Relationship between signal strength and antenna gain

I recently did a comparo between a corner reflector antenna based on the EZ-10 (of quoted gain = 10 dbi) and a 30 cm diameter stainless salad dish /usb adapter reflector. Both 54G.

The weak reference signal was 100 metres away.

The corner reflector hanging off a Belkin PCI card and very short pigtail pulled in the chosen signal at 18 - 20%.

The dish in the same position using a USB Belkin adapter pulled it in at a constant 40%.

The internal antenna on the laptop used for the USB didn't pick up anything.

So can I make an assumption on the gain of the USB dish, based on the supposed gain of the EZ-10, given the different transmitters?

The EZ-10 clone is actually my EZY-F freestanding unit - but the specs are identical.

The salad bowl is a perfect half sphere except for the 10 cm flat base, so the beamwidth should equate closely to a half sphere.

I had a browse on the net but it wasn't very helpfull.

Can I assume the dish is greater than 10 dbi?



Reply to
me here
Loading thread data ...

Oh-oh. I think I know where you found the salad bowl.

Problem.... The earth is inside the Fresnel Zone which will cause some weirdness in trying to measure signal strength. See:

For 2.4GHz at 100 meters range, the end points have to be at least 1.4 meters above ground level. Get two ladders.

It's going to be rather difficult trying to guess the gain using percentages. Percentage of what? The numbers don't mean much except for relative improvements.

Nope. If you had numbers in dBm or milliwatts, it might be possible. Can you run Netstumbler?

Incidentally, this floated to the surface of PC World Magazine recently:

Find a dark room and a light bulb (LED with wire leads works) and see the pattern on the wall. Instead of a concentrated spot, as you would expect from a parabola, you get an annualar ring. It's still useful, but there's a substantial dip in the signal level at dead center. I've never bothered to measure the salad bowl gain.

However, it is possible to guesstimate the maximum gain for a given diameter parabolic dish antenna. See:

Nope. Besides the hemispherical shape and weird pattern, the USB dongle is not exactly an ideal feed element. Much of the RF radiates in directions that do NOT hit the salad bowl. My guess is about half, for a net loss of -3dB. In addition, the tiny ceramic antenna inside the USB dongle has a gain of perhaps -4dBi, which is why it didn't hear anything. It's difficult to guess the gain in absolute terms. Best to have a test antenna, with a known gain, and do a comparison.

Reply to
Jeff Liebermann

Hi Jeff,

Thanks for the indepth response.

Some interesting stuff in those links.

My salad bowl and your salad bowl must be related :-) Yep, they're identical. Hadn't seen that photo before.

I mounted the dongle differently, I pop rivetted an L shaped tab at the back of the flat section (underside) of the bowl and held the back end of the adapter against it and in the focal position with two stretchy "O" rings. Works well.

I can run Netstumbler on both the PCI based PC and also the USB based laptop.

However, I get strange happenings with Netstumbler and BeLkin wifi.

Often Netstumbler will not pick up all the networks that the Belkin software shows.

I am well aware that Belkin and windows wifi management do not get on well together and you must tick box to turn it off :-).

Netstumbler suspends the Belkin process when it runs, and I suspect there's a conflict there somewhere. It seem to turn Windows management on again, so I doubt thats the problem.

The end result of the comparo is that the dish easily outperforms the corner reflector, even if the USB signal pattern is dodgy.

Incidentally, before I rigged up the bowl, I tried to replicate my EZY-F corner reflector setup with the USB adapter instead of the dipole.

Absolute failure. I suspect that as you suggest, the USB dongle just sprayed signal everywhere and the reflector was useless - couldn't focus it.

With the bowl, the USB adapter works better with one side against the locating tab than the other. Could be some interference on one side of the adapter from the tab.

The bowl however definitely gets stuff the corner reflector can't.

So is it worth a try of the USB adapter with the PC World parabola? Or is the USB going to get more lucky spraying signals within a dish.



Reply to
me here

Do the above.

Metal in the reflector area is a bad idea. If it doesn't block the signal, it sends it off to who knows where. The reason I used a PVC tube was so that I could slide the USB dongle up and down the tube to get the exact focus point. Also, to keep the USB cable from getting into the RF pattern.

Not that odd. The Belkin site survey software does NOT require a response for the access point that it hears. It just scans for AP's and lists everything that it hears. Netstumbler requires a response from the AP. It may also be that Belkin is including AP's that are not broadcasting their SSID. Dunno.

In addition, it's quite easy to have different xmit and receive gains with a dish. In this case, the USB dongle "feed" will be spraying quite a bit of RF (my guess is at least half) in all the wrong directions. That drastically reduces the xmit gain. However, in receive, the same antenna will reflect most of what hits the dish, directly to the USB dongle, resulting in no loss of gain. Therefore, your xmit gain and receive gain will be quite different. With a low xmit gain, it's possible that the AP's are not hearing you, which is why Netstumbler doesn't show much.

I'll spare you my opinion of Belkin hardware and software.

That's normal. Netstumbler needs control of the NDIS driver. Killing WZC is about the only way to do that. Most other sniffer software does the same thing:

I think you'll find that you'll get about the same results with a flat sheet of aluminum foil on cardboard.

Maybe. It's difficult to tell how much RF the USB dongle wasted. With a DBS dish antenna, its something like 90% of the RF wasted. However, note (again) that it's only wasted in xmit. In receive, all the signal that hits the dish, hits the USB dongle.

The metal locating tab is causing some kind of problem. No clue what but it certainly will have an effect on the pattern.

The bowl is much larger (large effective aperature) than the corner reflector.


Reply to
Jeff Liebermann Forums website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.