RF Linx Corporation Wireless Networking

Wireless Solutions for Wireless ISPs, Hot Spots, Military, Commercial

Home Phone: (513) 777-2774

FAQs

Amplifiers

What is the P1dBm Compression Point and why does it matter?

Every amplifier has gain and a compression point.  The TX gain on an amplifier is a measure of the amount of amplification and is measured in (dB).  This gain holds true for RF input power up to a certain point.   By increasing the input power more and more the amplifier starts to become non-linear and compress, and the amplifier starts to actually lose gain.  When the input RF power is high enough to cause the TX gain to drop by 1dB, this is the point called the P1dBm compression point.  This is also where an amplifier is typically rated for maximum usable power.   When selecting a WLAN amplifier one should always consider the P1dBm compression point of the amplifier; it's a measure of how much horse power is under the hood.

When using any type of fixed gain WLAN amplifier one needs to pay close attention to not exceed the rated P1dBm point.  Compressing amplifier beyond the P1dBm point can potentially decrease the operational life and damage the device.  In addition overdriving and compressing an amplifier well beyond its P1dBm point will cause unwanted distortion products and harmful interference to both in-band and out-of-band signals.    RF Linx Amplifiers are AGC style amplifiers, whereby they automatically adjust their TX gain to maintain linearity and thus retain a fixed average RF power output at the Antenna Port for a wide RF input range.   RF Linx AGC Amplifiers are rated at both an Average and Peak value, and thus work optimally with OFDM modulations.    Our Tunable Amplifiers allow for full user adjustability so one can optimize the RF performance of both Average TX power and RX gain in their links.    

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What are the benefits of a Digitally Tunable Amplifier and how does this help me?

The LT, DT & PT Series of Tunable Antennafiers can be adjusted in the field using a USB connection and a simple PC software GUI.

The field tunable features offer incredible versatility never before seen in a device of it's kind.  This option gives you the control over circumstances that can change in the field.  Rather than having fixed devices that you must physically change out when your RF environment is negatively impacted you can simply change settings at your leisure with no additional expense or costly and dangerous tower climbing.  This is crucial in the winter when tower climbing is difficult or impossible.  You can adjust your TX power in the field to give you maximum performance with a variety of antennas (EIRP regulations apply) and cable loss conditions.  You can also adjust your RX gain to provide the perfect amount of receive amplification without raising the noise floor unnecessarily.   With our DT Series you also get the benefit of an integrated High Q Tunable Filter providing an incredible -50dB adjacent channel rejection and the ability to select and change your operating channel from the ground.  This means that if a competitor or a change in your infrastructure requires selecting a new channel on this system you could do so without climbing the tower or switching out a fixed tune channel filter. 

 

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Advertised 802.11g / 802.11a output power and how do I compare from one manufacturer to another?

By definition the IEEE 802.11g / 802.11a standard uses OFDM signal modulation.  This type of modulation called OFDM requires addition headroom or Peak Envelope Power (PEP) for field usable output power.  Many other manufacturers are advertising only the peak power of their 802.11g / 802.11a amplifiers. This means that your field usable or average RF output power could be significantly much less than advertised.   It also means you donít know what the Peak to Average power rating is and this could have a dramatic impact on data throughput and range. 

We advertise and rate our amplifiers for both average and peak power, we have nothing to hide from the customer and believe they should know what performance they paid for.  In addition our advertised peak power is rated at the amplifiers P1dBm point and many other manufacturers are rating their amplifiers at saturated power sometimes at itís P3dB point.   Thus, a competitors amplifier might say ďsaturated TX power= 2W (33dBm)Ē and not give the specifics of compression point or average usable power (dBm).   If RF Power is measured at the P3dB point itís about 2dB less or in this chase 1.25W(31dBm).    This is false advertising, donít be fooled ask for both the amplifiers P1dBm point and Average Power. 

 

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Should I mount the amplifier at the tower top or tower bottom with the radio?

This is an often asked question and there are several correct answers.  In a typical cellular telephone industry active RF equipment is mounted at the base to provide ease of access to components like amplifiers and radios.  However it requires very expensive low loss RF cable to preserve RF performance.   In most WLAN applications the cost of developing a base mounted system is sometimes cost prohibitive due to high RF losses thatís where a split system where the radio is at the base and the amplifier is at top of the tower close to the antenna is the favored approach.

