Dual Antennae on Wireless Routers

Obviously, the make and model of the wireless router is of no importancd. All wireless routers are identical, and interchangeable.

So far so good. I suggest you look into dish and panel (patch) antennas instead of a yagi. The cost of a yagi for the amound of gain delivered is in my opinion excessive. If the range is fairly short, a simple patch, biquad, or reflector antenna will work.

Also, if you use an ordinary CPE radio (client mode) at the neighbors, they will only be able to connect ONE computah to the CPE and have it work. If you want more computers at the neighbors, they will need either an additional router runing NAT or setup a wireless transparent bridge system, which involved additional radios. Another way is use WDS (wireless distribution system) which requires compatible (and specific) wireless boxes.

Yes, with a very minor catch. If you're using wireless in your house from perhaps a laptop, the thruput between the laptop and the neighbors will be horrible. That's because the access point has to switch between the side and outside antennas. Some, not all, wireless routers and access point take excessive time to do this.

Diversity receiption. 802.11a/b/g is suceptible to errors caused by reflections and multipath. The path from a client radio and the access point may not be perfect in all locations. However, by adding a 2nd antenna, and switching (scanning) between the two antennas, at least one antenna has a better chance of hearing the client. It's a cheap improvment in reliability.

Reply to
Jeff Liebermann
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snipped-for-privacy@privacy.net (Nick) wrote in news:42691892$0$65730$ snipped-for-privacy@authen.white.readfreenews.net:

Maybe. Mine only has one. What will happen if you connect another distant and directional antenna in place of one of yours, I don't know.

Space diversity. In theory, having two antennas gives the receiver some immunity to fading due to multi-path signal propagation.

This is why I wonder what will happen when the two antennas are really far apart, and one is directional.

When you're talking about radios, the plural is "antennas"; when you're talking about bugs, the plural is "antennae".

Reply to
Bert Hyman

I want to run a wireless router with only one of its antennae.

The other antenna port on the router will be connected to an external yagi for a link to a neighbouring property.

The other property will have a WAP configured in AP Client mode to act as a receiver.

Will my wireless router still work with only one antenna?

Why are they fitted with two antennae anyway?

Any help/advice/comments would be welcome.



Reply to

It was not obvious that you had not purchased anything yet. On the assumption that you might have some existing equipment, I asked for the manufactory and model numbers. (Hint: Tell us what you are trying to accomplish and what you have to work with).

If you look carefully at the various boards, most of them come with a diversity switch and two antenna connectors. That's because the MiniPCI boards found in many wireless routers are also use in laptops and client radios, where the packaging determines whether one or two antennas are used. I'm looking at some wireless PCI cards and note that the 2nd antenna is terminated with a 50 ohm chip resistor. There's nothing in the design of an "ADSL router" that requires either one or two antennas.

Also, permit me to disuade you from purchasing an "all in one" unit with ADSL modem, router, and wireless in one box (even if they are somewhat cheaper). If you move, and your new location has a cable modem instead of DSL, you get to throw the whole box out. The wireless part tends to add a few acronyms every few months making obsolescence a real problem. However, the worst issue is over the location of the box. The modem and router parts want to live near where all the wires come together. AC power, phone, and CAT5 LAN cables all want to live near the floor, under a desk, in a closet, or in the basement. However, the wireless part wants to live in the open, up high, with a maximum view of the coverage area. These are incompatible. You can place the "all in one" unit on the floor and use a coax cable to relocate the antenna(s), but the coax losses are horrendous. Anyway, I suggest you purchase seperate boxes for the DSL modem, the router, and the wireless access point.

I now have 3ea older unreliable Belkin 802.11b access points. I don't have the model number handy. The problem was that Belkin does not seem to be interested in updating the firmware in older products. If you look at the previous generation of Linksys, Netgear, and DLink products, you will see continuous product development well after the product is considered to be obsolete. Belkin and some others do not bother to do this. The reliability issues I had with Belkin are apparently known, but because development stops immediately after the product release, the issue becomes permanent. I suggest you keep looking.

