Amplifiers ?

During daylight hours are best. Loud amplified music at night will cause the neighbors to complain.

What are you trying to accomplish or what problem are you trying to solve? What equipment, location, topology, and hardware do you have to work with?

The more I read newsgroups the more I find people who haven't a clue about life. Dusan is one.

Specific information about your system, your needs, hardware, etc., would be good info to include in a request for additional info. Get a brain.

EW

The things i've read about amplifiers maked general impression on me that they are introducing more problems then they solve. Is that correct? How about using amplifier in the town with a lots of concrete buildings together with the sector antenna (equipment would be a 3 hyperlinktech sector antenna HG2417P-120° 17 Dbi + 3 Cisco AIR-AP352E2R Access Points)? Is the amplifier only good to really long distance point-to-point links, and as compensation for cable losses?

url for the antenna:

ps.

I've deserved your sarcasm, I should write that question better. Hope this one is better? Personally I appreciate your postings in this newsgroup.

Amplifiers are generally a waste of time and effort for such a system. The Cisco 350 series xmits 100mw (+20dB) which should be enough.

The only place where an amplifier is necessary is to eliminate losses created by long coax cable runs. I was involved with a WISP system that had two Teletronics 1 watt amplifiers on a mast to cover about a

1/2 mile radius. The problem was that the radios were on PCI cards on the ground floor. There were two floors and the mast was 30ft high. About 80ft of LMR-400 and LMR-600 coax cable. To compensate for the coax loss, I used an amplifier. I selected one that had AGC (automatic gain control) because I had no easy way to insure that the drive levels to the power amplifier input was within the correct range. A fixed gain amplifier would not have worked.

We immediately ran into problems. You could hear the access points anywhere in the area, but trying to get a reliable connection was a problem. I could improve the situation by tinkering with preamble length, flow control, fragmentation, and such. When I dragged out the test equipment, the problem was obvious. The switching time between transmit and receive on the amplifiers was excessive. It was fine for

1 and 2 Mbits/sec with a long preamble, but too slow for 11Mbits/sec with a short preamble. The receiver was also comatose for a few msec when going from transmit to receive, which clobbered part of the short preamble of a returned signal. Xmit linearity at 1 watt didn't look all that great resulting in the amplitude portion of the 5.5 and 11Mbit CCK modulation being distorted. 1 and 2 Mbits/sec was all FM with no AM component so those worked just fine. I decided to take the short cut, tweaked the amplifier a bit for lower power and better linearity, and fixed the speed at 2Mbits/sec.

My opinion of the amplifiers went further downhill from there. Water is the sworn enemy of 2.4GHz. All of the outdoor amplifier do their best to be waterproof. All I've looked at fail or should fail. (I used to design marine radios for Intech Inc). Constant failures of the amplifiers immediately after winter rains was traced to water incursion. I just drilled a small hole in the bottom of the case, added a power resistor heater to raise the dew point, and sprayed the boards with Humiseal (polyurethane waterproofing). End of water problems.

Enough with the horror stories and on to theory vs practice. The problem with tx amplfiers is that they are asymmetrical in a link. If you had a point to point link, and installed an ampflier at only one end, then your OVERALL range would not improve. Such arrangements create an alligator, an animal with a big mouth and small ears. Everyone can hear the amplified central access point, but the access point can't hear any replies from the clients because they are still running with the usual +15dBm xmit power.

Worse, since the transmitter now carries well beyond the useable coverage area, it effively creates a jammer capeable of interfering with neighboring systems in areas that it cannot effectively communicate. Such things seem to be common in mesh networks (i.e. Tropos) that use 1 watt radios at all nodes. If you want to improve range with tx amplification, then it must be done at both ends of a link. That also applies to point to multipoint type of WISP systems. There are WISP's that supply amplifiers to their long distance clients. For such systems, a similar amplfier at the central access point would work just fine. But not at one end only.

Incidentally, all new FCC approved communications schemes authorized in the last 10 years have required automatic transmitter power adjustment. The idea is to only use as much tx power as required to maintain a useable S/N ratio at the receiving end. Cell phones work this way. FCC 15.247 appeared before the FCC recognized the problem with overpowered xmitters.

I don't want to get into determining if your selection of antenna and access points are proper for the intended purpose. I would need to see the topology, layout, intended coverage area, building construction, folliage, weather, and local aesthetics restrictions, to determine the appropriate selection. Way too many factors to pass judgement here. However, I can tell you what to watch for.

Mention of concrete buildings implies that you're either trying to go through the concrete, deal with reflections from the concrete buildings, or attempt to do NLOS (non-line of sight) communcations. All will fail. 2.4GHz does not go through anything with water inside and concrete is about 30% water. Reflections are a serious problem. At best they will cause deep fades that tend to come and go. NLOS is in my opinion science fiction. You can always establish a link via a marginal path, through whatever obstructions happen to be in the way. What you cannot do is maintain that link in a reliable manner. Commerical microwave links and dual antenna access points use spacial diversity to overcome such path problems. If the path to one antenna craps out, chances are good that a nearby antenna will still have a useable path.

