SWR meter - Do it yourself project

[guy1a]

Yes. I refer to this as the "feed gap". Take-off angle, closure angle, and feed gap are all IBCrazy terms. They aren't formally used in the rest of the RF World.

-Alex

ok. Thanks. Will try better at weekend.

:beer:

[roberto]

Today I found out 2 things.
1) Michaelīs board works like a charm.
2) My antennas suck.

[NDw]

So tried using about a 2 meter extension when testing my 433mhz antennas and getting basically the same results still.

NAGOYA NA-771 SF
forward power 1.8,
reverse 1.25
swr 5.54

That antenna is suposed to have a max swr of 1.5!

[Mictronics]

I have ordered a Nagoya NA-771. When arrived I will do a measurement and post a plot.

[Joesa3rd]

So tried using about a 2 meter extension when testing my 433mhz antennas and getting basically the same results still.

NAGOYA NA-771 SF
forward power 1.8,
reverse 1.25
swr 5.54

That antenna is suposed to have a max swr of 1.5!

What are you using for your rf source device and what's the power output of the device? Do you have any other 433MHz antennas you can use as a reference or baseline?

Joe

[NDw]

What are you using for your rf source device and what's the power output of the device? Do you have any other 433MHz antennas you can use as a reference or baseline?

Joe

I'm using a chainlink tx which has selectable power output of 200mw and 500mw.

That was on 200.

I've a few other 433mhz antennas that are not home made.

These are numbers for 2
a)2.09 fw, 0.75 rev., swr 2.11
b)2.24 fw, 1.2 rev., swr 3.3

I have ordered a Nagoya NA-771. When arrived I will do a measurement and post a plot.

Great. JUst wondering should you get good results, what would you think is my problem? I've tried doing it outside in the middle of a lawn and got similar reults.

[IBCrazy]

I would think 433 would be easy to get good SWR. Maybe I'm wrong. The higher the frequency, the more difficult it is to attain proper SWR.

I guess this is why my Vee tutorial was so popular...

-Alex

[NDw]

Well yes. That makes the results I'm getting strange. You'd imagine a commercial antenna designed for very close to the target frequency would have a much better swr than 5.5

[roberto]

I got 5 for my SPW and have been depressed ever since.

We can maybe do some standard measurements for calibration. I will try and do the stock lawmate 1.2 whip for instance.

[Pollux]

Hi,

I missed this awsome thread and I come here a bit late... what a wonderfull job !! congratulations for this swr meter !
I tried to do one some month ago, but I chose a BAT24 as detector, and thess things were to tiny for me, I burnt them all and I gave away...

I don't know if this could help, but I found this trick to get a bad swr meter to work better:
http://www.arrad38.fr/bidouilles/ros...metre_23cm.htm
sorry, it's in french... but the little wire over the coupler seems to add some capacitance and may reduce the resonnance problems...
I'm novice in RF and electronics so I can't really understand how this works, but I'm sure IB or Mictronics will explain this in two lines.

Also, I have a question: would it be a good idea to put the coupler in a metal shield?
this question poped in my mind when I saw (on a picture, in the middle of this thread) a 1.3GHz CL perfectly aligned over the coupler... since the coupled lines and the antenna ground wires are the same lenght, I guess a coupling is possible....

I'll surely buy one of these coupler soon... once I'll get over the 25 posts... for the moment, I can't access the page.


Pollux.


EDIT:
I forgot to mention a trick I saw on many "homemade swr-meter" pages... they never end the coupled lines with one 50 Ohm resistor but with two parallel 100 Ohm... I think that the purpose is to avoid excessive capacitance from the resistor (and avoid resonance).
example: http://www.marwynandjohn.org.uk/GM8O...irCoupler.html