Are You Dual Polarized

[phendry]

Has anybody had experience with dual polarized antenna's?

The idea is that a single antenna has 2 feeds. 1 to send/receive a horizontal signal and 1 to send/receive a vertical signal so in theory as long as you fresnels are all good you get 1 antenna doing the job of 2 and therefore can save a few quid on mast installs and rental.

I am contemplating getting a couple from Radio Waves Inc but they aren't cheap and I wanted to hear if anyone had good / bad feedback on em.

Cheers,

P.

[Skaught]

I used them as they were built into the AB-Access Units.

Unless you have some special case need for them I would not waste my money.

I have some 29dbi 5.8 dishes from Hyperlink. I can change polarization on them faster than I can unplug and replug a dual polarized antenna.

Trango gear might stand to gain as they have 2 antenna ports that can be swapped via software but for most cases it is not worth the cost.

[phendry]

Not looking at these to swap the ports but to use both port simultaneously (which is what they are designed for). This way we effectively have 2 antennas for the radio tower annual rental price of 1. This can only be a good thing right?? Interested to see if anyone has used these type of antennas and had problems.

[MerTech_Wes]

From what I understand, dual polarized antenna's aren't designed to be used by 2 radio's simultaneously, rather they should be used by one radio that supports diversity.

[phendry]

Really ?? That's not what the sales people at RadioWaves are saying. Mind you that could be because they are sales people. Is there any reason why they couldn't be used by 2 radio's simultaneously?

[MerTech_Wes]

My reasoning why one could not run two radio's on a dual polarized antenna would be that even though they [antenna elements] are seperated by polarity (i think physics say theres 30dB of seperation) they are still very close to each other, so they would cause out-of-band interference. This is why even when using polarization for channel reuse, you still keep the antenna's a "safe" distance away from each other.

If someone wants to chime in a explain this better, feel free
If someone can prove me wrong, go for it.. this is just what i know.. :D

[bairdc]

If you were using two radios, on different channels, going into a dual polarized antenna, I would think that this would work fine. On the same channel or a nearby channel I think you'd probably see problems.

Having said that, we just installed an Orthogon link, and they are supposed to use either dual polarized antennas or two antennas polarized opposite of each other. And from what I can tell, they use the same channel. The Orthogons have two radios in them, and they use the multiple beams as part of what they call "Space Time Coding", which supposedly helps them overcome NLOS.

Craig

[phendry]

Interesting. Do you have a link to the Orthogon site?

[bairdc]

Interesting. Do you have a link to the Orthogon site?

http://www.orthogonsystems.com

Pretty awesome radios, especially where LOS is questionable. But believe me... they're not cheap.

Craig

[mp3turbo]

hi,

http://www.orthogonsystems.com/ should do it.

keep in mind that these systems are DESIGNED to work so and that makes kinda' difference to standard 11b / 11a ...

bye, mp3turbo.

[phendry]

How cheap is not cheap? And wouldn't this suggest that dual polarized antennas are designed to work with 2 feeds running simultaneously? Had a look at the andrew antenna catalogue and there UMX range is dual polarized and dual band and so has 4 inputs which also suggests that as long as the 2 channels you are using are different (or at least seperated by a few channels) then there is no reason why you can't use 1 dual polarized antenna to run 2 links with no problems.

I wonder if you could use the Andrew UMX antenna's to run 4 links simultaneously or do you think that would be asking to much ;)

[bairdc]

Around $9,000 US per link (2 radios).

petrzalka.net is right about the Orthogons being designed that way. I think you could use 802.11a/b/g this way so long as you used distant channels on each polarization. For example, you could do 5180 on Horizontal, and 5825 on Vertical--assuming your antenna could do 5.2 - 5.8. However, if you run the same channel on both polarities, I think you'll end up with self-inflicted interferrence. Opposite polarity does give you some isolation, but not *complete* isolation.

Craig

[phendry]

I think I may have to try this ;) What do you think would be a safe gap between the channels? What sort of gap is used between channels on the Orthogons?

[bairdc]

I'm not quite sure how the Orthogons do it. They use 12 MHz of bandwidth, but whether they use that 12 on both antennas simultaneously or not, I don't know--maybe they use the bottom 6 MHz on one, and the top 6 on the other. Being a proprietary product, you really can't assume anything.