Methinks it would be nice if you describe exactly what you are trying to accomplish and what you have to work with. There are far too many topology options in wireless to provide universal solutions.

At 30 meters, almost any external antenna will work. You should have a good strong signal with your unspecified gain yagi antennas. However, there's always a way to screw things up. One of my installations involved a pair of 19dBi dish antennas at about 100 meters. Signal was very strong and I was getting about 25Mbits/sec thruput. No problems until someone installed an access point that was directly in line with one yagi, but somewhat furthur behind the yagi's intended target. Because it was in line, the gain of the antenna made the interference problem much worse. Oops. They refused to move to another channel (because the others were polluted) so I reduced the gain of one of the yagis and repositioned it somewhat until the interference was somewhat less of a problem. Final thruput was about

15Mbits/sec with no interference and about 10Mbits/sec when the interfering access point is live. Watch where you point high gain antennas.

Working better measured how? More signal? Better S/N ratio? Better thruput? What are you measuring?

I much prefer patch (panel) antennas over yagis. It's not just the cost per dBi of gain issue, but also bandwidth, sidelobes, and sensitivity to environmental issues, that make the patch superior. The yagi's rotten sidelobes and f/b ratio will cause reflections and interference pickup. Also, the yagi is MUCH more sensitive to mounting issues than a panel antenna (with a solid ground back surface). Yagi's also have a practical limit on gain. For every 3dB of gain on a yagi, the antenna becomes twice as long. 15dBi is about the practical limit for yagi's, while panels will go to 19dBi and dishes to 24dBi. (Yes, I know there are higher gain dishes, panels, and yagi's, but if you read the fine print, they will be very narrow bandwidth and not cover the entire 2.4GHz band).

It's been a while since I've played with the numbers so let me do some digging (mostly on the fab-corp.com web pile)

Type specified -3dB bw Cost Cost per gain dBi degrees $US dB gain

Patch Rootena RT24LP14 14dBi 35 $42 $3.00 Maxrad Wisp 13dBi 35 $39 $3.00

Dish PacWireless 15dBi 19 $35 $2.33 PacWireless 19dBi 17 $41 $2.16

Yagi Maxrad 15 15dBi 30 $59 $3.93 Antennex 14.6dBi 30 $65 $4.45

Yep, the yagi is still the most expensive (per dBi gain).

Splitters and hybrid combiners have loss and loss is a bad thing. If the path length to the two antennas is different, there's a real chance that the signal phase at the two antennas will be different. If 180 degrees, they will cancel. The result will be a rather nasty series of nulls. There are systems that do use combiners and splitters, but only with isolated antennas, where there's no chance that a given signal will be heard by both antennas. On the other foot, the switch does not have any (major) interaction between antennas and can be run without worrying about creating nulls (i.e. dead spots).

I'm still trying to decode what you are trying to accomplish. If you are worried about security between wireless clients, then please consider the WRT54G router. It has a feature misnamed "AP protection" which is really "client isolation". No wireless to wireless bridging is allowed.

No, it's much simpler than that. The access point stores which antenna heard a specific MAC address successfully last. If it fails to hear another packet within a specified time (varies from 100msec to several seconds), then it switches to the other antenna and waits for the resends. Note that it does NOT work like a radio scanner and does NOT make any decision as to which antenna has the best signal strength of S/N ratio. There also a mess of other algorithms and variations. Some AP's scan between antennas when there's no traffic. Also, versions of 802.11n (and Pre-N) are completely different and have seperate receivers on each antenna to try to reconstruct the data from multiple antennas.

See the call sign in the signature. Yeah, I have the ham radio disease and suffer accordingly. Most of those I know that are involved in wireless design and development have ham licenses. Most of the sane ones are inactive. The big difference between ham radio and product design is that the typical ham can make *ONE* of anything work. I have to make my stuff produceable. (Now back to doing battle programming a Motorola GTX-900...).