A split system uses a radio and DC Injector, located at the tower bottom, that injects DC power and Radio RF energy onto a single coax cable.   The other end of the coax, located at the tower top, consists of an amplifier and antenna.  The benefit of this system is that it allows one to use a cost effective indoor commercial radio and yield the high performance of an outdoor solution.   In this case a weatherproof amplifier sets the critical RF performance at the antenna (Low Noise Figure in RX and maximum boosted TX power).    It is at the antenna where you want to preserve these important system parameters.   

The traditional cellular approach limits all outdoor components to passive elements (non powered devices).   This is what cellular companies do, yet it requires very expensive low loss coax to be used from the amplifier to the antenna.    Every dB of loss associated with this cable is directly added to your critical RX Noise Figure and robbing your TX power.    We recommend no more than 3dB of loss in these applications, since 3dB is 1/2 your TX power and equates to about 25% reduction in your overall system range.

 

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What is a POE Amplifier?

POE stands for Power Over Ethernet.   A large majority of radios manufactures are offering POE for the ease of installation and elimination of high cost and RF losses associated with the traditional coax installations (Radio at the bottom and antenna at top of tower).  A POE radio maximizes RF performance because it eliminates or minimizes the RF cable to the antenna, thus preserving RX NF and TX power output.    

An outdoor POE amplifier works in conjunction with a POE radio and is also mounted at the antenna.  The POE radioís RF output is connected to the POE amplifier, where the signal is boosted and sent to the antenna.    A POE amplifier contains a wide input DC-DC Converter (+18V to +50VDC) and an Amplifier.   DC power is supplied to the amplifier via a standard CAT-5 cable and an included +24V POE power supply.   DC power is injected onto the spare (4) lines located on the CAT-5 cable.  Lengths of CAT-5 cable runs can be substantially long (150í to 250í). 

 In some applications our supplied +24V POE power supply and a single CAT-5 cable can be used to power both radio and amplifier.  Or as in most applications one can simply use two CAT-5 cables, one for the radio and another for the amplifier.

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A radios operating distance is determined by many factors, frequency, path loss, NF, Fresnel effects, cable losses, and antenna gain; all of these parameters effect the overall system link margin and S/N Signal to Noise.    In establishing a good system link margin one must overcome all these RF losses and arrive at a desired S/N ratio.    Raising a systems TX power in a spectrally clean fashion can overcome these RF path losses.   As a rule of thumb every 6dB increase of TX power doubles oneís free space operating distance.      

Another method to increase oneís operating range is to provide a low system receive NF followed by an appropriate amount of RX gain to preserve the NF and present a clean signal to the radio receiver.  A radios receive performance is rated by many factors, but typically the most important being RX sensitivity.   Using an amplifier mounted close to the antenna with a low NF will provide a spectrally clean signal to the receiver.  

If your system RF performance (TX Power and RX NF) is being robbed by high cable losses from your radio to antenna youíre getting hammered with a double whammy hit from a link margin standpoint.     Improve you link margin significantly by inserting a tunable amplifier mounted near the antenna to provide spectrally clean TX output boost and highly selective RX gain. 

 

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What does Automatic Gain Control (AGC) really do and why should I use it?

Automatic Gain Control or AGC as it is commonly called offers a plug-and-play type functionality.  The AGC feature of the amplifier will adjust the gain up or down so that it always maintains a steady average RF output power equal to it's rating regardless of changes in the input power or cable loss between the radio device and the amplifier.  Please note for the AGC to function properly you must have an RF input signal at the amplifier between +2dBm to +27dBm to switch from receive to transmit mode.

Example:  If you use a standard 32mW (+15dBm) radio device you have +15dBm of Radio signal strength.  If you have 7dB of cable loss (equal to 100' of LMR 400) between the tower bottom mounted radio and the tower top mounted +30dBm (1W) Amplifier you would be providing the amplifier with about 8dBm of signal.  This is more than is required to switch the amplifier from RX to TX mode.  The amplifier will sense the signal strength of +8dBm and then add +22dBm to it to bring the signal power output to +30dBm or 1W as rated at the antenna port of the amplifier.

