A VHF Band Scanner

rhe pan adapter has received a considerable amount of publicity and does a fine job for the HF (160-10 meters) ham. The pan adapter that covers 250 KHz is usually sufficient. The VHF ham, however, may want to observe 2 MHz or more of the band.

The band scanner to be described will cover the full 4 MHz of the 6 and 2 meter bands, if desired. The band scanner is sometimes referred to as a spectrum analyzer. Either term is ok but I prefer band scanner.

Pan Adapter vs Band Scanner ere is very little difference in the units. The pan adapter has the same input frequency as the if of the receiver and is usually connected to the receiver mixer plate. This arrangement keeps the received frequency centered on the shield of the cathode ray tube. The selectivity of the rf stages of the receiver reduce the signal strength of the signals either side of the center frequency. The pips, or displays, are progressively smaller as they are farther from the received frequency.

The band scanner uses a typical crystal controlled VHF converter with the same output frequency as the band scanner input. A two or three stage // amplifier is usually required between the converter and the band scanner to give a good distinct display of weak signals. An rf tap for the receiver can be made at the input or output of the if amplifier. The band scanner display is independent of the receiver tuning- If properly adjusted, the scanner will have nearly equal sensitivity over the entire band, Trie scanner cathode ray tube shield can be calibrated for direct frequency reading.

The Units

Band scanners are available on the surplus market so cheap that there is no advantage in constructing one. The scanner is easily repackaged to give it a neater and 'civilian' look, if desired. The conversion information

VHF band scanner. On top from front to rear are the converter, if amplifier and power supply, that is sent with the IP 274/ALA10 or ÏP 69/ALA2 band scanner covers changing the power supply to 60 Hz (the original is 400 Hz) and changing the input to 14 MHz, All my VHF converters have 28-30 MHz output so I did not change the scanner input frequency, The IP 274/ALAIO and IP69/ ALA2 scanners have the four standard oscilloscope controls: intensity, focus, horizontal position and vertical position. The scanner controls are; sweep limit, width, center frequency and gain. I removed the front panel power plug and replaced it with a plate and an ac outlet. The outlet is connected through the switch so the converter and if amplifier will be turned on and off with the scanner-

If you want to cover only 2 MHz of the band (I scan 144-146 MHz) the center frequency conirol may not center the scan. I use a converter output of 28-30 MHz so I injected a 29 MHz signal into the scanner input then turned the center frequency control to bring the pip as close to the center of the shield as possible. I backed the control

Block diagram of band scanner and associated equipment.

off about % turn to allow for calibration then adjusted L 104 (scanner sweep frequency coil) to center the 29 MHz signal on the cathode ray tube shield- Touch up the scanner input and // coils for equal sensitivity at 28 and 30 MHz,

The converter can be constructed or purchased. Articles are readily available in past issues of 73 or the handbooks for those desiring to construct the converter A poorly built converter can give false pips as the scanner will show any rf signal in the sweep range. The most common sources are self oscillation of a tube or transistor or excessive oscillator injection. Be sure your converter is adjusted for flat response over the sweep range.

I used a surplus 30 MHz if amplifier If you prefer to construct your if amplifier 73 has a construction article on an integrated circuit 30 MHz amplifier on page 52 of the July, 1967 issue. Stagger tuning this amplifier should give it sufficient band pass and

still have enough gain to operate the scanner There are transistor and tube 14 and 30 MHz preamplifiers in the handbooks. These pre-amps will, of course, work as an ¡/amplifier The if amplifier requirements will depend, to a considerable extent, on the quality of the converter. he if amplifier should be tuned for flat response over the sweep range. Generally, an amplifier with 20-30 db gain is sufficient. The schematics show both capacitive and inductive coupling for the receiver rf tap at the if amplifier input and output. I use capacitive coupling at the output. If your receiver does not have an rf stage the local oscillator may radiate enough signal into the antenna input to give a false pip on the scanner. Connecting the receiver at the if amplifier output will minimize this condition. The receiver rf tap at the amplifier input would be used by those that have a high quality receiver and the if amplifier would not aid the receiver.

Assembly

Due to the variations in the converters and if amplifiers that can be used with the scanner, and the simplicity of hookup, step by step instructions would be of little value. The interconnecting rf cables should be as short as possible. Use good VHF practice and no difficulty should be encountered.

Power supply requirements will depend on the units used.

Operation

A band scanner will show the band activity, or lack of it, without tuning around and searching. The scanner can be used for analyzing signals as described in the panadapter articles, I find the scanner more convenient than my oscilloscope for aligning converters.

If you liave more than one converter and antenna for a VHF band you may want to use separate converters for receiving and scanning, I use this system. The receiver coupled to the scanner lets me check out any other signals that I see without interfering with the QSO in progress. 1 put a relay in the scanner antenna lead and remove the converter B+ voltage when transmitting. The antennas are quite close together and the transmitter has a maximum output of 200 watts so I wanted to protect the converter from possible damage.

Some transmitters used in our local two meter net are crystal controlled and slightly off frequency so we have to tune for them. The scanner lets me see when they are on without wasting time tuning between transmissions to see if they are on.

Other Uses

For our CB friends who might be interested in a band scanner I would recommend a good if amplifier as no converter is needed. If, however, you plan to build a fancy scanner and calibrate the shield to show the

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