Reply to
Jeff Liebermann

I forgot to mention some detail on splitters. In receive, the splitter with take signal from either antenna and deliver it to the receiver with about 1dB of loss. That's not too bad. However, in transmit, the splitter splits the signal equally between the two antennas for a -3dB loss per antenna plus a -1dB loss per port. Therefore, if you have two independently located antennas (outside yagi and inside omni), you will have a -4dB tx power loss at each antenna. Yech.

Reply to
Jeff Liebermann

Thank you for your thoughts Bert.

I guess I'll just try it. I have a fallback which is to connect a WAP via a crossover cable into the back of the router and use that for the external link.



Reply to

I had more or less come to that conclusion. Most of these ADSL modem wireless routers only have one antenna. The Belkin seems to attract the least number of negative comments on the shopping sites and it has two antennas, so I guess it is try it and see. The fallback would be to connect a WAP to the router via a crossover cable and use that to feed the external antenna. I imagine this would overcome the "throughput problem" you mention below.

Well the yagis are already fitted both ends from a soon-to-be-defunct community wireless project. The LOS is only about 30m. Elsewhere on the network we have used small patch antennas to good effect. They seem to work better than the 7-ele yagis, particularly in a multipath environment. I don't really understand this because I would have thought a narrower beamwidth would work better in the latter case and the patch antennas are much smaller than the yagis by volume!

There is already an additional router in the neighbouring property connected back to back with the AP client box via a CAT5 crossover cable.

That could be a benefit from a security POV. There is no requirement for client machines in the two properties to exchange data. I didn't realise that the WAP actually switches between the two antennas. I would have thought they would just be connectected to the electronics via a hybrid splitter.

They must scan between the antennas synchronously with the tx/rx packet stream. Clever stuff.

BTW I just looked at the pictures on your web site. It bears an uncanny resemblance to mine i.e. radio and computer junk everywhere. Guess you must be a ham too?



Reply to

Hee hee, I wasn't going to pick on him...

All of the above is why space diversity only works well with a baseband combiner. That requires two complete receivers, not just a pair of antennas.

If the demodulated signals from each of two receivers sent to an analog combiner equal signals will contribute equally to the output, rather than one better signal being switched in, and there is effectively a 3 dB *increase* in signal to noise ratio (the signals are added but the noise isn't). The control voltage would normally be an out of band noise slot, which will have less noise as the input signal increases.

Such an arrangement is required when there is no error correction or flow control of digital data (for example, if the data is analog!), but is also significantly more expensive to implement than the hot switched antenna algorithm being used by

802.11 radios.
Reply to
Floyd L. Davidson

Jeff was being sarcastic. His point was that its impossible to say how a router will behave if you don't say /which one/ it is.

Reply to
Mark McIntyre

Argh. Y'er right. I screwed up. It wasn't a long day. It was a long night doing battle with one of my pet projects, preceded by me destroying my diesel engine last weekend, and buying a new (used) car. The loss in the splitter is the same in both directions and not different as I proclaimed. What's scary for me is that I made the same exact mistake in the NEC-LIST mailing list back in 7/2001. I just found the posting. Maybe vacation...

Per week? I make that many mistakes? I thought I was (almost) perfect.

Yep. I agree. The jury will kindly disregard my stupidity.

Reply to
Jeff Liebermann

Two separate antennas. Amplitude and phase are not significant, and will vary at different times on the two signals.

Baseband is whatever signal is recovered from the initial demodulation of the RF carrier. Typically that is a signal which multiplexes several other separate signals together (and might be an AM or FM or whatever carrier itself).

An "analog" device is continuously variable, while a "digital" device has a discrete set of values. Hence a "hot switch" system such as that being used to switch a single receiver between either of two antennas might be called a "digital combiner" (a binary, or two level switch).

But something which continuously adjusts the output to different proportions of signal from either of two devices would be an "analog combiner".

A hybrid that connects two antennas to one receiver would also be an analog combiner, but since the phase differences would result in unwanted signal variations, it doesn't work well (as Jeff correctly described) and merely amounts to guaranteed multipath interference.

(Note that this can also be done with spectrum diversity, using two transmit signals and just one antenna.)

