Omni antenna help please

snipped-for-privacy@here.com hath wroth:

About 15.5dBi for 2.4GHz. 70" long:

Yes, they're usually fine antennas.

In my never humble opinion, omni's are a bad idea. The vertical half power beamwidth is conveniently missing from the data sheet. My guess is that it's probably about 3 degrees or less. That's VERY narrow.

Let's play with the numbers. With a 3 degree downtilt, and mounted

50ft off the ground, your usable coverage area is from 0.13 to 0.36 miles (686ft to 1900ft). Hopefully, that's not what you want.

Plug in your own numbers and see what you get.

That also means you have to mount the antenna absolutely perfectly vertically.

There's also the problem of bandwidth. Very high gain antennas tend to be very narrow band. I doubt that the antenna will cover the entire 83.5MHz of the 2.4GHz band with a decent VSWR. It's hard to tell because Comet doesn't supply any test data.

Hmmm... The 15dBi model isn't even listed on the Comet web pile:

Jeff Liebermann hath wroth:

(...)

The plot thickens. I can see why it's missing. The data sheet for the 12dBi model is at:

The overall length from the data sheet and the above URL are both 70". That's the same as the Fab-Corp 15dBi antenna. That's impossible because omni antennas double in length for every 3dB of gain. What that means is that the 70" antenna is NOT a 15dBi antenna and is really a 12dBi antenna. That also explains why there's no 15dBi model on the Comet web pile.

Everyone lies, but that's ok, because nobody listens.

Thank you Jeff I really appreciate the honesty. I am currently using an antenna I bought from Pasadena, it's a 40" omni they claim to have 14db but I don't see it for sale any longer. I assume the Comet omni units are vertical polarization, correct? Thank you.

Jeff Liebermann wrote:

snipped-for-privacy@here.com hath wroth:

I suspect you can call or write them and get the specs on whatever you have. 40" and 14dBi seems a bit high. I have an Antenna Specialists ASPT2976 omni antenna handy. It's about 2ft long and has a measured gain of 8dbi gain. I've measured this several ways and it's within

0.5dB of 8dBi.

If I use this antenna as a reference standard, and the antenna guts were the same construction, and I assume that the gain doubles (3dB) for doubling the length of the antenna, then the gains would be:

Length gain radiation angle: 24" 8dBi 11 deg 48" 11dBi 5.5 deg 96" 14dBi 2.8 deg

Draw your own conclusions, but it looks like 70" lands at about 12dBi as the data sheet suggests. 15dBi rediculous.

I don't think my previous comments sunk in. So, I'll be blunt. High gain vertical omni antennas are awful for just about all but a few specific applications. You haven't bothered to specify yours, so I can't tell if this is going to work for you.

Also, yes. If it's a fairly thin vertical fiberglass tube, it's undoubtably a vertically polarized antenna. Horizontally polarized omnis are basically slotted waveguides and are much wider.

1:30AM Yawn...

Jeff Liebermann hath wroth:

I forgot to plug my favorite antenna system. Look into Franklin or AMOS sector antennas. See:

Lots of gain, very small vertical radiation angle, fairly wide horizontal coverage (90 to 120 degrees). They're easy to build, but there are commercial versions if prefer.

I use them on my boat for hitting shore based APs. Since the boat is constantly swinging at anchor an omni antenna is absolutely essential, and the higher the gain the better. I have sometimes been able to connect with good outdoor APs that were over 4 miles away.

This is the one I'm using at the moment:

High gain omnis are also useful at APs where wide outdoor coverage is needed.

That is the antenna that I want to buy. I bought the 14db, 40 inch from them and I am using it now and it seems to work great.

Wayne.B wrote:

Here is the one I bought and it works great for me:

I would like to buy the commet you mentioned though.

Wayne.B wrote:

• I'm glad the antenna of such as gain has worked well for you. I would think that it would be possible (see beamwidth earlier by Jeff) to miss some access points or hotspots altogether because you either over or under shoot them on transmit and receive. I think 8.5dBi is about average recommended especially for marine.

Wayne.B hath wroth:

If you have the room, set two anchors. One at the bow, the other at the stern.

Almost identical to the Comet antenna, but with the correct 12dBi gain.

