SWR meter - Do it yourself project

[terry]

I don't want to duplicate my post so those who could be interested by some experimental results can start from here

[swoop_g]

Well I just finished testing my antennas and I hope I am doing something wrong. Most of my homemade antennas come out to be around 9-12 for my SWR! My stock lawmate "rubber ducky" comes in at 5! Is there something I am missing? I take the reverse voltage and divide by the forward, I get p and then p +1/p-1 = SWR. My forward voltages are all around 4.4 and my reverse voltages are all between 2.3 and 3.5. I have my multimeter set to test normal DC voltage and I am not using any filters. I did not expect perfect SWR but 9! I was hoping for a little better than that. Anyone have any ideas? I have tested 3 CL, 2 SPW, 2 rubber ducks, and 1 fast patch. All but two have SWR's approaching 10 or more. One SPW is about 3 and one duck is about 5. Talk about let the wind out of your sails!

[Mictronics]

What frequency you talking about?

And Vf of 4.4V sound like a high power transmitter, 1W?

[swoop_g]

Yea its the 1W lawmate 1280mhz.

[swoop_g]

Ok now I am really confused. My voltages have changed! Now I just measured 0.49 forward and 0.28 reverse! That makes that antenna about 3.65:1 SWR now. Why on earth would the voltage change like that? I haven't dropped anything or shorted any leads. I have checked a few batteries and the measured volts match some lipo alarms I have. The solders on the SWR test lead look fine with no places that look like they would lose cont. I'm lost. Could it be the dummy load? Should I buy a different one?

[IBCrazy]

^ damaged detector diode. Max reverse is 3.3V. You had 4.5V, so chances are that diode is shot. Replace the diode and use a 6 db attenuator and you should be ok.

-Alex

[swoop_g]

Well actually I did not get any value over 3.3 in REVERSE. I did get around 4.4 FORWARD though. But now I am getting around .5v forward and .3v in reverse so either way something is amiss. Do you think that I overpowered the circuit with my 1W TX? Is that why you suggest the attenuator? Anyhow I found a 10-pack of the diodes you used on ebay for a decent price so I will get them and try to solder it. I will prolly need 10 to get one that works!

[MASHTON1138]

Can you post a couple photos of your antennas and SWR meter during testing? Maybe the guys can see something you are missing in the pictures. 0.5 Vf and 0.3 Vr is a 4.0:1 SWR. There are tons of things that could be going on there as far as that goes. If you move the RG316 with the diode and all on it does the voltage go nuts? I think pictures may be of benefit here.

Marc

[swoop_g]

I posted the pics of Vf and Vr here:
http://www.rcgroups.com/forums/showt...0#post20779043
I am not sure how to post pics here and don't have time to find out right now. I wish I knew that my Tx would fry the diode before I used it! Oh well I ordered more.

[Mictronics]

First question I would ask is for what you need a 1W TX? Half the power is more than suitable for almost any FPV purpose.

Second, with high output power the radiated RF fields will drive your measurement setup nuts, especially when long wiring is used between detector and meter.