One transmit signal and two paths to two different antennas separated by physical space, is called "space diversity". The two separate signals will each suffer from various degradations, but almost always at different times and/or to different degrees. A multipath fade on one antenna will not likely happen at exactly the same time as a multipath fade on a second antenna located a significant distance away (significant in terms of wavelength, so 4 inches is significant for 2.4GHz equipment).

The problem with using a single receiver with two antennas is that those multipath fades are associated with a phase shift, and if the two signals from the two antennas are combined at RF (before demodulation), they will varyingly add or cancel, and thus defeat the entire purpose of using diversity.

Well, yes it is certainly simplified! I suppose this topic could fill a chapter, at least, in a fairly difficult text... :-)

They *should* have a relatively constant amplitude and phase difference. The phase shifting of the RF signal would not cause a linear change to the phase and amplitude of the demodulated signal. The better the signal, the less variation... and whatever there is is *noise*, not signal. I.e., with a good signal the baseband output will have a high signal to noise ratio.

The combiner takes advantage of that by combining two signals that are *coherent* (or, they were to start with, when they came from one transmitter), and thus the signals will add. The noise is not coherent (the noise on each signal is from a different source) and thus does not add. That results in an effective increase in the signal to noise ratio.

Only at an instantaneous moment when multipath signals are minimal. Which is to say, if the antennas are close enough together, yes. But of course then any instantaneous fade that affects one will also affect the other, and there is no diversity because it is effectively just one antenna.

That is one possible implementation of what is being discussed, and would be an extension of the existing system used by most

802.11 radios today.

Jeff has worked for years directly with "wireless" systems. He is far and away the most expert person posting in this newsgroup.

You are probably senior to both of us! You will no doubt get a good chuckle from what I am basing the above discussion of space diversity on! I came to Alaska in the 1960's to work on the White Alice Communications System, which used L-Band (700-1200Mhz) forward troposcatter radio links that implemented space diversity exactly as described. A typical link used a single transmitter with an output of perhaps 3Kw and the receive end employed two antennas and two receivers with exactly the type of combiner described. The antennas of course were 75 foot high billboards with 60 foot parabolic reflectors (something like 45 dBi gain if I remember right).

I've been twisting knobs on microwave systems since the early

60's and those troposcatter systems were the only combiners like that (and the most fun radio systems too) that I've ever worked on. The manuals had a set of routine maintenance tests that one could do, which might take a couple days to accomplish on each set of receivers, that would indicate if the receivers were operating correctly or not... But what we did was hang a VTVM on the output of the combiner control voltage amplifier, and terminate the receiver antenna input. If the resulting maximum possible combiner control voltage was up to snuff, that *had* to be a good receiver, and we could sign off the routine and go play pool, catch fish, or chase girls, for another day or two... ;-)
Reply to
Floyd L. Davidson

Now just wait a minute... We've got a rifle team showing up at the wall for a bit of shooting practice at sunrise.

If we let you cop a plea, who they gonna execute?

Reply to
Floyd L. Davidson

Woops - Once in a million posts even the group expert misstates after a too long of a day. You are still world class though since we allow you a few every week! Above splitter loss is identical in both directions (about 4db). In the receive path 1/2 (-3db) of the signal is dissipated in the internal matching termination.

Reply to
Jim Beam

Sorry but I completely missed your point. I assume *space diversity* means two isolated (or at least different amplitude/phase) antenna patterns relative to the linked client/clients.

I can not picture the configuration of your system with dual receivers and an analog baseband combiner. Of course I don't know exactly what you mean by "analog" or "baseband" in a system like this.

How are the two receivers and single transmitter configured relative to the two diversity antennas? Certainly not with a power splitter/combiner.

Your explanation seems a little over simplified or "twisted" because the two *diversity* analog demodulated signals from each of two receivers probably will not be equal amplitude or in phase so there probably will be no 3db S/N improvement when combined.