You should use an AGC amplifier when you want the ease of designing your system power level or Effective Isotropic Radiated Power (EIRP) based upon the constant AGC controlled output.  You may also find that having an AGC amplifier offers a nice standardization where you can mix and match radios and cable types / losses and still use the same amplifier from installation to installation.  This allows you to keep a spare unit that will work as a drop in replacement for all or many of your systems depending on the antenna used and the EIRP expected.

 

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When you say the unit has an average power limit and a peak power limit when using OFDM what does that mean?

OFDM (Orthogonal Frequency Division Multiplexing) is the type of modulation used in 802.11g and 802.11a devices.  OFDM modulation with its multiple carriers requires a high degree of Peak to Average RF power to perform properly.  This wide power range is for the multiple carrier bursts and often described as headroom and measured as Peak Envelope Power (PEP).     

When using OFDM modulation we recommend a 4 to 6dB back-off from the amplifiers P1dBm point.  This will provide the additional headroom for the OFDM modulation and yield maximum data throughput. 

 

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Does it matter what radio power I use with a fixed gain amplifier?

The radio power, as well as the cable loss between the radio and the amplifier, is critical when using a fixed gain amplifier.  You must be aware of and consider the RF radio power, pigtail losses and amplifier TX Gain before selecting a fixed gain amplifier.  If you provide the fixed gain amplifier with too much power you will overdrive it potentially damaging the unit and creating system performance issues.  Too little power and you will not obtain the intended results.  Our AGC amplifiers handle this in the background by reading the radio input power and adjusting the amount of TX gain applied to ensure that the rated average RF output is reached with a wide RF input range.

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What channel should I use and how does the channel filter help me?

As I am sure you are already aware the WLAN bands are getting more and more crowded and will get worse before they get better.  Channel filtering is the next logical step beyond channel selection.  Logical channel selection will help you deal with normal co-location issues and general network layout.  Remember competitors will want to use the "good" channels too. When you have co-location or interference issues that cannot be solved by channel selection you must combine filtering with good channel selection.  RF Band Pass Filtering is a method which narrows your receivers passband and gives you rejection to unwanted out-of-band interference. 

By filtering out unwanted out-of-band interference receiver is able to now distinguish the intended signal from the noise and interference.  Please note that you cannot filter out in-band interference or interference that is already in your channel.  If you are on Channel 6 and are getting interference from a competitor also on Channel 6 you only option is to change channels or convince your competitor to do so. Working with other local WISPs when selecting channels and filters will benefit them as well as you.  The channels can be logically divided and protected with filters so that channel re-use can occur and everyone can share the limited channels and prosper.

The diagram below provides a graphical rendition of the 802.11b/g channels and is provided to help you picture how specific channels will react to other channels in terms of potential interference and co-location problems.

As I am sure you are already aware the WLAN bands are getting more and more crowded and will get worse before they get better.  Channel filtering is the next logical step beyond channel selection.  Logical channel selection will help you deal with normal co-location issues and general network layout.  Remember competitors will want to use the "good" channels too. When you have co-location or interference issues that cannot be solved by channel selection you must combine filtering with good channel selection.  RF Band Pass Filtering is a method which narrows your receivers passband and gives you rejection to unwanted out-of-band interference. 

By filtering out unwanted out-of-band interference receiver is able to now distinguish the intended signal from the noise and interference.  Please note that you cannot filter out in-band interference or interference that is already in your channel.  If you are on Channel 6 and are getting interference from a competitor also on Channel 6 you only option is to change channels or convince your competitor to do so. Working with other local WISPs when selecting channels and filters will benefit them as well as you.  The channels can be logically divided and protected with filters so that channel re-use can occur and everyone can share the limited channels and prosper.

The diagram below provides a graphical rendition of the 802.11b/g channels and is provided to help you picture how specific channels will react to other channels in terms of potential interference and co-location problems.

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Can you change an amplifier from AGC to Fixed Gain?

