PostPosted: Mon Dec 02, 2013 7:33 pm    Post subject: Can I convert my 900MHz Grid to a 750MHz Antenna? Reply with quote

I have a 15dB 900MHz parabolic grid antenna that I purchased from L-Com a number of years ago to boost my 3G service. Since Verizon went to 4G in my area I left the old antenna in place and plugged into the UML290 modem. Signal and speeds seemed decent, everyone was happy, Until the bills came. Verizon 4G data is expensive and it goes quickly.

Long story short we switched over to Millenicom, For some reason the SIM does not work for crap in the UML290, and the hotspot we got from then does not perform much better. (performance on either device is with or without being hooked up to the existing antenna. Note: the antenna has not been moved at all) If your browsing the web the ISP service seems to drop almost every minute at some point. But the weird thing is that when I have a terminal window open all the pings to www.google.com go through and an audio stream will not be interrupted. But all this would all be for a different post...

First I'm looking to get an antenna tuned to Verizon's 4G band and was wondering if I could modify my existing antenna? I dug up a post I read back in the day about converting a 2.4 GHz antenna to a 1.9 GHz and was thinking what the heck. Maybe it would work for mine going from 900 to 750 MHz.

I am no antenna expert so I am looking for help with this. What do you think, possible? Is all I'd need to do is move the feed horn further away from the grid? Any idea of what the difference should be or how to calculate it? Metallic or non metallic spacers?

I really need to get better signal strength before I can diagnose any other potential equipment issues. And I need to do that quickly before my wife cracks me over the head with her MacBook!

Then maybe buy a second antenna to do the MiMO thing that all the cool kids are doing these days Laughing

Thanks!

Bill
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pilot zenith
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Joined: 12 Jan 2011
Posts: 33

PostPosted: Thu Dec 05, 2013 9:24 am    Post subject: Reply with quote

I'm using same uml290,with Millenicom tied to my MBR95 Cradlepoint router.
Have A flat panel antenna connected to back side left connector and the 290 works perfect for 4G. I wonder if you got A bad SIMM card adapter thats causing the problems?
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Wild Bill
EVDO User


Joined: 06 Dec 2007
Posts: 36
Location: Erin, NY

PostPosted: Thu Dec 05, 2013 9:58 pm    Post subject: Reply with quote

My UML 290 is toast. Not sure what happened but its stuck in a boot loop. No matter what its plugged into, no matter what SIM is in it, it just keeps rebooting. New one from e-bay should be here tomorrow.

Back to the antenna thing. My question about adjusting the feed in or out is a bust, The antenna will have the same focal point no matter what the frequency. I believe I would need to modify the reflector / driven element.

Here is a pic of my antenna.



Here are the specs on the dish.
"900MHZ" antenna, 824-960MHZ actual
Max Dia - 992mm
Height - 597mm
Depth - 192mm
Focal point - 320mm (driven element location)
f/D Ratio - .32
driven element width - 122mm (3/8 of 900MHZ wavelength)
reflector width - 164mm (1/2 of 900MHZ wavelength)
reflector distance - 367mm
Driven element offset from center - 25mm

Reflector distance minus the focal point = 41mm (1/8 of 900MHZ wavelength)

So my question is... are the measurements and their indicated relationships to the 900MHz wavelength the only pieces of the puzzle that I need to modify? Is the relationship coincidence?

If I AM on the right track I think I would need to make the following modifications: given Verizon 4G LTE runs over 746-787MHz and I'd tune the antenna for the median frequency - 766MHz

driven element width - 147mm (3/8 of 766MHZ wavelength)
reflector width - 196mm (1/2 of 766MHZ wavelength)
reflector distance - 369
Reflector distance minus the focal point = 49mm (1/8 of 766MHZ wavelength)

Anyone have any thoughts? I've been trying to educate myself and things can get a little confusing for the lay person. Been looking mostly at Parabolic Antennas and their Feeds and www.antenna-theory.com
Note: my assumptions on the above dimensions were not sourced/corroborated by any information that I've found on the internet, it is strictly empirical.