I realize of course that the outputs of two receiver channels with identical antenna patterns will combine with a 3db S/N gain. And I accept that two outputs of two complete (receiver, transmitter, antenna) systems can be combined digitally using flow control but that doubles the expense. That is not the system being discussed though.

BTW, I realize that you are also a "group expert" but Jeff has seniority and a higher post count so hence the title ;-)) Both of you guys really impress an old senile retired RF design engineer!

Reply to
Jim Beam

Sorta. The two receivers might have a common local oscillator if it's a heterodyne system, so the demodulated data will fairly close to in phase. However, most modernish 2.4Ghz radios are direct conversion, so that's not an issue. Let's play with the numbers.

If we measure the phase difference or time difference of arrival of a signal between two antennas, the difference could easily be a full wavelength or more. If there is a reflection involved, or the path to each antenna is different (one direct, the other reflected), then it can be even more. If I separated two antennas by exactly 1 wavelength (12.5cm), and combined the signals with a resistive combiner or 0 degree power divider, then as I move my transmitter around the antennas in a circle, there will be 2 very deep RF nulls when the transmitter is inline with the antennas. Obviously, this is not the way to improve reliability.

With 802.11b/g, the base phase data rate is about 2Mbits/sec or 500 nsec per bit. The rest of the speed comes from moving the phase around and amplitude modulation to form 8 or more points on the constellation diagram, plus multiple data carriers in the case of OFDM. If I space the antennas again exactly 1 wavelength, but this time use two receivers to demodulate the data, the antenna spacing necessary to created a null at baseband when the transmitter is in-line with the antennas is: 1/2 * 500 nsec * 3*10^10cm/sec = 75 cm That's now 6 wavelengths instead of 1 wavelength. A data de-skewing register will resynchronize the decoded data and prevent a data null. This is very very very roughly what a "RAKE receiver" does when dealing with multipath. I vaguely recall a nifty Intersil application note that described exactly how it worked but I can't find it.

If we demodulate the data to a lower frequency (as above), the delays required to produce nulls are much larger. Given the fairly narrow antenna separation on the common access point, the separation at the modulated data rate is a sufficiently small percentage of data bit time, that simple summing of the demodulated data would yield an improvement in reliability. With de-skewing (bit alignment) the phase error can be mostly eliminated.

Note that I haven't mentioned anything about inter-symbol interference caused by excessive delays and reflections. There's a limit to how long a delay can be accomidated at baseband.

There's no 3dB improvement in signal to noise ratio with two receivers. That's because each antenna picks up the signal at a fixed S/N ratio. Combining these two signals in any manner will result in the noise increasing along with the signal. What it can do is select the antenna with the *BEST* S/N ratio and thus prevent dropouts, nulls, fades, and multipath effects.

A simple analogy would be if I placed 100 identical antennas and data receivers on my kitchen table. As I move the transmitter around, the BER (bit error rate) from each receiver will vary from whatever works out as a maximum at the distance and connection rate, down to totally gone in the case of reflective cancellation. Each of the 100 receivers will vary exactly the same way, but not exactly at the same time or when I am at the same exact location. If I had a way to combine the data output from each of the 100 receivers (and discard corrupted packets), the BER would whatever is coming from the best receiver at that instant. There will always be at least one of the

100 receivers with a good signal. However, the BER will never be better than what is offered by the worst case path loss at the connected data rate. If my transmitter is very low power or the distance is large, then all the 100 receivers will hear a weak and rotten signal and respond accordingly.

Incidentally, OFDM is frequency diversity. With multiple carriers (802.11a/g are 48 carriers), on slightly different frequencies, the chances of at least one of the carriers not ending up in a situation that creates a null is dramatically reduced. (Null's are very RF frequency dependent). Add two antennas and a diversity switch, and the typical 802.11g access point has both spacial and frequency diversity features.

I beg to differ. Because noise is not coherent, it cannot be removed or cancelled by summing. Noise plus noise equals twice the noise no matter what form of combination or summation is used. If I take two identical signals at any frequency with perhaps a 10dB S/N ratio and simply combine them with a resistive combiner, I'll end up with a 10dB S/N ratio output. If what you say were true (noise does not add), then it should be possible to take one noisy signal, split it between two ports, cutting both the signal and the noise in half, and then combine it back to yield twice the signal and half the noise. This is obviously impossible.

