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Old Jul 05, 2011, 06:51 AM
FPVEEE ED
ecampos's Avatar
Quebec, Canada
Joined Jul 2010
293 Posts
Help!
Lawmate: SWR meter showing low forward power but good SWR (low reflected power)

Hi all !


I just got my SWR meter (Daiwa 801-SII) and after playing with it a little i have a strange behaviour as when testing my homemade Vee i get a forward power reading of 800mW for a low reflected power reading (sorry i'm at the office and i don't remember the exact reading)... Those gave me about 1.2:1 SWR but i can't figure out why forward power is low as it's supposed to be around 1.2W as tested with my other antennas... My VTX is a Lawmate 1.2Ghz. Any help here will be very appreciated Thanx
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Last edited by ecampos; Jul 06, 2011 at 01:33 PM.
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Old Jul 05, 2011, 07:11 AM
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Hugeone's Avatar
Canada,Quebec
Joined Jan 2005
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Maybe the adapter sma>N on the lawmate tx is bad...
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Old Jul 05, 2011, 07:17 AM
FPVEEE ED
ecampos's Avatar
Quebec, Canada
Joined Jul 2010
293 Posts
Thanx ! I'll try swapping the adapter... But why my other antennas give me normal readings ? What factors can cause a power drop like this ? THanx !
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Old Jul 05, 2011, 09:49 AM
Gamble with Gravity
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Joined Jun 2011
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Environmental factors, probably. Proximity to other objects (ie the SWR meter case, your hands, ect) can have an effect on SWR. Wave your hand around the Vee and watch the SWR jump all over the place. I've made two nearly identical antennas and tested them with my SWR bridge and found the output power of my TX to be very different between them despite both having acceptable SWR.
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Old Jul 05, 2011, 12:51 PM
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ecampos's Avatar
Quebec, Canada
Joined Jul 2010
293 Posts
Just tested again the antennas and i get different readings depending on position or if i touch the antennas with my hands... What are the best practices to test an antenna ? Do i need to mount the antenna on a support the same way it should be installed on my plane ? I actually get different readings when my vee lays flat on the table and when i put it on a vertical position. Also, i actually plug the VTX directly to N-SMA adapter and Vee to the other one (output), is that right (avoid loss) ?

Thank you again !
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Old Jul 05, 2011, 02:15 PM
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Mount it on the plane and test in place. This way you get real results.

Here's an interesting thing I found: move the coaxial cable in different orientations and you'll see the power output and SWR change. It appears as though in certain orientations the cable resonates.
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Old Jul 05, 2011, 02:45 PM
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Quebec, Canada
Joined Jul 2010
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I'll try that... But how do i get constant results with that thing ?
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Old Jul 06, 2011, 06:29 AM
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Good question. I don't know. I just know that environmental factors affect dipoles significantly.
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Old Jul 06, 2011, 08:32 AM
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Quebec, Canada
Joined Jul 2010
293 Posts
I've done some more testing yesterday... Tried moving to the middle on my living room as there's no metal or obstacles around. Then tested the same antennas and results are better, but still quite different depending on how the antenna is placed. So i made the test using 4 different antenna positions and 2 different frequencies and i calculated an average SWR and i now use it as the reference for tuning. Seems to work ok as i get quite good video, but i'll do a distance and banking test to see how it performs.

Now, if i use the SWR as the reference for tuning, for example, the inverted Vee, i don't get the same measurements as stated in IBC howto... The measurements seems to be quite higher, about 3mm more per element in some cases on 1120Mhz. I used 14 and 12 gage copper wire and RG-316 coax/ebay SMA connector. Based on the formula , i get 60,05mm per element for 97% degration factor (angle: 125degres) and about 63mm if using SWR meter. 60,05mm antennas give a SWR of 2 and even more in some cases ! ??

So, can i trust SWR readings or do i go with calculated method ?
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