It is possible to change any of our AGC amplifiers to Fixed Gain.  We simply disable the AGC circuit and tune the amplifier to a customerís custom specifications.    Our digitally tunable amplifiers can also be converted to allow a user to tune TX gain (dB) as opposed to TX power (dBm).   Caution should always be used when adjusting an amplifiers TX gain, since itís easily to compress an amplifier beyond itís P1dBm compression point. 

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Can you modify an amplifier to use less DC power?

Yes we can reduce the DC power consumption or change the operating voltage.  This typically involves removing RF gain stages and performing additional modifications.   We can easily perform custom modification to our amplifiers since we design and manufacture these products at our facility.   Please give us a call to see how we can assist.

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What are IP Ratings and what do they mean?

The IEC standard #60529 defines waterproofing ratings (IP Codes) for various levels of protection against water ingress. This standard is used to define everything from electrical connections to enclosures.

There are several different uses of IP Codes, as described in IEC 60529. IP Codes can have the following arrangement:

The first character indicates the degree of protection against the ingress of solid foreign objects.  First character definitions are as follows:

0 - Non-protected
1 - Protected against solid foreign objects of 50 mm diameter and greater
2 - Protected against solid foreign objects of 12.5 mm diameter and greater
3 - Protected against solid foreign objects of 2.5 mm diameter and greater
4 - Protected against solid foreign objects of 1.0 mm diameter and greater
5 - Dust-protected
6 - Dust-tight

The second character of the IP Code indicates the degree of protection against the ingress of water without harmful effects. Second character definitions are as follows:

0 - Non-protected
1 - Protected against vertically falling water drops
2 - Protected against vertically falling water drops as the enclosure is tilted 15 degrees
3 - Protected against spraying water
4 - Protected against splashing water
5 - Protected against water jetting
6 - Protected against powerful water jetting
7 - Protected against temporary immersion (1meter, 30 minutes)
8 - Protected against continuous immersion (1meter, > 30 minutes)

 

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Antennas

How much base station antenna isolation is required?

We would like to see at least 30dB of isolation between base station antennas.  Antenna isolation increases with distance.  Antennas spaced at 3 feet horizontal spacing will provide about -39dB loss (isolation) between them, this increases to about -42dB at 4 feet and -44dB at 5 feet.

As with anything RF, there are many factors which can affect system performance.  A big one when dealing with antenna isolation is that coupling to tower structures can reduce effective path loss between antennas.  This could change the necessary spacing for any given installation.

To calculate isolation for directional antennas like sector antennas, you have to add the front to back/ sidelobe spec to the gain (dBi).  For instance; with a 16.5dB sector and a front to back spec of -14Bi, you have effective gain toward the side or back of the antenna of 2.5dBi.   You'll see that you get the best isolation if you can space the antennas with some vertical separation.  Vertical separation is measured from antenna center to center in feet.

Please see our calculations page for additional information.

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Filters

Can RF Linx meet my custom filter requirements?

RF Linx specializes in providing "solutions" products for the wireless industry and can provide custom filter designs to augment our production offerings.  We can provide Bandpass, Lowpass, Highpass, and Bandstop / Notch filters as well as Multiplexers.  Custom filters may require additional design cost and manufacturing time.

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Why do I need a RF Linx Ultra High Q Filter?

A RF Linx Ultra High Q Filter is used to reduce interference which improves the performance for co-located equipment and radio reception in general.  Interference is caused by transmission sources near the channel you are using to transmit.  These unwanted transmission can interfere or cover the intended signal.  Please note that a filter cannot remove intraference, or interference on the same channel you are using.

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What is the difference in the number of poles in a RF Linx Ultra High Q Filter?

Each pole is a filtering circuit.  The more poles a filter has increases it's rejection or strength.  Generally all RF Linx filters have 8 or more poles to provide the best performance against high level interference.  Some competitors offer 4 pole filters but in our opinion they are not strong enough to provide the intended outcome.

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Orders

Do you offer additional discounts for large orders?

Yes we do.  We look at every order individually.  When large quantities are being ordered addtional discounting may apply.  We will review your unique project with you and let you know where you can save.  Please give us the opportunity to quote your amplfier project directly.

 

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