2.4 Ghz SWR Meter

I'm thinking about building a device to measure the input impedance of WiFi antennas. The simplest device I see is shown here

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's an SWR meter that uses a uses a wifi signal to drive the antenna. Any thoughts or suggestions before I order parts? Mike

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I use a sweep generator HP8620a or Wiltron something, a directional coupler or reflection coefficient bridge, and a scope. Without a sweep generator or network analyzer, the VSWR bridge is kinda wasted. For antennas, you want to know the VSWR across the entire

2400-2583.5MHz band, and then some.

Nope. It works. I've built two. Adjusting the balance cap is a problem as it changes when I move the bridge, install the cover, and move things around. Build it solidly and inside a case so you won't lose the null.

I didn't use a meter. The output went to a scope.

If you can find scrap polysulfone PCB material, use it instead of G10/FR4. Less lossy.

If using a Wi-Fi router for a signal source, you'll have problems seeing anything on the meter without the peak detector circuit.

There's VSWR bridge in the middle of this page, somewhere, maybe:


The commercial Wiltron versions:

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Jeff Liebermann

Once again Jeff, are you ready to help the cause by acquiring (by hook or crook) the airsleuth stuff? They have code to do a sweep generator using the Proxim HomeRF stuff as well as their spectrum analyzer. Since there is more hardware out and about unmarried to their s/w, it would be nice to know what may be necessary to induce nuptials.


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Thanks for the input Jeff, I don't have any Ghz equipment so when I build an antenna I don't really know what I hav other than it works. I've compared signal strengths on the computers wireless utility so I know which is better, but not what makes an antenna match better.

When you mention balance cap, do you mean the cu foil that you trim or add. If so? I think you adjust this with a 50 ohm dummy load, to zero the output? I don't really see this? I think we're adjusting the 49.9 ohm resistors capacitance to ground. Tell me what you can, I'll have to study it more. Are you recommending a case on the head unit? PCB material ok?

You monitor the dc level with your scope? Never mind, I see that is a pulse coming out of the head unit.

I have some Rogers Duroid .031", 5880 I think

Interesting, why do you think the author says it (the peak hold switch) is not needed? I think we're talking about the same thing!

Did you use the BAT 62-03W diode? If not what did you use? Some one recommended the BAT 62-02W it's smaller so should be ok, and I can get it.

Thanks again, Mike

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Nope. Not interested, too busy, too lazy, it's summer, I got better things to do, no money in it, no fun, I hate grinding code, HomeRF is dead, etc.

Reply to
Jeff Liebermann

That's what eBay is for. Most of this junk came from eBay:

Note the pile of HP 8620 sweep generators and plugins. If you want one that works, buy three. Fix the best, use the worst for parts, and save the middle one for when the best one blows up.

The 2 gray boxes on the left are Wiltron 610(?) sweep generators and plugins. Good stuff but you gotta know how to fix them.

Use an antenna modeling program first. I suggest 4NEC2.

The real antenna will never work better than the theoretical model. Compare test results with the model and you'll have a clue what to do to fix the antenna.

Not good enough. You need to sweep the VWSR over the frequency range to get it right. Never mind gain. If you have the matching nailed, the gain will be close to theoretical with no inconvenient bumps in the gain curve across the freq range.

Incidentally, if you want to build your own, I suggest you look at AMOS or Franklin antennas. Same URL as the VWSR meter:

There are others but I'm too lazy to dig them out.

Yep, that's it. Microwaves use very small value cazapitors.

Yep. Good luck finding a dummy load that's exactly 50 ohms at 2.4GHz.

Not now. It's easier to build it and see what it does, than to explain it. Besides, I'm not sure I can explain how it works and will need to hit the books.

If you have time, just build a lower frequency version of the bridge. Use whatever resistors you can. Scale the capacitor values. Almost any decent diode will work. Use it a 50 or 150 MHz where you should be able to scrounge some usable test equipment.

Yep. The ones in the photos are fine. Just make sure that things do not move around, which was my problem. Also, you will have the null change when you install the covers, so be prepared to adjust the null cap through a small hole.

See my comments about FR4/G10 PCB material. Use Polysulfone if possible. However, G10/FR4 will work for a first attempt.

You can integrate the DC output if you don't like pulses. That's what the peak detector does. I don't use a Wi-Fi radio as a signal generator so I don't have the pulse problem.

er=2.2. Low loss glass and teflon composite dielectric. Loss tangent of 0.0009 at 10Ghz. Probably overkill but I would certainly use it if you have it. That's better than polysulfone. However, I would still use G10/FR4 for the first attempt, which is by definition, doomed to failure. Learn By Destroying(tm).

Dunno. Probably because he's also not using a pulsing wi-fi router as a signal source. Otherwise, his meter is fed by a rectifier and integrator which gives tolerable output. I'm just guessing.

I have no idea what I used. I have a mess of unlabelled diodes of undetermined origin, that seem to be suitable for microwave applications. No logo, no number, no code, not even a color dot. Probably production rejects, but they're good enough for what I'm doing. Sorry, no recommendation for substitutes other than it be a shottky diode with low cazapitance and a sufficiently high reverse voltage to prevent vaporization.

About $5/ea and you'll probably have to buy a bunch to get over the minimum. The 62-02W is very tiny. I hope you have the proper handling tools (toothpick, superglue, and paper clip around the soldering iron tip).

If you need a substitute, I can dig something out of Digikey, Mouser, and accomplises. You can possibly also bug the distributors for "samples".

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Jeff Liebermann

I've looked at a lot of antennas to build and built a few, cantennas with dongle mounted inside, biquad, patch, yagi, I've put a couple of these at the focus of a dish and worked that. Its been suprising that they all work. I bought an MFJ-1800 and that is the one I use. It seems roughly 6 db better than my biquad or patch.

I figured I'd build one like on the SWR meter page, (a couple of 100 ohm smd's mounted on a connector)

I have several large sheets of the Rogers, so I'll use it.

Thanks for the tip about the paper clip. Mike

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