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Old Jan 02, 2012, 02:33 PM
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TexasAggie's Avatar
United States, TX, College Station
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The wire length he lists for 1280 (18.49"/469.8mm) and the quarter wavelength (2.31"/60mm) do not work out. He says the 60mm should be an eighth of the wire length. I'll try 235mm for each leaf (I was using 240, which the CL dimension) this time and see how that ends up.
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Old Jan 02, 2012, 02:43 PM
I think it'll work this time!!
Chairwolf's Avatar
399 feet above Roanoke, VA
Joined Jan 2007
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Originally Posted by TexasAggie View Post
The wire length he lists for 1280 (18.49"/469.8mm) and the quarter wavelength (2.31"/60mm) do not work out. He says the 60mm should be an eighth of the wire length. I'll try 235mm for each leaf (I was using 240, which the CL dimension) this time and see how that ends up.
Sounds good! I hope your trash can has less wire in it than mine!!
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Old Jan 02, 2012, 02:46 PM
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Well I bought a 2lb spool of welding wire, so it has the potential to be stuffed...
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Old Jan 02, 2012, 02:56 PM
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Here's where the discrepancy exists: his quarter-wave metric calculations are screwy. 75168/1280 = 58.7 NOT 60. The lengths in his photos also don't agree with his computations.
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Old Jan 02, 2012, 03:05 PM
I think it'll work this time!!
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399 feet above Roanoke, VA
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Quote:
Originally Posted by TexasAggie View Post
Here's where the discrepancy exists: his quarter-wave metric calculations are screwy. 75168/1280 = 58.7 NOT 60. The lengths in his photos also don't agree with his computations.
Don't forget to add a little for the hooks and the tail on the ground side.
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Old Jan 02, 2012, 03:07 PM
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I'm leaving the hooks off of this one, and the tails don't matter since they're attached when the center conductor is shielded.
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Old Jan 02, 2012, 06:29 PM
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Switzerland, LU, Buchrain
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Originally Posted by TexasAggie View Post
My first SPW attempt turned out pretty nicely. I'll put up pictures in a little while. One question: In the below video, the angles that are supposed to be about 17 degrees turned out to be about 40 degrees in mine. Is this okay or should I redo it?
40 degrees is a lot. Try it out. If they perform good enough for what you are after - who cares! If not, you found one point to improfe.

It seems like the relation of the arc length compared to the two legs is not ok then. Usually the arc should be about half the wafelength, the legs about a quarter. Note though that the correction factor to apply differs between the SPW and the CL antenna.

HTH

Markus
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Old Jan 03, 2012, 08:39 AM
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Amherst, VA
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TexasAggie - You might find the little hooks help a lot. They won't harm anything at 1.3GHz. I make them as long as the feedpoint gap so the overall lobe lenght is consistent.

The lobes should be .5% longer than 1 full wavelength depending on the wire thickness you use. Wire thickness will change resonant wavelength, so the little details will change. Generally the thicker the wire, the higher the resistance (I know this sounds backwards but we're talking RF, not DC). My calculations were done with .035" wire. if you use thinner wire (say .030 or .025) you will need the lobes to be slightly longer than my calculations.

-Alex
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Old Jan 03, 2012, 09:19 AM
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I've got .030 welding wire. So do I need to add the length of the hook to the lobe length, or get the lobes correct and then just bend the last few mm into the hook? Also, how important is that 105 angle? Without another dimension, I find that it can be physically anywhere in a fairly large range. Seems 115 gives a nice-looking lobe. That dashed line on the left is tangent to the arc at the corner.

Edit: I'm going to be doing the trick in your YouTube video with the legs, so I'll make 4 independent lobes.
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Old Jan 03, 2012, 09:28 AM
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Since you're here, Alex, can you take a quick look at my FPV test flight from yesterday and give any troubleshooting tips you can think of? 1280 MHz. Tx is a generic 400 mW from RMRC. Rx is Lawmate unmodified. Both antennas were cloverleafs. The transmitter antenna was slightly better-made than the receiver antenna. I highly doubt the problem is due to the Tx turning off in flight, since I replaced all my wiring.

www.youtube.com/watch?v=l7lb99DZL4I
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Old Jan 03, 2012, 11:50 AM
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Texas Aggie - I bend my hooks after I strip the cable. The length of the hooks = feed gap length. The reason is that the feed gap counts as part of your lobe length. Therefore by bending the hooks in the end you remove that length from the lobe itself but it gets added back in the soldering stage due to the feed gap. If you had no feed gap at all you would need to add a little wire for your hooks.

The arc isn't too critical. Just be sure it doesn't touch the adjacent lobe.

When I look at your video, your problem appears to be external interference. Perhaps a pulsed transmission? I can't tell. Your video is rock solid and then suddenly loses the frame sync. This would indicate an outside source.

-Alex
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Old Jan 03, 2012, 11:56 AM
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Thanks for the tips. I really appreciate your help. I'm kind of surprised at video interference since that park is basically in the middle of a forest with no built-up areas for several miles.

Also, when you say "slightly longer" for the .03 wire, can you quantify?
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Old Jan 03, 2012, 12:44 PM
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Tucson Avra Valley, Arizona, United States
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TA

Here in Tucson the public services all use the 1280 band. They have a lot more power and can interfere with FPV setups.
Hope this helps.

Neal
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Old Jan 03, 2012, 12:50 PM
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How did you find this out?
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Old Jan 03, 2012, 03:46 PM
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I have friends in Police, Fire and EMT.
Neal
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