Cantenna improvement: A+++

cst · 02-01-2004

I've just recently started experimenting a bit with my 802.11b network, and nabbed a cantenna with a rebadged Orinoco card to play with Netstumbler. This cantenna seems to have been working well for me, even though at 4.1" diameter it is larger than it should be for optimum use in the 802.11b band: that size gives a TM01 mode cutoff of 2200 mHz.

On one cantenna web site, I saw where someone went to Lowe's home improvement store, and bought a simple 4" to 6" reducer to stick on the end. They estimated it should provide up to 3 dB gain, but on my cantenna it came to a solid 4 dB and maybe 5 dB on channel 6.

Wow! For $5 and a bit of aluminum tape, thats a pretty impressive increase. I actually thought it might hurt, since the 4" end fits inside the can such that the inside of the waveguide is no longer an edge free conduit, but even before I taped it securely in position I was getting a 19 to 20 dB S/N ratio on an AP straight out my window that was at best 15 dB before.

Maybe its disproportionate improvement has something to do with 'correcting' my oversize cantenna somewhat. The physical aperture size increase is about 125%, which would translate to about 4 db if physical aperture size correlated well with electrical aperture size.

Anyway, the $5.00 4" to 6" reducer at Lowes: Recommended!

Eyecannon · 02-01-2004

Yep, I can attest to this as well... though the 6" cylindrical part does nothing to help signal and can perhaps even hurt it. I took that part off and I always use it now.

cst · 02-03-2004

Eyecannon -

I did a bit of testing today with the cantenna and the add-on 4" to 6" reducer.

On channel 11, taking off the 6" diameter cylindrical section (about 1.5" in width) makes no difference in SNR compared to the unaltered reducer.

On channel 6, however, that extra cylindrical section is good for 1-2 dB of SNR over the modified reducer.

On both channels, the 4" to 6" reducer was good for a solid 3 dB improvement in SNR over the straight cantenna.

So for handiness, I will use mine as you have and remove that 6" cylindrical section, but in a fixed application it might be worth it to leave it on.

Now I'm kind of curious to find out what would happen running the same tests on channel 1.

cst · 02-14-2004

I've had a chance to experiment with four different directional antennas, and thought I would post my findings here. Three are cantennas and one is a commercial Yagi. The tests were conducted using a 2WIRE 802.11b AP in a neighbor's house about 300 feet away on channel 11. On my end, a Toshiba laptop running NS 0.3 with a Lucent USB (Orinoco) wireless adapter, attached to each antenna via a 19" LMR-100A pigtail. Results:

1. EtherDesigns cAntenna III - this is a based on a 3.25" diameter can, 10" long. Signal -85dB

2. 4" diameter by 6.5" long cantenna from Hugh Pep on ebay (ID is hughpep ) Signal -85dB

3. 4" cantenna above with 4" to 6" reducer stuck on the end with the 6" cylindrical section removed: -82 dB

4. Cuschcraft 13.9 dB 15 element Yagi (PC2415N) -79 dB

If the Cushcraft is used as a reference point as a 14 dB gain antenna, that gives the following empirical gains for each:

1. EtherDesigns 3.25" cAntenna: 8 dB gain
2. Hugh Pep 4" cantenna: 8 dB gain
3. Modified 4" cantenna: 11 dB gain
4. Cushcraft Yagi: 14 dB by definition

Looks like most cantenna gain claims (I've seen as much as 18 dB claimed by some sellers) are a wee bit optimistic.

Results could vary somewhat for other channels.

I should soon have the good Fab-Corp 5 dB omni antenna to use as a benchmark, and I'll retest with it too.