I think if I were going to try to use a dual polarized setup with Atheros cards, I'd leave at least one empty channel between them. For example 5180 and 5220 (leaving 5200 empty), or 5745 and 5785 (leaving 5765 empty). Of course, keep in mind that I've never tried this. This is just what I would try. If you don't get good results, try spacing them further apart.

Craig

[georgew]

The Orthogons use multiple radios simultaneously transmitting data on thousands of channels at really low bit rates. the channels are extremely narrow, and several channels are used as alignment carriers without data.

OFDM with extreme parameters like with Orthogon, can actually use things like multipath to the advantage of the radio.

To transmit on two antennas in the same enclosure, you must have a very special radio set to do this in a useful manner. 802.11 is NOT going to do good on a shared antenna. Did the salesman know what kind of radio this was useful for? Radios supporting diversity antennas can make use of it only if they support both diversity send and receive. Most diversity radios only use receive diversity, so one side would never be used. For most diversity systems, polarity is not what of the variables you want to employ, unless you have a severe multipath problem. On the other hand, the atheros radios let you switch antennas for both send and receive, so you coud use it to try different polarizations on a link... but honestly you might do this testing once, and then stick with the one that worked best. At best it is an expensive gimic.

It might make for a good site survey antenna where you have towers with different polarizations.

Using both antennas for receive diversity will increase sensitivity to being blocked by interfering transmitters...

[phendry]

To transmit on two antennas in the same enclosure, you must have a very special radio set to do this in a useful manner. 802.11 is NOT going to do good on a shared antenna.

In what way would the radios need to be special? What do you mean by "two antennas in the same enclosure"? Surely most radio installs are single units with at least 2 radios in them. Why would 802.11 not be good on a shared antenna? Why exactly would this not work if you used 2 separate radios to connect to the 2 feeds of the antenna?

I have seen many sectors being sold that advertise this exact thing by many different companies and cannot see any special requirements being mensioned.

[georgew]

Since 802.11 is not syncronous, it is likely that one radio will be transmitting while the other is trying to listen. Even when the radios are on different channels, the one transmitting will overwelm the receiver circutry of the other radio that is trying to listen. The closer the two antennas, the more likely this is to happen.

Yes, I see antennas designed for wifi operators that have two antennas in one radome. Just because someone makes it doesn't make it good. The ones I see are not designed by engineers, and are not certified and do not have fcc id's. Now I'm sure there are more out there than the ones I know about, and I'm sure they are being sold... but that doesn't make them any good...

You should have 10' distance between any two antennas in the same band, and for optimum performance 100' is even better.

The point is that you are not helping yourself by using antennas that break the 10' guideline. Anyone that tells you to do this is not working in your best interest, unless they are telling you to use a syncronous radio system, which is not 802.11.

People sell bad antennas all the time. The cantennas are good examples, the pringles can antenna is an even better example. I get great range from stripping coax in a special way, forming a 1/4th wave whip...if I stick that whip in a pringles can and it still works, it doesn't make the pringles can a valid design... but this is exactly what I see people doing. My point being that just because someone sells an antenna doesn't make it a good antenna.

I spent a lot of time studying antennas, including building my own dual-polarized dual-sector antenna. My antenna worked great when only one side or the other was in use. Together it caused problems.

As an engineer, I can tell you it is a bad idea because I tred it myself. But as an ISP that has spent the last 17 years buying networking hardware, not everything being sold works. at least 50% of the expensive hardware I have purchased was retired after being tested. Just because it is for sale doesn't make it good.

How do you tell if a salesman is lying? The first clue is his lips are moving. the second clue is you can hear him talking. The third clue is he has paperwork to back his claims.

I see a lot of claims on antennas that just don't make a lot of sense. And I have seen known trusted brands outright lie about performance.

Now I have talked to an honest salesman before, and I have seen whitepapers and specsheets that are not a fabrication. Not everything you hear is a lie... but even if it is printed in a trusted book, it may still be less than factual.

I remember o'reilly printing instructions on how to increase the output of a 30mw linksys radio to 250mw. They forgot to mention that when putting out 250mw, the signal is so muddy that the range was reduced and no improvement on the range of the original 30mw was ever seen.