Bill
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dmc271
EVDO Heavy User


Joined: 31 Jan 2011
Posts: 115
Location: Oklahoma, USA

PostPosted: Sun Dec 08, 2013 12:15 pm    Post subject: Reply with quote

Hi Bill,

You're on the right track. I actually did something very similar this past summer, but I haven't yet had a chance to test it. I removed the original feed, and then built a 3-element Yagi using the existing square metal pipe as the boom. The Yagi uses a folded dipole and points towards the grid.





To properly design the 3-element Yagi for the 747-787 MHz range, I used MMANA-GAL modeling software. Because the config was so complex with the presence of the grid, I could never get the software to do its own optimization. Instead, I optimized via trial and error over the course of probably 30 hours total. A painful process, but good results in the end (at least in simulation):



Here are the resulting SWR, gain, and radiation pattern (far-field) plots:







Note that the far-field behavior is expected to be better than the simulation shows, simply because MMANA-GAL has a hard time modeling a folded dipole.

I'd be happy to share the design specs with you. I just need the dimensions of your square metal tube. You can then follow the procedures listed here http://bcbj.org/antennae/lte_yagi_diy.htm for the construction of the Yagi (very easy with only three elements).

Like I said, I haven't had a chance to test the grid, but I expect it to work well. The modeling software has always yielded excellent results for me in the past.

Cheers,
Damon


Last edited by dmc271 on Sun Dec 08, 2013 7:06 pm; edited 1 time in total
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Wild Bill
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Joined: 06 Dec 2007
Posts: 36
Location: Erin, NY

PostPosted: Sun Dec 08, 2013 3:42 pm    Post subject: Reply with quote

I was kind of thinking that a yagi type design would be a likely candidate but I had no idea how to enter anything like this into a calculator.

Quote:
I'd be happy to share the design specs with you. I just need the dimensions of your square metal tube.


I would be greatly appreciative if you'd share your design specs.
My tube is 1" square. I could also fab up a tube of another size if needed.

Thanks!

Bill
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Wild Bill
EVDO User


Joined: 06 Dec 2007
Posts: 36
Location: Erin, NY

PostPosted: Sun Dec 08, 2013 4:09 pm    Post subject: Reply with quote

Found a 4' LMR240 Rp TNC Male to N Female cable on Amazon for$12

Would that cable be acceptable to make the balun out of?
Just cut off the TNC Male connector...
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dmc271
EVDO Heavy User


Joined: 31 Jan 2011
Posts: 115
Location: Oklahoma, USA

PostPosted: Sun Dec 08, 2013 7:00 pm    Post subject: Reply with quote

Sure thing, here's the design...



My only concern with the cable you mentioned is that its outer braid might not be made of copper or other material that can be soldered.

3GStore sells some high-quality RG58 that you might want to use instead. It's such a short run that RG58 vs. LMR240 isn't going to matter much here. But, don't buy cheap RG58; I recently bought Made-in-China RG58 where the outer braid consisted of six total strands! Like I said, I've had good luck with the RG58 from 3GStore. Belden and Times are other reputable brands.

The design assumes a 3 mm element diameter. Anything within +/- 1 mm of this should be OK. If you're using something beyond 2-4 mm, let me know and I can tweak the lengths/positions.

Remember, my design was tested only in software. I won't be able to physically test my build until spring. I'm hoping you're in a warmer climate or more tolerable of cold, and are thus willing to serve as the guinea pig.

Cheers,
Damon
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Wild Bill
EVDO User


Joined: 06 Dec 2007
Posts: 36
Location: Erin, NY

PostPosted: Wed Dec 11, 2013 8:30 pm    Post subject: Reply with quote

Well I have all the required parts except the cable for the balun. Unfortunately that won't be here until next Mon. Sad

My questions:

1) I don't see a width on the folded dipole. Is it the same 37mm as listed at http://bcbj.org/antennae/lte_yagi_diy.htm ?
2) I am not doubting your calculations but I am wondering why everything is so short. The focal point of the antenna is at 320mm but in your design the reflector is only at 229mm. Seems odd...
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dmc271
EVDO Heavy User


Joined: 31 Jan 2011
Posts: 115
Location: Oklahoma, USA

PostPosted: Thu Dec 12, 2013 4:58 am    Post subject: Reply with quote

Wild Bill wrote:
1) I don't see a width on the folded dipole. Is it the same 37mm as listed at http://bcbj.org/antennae/lte_yagi_diy.htm ?