Bah. Quantity is not a very good replacement for quality. I make all too many mistakes but figure that if corrected quickly enough, that's acceptable. The problem is that I haven't done full time RF design for about 20 years. I'm catching up quickly, but discovering that I have some holes and misconceptions to deal with along with the usual age related memory problems. I'm also weak in the all important protocols area, probably because I hate reading RFC's and IEEE specs.

Quantity test for alt.internet.wireless: |

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postings. No wonder I need a vacation.

Thanks. However, you've caught me at two giant goofs and will probably find more if you dig deeper. Experts should be infallible and I'm certainly not. Perhaps a lesser title might be more appropriate.

I'm 57 and am nowhere near retired. The family tradition is to drop dead at work, and I'm doing my best.

Without the antenna, you're just measuring the front end noise level. For a system test, that's a good go/no-go test, but can be easily screwed up by some common failure modes. For example, an oscillating RF stage will produce more noise, instead of less noise, and end up with a deaf receiver. Same with the then common carbon comp resistors becoming a noise source. It also won't test the antenna since it's disconnected. The good news is that a failure anywhere else, except the front end, will result in a dramatic drop in detected noise, so this is probably an acceptable test.

VTVM is a "Vacuum Tube Volt Meter". I just noticed I still have an Eico VTVM in the pile. Why, I have it, I dunno.

Reply to
Jeff Liebermann

I assure you that is not true. The received signals in the two channels are coherent, but the noise is not. There will in fact be an improvement in SNR *of the baseband*. That is a *very*

*different* thing than the received SNR for RF signals at the receiver inputs. The two are related only to the degree that a better signal (both in terms of absolute level or in terms of SNR) at the antenna will (within the dynamic range of the receiver) result in a higher SNR at the baseband output.

Not that phase distortion of the RF signal won't cause inter-symbol interference and reduce that SNR. But it will always be *at least* as good as the best signal and when both signals are equal the SNR will be 3 dB better than it would be with only one signal.

That's a good description. However, that is a digital combiner, not an analog combiner. What you gain there is entropy, not SNR. Which is to say that you could have 50 of the

100 receivers out of service and the BER of your data would not change! All 50 of them are just passing information that is redundant with the other 50.

That's precisely true. I'm sure you've seen an HF RTTY signal using 2 tone Frequency Shift Keying, where the two frequencies less than 200 Hz apart fade in and out totally independent of each other. Watching RTTY on a scope is about the best demonstration of frequency diversity I've ever seen.

Exactly. Summing the incoherent noise will *not* produce the same result as summing the two received signals, which are coherent.

If the two noise sources are coherent, that will be true... and won't be if they are not.

And that is obviously true because those two noise signals *are* coherent!

I agree with you about "post count", but even so... you've still got at least twice as much hands on experience with wireless as probably any other 3 or 4 of us put together here.

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1400 postings. No wonder I need a vacation.

You ain't doing bad. Google says 2710 posts total, to all newsgroups, over the past 5 years.

Looks like a simple case of Usenet addiction to me. Maybe you can get an appointment with my shrink...

Nah, experts make goofs. The only reason your "two giant goofs" look giant is because you made them. But, lets face it... it literally takes someone who has been doing this stuff as long as you to see half the nuances of what you are saying most of the time, never mind to correct some part of it.

Well, Jim is almost certainly older than both of us then. I'm a couple years older than you. But I bet I'm a *lot more retired* than even Jim is! My most stressful decisions these days involve which cat to pet first... or maybe what to eat for dinner. Or whether I want to take a nap now, or later, or both!

It's an FM radio. The control voltage is developed from an "out of band" slot above the normal baseband bandpass. The noise in that frequency slot goes up as the amount of quieting decreases, and inversely proportional to the signal input. No signal... lots of noise. Just how much is a very good indicator of how functional the entire radio is.

