Testing tower top amplifiers

The proliferation of the cellular telephones, PCS telephones, low power two-way radios, and wireless LANs, has created a need for tower top amplifiers (TTA). Before TTAs were in the field, radio engineers had to allow for more receiving sites than transmitting sites for many of the systems that they designed, or put up with the talk out range exceeding the talk in range. This was due to the fact that the base station transmitter is 10-300 watts in power, while the mobile and portable radios are 50 microwatts up to 50 watts in power. In the old ham radio days, we would call the system an alligator, which meant that it was all mouth and no ears. The site can could transmit to a mobile or portable station much further than the site can receive from radios in use.

If there were a way to increase the sensitivity of the base site receiving equipment, much of the problem would disappear. The radio equipment manufacturers did meet the challenge, and the Tower Top Amplifier came into being. TTAs cannot just be placed at a site and have everything fall into place. There are a few laws of physics and other parameters that must be considered before adding an amplifier in series with a coaxial cable line.

Once a Tower Top Amplifier is introduced into a system, the testing of the amplifier does become a problem. This Application Note discusses the fundamentals of Tower Top Amplifiers, the application of TTAs, and the testing of them. With the correct test equipment, the testing can be done with little effort, and can be included with the normal maintenance of a site, or easily tested when there is a trouble report from the area around a site.

Fundamentals

The next few paragraphs are written to acquaint the reader with some of the basics of radio and physics. If this is old hat to you, you can skip this section on basics.

Fundamentals of receiver sensitivity

Sensitivity is the property of a receiver that gives a quantitative number on the ability to receive a weak signal. The more sensitive that the receiver is, the weaker that the signal can be heard and decoded.

Receiver sensitivity is usually expressed in one of two ways. These are:

The two terms can be directly changed from one format to the other by looking up the corresponding dBm level for a given microvolt level.

Some receivers have the sensitivity specification in dBm, while others use microvolts.

The way to measure receiver sensitivity is to connect a signal generator to the receiver antenna port and generate an “on frequency” signal to the receiver at a calibrated level. The signal generator output is read when the decoded signal meets a certain signal to noise or Bit Error Rate (BER) level.

Fundamentals of coaxial line loss

The coaxial cable that connects the radio to the antenna has a few parameters that also must be taken into account when a system is designed. The Radio Frequency signal will suffer a loss of amplitude as the signal travels along the cable. This is due to resistance of the conductor, and capacitance that allows the signal to be shunted as it travels down the cable. The loss is commonly expressed as _xxx dB loss per 100 feet.

To counteract the loss, the design engineer will select larger cable, as the larger the cable, the lower the loss. All of the manufacturers of coaxial cable publish a list of the loss of the RF signal for their cable. The higher the frequency, the more loss of signal for a given cable type or size.

Fundamentals of noise floor levels

Many site locations have electrical noise or other radio station interference, so the receiver sensitivity is not the only factor that determines how low of a signal that can be heard by the receiver. If the noise floor is even slightly elevated, the sensitivity of the receiver is severely affected.

Fundamentals of noise figure paramenters

If you are lucky enough to have a low noise floor, and there are no other radio stations around you to cause interference to your station, then you have a site that can benefit from using an amplifier to increase your talkback range.

The amplification of radio signals is possible. The main parameter that limits the ability to really hear very far down in signal level is a phenomenon called Noise Figure. All electronic circuitry and devices have an inherent noise due to the flow of current in them. If you are trying to amplify a signal, you may be just amplifying this noise. Specially designed low noise transistors and operational amplifiers have been designed that have a very low noise figure and these are what the design engineers have used in the amplifier stages that are used in the Tower Top Amplifiers.

Interference

Radio systems can generate interference, receive interference, or make their own interference. When an amplifier is added to a system, the probability that interference is present is increased, along with a type of interference called Intermodulation (IM). This is where two or more radio signals combine, and new radio frequencies are generated that cause much grief to radio systems. Good filters help greatly to reduce or eliminate this problem.

By eliminating one of the components of the intermodulation mix with a filter, the IM mix does not occur.

Where is a tower top amplifier used?

A Tower Top Amplifier will increase the talkback range of a system if the site has a low noise floor and does not cause the receiver to hear interference such as IM or other types that may be below the normal receiver sensitivity. The Tower Top Amplifier will allow a signal to be amplified effectively, and overcome the loss in the coax from the antenna end of the coax to the receiver. This can give a substantial boost to the range of the system.

Some of the systems that do use Tower Top Amplifiers include:

In these systems, the mobile or portable units have low power transmitters than the the base transmitter units. This is why most of these systems are alligators.

What are the components of a tower top amplifier system?

A Tower Top Amplifier consists of more than just an amplifier and a housing for it. In fact, the sub-components include:

Each will be discussed below.

Preselector

In order to keep the intermodulation from making the amplifier useless, a very sharp filter that only lets in signals within the radio spectrum band that the system occupies is required to be the first stage. This filter normally covers all of the frequencies within the operating band, and is called a Preselector.

Low noise amplifier

The amplifier itself is always a low noise design. This amplifier normally has 15-20 dB of gain, but the preselector usually has 3-6 dB of loss, so the net gain of the TTA is only 10-15 dB.