AFAIK, folded dipoles are supposed to be robust to variations in width; my simulations and previous builds seem confirm this. Just make it 1.5-2 inches, wide enough to clear the boom.

Wild Bill wrote:
2) I am not doubting your calculations but I am wondering why everything is so short. The focal point of the antenna is at 320mm but in your design the reflector is only at 229mm. Seems odd...

I haven't done any calculations, just simulations, so don't hesitate to raise doubts. Unfortunately, once the design includes more than a couple of passive elements, the interactions are so complex that all one can really do is simulate.

In the design that I posted, I actually pushed everything back so that I didn't have to fabricate a new boom. I recall testing near 300 mm, but the results were only marginally better than the shorter design (only an ~1 dB difference in gain). But I agree with you that the current design seems too short! Perhaps part of the reason this is the case is that a Yagi is not just a point-source radiator, so the optimal location depends on the interaction of the grid and the Yagi's radiation pattern. I don't know.

What makes the design process so hard is that changing the position of the Yagi also changes the impedance, and thus the SWR. There were a lot of locations that gave great gains, but then the SWRs at 747 and/or 787 MHz would be so high that the antenna would basically be unusable without some custom impedance-matching network (and I don't have an SWR meter to build such a match properly). When this happens, to correct the SWR, you can move the Yagi's elements as a group, move individual elements, and/or change the lengths of the individual elements. As you can imagine, testing all of these possibilities quickly becomes painful. I tried many, many of these variations, and ultimately settled on the shorter design.

If you want to try tweaking on your own, I can send you my MMANA-GAL file. I have no doubt that there's a more optimal combination of the position of the elements as a group, relative positions of the individual elements, and element lengths that yield a higher gain, while simultaneously yielding a good SWR across the 747-787 MHz range, while simultaneously yielding a clean radiation pattern. Just be warned, it's a painful joint optimization! Smile
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Wild Bill
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Joined: 06 Dec 2007
Posts: 36
Location: Erin, NY

PostPosted: Sun Dec 15, 2013 8:28 pm    Post subject: Reply with quote

Damon,

Same 126mm loop on the balun as described here?

I ended up getting the LMR240 cable due to it having a N-Female connector on the other end. I don't see that cable as an option on K7MEM's site. Also, the dielectric is white, but not necessarily like foam. More plasticy.

Per this site LMR240 has a velocity factor of .84.

The closest thing that K7MEM has is "RG-8/U Foam" which is .80
If I select the "RG-8/U Foam" I get a length of 156mm.

Do you think that would be close enough?
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dmc271
EVDO Heavy User


Joined: 31 Jan 2011
Posts: 115
Location: Oklahoma, USA

PostPosted: Mon Dec 16, 2013 7:10 pm    Post subject: Reply with quote

Bill: Use 164 mm. This is a 1/2 wavelength assuming 0.84 VF and 767 MHz frequency.
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Wild Bill
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Joined: 06 Dec 2007
Posts: 36
Location: Erin, NY

PostPosted: Tue Dec 17, 2013 8:12 am    Post subject: Reply with quote

Does the braid in the balun get attached to the mast. i.e. grounded?
I didn't see where it was in your picture.

Mast and elements are done. Balun is cut. Just a little work with the balun and feed and this thing should be ready to go.

Tonight...
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dmc271
EVDO Heavy User


Joined: 31 Jan 2011
Posts: 115
Location: Oklahoma, USA

PostPosted: Tue Dec 17, 2013 9:00 am    Post subject: Reply with quote

You seem to be on a roll! Fingers crossed that it works!

No, there's no need at this point to attach the outer conductor to the boom.