02-01-2004	#1 (permalink)
cst Registered Member Join Date: Jan 2004 Posts: 28	Cantenna improvement: A+++ I've just recently started experimenting a bit with my 802.11b network, and nabbed a cantenna with a rebadged Orinoco card to play with Netstumbler. This cantenna seems to have been working well for me, even though at 4.1" diameter it is larger than it should be for optimum use in the 802.11b band: that size gives a TM01 mode cutoff of 2200 mHz. On one cantenna web site, I saw where someone went to Lowe's home improvement store, and bought a simple 4" to 6" reducer to stick on the end. They estimated it should provide up to 3 dB gain, but on my cantenna it came to a solid 4 dB and maybe 5 dB on channel 6. Wow! For $5 and a bit of aluminum tape, thats a pretty impressive increase. I actually thought it might hurt, since the 4" end fits inside the can such that the inside of the waveguide is no longer an edge free conduit, but even before I taped it securely in position I was getting a 19 to 20 dB S/N ratio on an AP straight out my window that was at best 15 dB before. Maybe its disproportionate improvement has something to do with 'correcting' my oversize cantenna somewhat. The physical aperture size increase is about 125%, which would translate to about 4 db if physical aperture size correlated well with electrical aperture size. Anyway, the $5.00 4" to 6" reducer at Lowes: Recommended! Last edited by cst : 02-01-2004 at 08:16 PM.

02-01-2004	#2 (permalink)
Eyecannon Ward River Join Date: Aug 2002 Location: Santa Monix Posts: 679	Yep, I can attest to this as well... though the 6" cylindrical part does nothing to help signal and can perhaps even hurt it. I took that part off and I always use it now. __________________ No, officer, YOU are under arrest! http://eyecannon.com/wardrive.html

02-14-2004	#4 (permalink)
cst Registered Member Join Date: Jan 2004 Posts: 28	Antenna experiments I've had a chance to experiment with four different directional antennas, and thought I would post my findings here. Three are cantennas and one is a commercial Yagi. The tests were conducted using a 2WIRE 802.11b AP in a neighbor's house about 300 feet away on channel 11. On my end, a Toshiba laptop running NS 0.3 with a Lucent USB (Orinoco) wireless adapter, attached to each antenna via a 19" LMR-100A pigtail. Results: 1. EtherDesigns cAntenna III - this is a based on a 3.25" diameter can, 10" long. Signal -85dB 2. 4" diameter by 6.5" long cantenna from Hugh Pep on ebay (ID is hughpep ) Signal -85dB 3. 4" cantenna above with 4" to 6" reducer stuck on the end with the 6" cylindrical section removed: -82 dB 4. Cuschcraft 13.9 dB 15 element Yagi (PC2415N) -79 dB If the Cushcraft is used as a reference point as a 14 dB gain antenna, that gives the following empirical gains for each: 1. EtherDesigns 3.25" cAntenna: 8 dB gain 2. Hugh Pep 4" cantenna: 8 dB gain 3. Modified 4" cantenna: 11 dB gain 4. Cushcraft Yagi: 14 dB by definition Looks like most cantenna gain claims (I've seen as much as 18 dB claimed by some sellers) are a wee bit optimistic. Results could vary somewhat for other channels. I should soon have the good Fab-Corp 5 dB omni antenna to use as a benchmark, and I'll retest with it too. Last edited by cst : 02-15-2004 at 07:54 PM.

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02-03-2004	#3 (permalink)
cst Registered Member Join Date: Jan 2004 Posts: 28	Eyecannon - I did a bit of testing today with the cantenna and the add-on 4" to 6" reducer. On channel 11, taking off the 6" diameter cylindrical section (about 1.5" in width) makes no difference in SNR compared to the unaltered reducer. On channel 6, however, that extra cylindrical section is good for 1-2 dB of SNR over the modified reducer. On both channels, the 4" to 6" reducer was good for a solid 3 dB improvement in SNR over the straight cantenna. So for handiness, I will use mine as you have and remove that 6" cylindrical section, but in a fixed application it might be worth it to leave it on. Now I'm kind of curious to find out what would happen running the same tests on channel 1.