Ok, let's play with the numbers. See data sheet at:

3 degrees downtilt and 5 degrees vertical beamwidth That means the half power points are between -0.5 degrees and -5.5 degrees. Signal levels outside this range are usually kinda marginal or just plain gone.

Duz your vessel pitch and roll at anchor? If so, does it rock and roll more than +/- 2.5 degrees? Unless your vessel is gyro stabilized, it's going to spend quite a bit of time outside this range. With the 3 degree downtilt (a lousy idea anyway for antennas near the water line) even if your antenna was perfectly vertical, most of the signal will hit the water and not go horizontal.

Kindly disclose the antenna height above the water and I'll grind the numbers. Or, you can do it thyself at:

So, why duz it work for you? Three possible reasons:

1. Note the antenna pattern on the data sheet. There is substantial leakage (side lobes) outside of the main lobe. If you tilt the antenna to the first side lobe at 10 degrees, you lose about -12dB of signal. That's a big loss, but with luck, it should help you retain the connection. (Note: for each -6dB loss, the range gets cut in half).
2. With the -3 degree downtilt, your main signal is bouncing off the water. That works well with calm seas, but probably will be flakey in any chop.
3. If you happen to have installed the antenna above a ground plane, then the radiation angle will move upwards. That will put the main lobe closer to horizontal, which will certainly help.

I don't know if this will be helpful, but this is the PVC antenna mount I contrived to help someone clear some superstructure on their power boat.

Most WISP (wireless internet service providers) use sector antennas and multiple radios. The only lunatics that use omnis are mesh and metro network providers. The situation is much the same with cellular providers. Lots of advantages to sector antennas which I'm too lazy to itemize. Incidentally, most of the omnis in my collection were removed from WISP installations that switched to panels or sectors.

The antenna I bought from lan parts is 40" long. The antenna here

70". It is a true 15db gain?

Jeff Liebermann wrote:

snipped-for-privacy@here.com hath wroth:

I don't know for sure but probably more like 12dBi. I did some guessing in:

My guess(tm) is that a 70" antenna is about 12dBi and a 40" is about

10dBi. However, this is pure guesswork based upon similar antennas of known construction and gain. There's no guarantee that this is correct or applies to these particular antennas.

Let me *AGAIN* point out that the URL at:

lists this antenna as 70" long and 15dBi gain. However, the PDF data sheet referenced on the same page at:

shows 70" long, but only 12dBi gain.

If you have a 40" antenna, do you also have a model number, data sheet, URL, or clue as to what you purchased? I enjoy detective work and like to guess(tm), but I gotta have something for the bloodhounds to sniff.

Jeff, here is a link to the 40" unit I bought

I don't have a data sheet, sorry.

Jeff Liebermann wrote:

snipped-for-privacy@here.com hath wroth:

This is getting rediculous. First, there's the 70" antenna which claims to have 15dBi gain but really only has perhaps 12dBi. Now, you have a 40" antenna, that also claims to have 15dBi gain. No way. Something is really wrong with the numbers. My guess may also be wrong as I calculated that a 48" antenna would have 11dBi gain. That's not going to work because I claimed a 70" antenna has 12dBi gain. So, my calcs and guesswork are both wrong. Grumble.

I don't know what to suggest. I know that there are sufficient number of discrepancies in the web pages and data sheets to suggest that something is very wrong. Howver, short of doing some real work on this, I don't have a clue what gain these things offer. Given a choice, methinks the PDF data sheet is more accurate than the web pages.

I was hoping the the 70" would have more range than the 40" that I now have. Since I like the way the 40" is performing, do you think there is any chance at all that the 70 will be worse instead of better? Thanks alot Jeff.

Jeff Liebermann wrote:

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I have that one also and it works well.

I've heard that before but have not experienced the issue. Higher gain has always been better, perhaps because the boat is relatively stable and does not rock all that much. Certainly on a smaller boat prone to rolling narrow beam width could be an issue.

Not generally - 49 ft boat, 60,000 lbs.

It is stabilized but normally we're anchored when using WiFi. My Sprint aircard is usually the better choice when underway although it is slow compared to a good WiFi connection.

You really need to try them in your location. Just as important as the antenna is using good low loss coax and keeping it as short as possible.