Finer Points of Operation

By Jeff Schmidt N5MNW

This is a small glossary of various modes of operation (besides AM/FM/SSB/CW, etc) that are common in Amateur Radio. It is meant as an overview of some of the more common ways to make your radio do its thing.

VFO operation:

Free-tuning of the radio by the “big knob on the front”. This is similar to the tuning knob or control on your AM-FM entertainment radio. The exception, of course, is you are also changing a transmitter’s frequency along with the receiver. While in VFO mode, one “dials around the band” or punches in the desired frequency. Before transmitting, one must remember where the band edges are (one reason for the exam) to keep one's emissions, including sidebands, within the assigned band.

VFO A and/or VFO B:

Many Amateur radios have two (or more) separately tunable VFOs, selectable by a front-panel button. This allows one to rapidly “QSY” or change frequencies without disturbing the exact settings of the other VFO. One VFO may be selected as the Transmit frequency and Receive on the other (see Half-duplex or “Split”).

Memory operation:

Most Amateur Radios do not come with pre-programmed “channels”. We select our frequencies of interest and store them in numbered memory locations of our choosing. Once offsets, PL tone, etc. are selected, one “enters” the information in a selectable numbered memory location for later automated retrieval.

Simplex or "Direct":

This is a two-way mode that uses the same frequency to receive and transmit. It is the most common mode on HF, as there are no repeaters allowed below about 29MHz and most stations tune to the exact frequency of the station they wish to contact. Most bands have a calling frequency. On 2m FM, 146.52 is reserved as the “National FM Simplex Calling Frequency”.

Duplex:

A two-way mode that uses one frequency to transmit and a different one for receive.

Half-duplex or “Split”:

Using two different frequencies, one for receive, one for transmit. The other station must use opposite frequency pairing for transmit and receive. On HF, this is most often used by remote DX stations to reduce crowding and interference during contesting. The DX station will announce "listening XX" to inform contacting stations where to transmit. (see VFO A and/or VFO B). On VHF/UHF, this is used to offset the transmitter from the receiver to match the repeater input/output frequencies.

Full Duplex:

The -simultaneous- use of two different frequencies, one for receive, one for transmit. Both parties can hear the other while transmitting. Full Duplex is the normal mode of a repeater, the receiver feeding audio to the transmitter for immediate rebroadcast. Many dual-band FM rigs are capable of receiving on one band while transmitting on the other.

RIT:

Receive Incremental Tuning. This feature allows moving the receive frequency slightly without changing the transmit frequency. This feature allows the operator to adjust the receiver to satisfy the ear, without moving the transmitter's frequency, which would change the apparent pitch to all listening.

XIT:

Transmit Incremental Tuning. This feature allows moving the transmit frequency slightly without changing the receive frequency. This feature is seldom used but compliments the RIT function. A practical application would be- tuning the transmitter to a specific (split, net or standard) frequency while leaving the receive frequency adjusted to the operator's desired one.

DTMF:

Dual Tone Multi Frequency. A signaling format that uses two simultaneous audio tones to activate telephone or control circuitry. A sixteen-button pad has four rows and four columns. Column tones are 1209Hz - 1633Hz and rows are 697Hz- 941Hz. Two tones are outputted when any one button is pressed, corresponding to the intersecting row and column. A “1” (one) would be signaled when 1209Hz and 697Hz corresponding to row 1, column 1 were output together. Also known as “Touch Tones”, the same tones as heard on your telephone.

Encode:

The process of modulating your transmitter (outgoing signal) by a specific pattern or signal to be identified by a selective circuit at the receiving end.

Decode:

The process of selecting a certain modulated pattern or signal and using it to select only those received (incoming) signals to be heard by the operator. Could also describe interpreting a transmission and displaying a message.

Subaudible Tone or “PL” and DCS:

A signaling system most commonly used on FM that uses continuous, low-frequency audio tones (67-250Hz) modulated or “encoded” on the carrier. The tones are detected by a “decoder” and used to filter out noise or co-channel activity. This is also referred to Continuous Tone Coded Squelch System (CTCSS). DCS stands for Digital Coded Squelch. It basically encodes a short three-digit number over and over in a low-speed, low deviation format- similar to PL but a wider (2Hz-300Hz) bandwidth. The “decoding” receiver “listens” for the number and rejects all signals without it. Many times more code numbers are possible with DCS than PL, as there are theoretically 1000 different codes in a three-digit number and only 50 or so PL tones. In practice, there are 104 standard DCS codes.