(Functionality-wise, it makes no difference. However, if/when you later decide to add a lightening protector to the coax, then it's probably a good idea to ground the outer conductor.)
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Wild Bill
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Joined: 06 Dec 2007
Posts: 36
Location: Erin, NY

PostPosted: Wed Dec 18, 2013 8:20 am    Post subject: Reply with quote

Here's how it turned out.


So how did it work?

Equipment used:
Modified 15dB 900MHz parabolic grid antenna purchased from L-Com.com @1700’ in elevation
10’ LMR240 N-Male to N-Male
N-Female to FME Female adapter
FME male adapter cable
Navatel 4620LE Hotspot

All testing was done with the antenna inside the house pointed out glass doors or windows toward towers.

The cellular landscape:
Northern Tower 47* 4.94 Miles @1705’ Trees & Earth between
South Eastern tower 150* 8.8 Miles @1835’ Trees between
Southern tower 223* 9.43 Miles @ 1945’ Trees between

The results:
Hotspot in house w/o any antenna:
-109dBm RSSI 6dB SINR 4.70 Mbps Down 1.30Mbps Up 84ms PING


Hotspot hooked up to new antenna (best/closest tower):
-104dBm RSSI 8dB SINR 12.20 Mbps Down 8.35Mbps Up 81ms PING


The other 2 towers had similar signal strength and the download averaged around 8 Mbps

Unfortunately I forgot to test / include data from the stock 900 MHz antenna. That is kind of important here.


Interestingly enough I got at least the same speeds (actually they were typically better) by resting the hotspot on top of the antenna boom without having it hooked up to the antenna. This setup is certainly a lot easier but only works if you are going to put the antenna indoors.


A recap of the build
Materials:
Boom Material: 1" aluminum square tubing from Lowes
Bracket Material: 1" aluminum angle from Lowes
Reflector/element/director material: 1/8" brass bar stock from Lowes
Balun/feed material: LMR240 cable with attached N-female connector from Amazon
Standoff: PVC threaded hanger thing from Lowes
Insulation under balun/element: Splice tape from Lowes

Dimensions used:
Boom Length 305
Reflector Length 195
Reflector Location 229
Element Height 164
Element Width 37
Element Length 402
Element Location 177
Director Length 165
Director Position 143
Balun Length 164

I referenced the following web pages during the build:
http://bcbj.org/antennae/lte_yagi_diy.htm
http://www.k7mem.com/Electronic_Notebook/antennas/yagi_vhf_feed.html

Build steps:
-Boom and 2 pieces of the 1" angle were cut to length with a reciprocating saw.
-Mounting holes were drilled in the angle so that they would attach to the antenna, then they were riveted to the boom with 1/8" dia x 1/4" long rivets. Angle brackets were mounted 1-1/2" in from the end of the boom.

-1/8" holes were drilled in the boom for the reflector and director
-Reflector and director were cut to length using a Dremel tool with cut off wheel
-Reflector, director, and boom were roughed up / cleaned to (sort of) accept solder, elements were centered and soldered into place
-PVC insulator was cut down so that it did not interfere with the director, 1/8" hole was drilled for the folded dipole, and it was riveted to the boom with 1/8" dia x 3/4" long rivets
-Folded dipole element was inserted into the insulator and bent into shape using pliars


-Exposed boom was insulated with splice tape

-Cable for balun was cut 2" longer than required, jacket was removed to make the cable the specified length, un-braided the outer conductor and trimmed the dielectric insulation. Same for feed cable.

-Soldered outer conductors of balun and feed together

-Roughed up ends of dipole element to accept solder and soldered the inner conductors to the two ends

-Mounted the new boom into the parabolic grid and began the testing.
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Wild Bill
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Joined: 06 Dec 2007
Posts: 36
Location: Erin, NY

PostPosted: Wed Dec 18, 2013 8:37 am    Post subject: Reply with quote

So in the end -109 to -104dB is not much of an improvement but the speed improvement is incredible.

The SINR was all over the place 2dB to 12dB depending on what... which way the wind was blowing I guess.

For now I'll run it and see if it drops out at all.

Any suggestions on improving the signal strength? Did I screw something up somewhere?
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