Because the noise slot is at a higher frequency than the normal range of the baseband the noise will be reduced if the bandwidth of the receiver is too narrow, and will be reduced if the gain of any of the receiver's RF, IF, or noise channel is too low.

In this case the control voltage was supposed to be adjusted to something like -42 volts when there was no signal input to the receiver front end. With a receiver that had just been totally overhauled it could be adjusted as high as maybe -50 volts. So we liked to see anything from about -46 to -50 as the max possible. If it was less than that, it was time to work for a living.

(These radios were made to Western Electric specifications by REL, Inc., which if I remember right stood for Radio Engineering Laboratories. I have no idea who actually designed them or who was behind REL. It was all early 1950's technology. Loads of fun...)

The whole system has to be optimum to get that much noise in the out of band slot. Tune the IF too narrow... no noise. Cut out the RF or 1st mixer... no noise. Basically when the antenna is removed it causes every AGC loop to operate at full gain with the single exception of the combiner junction point, which will be at cutoff or squelched.

Resistors are in fact a noise source... but far less than anything these receivers were going to sense! We're talking -95 to -98 dBm input for 20 dB of quieting. The front end was a

416B triode in grounded grid (actually, a specially selected 416B called a 6280WA which back then cost about $150 each).

Testing 60 foot parabola's isn't practical.

But in fact at one point we *did* have a special crew go around and test feedlines and antennas, because we had one that had something like 20 dB extra loss on the transmit side. Which near as we could tell, once we had equipment to tell us which part to dismantle, was caused by improper installation and had resulted in some burning in a 4" rigid coax section that probably took place the first time they applied RF power to it.

The problem was that we could only figure out where about 10 dB of that loss was, and until the day it was turned down that one antenna had significantly less gain than the others. (We used it for the backup transmit, which was a 10 Kw klystron running at only 500 watts output. Low enough not to interfere too much with the 3000 watts on the other antenna, but enough that if the main side failed completely it would still hold the path. On the other antenna only 5 watts would have been enough.)

Exactly. And in fact it would catch a flat front end too. Instead of -47 volts, the max output would be maybe -43. Not much, but it definitely gave an indication.

Reply to
Floyd L. Davidson

- snip - Lots of good stuff but the discussions are not on exactly on the same wavelength nor discussing the same topology, conditions, assumptions, etc.

Jeff -

I think Floyd was referring to the difference between combining/summing non correlated noise *vs* correlated signals. The absolute gain/loss reference got lost in the discussion! Keep in mind that these combiners combine/sum instantaneous *voltages* rather than power.

Re-visit the "splitter" used as a receive combiner in your post that started this discussion. (For discussion assume an in phase combiner with 0 dissipative losses) Then:

Any single signal or two non correlated signals will combine/sum with 3 dB individual signal losses. Thus total output power will be 3 dB less than total input power. For example two 0 dBm input noise signals will result in a 0 dBm output signal.

BUT two *correlated* input signals' will sum depending upon voltage phase. Total output power will equal total input power for "in phase" correlated signals (voltages will add). For "out of phase" correlated signals the voltages will subtract and output will be zero (the null you referred to) for equal level input signals. For example two 0 dBm input "in phase" correlated signals will result on a +3 dBm output signal.

Thus the 3 dB S/N improvement he referred to.

This discussion is not completely "apples to apples" so it results in lots of different answers.

Thanks to both You and Floyd for all the help you provide here. I have followed this group since it was started and the present level of technical expertise by far the best ever. And the ass hole flame level is the lowest too!

Back to lurking!

Reply to
Jim Beam

Satellite is usually quite slow and has sky high latency. ADSL should be a substantial improvement. However, since there is apparently an existing wireless system nearby, take care to select your channels to avoid mutual interference.

I don't really need these to answer the antenna questions but it would be nice to know: How far away are the properties? Any line of sight problems or obstructions? Since you have existing equipment, what are the model numbers? How much antenna gain on the yagis?

