SWR meter - Do it yourself project

[Mictronics]

This cable will not cause the readings you get.

[catamar]

Thank you Alex, I asked you in another thread in RCGROUPS, about a new cloverleaf I made (I have done 6), because I changed to another frquency, 1360 mhz. Thing is that the reflected power I have with the cloverleaf is 1.3 (big deception, because I read you obtain easily a 0 reflected power with cloverleaf). But I don't know how to change the cloverleaf swr, or how to make a new one with good swr.

Catamar: 1.65 SWR is 96% efficient. 1.06 is 99.7%. I'd consider that excellent.

-Alex

[DennisFrie]

I received my directional couplers and RF-meters from Mictronics a few days ago and just got time to get it up and running.

As i would like to sweep the entire frequency-range I have used an OpenLRS to sweep from 240 mhz to 470 mhz and an Atmega328/Arduino to measure the reflected power. That should make it possible to get pretty accurate readings and do automatic frequency-sweeps with a great resolution.

A quick test-setup:




First, here is a sweep of the reflected power with Nagoya 771 430 mhz UHF antenna.


While it does show "something", it doesn't look like the normal frequency-response from an antenna. The RFM22B RF chip is designed for 433 mhz, resulting in a pretty unstable output-power when pushing the frequency-limits.

Here is the output-power/forward power plotted with the reflected power:



The difference should give the transmitted power:


- and it's now possible to see a good output-power around 400 mhz and 425 mhz. (Will check VSWR later)

I also did a quick test with the little coil-antenne from flytron:


A nice peak around 428 mhz.

The frequency-sweep is running at 50 khz steps giving plenty of resolution (4600 steps from 240 mhz to 470 mhz).

[DennisFrie]

- And a plot of VSWR from Nagoya 771. Please note this was a quick test with a bad antenna-placement etc. Mainly "proof of concept" with automatic sweep.

[Mictronics]

Would be nice to see a plot with a 50Ohm load on the output, cause this is the only thing which shall be considered as a reference.

Oh and hopefully you didn't run the measurement with the antennas laying around like seen on the pictures.

[DennisFrie]

I'm not home at the moment and don't have the data, but I will post the plot with 50 ohm load later

[DennisFrie]

As promised, here is the plot with 50 ohm load.
Forward voltage:


Only about 1 % deviation. The interesting thing is, that the forward voltage drop dramatically when a mismatched antenna is attached. I though it was caused by the RFM22B RF chip.

The reflected power with 50 ohm load is as expected more or less, 0 all the way. A reading of 13 ~= 0.06 volt.

[guy1a]

hi

I received my directional couplers and RF-meters from Mictronics today.
watched the videos in the 1st page several times ... and tried

i can't understand some things.

in the videos, the directional coupler has a direction. this one does not ... does it matter ? or what matters is the location of the RF-meter related to the tx & antenna ?

another thing, i tested few stock antennas that gave readings other then 0.0 ... some gave reading higher then 2.3
my home made CL gave 1.oo in one way and 2.22 in the oposite way ...
is there a manual to understand those things ?

thank you

[volto]

It's bi-directional, so you can have the transmitter on either side and the antenna on either side. When measuring, if you have the detector on the transmitter side you will read the forward power, on the antenna side you will read the reflected power. That's the way I understood it anyway, I haven't actually used mine yet, still don't have a detector circuit.

[IBCrazy]

Thank you Alex, I asked you in another thread in RCGROUPS, about a new cloverleaf I made (I have done 6), because I changed to another frquency, 1360 mhz. Thing is that the reflected power I have with the cloverleaf is 1.3 (big deception, because I read you obtain easily a 0 reflected power with cloverleaf). But I don't know how to change the cloverleaf swr, or how to make a new one with good swr.

Catamar - One thing many people do not know about my tutorials: I am selective in how much information I give out. I give out just enough information for a Hobbyist to make a good antenna, but not enough so someone can build a commercial product to compete with me (which a surprising number of people have done).

That said, it's hard to say where your cloverleaf is deficient. Your wire thickness must be .035" or close to it. If your element length line up with the ones in my tutorial, you should have a good antenna with 1.1 SWR or better. Your angles out of the canter element should be 35-40 degrees from vertical which os one of the specific details I never published.

-Alex