SWR meter - A DIY project - Page 48 - RC Groups
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Jan 01, 2013, 08:29 PM
KK4NZS / RP sUAS
Nelapaty's Avatar
Quote:
Originally Posted by Pisces
Hmmm... seems your calculations give a different answer than the formula given by IBCrazy in post #1.. quote

"SWR is calculated as follows:

p = Vr/Vf

SWR = 1+p/1-p "

Then
p = 0.29/4.35 = 0.0666

VSWR = 1+0.066 /1-0.066 = 1.066/0.934 = 1.14

VSWR = 1.14

IBCrazy of course is the correct way to calculate VSWR. Voltage Standing Wave Ratio

Efficiency for VSWR 1.14 is > 99%.

PS..the IBCrazy method as with most VSWR meters measures Voltage not Power. The direct power reading is unknown but of course is related by the formula Power = Voltage^2/Ohm ( P=E^2/R )

Hope this helps
Thanks that really clear things out.. Keep it simple
Last edited by Nelapaty; Jan 01, 2013 at 08:35 PM.
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Jan 06, 2013, 09:18 AM
KK4NZS / RP sUAS
Nelapaty's Avatar
Pisces,

Is there a chart that shows efiiciency?
Whats the efficency for 1.71 VSWR
Jan 06, 2013, 04:07 PM
The Dreamer
Pisces's Avatar
Quote:
Originally Posted by Nelapaty
Pisces,

Is there a chart that shows efficiency?
Whats the efficiency for 1.71 VSWR
There are charts and if I come across it on-line I will post the link. I have the link saved here somewhere but cant find where i have saved the link in my favorites folder.

But, you can DIY using a on-line VSWR efficiency calculator.

http://www.csgnetwork.com/vswrlosscalc.html

Example,
Enter VSWR 1.14
The Loss in power W = 0.428%

Therefore the Forward Power = 100 - 0.428 = 99.57%

Also on interest in the calculator is Input power 100W
Shows Loss in Power = 0.428W
Therefore the Forward Power radiated by the antenna is 99.57W.

PS..Enter VSWR 1.5 and the calculator gives Power forward = 96%.
This is why a VSWR of < 1.5 is considered a good antenna. 4% of input Power W is lost due to impedance mismatch. Impedance mismatch is measured as VSWR. One and the same.

Hope this helps
Lindsay
Last edited by Pisces; Jan 06, 2013 at 04:15 PM.
Jan 16, 2013, 03:54 PM
Registered User
I bought Coaxial cable from ebay, but the smalest I could find was 2.5m long.
It has SMA male on both sides. Can I cut the cable, shorten it, and resolder it so I get the short coax cable for the SWR meter? Or will the solder point ruin the whole calculation system?
If so, does anyone have an ebaylink to a short RG316 jumper?
thanks in advance
Jan 16, 2013, 04:01 PM
Registered User
Nobert's Avatar
Why not use a SMA Male to male connector like this one:
http://www.ebay.com/itm/SMA-male-To-...item2576bda426
Feb 07, 2013, 06:22 PM
Registered User

DIY SWR meter


can i use this coupler
http://www.ebay.com/itm/Mini-Circuit...item4839f26993

for 1.3 ghz. diy swr meter.
or if you have a link to on that work better '
Feb 08, 2013, 10:14 AM
Registered User
Nobert's Avatar
Gazz

That will work great and the price is right.

Neal
Feb 08, 2013, 02:20 PM
Registered User
a9amg's Avatar
Just build the smallest Diode, Resister & Capacitor circuit...
But think I shorted out pins 4 & 5 on the Diode, will this matter?
Feb 08, 2013, 03:08 PM
Wallop!
Martin7182's Avatar
If you have the K-package of HSMS-286 then it shouldn't matter (pins 2 and 5 are just for isolation between the two diodes). But it introduces a little extra capacitance.

Martin
Feb 08, 2013, 08:13 PM
Registered User
Quote:
Originally Posted by Nobert
Gazz

That will work great and the price is right.

Neal
thanks Neal
Feb 13, 2013, 03:52 PM
Wallop!
Martin7182's Avatar
Finally my SWR meter has been completed. It has everything I wished for; usable for 2 - 8Ghz, sensitive; good response with only 14dBm signal, analog scale, small; about the size of a pack of cigarettes.

My printer is really horrible, so the scale is unreadable. I don't mind as I'm the only user and I know what's printed. The little green led is what takes most current, I should solve that later .

Thank you Alex for your great ideas!
Martin
Feb 22, 2013, 11:18 PM
Mobius Cables... I got em'
chanyote66's Avatar
Quote:
Originally Posted by Martin.
Finally my SWR meter has been completed. It has everything I wished for; usable for 2 - 8Ghz, sensitive; good response with only 14dBm signal, analog scale, small; about the size of a pack of cigarettes.

My printer is really horrible, so the scale is unreadable. I don't mind as I'm the only user and I know what's printed. The little green led is what takes most current, I should solve that later .

Thank you Alex for your great ideas!
Martin
Ok... im confused, how are you getting SWR from a single reading? doesnt it need forward and reverse?
Feb 22, 2013, 11:26 PM
Mobius Cables... I got em'
chanyote66's Avatar
Hey IB... or anyone reading...

If i get the below results is there any problems with the setup?
Name: sensor.jpg
Views: 203
Size: 175.7 KB
Description:

Meter built from:
HSMS-286k
1k resistor
0.01mF 50v capacitor
Multimeter used: Fluke 77IV

Readings:
Stock antenna
Vf=131mv
Vr=-3.1mv
P=-0.023664122
SWR=0.953765846

LHCP Mad Mushroom (Self built)
Vf=121mv
Vr=-.9mv
P=-0.007438017
SWR=0.985233798

RHCP Crosshair
Vf=131mv
Vr=-1.1mv
P=-0.008396947
SWR=0.98334595

DIY SWR (7 min 19 sec)
Last edited by chanyote66; Feb 25, 2013 at 11:48 AM.
Feb 23, 2013, 08:03 AM
Wallop!
Martin7182's Avatar
Quote:
Originally Posted by chanyote66
Ok... im confused, how are you getting SWR from a single reading? doesnt it need forward and reverse?
You're right, in principle it needs Vf and Vr. But Vf is quite stable, it varies about 10% for different loads (antennas, resistors) if I remember correctly. So given the formula VSWR = (1 + Vr/Vf) / (1 - Vr/Vf), my VSWR reading is just a little bit off. But when calibrating the meter with a known load, it is spot on. So this is what I normally do: I connect a 6dB attenuator as load and set the reading to VSWR = 1.66. Then I replace the attenuator by the antenna and try to tune it at or below VSWR 1.30. Then I verify the reading with my DIY 60Ω dummy load.

I think for low VSWR values, the error in the diode's non-linear region is bigger than the error you make by assuming that Vf is constant. So theoretically you're absolutely right, but practically there's nothing wrong. Calibrating the meter with known dummy loads cancels out all errors, provided your measured antennas give about equal readings on the meter.

More details can be found here: https://www.rcgroups.com/forums/show....php?t=1718928

Martin
Feb 23, 2013, 12:47 PM
Mobius Cables... I got em'
chanyote66's Avatar
Ok, Ill see if i can find a Dummy load for 1280/1258 and see what that gives me. I roughly based my tests off the factory antenna, but we all know they aren't always the best...

Chris


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