802.11b might be a problem if you purchase an ADSL line of 3Mbits/sec or faster download speed. We have 6Mbit/sec service available locally. You'll probably need 802.11g to take advantage of the full speed.

The load resistor rarely gets tx power. The way the diversity switch works is that the radio selects the antenna that received the last successful packet. Both the transmitter and receiver are switched. Since the real antenna is getting all the successful packets, the radio spends all its time connected to the real antenna. It may occasionally scan the terminated antenna port, and perhaps transmit a beacon or two into it, but the greatest majority of transmission go to the real antenna. Not a problem.

I don't see the problem. It's much easier to run one pair of telephone wire from the loft to a tolerable radio location, than it is to run coax cable for an antenna, or CAT5 for ethernet. Just locate the ADSL modem in a convenient location but concentrate on putting the radio part (CPE) in an RF ideal location. That's much easier in a separate box. It's easy enough to enclose a wireless access point, powered with PoE, in an outdoor box, on a pole, and next to the existing yagi. As for price, it's usually more expensive to initially purchase separate boxes, but far less expensive when only one part of the puzzle needs to be replaced.

It's impossible to generalize solely on the basis of manufacturer. Almost everything comes from assorted manufactories in China or Taiwan. Dlink, Linksys, Netgear, etc all just put their names on some obscure factories products. Many are almost identical. For example, look at how many are similar to the DWL-900AP+ wireless bridge:

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are basically a list of TI ACX-100 boards. Each manufacturer also has winners and losers all mixed together. What differentiates the vendors is not the quality of the hardware, packaging, or construction. It's support and firmware. The apparent inability of Belkin to support its *EXISTING* customers with continuous firmware updates is why I think don't like Belkin. The same products, with the same chipset, possibly from the same factory in China, but from a different vendor, does get updates, fixes, and support. Before buying something, I suggest you check into the availability of firmware or driver updates or the date of the last update. If there's only one update, find something else.

Also look into alternative firmware for the WRT54G. Sveasoft, HyperWRT, etc.

Close, but not exactly. The radio decides which antenna works best with specific client radio. Each radio is identified by its BSSID also known as its MAC address. In this case, the MAC address is simply a radio identifier and has nothing to do with traffic handling. Incidentally, *ALL* wireless is based on bridging.

In a bridge, the system sniffs the traffic to extract a table of MAC addresses heard on each port. The bridge then decides if a packet needs to cross the bridge based upon the destination MAC address. One distinction is to select antennas, there does not need to be any traffic or sniffing. Both the antenna and MAC address table expire, but the antenna selection has a much shorter expiration time because the path might change much faster than perhaps a client radio roams to a different access point.

Good plan and good luck. Do a "site survey" to make sure there aren't any other users around.

Reply to
Jeff Liebermann

The objective is to share an ADSL connection between two properties to replace a community wireless solution for internet access. Both properties have external yagis connected to WAPs configured in AP client mode and then via a crossover cable to wireless routers. The yagis currently point to a WAP about 50m away which is connected to a satellite internet feed via a firewall. Internally each property has its own wireless network with Channel, ESSID and WEP key. Wireless laptops are used in each property. The existing wireless kit uses Pheenet APs and Linksys 802.11b wireless routers.

I never took the lid off a wireless router and have never noticed that on PCI cards. >

Agreed. But I imagine that two antennas might give an 'average' 3dB improvement in SNR instead of heating a chip resistor.

Point taken. But in this case the PSTN line is in the loft and so are all the power and network connections. The price of an 'all-in-one' unit is almost the same as a separate ADSL/ethernet modem here in the UK.

The Belkin seemed to attract the least negative comment from other purchasers on the etailer's site I looked at. I'm also looking at 3Com, Dlink, Netgear and Linksys.

Thanks. I will look at the WRT54G.

That sounds like a bridge to me.

I have pretty much decided to connect the new router to a separate AP on a different channel i.e. a separate wireless network to link the two properties. That way any nasty interference effects between the external yagi (with its sidelobes) and the router antenna(s) in the loft would be avoided.



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