Angle Linear Dec1.14 mod apr.17
Duplexers are merely a three port version of a resonant device that allows simultaneous receiving and transmitting on a common antenna but at two separate frequencies. The separation of the two frequencies is commonly referred to the "split" and is usually determined by the practical physical limit of construction of the components used to construct the duplexer.
Duplexers are usually made from all-pass type resonators in conjunction with a "notch" to provide the attenuation necessary to allow this simultaneous transmitting and receiving. They can also be made using regular band pass filters provided there is adequate skirt rejection. This is usually not the case at lower frequencies, less than 800 MHz, because the typical receive and transmit splits are so small, <1/2%.
Generally the bigger the diameter the better when it comes to filtering as the losses will be lower (not always) and the quality of the resonator will be better. Many people believe their duplexer will actually provide some sort of filtering for their receiver which is typically not possible at VHF frequencies. All the duplexer will really do is attenuate your transmitter and anyone else who is very close to your transmit frequency while using a single.
Interference Data Collection:
The best way to assure interference free operation for any antenna connected receive system is to "prove" what interference you need to eliminate. This interference information or data can be obtained with a spectrum analyzer. Measure all signals from at least 50 MHz to 1000 MHz and record all signals greater than -35 dBm. The objective of the filtering is to reduce all "out of band" signals to below -35 dBm. That includes your transmitters. With the correct amount of filtering you can achieve this and have virtually interference free system. The minimum filtering we recommend is two (2) band pass resonators on the receive port of any duplexer regardless of who makes it or what fancy name it has. They all need band pass filtering to get the out of band unwanted signals down to a tolerable level. Some locations may require three (3) resonators on the receive port of the duplexer to survive; it all depends on your location and circumstances. Remember: no guessing….. prove everything!
Duplexers are usually assembled from notches, two or three on each frequency. This gives maximum attenuation at only one frequency and have minimum insertion loss at one pass frequency. The bandwidth of these notched frequencies can vary but generally they are a few hundred kiloHertz at most.
By combining two deep notches in series, the characteristics of each will add and the result will be twice the attenuation at the notch frequency and double the insertion loss at the pass frequency. Usually 90 to 100 dB of attenuation is adequate to eliminate all affects of the transmitter on the receiver, overload and noise, etc. In some high power cases (>100 Watts) and additional 10 or 20 dB of rejection (notch) may be required depending on levels of out-of-band transmitter noise.
Many power amplifiers suffer from poor design and will generate out of band noise. The noise may fall on the receive frequency thereby masking the true receiver noise floor. In Those cases, repair the power amplifier, usually deficient low frequency decoupling or oscillations.. The addition of a low insertion loss band pass filter, <0.2 db to the output of the amplifier.
By combining two pairs of notches, one pair for receive and one pair for transmit, one type of duplexer can be fabricated.
Alignment is simple:
MARK ALL cables… ALL cable lengths are CRITICAL.. DO NOT Change.
Separate all resonators and individually tune (and prove) on a good network analyzer for best pass frequency return loss. When this is done, the resultant insertion loss will be minimized.
Then and ONLY then prove that the notch attenuation is optimized on the correct frequency. Then reattach the cables WITHOUT retuning anything and measure total duplexer performance. If the are adjusted correctly and the cable lengths are correct the performance will not change when things are recombined. The depth of the notches and the insertion losses will add correctly and most importantly, the return loss will not change on the alignment frequencies. Always terminate all unused port in a known good 50 Ohm termination when testing.
Most of the real RF world can easily pass into the front of your receiver virtually unattenuated through any VHF duplexer. There is an average of only a few dB attenuation away (out-of-band) from the operating frequency. If you are using a preamplifier then there is no protection for your receiver whatsoever from being overloaded by signals even several hundred MegaHertz away, FM,, TV, wireless internet etc.. They can even be miles away and still cause interference. The inclusion of a two or more of band pass resonators on the receive port of the duplexer will eliminate virtually all of the interference from out-of-band signals. In some cases there may be more needed. That can be proven by measuring out-of-band signal levels on a spectrum analyzer on the receiver port of the duplexer.
Preamplifiers can be a major source of problems, if not used correctly. Many manufacturers produce amplifiers which have entirely too much gain. This excessive gain can overload the typically low dynamic range of most receiver front ends. Many receivers use low current bipolar or FET front end preamplifiers and mixers. The rule is " the farther you get into the receiver the higher the required signal handling capability of each stage". So be kind to your receiver and don't put a 40 dB gain preamplifier in front of it. In high density RF environments there may be too much man made noise already and in many cases you are only going to make it worse by including a "high" gain preamplifier. Remember the earth is warm and produces "noise".
When antennas are elevated the noise will drop by typically several dB if you are not pointing at the noisy galaxy. Keep the gain down to 15 to 20 dB or so and some of your problems will be prevented. If you need to reduce the gain, use a 3 or 6 db 50 Ohm attenuator after the preamplifier, not before. Attenuators are usually 50 Ohms, require no DC power, won't oscillate and are predictable in performance. Run crossed polarization antennas diagonal in physical orientation, not vertical & horizontal. This will give identical elevation of the antenna main lobes due to ground effects, retaining circularity.
Now that you have gone to all of this work to get your system filtered and clean don't throw some of your efforts away by using interconnect cabling that reduce its performance. Double shielded cable is a must. Prove the performance of ALL cables. Low insertion loss is also important. Look at the specifications on most cheap cable today and you will find it has effective braid shielding of something less than 98%. What about the 2% left over? It's all holes......... That's where the problem occurs, it can leak going in and out. Double braid or foil with braid is the minimum that should be used for any interconnect cable. Deficient connector assembly can also produce RF leaks. Type N connectors can handle several KiloWatts easily and are rugged.
If you wish to discuss any of the foregoing, please contact us at 310.539.5395