Just be aware that there is more bad advice and broken promises out there than there is truth and provable results. I have hundreds of thousands of dollars in poorly spent money to prove that... And I am an experienced engineer.

But don't just take my word on it... Buy the antenna and put it through some tests. Build two links with it and do thruput tests first on one, then on the other, then on both. Do the tests to remote links far enough away to actually test the limits of the antenna. On each test be sure to test data flowing in each direction, there should be a good 12 different combinations of tests to perform. Test the antennas at the full power you plan to use through them, and select test links that have marginal s/n ratios so that you are testing the antennas ability to perform to spec simultaneously. I would be very suprised if your can perform two bandwidth tests in two directions without seeing a huge impact on performance.

[phendry]

Cheers George for the explanation.

Based on "when 1 radio transmitting will overwelm the receive circutry on the other" would you say that there is no way the very expensive dual band antennas that Andrew sells will perform? The very very expensive UMX series for example provide "simultaneous dual-band, dual-polarized (4-port) operation in various frequency bands. These antennas can save considerable cost by reducing the installation expense and the amount of equipment required on systems with multiple frequency requirements. UMX antennas allow easy single to multiband transition without compromising high performance" (end quote).

Also, if the receive circuitry was so suseptable to this wouldn't it mean that it would be difficult for more than 1 wifi operator to operate from a shared radio mast?

[MerTech_Wes]

phendry,

You have to understand that this andrew UMX series antenna was never designed for the 802.11 MAC (which is not syncronous) it was designed for cellular applications (CDMA, TDMA, GSM), which are syncronous.

In regards to receive circuitry being overwhelmed; george is specifically talking about the effects of having co-located antenna's too close to each other (under 10'), at this range, the front end circuitry would be seeing signal levels less than -30dB (regardless of channel) not only would this "de-sensitize" the radio, it would eventually damage it. This is why you never see co-located antenna's very close to each other. i.e. when you co-locate on a radio broadcast tower, you wouldn't be able to place your antenna's any closer than 40 or 50 feet from the FM element (and FM is 87MHz to 108Mhz which is at least 2300Mhz away from the 2.4Ghz ISM band).

Best thing you can do is keep co-located antenn's facing away from each other, keep them as far away from each other as possible, use antenna's with good f/b ratios and low sidelobes, dont put adjactent sectors on adjacent channels (dont have sector "A" on channel 1 and sector "B" which is right beside it on channel 6.. if at all possible) and use band-pass filters to block out any out of channel interference.

[phendry]

Hmm, but aren't sectors normally mounted right next to each other? i.e 3 x 120' to give a full 360 coverage? I used the UMX series as an example. Andrew also do other antennas that are dual band and are aimed at wireless broadband providers operating in the 5Ghz bands so how could these work if what you say is true? Are you guys speaking from experience? Has anyone actually tried this in the real world?

[MerTech_Wes]

I'm speaking from experience.. Yes you can mount sectors back to back... BUT if you can.. always give them as much seperation as possible. real world examples: our first site was a 100' tower, operating in 2.4Ghz band with Proxim Tsunami gear, the sectors (120 deg)were mounted to the supports of the tower with roughly 12" (after mounts etc.) of seperation. We're going to be retrofiting the tower with larger standoffs as soon as we have another tower in that area. Reason: the noise level on base station radio's is -85dB.. we have other towers with the same setup but with larger stand offs and band-pass filters, and we get -95dB or better.

[georgew]

In cell towers, very special radios are used. That all transmit at the exact same time, and that all receive at the exact same time. No one's receive circuit is engaged when a nearby transmitter is fired up.

Not only that, but strict engineering is applied to who is using a given tower at what frequencies.

The free-for-all typical of wifi applications is not how the licensed operators do it. When wifi colocations seem to work trouble-free, it can only be because no one operator is using all of the airtime.

Two things happen when you have a transmitter and a receiver near each other but on different channels. First, the amplifier that comes before the tuner sees the whole band, so the receiver either saturates, or attenuates. The second thing that happens is energy is detected by the receiver that will prevent it from going into transmit mode.