Bypass switch

In the event that the amplifier does malfunction, almost all of the manufacturers put in a series of relays called a Bypass switch. This allows the antenna to be connected directly to the coaxial cable without going through the amplifier when the system operator so desires. This is accomplished by having the signal always bypassing the amplifier unless power is applied. Anytime that power is applied to the TTA, then the bypass is removed and all signals will go through the amplifier.

Some TTA units have the RF Preselector Filter present whether the Bypass mode is on or off. In these systems, the bypass switch is between the RF Preselector and the amplifier.

Bias-T coupler

BIAS-T couplers are used within the TTA and at the bottom of the coaxial run, either external or internal to the receiving equipment. What the BIAS-T does is to couple the DC energy required to operate the amplifier onto the coaxial line between the TTA and the receiving equipment. It does this using capacitors to block the DC voltage, and RF CHOKES to block to RF signals in the appropriate directions

Normal operation of a tower top system

The DC voltage that operates the Bypass relay also provides power to the amplifier in the TTA. The absence of the DC voltage is what puts the TTA in the Bypass Mode. The application of the DC Voltage is what allows the Bypass relays to be activated and the amplifier to become active.

Under normal operation, the received signals enter the TTA, are amplified, and sent down the coaxial line for receiving. If you suspect that the TTA is not working, then remove it from the circuit by removing the DC voltage going to the BIAS-T. If the signals are stronger when the voltage is removed, then the TTA may be defective. It could also be other factors at the site. YOU NEED TO TEST THE TTA AT THIS POINT.

Testing a tower top amplifier system

Since a TTA is a critical part of a system once it has been deployed, it should be tested before it is introduced to the system, and also, periodically to insure that it is performing as the system engineer has designed. This is easily accomplished with the correct equipment.

Bench-top gain measurement

To test a TTA on a workbench, you need a Signal Generator and a Spectrum Analyzer or a Tracking Generator, which is a combination of the two. You will also need a BIAS-T and a 12 VDC power supply to provide power to the TTA.

The procedure is as follows:

If this does not occur, then there is something wrong with the TTA or the test setup.

Once a TTA is in place, it is a little more difficult to test it. There are two ways to accomplish this, and they are both listed below.

Reference signal using a spectrum analyzer

Using your Spectrum Analyzer, find a signal that is present within the band of your system. Turn the BIAS-T voltage on and off, and see if the signal that you are watching goes up and down by the 10-15 dB gain of the amplifier when the system is not in the bypass mode.

The problem with depending on an existing signal is that there may not always be a signal there to use as a reference, or your reference signal is moving or varying in signal strength.

In-place gain on a tower using a 2 port Site Master^™

Anritsu makes a 2-port Site Master^™ that will allow the user to look at the TTA while in place on a tower. It does this by providing the reference generator signal out of one port, and having a spectrum analyzer/return loss indicator on the second port of the Anritsu 251B/C.

The BIAS-T and a 12VDC power supply will also be needed here.

The procedure for using the 251B/C in this application is described below.

Save your readings and make a note as to the gain found and the date.

If you do not have an Anritsu Site Master, then a Signal Generator and a Spectrum Analyzer or a Tracking Generator can be used to perform the same tests. The diagram and test procedure is detailed below.

The procedure for using a Signal Generator (SG) and Spectrum Analyzer (SA) or Tracking Generator (TG) in this application is described below. (Refer to Fig.15)

Caveats of tower top amplifier systems

Tower Top Amplifiers are not the magic bullet that corrects all range problems. In fact, improperly used, they create more problems than they correct. Some of the more common problems are listed here:

These problems are detailed in the following paragraphs.

Intermodulation

The Intermodulation specification is always degraded when an amplifier is added to a receiver.

Noise floor limitation

If the Noise Floor is high at site, a TTA will amplify the noise, and actually decrease the talkback range of that site.

Too much range

One of the features of many systems is the ability to re-use frequencies, and too much range will cause co-channel interference that puts one call from another area on top of the call for the given site.

Amplifier overload

An extremely strong signal can cause the amplifier to be overloaded, and it will have an attenuation instead of an amplification on all of the other signals coming through that amplifier.

Shorted stub lightning protectors

If you use a TTA, you cannot use a Shorted Stub Lightning Protector. The Shorted Stub is a DC short at all frequencies other than the operating frequency band for which it was designed.

CONCLUSION

Tower Top Amplifiers can be a benefit, or a detriment to the range of a system, depending upon the site and circumstance. We hope that this Application Note helps clear up the use and testing of Tower Top Amplifiers.

Ira Wiesenfeld, P.E., is a consulting engineer who has been involved with commercial radio systems since 1966. He has spent time working in the broadcast, two-way, mobile telephone, paging, microwave, military, and public safety radio systems, and has consulted with most of the major manufacturers in the radio industry. Ira is the author of Wiring for Wireless Sites, available from Delmar Thompson/Prompt Publishing (www.electronictech.com).

Ira has a BSEE from Southern Methodist University in Dallas, Texas; a FCC General Radiotelephone Operator License; BellCore Certified Radio Technician; and is a licensed Professional Engineer in the State of Texas.

He can be reached at [email protected].