Height and Size does make a difference
A 2 GHz microwave receive system with an antenna at 150' requires much more filtering than one with an antenna at 1000'. The difference lies in the vertical radiation pattern of the PCS Cell Phone Site Antenna. The vertically polarized PCS antennas are typically mounted 100' above ground, and radiate the greatest amount of power (vertical beamwidth) toward the horizon. Most PCS sites do not have an electrical or mechanical downtilt. An ENG (or STL) site with an antenna height between ground level to about 400 feet above average terrain (HAAT), requires the greatest filtering.
Conversely, a system mounted on a 1000 foot tower needs less filtering. A system with a high gain, directional antenna, requires more filtering than an omni. The additional filtering is proportional to the increased gain of the antenna. If you use a typical 4 foot dish with a gain of 24 db, and compare it to a system that uses a 10 db gain omni antenna, you will need 14 db more filtering.
LNA gain also plays a critical role. The average CDMA PCS Cell Phone Site has an EIRP of about 500 watts, and a TDMA PCS Cell Phone Site checks in around 800 watts EIRP. You must calculate signal strength, and antenna gain, subtract the filter attenuation at the UNWANTED frequencies, and add the LNA Gain. The final result should leave about 20 db headroom for the LNA's rated 1db compression point. Anything less will result in intermod and a higher noise floor.
You must calculate the filter's rejection (or call us at 1-) to leave the ideal 20 db or more LNA headroom, and the filtering must be placed immediately after the antenna, ahead of any LNA or receiver.
How much filtering do I need?
The answer to this can be found in the following 2 formulas
This first formula determines how much filtering is needed for a double conversion receiver (two separate IF frequencies):
* factor needed to prevent PCS from causing intermod in receiver's mixer on weak signals
For a Millennium receiver with an LNA built in, connected to a 4 foot dish antenna, with 1000 feet of 7/8" Heliax at 2.2 db loss per 100', plus a 30 db LNA up on the tower, this works out to:
80 - 60 + 24 + 30 - 22 -10 or 42 dbm of filtering (attenuation) required for signals in the PCS band, 1930 to 1990!
This second formula is for single conversion receivers with 70 Mhz IFs. This applies to the majority of the receivers:
This gives the attenuation needed at the "C" block range. Now you must get rid of the IMAGE FREQUENCIES. These are the PCS signals that appear below the Local Oscillator, minus the IF frequency. If these signals are not filtered out ahead of the mixer, they will produce the same 70 Mhz IF signal, on top of and usually stronger than your remote!
This gives you a shape factor, or slope, to calculate how fast your filter should drop off, and what the attenuation should be. Rule of thumb: be 100 db down for a 1950 Mhz signal at the receiver input.
For an MA2GUX receiver with a 12 db LNA built in, connected to a 4 foot dish antenna, with 1000 feet of 7/8" Heliax at 2.2 db loss per 100', plus a 30 db LNA up on the tower, this works out to:
82 - 60 + 24 + 30 - 22 -10 or 44 dbm of filtering (attenuation) required for signals at the edge of the PCS band(1990 Mhz). The attenuation needed at 1950 Mhz climbs to 94 db ((44 db + 80) - 30)).
The problem you run into is the enormous quantity of stages that must be used to obtain proper filtering, not just get by. If you don't use enough filtering, weak signals will be masked! It may be FM, but any signal appearing at the mixer, which is nothing more than a diode, produces a little more rectification, and helps raise the noise floor. This reduces sensitivity, and makes weak signals noisy.
If you put all the filtering ahead of the LNA, the loss of the filter greatly reduces weak signals. If you amplify the signals first, the LNA itself produces intermod. We have solved this problem by designing a special LNA, with an IP3 of +36 dbm, and placing it after the first bank of filters! This LNA has a noise figure around 0.5 db, yet can produce about 250 milliwatts of power. Then the signal can be steeply filtered, without loosing weak ENG or STL signals.
We have a completely matched SuperFilter/LNA System for you! Each system is custom designed to your filtering needs and gain requirements. We have made these systems as lightning proof as possible.
We also have filters and LNAs for single channels.
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