So for example say you have two AP's and 1 backhaul, on channels 1, 6, and 11. The backhaul on channel 1 is busy because it is being used to feed video, using 100% of the airtime. The AP radios at channel 6 and 11 will never transmit because even though their own channel is quiet, there is enough off-channel noise to drive their receivers into saturation, and in that state of distorted overload they thik they see energy that is on their channel, so they refuse to transmit. The farther apart the antennas, the less likely this is to happen.

This does not happen to cellphones, because they are specifically designed not to do this. Cell tower radios are designed for colocation, in a manner where the colocation does not block any radio from transmitting. 802.11 was not designed for this. Putting two 802.11 AP's on one antenna might work if they were on different bands.

Motorola canopy radios are designed for 6 sectors, inches apart, on the same channel (optional). Each radio transmits at the same time, and listens at the same time. That is how it is done.

Take that picture a step back... Cell towers can see each other, and they operate on the same frequencies. The cell towers transmit thousands of times stronger signals than the cell phones. So how does a cellular radio on a tower hear a hand held cellphone in the basement of a building when there
are 4 cell phone towers within line of sight on the same channel?

All celltower radios on the same channel transmit at the same time. The first timeslot after that is not used to allow time delayed tower echo's to fade, then the cell phones each transmit a packet, on an assigned timeslot. This allows huge numbers of clients and towers to share the same channels.

When you use technology like this, you can put a lot of transmitters close together.

This is what the GPS sync thread is all about. Hopefully some day StarOS will do this.

[phendry]

WOW I never saw the GPS sync thread before. Very interesting and a shame we wont see it in StarOS any time soon :(

Georgew, you say that Mikrotik has a polling system (I guess as part of there Nstreme protocol). Does this mean that it may be possible to use both verticle and horizontal polarity of a dual polarised antenna simultaneously with 2 x Atheros cards using Nstreme? Or maybe with Canopy??

[georgew]

Not with polling.

Polling is not syncronous.

Polling is good, beter than 802.11, but it is not using gps sync. Only radios that support gps sync (or older forms of clocking which are all more expensive than gps... such as atomic clocks)

If you want something with gps sync, you will need to go with one of the proprietary systems, such as canopy. It is possible that you could add gps sync to a polling system, but I don't know if the radios will support it. Even if you can gps sync the polling, because the packet sizes aren't ficed, it may still not be syncronous enough to share antenna radomes.

The only one that I know for sure will support this is canopy. But that is only because I haven't seen how the others work.

[phendry]

So in theory you believe you could use 2 canopy rad's to transmit simultaneously on a single dual polarized antenna?

[georgew]

Yes, you could.

Only the 900mhz radios are connectorized, so a 2.4 or 5.x radio would need a pigtail soldered in. People do this, so it is possible.

Canopy AP's use short packets, and I think the backhauls use larger packets. So you can't mix backhauls and AP's unless they are on different bands. Typically you would use a 5.7 AP and a 2.4 or 5.2 backhaul. The work around is to use an AP subscriber radio pair rather than a backhaul, and set all of your radios for the same up/down ratio (such as 50/50).

[phendry]

Very interesting. Any idea how much a setup like that would be?

[MerTech_Wes]

To the best of my knowledge Canopy SU's go for NO LESS than $500 USD/each and that's when you buy 500 at a time.. :o

I dont know about what your deployment stratagies are, but my market won't support equipment that expensive... and it only offers throughput of about 2-3Mbps aggregate (half-duplex). Soo we're back a square one.. unless we..
a) bribe lonnie and tony to develop these features into star-os
b) use expensive proprietary gear (canopy) that has low throughput
c) use inexpensive gear that doesn't expand [into medium/high density] well
d) wait for WiMAX (2 to 3 years before it's affordable)

this is depressing... :?

[georgew]

A very good question I hope to get answered some day soon.

The 900 mhz stuff has only been out a short time, so there is a lot of demand, and not much supply... The retails are something like 1800 for an AP and 685 for cpe. The prices I heard about from someone with no stock was something like $1300 for the ap and $450 for the cpe.

[phendry]

Did anyone ever persuade the Valemount guys to look into GPS sync? Still the way forwards I think.

BTW, I did get those dual-polarised dishes and could not get 2 simultaneous links working however I am planning on getting some 5GHz band pass filters in the hope that they will attenuate the transmitted signal on one radio before it reaches the receiver on the 2nd.