Incidentally, it's very easy to tell if you're going to have a problem. First calculate the fade margin. Absolute minimum should be

10dB of margin. Then, measure it by shoving a 10dB attenuator in line with the client antenna. The system should still function. If it craps out, then try smaller levels of attenuation. Given the measured fade margin, I can estimate the number of support phone calls you'll get per year from that client.

The general solution to the amplfier (problem) is to locate the radio next to the antenna. That's the right way to do it, but has a problem. It increases the number of certified tower climber service calls that are required. However, on towers over perhaps 100ft high, long coax runs with amplifiers become impractical and you're forced to put electronics on the tower. Intermod, overload, blocking, and spurs from other users on the tower are a major problem. Same with lightning damage. If you have any electronics on the tower, you should have at least one working spare.

Selection of antennas also tend to create problems. I'm glad you selected sector antenna instead of an omni. Omnis are evil. The vertical radiation pattern of your proposed sector is 13 degrees. That's perfect for covering a large town area from a central tower. However, a similar size omni will have perhaps 4 degrees of vertical beamwidth, with possibly only a little downtilt. You could easily send the bulk of your signal over everyone's heads. It gets really bad if you're in hilly or mountainous terrain, where the signal has to follow the terrain. Omnis can't do that. Panels and sectors can be adjusted accordingly. For sectors, I use:

Anyway, good luck with whatever you're plotting. Do a site survey with a spectrum analyzer to see what interference you'll be getting. There's always some junk around. For better answers in WISP service, try the mailing list at:

Yes, your revised question is much better.

What about Cisco 350 series access points sold to european markets? Only 50mw output, would the picture still be the same? Btw, do you prefer Cisco too?

Do you know some not very expensive model which can do the job? I've have grown up with the impression that the spectrum analysers are not cheap.

Thank you very much for your wishes and your effort to answer my questions.

-Dusan

Tolerable. Here's the problem. You have central access point. It puts out +27dBm. Your client radios are putting out about +25dBm. The transmit and receive range, on both directions is about equal. Things are working, but you want more range. So, you buy an illegal

10 watt amplifier. That range is still the same. That's because you may have improved the access point transmit ot client radio range, but the return path from the client to the access point hasn't changed. Dumb idea.

In some cases, the return path range actually becomes WORSE with an applier. The problem is that these devices usually have a GaAS FET receive amplifier with about 10dB of gain. That's very useful for compensating for 10dB of coax cable loss, but an absolute disaster if installed next to the access point. What happens is the extra gain doesn't improved the sensitivity since it amplifies both the signal and the noise. But it does decrease the dynamic range of the same

10dB. It also amplifies any interference by the same 10dB. If your antenna is at a noisy site, that picks up junk from all over the countryside, then you're overall receive performance will be worse with an applier.

Now, back to hardware recommendations. I'm not going to suggest or recommend any hardware unless I have a good idea of how it's going to be used and over what topography. I like to calculate what's needed and pick my hardware accordingly. +27dBm may be adequate, or it may not, depending on range, topography, interference, antenna limitations, desired coverage, minimum data rate, type of service, monitoring, channel loading, co-channel users, reliability required, ad nausium. I'll happily help with the calculations. Transmit power is among the *LEAST* important considerations for selecting an access point. Also, don't accept recommendations from those that haven't used the specific hardware. I've only used a few Cisco radios and don't have a huge amount of experience. There are always surprises.

Cisco purchased Aironet which methinks makes some of the better wireless devices. The radios are nothing spectacular and are probably inferior to currently available radios. If you're trying to break the distance record, shoot across a crowded city, or engage in NLOS science fiction, Cisco 340, 350, 1200, etc are not the right choice.

However, if you're trying to build a conservative system, that is properly monitored (with SNMP), that is reliable, and that has some nifty security related options, methinks Cisco is a good choice.

Spectrum analyzers are expensive. I have one but it's not very portable. For when I need one in the field, I borrow one. Currently, it's a Tektronix something that quits at 1200Mhz. So, I how do I look at 2.4Ghz and 5.7GHz? Easy. For 2.4GHz, I use an MMDS downconverted that dumps the 83.5MHz of the 2.4GHz band into the 120-200MHz region, which can be seen on the spectrum analyzer. 5.2-5.9GHz is more complex as I'm building a downconverter and am having problems. I think I blew something up, so it doesn't currently function.

It's also difficult to see anything on a spectrum analyzer (for various reasons). You can do a partial site survey with Netstumbler or Kismet, but that will only show 802.11 type interference. I have more trouble with microwave ovens, cordless phones, Proxim Lynx, X10 TV extensions, and such, than with 802.11 interference. You need a spectrum analyzer to see those. Last time I dragged one up a tall glass office building, the display was a solid mess of interference. Little wonder nothing worked correctly.

I ranted on the subject of spectrum analyzers and alternatives using Proxim 7400 cards at: |

Forgot one: