SP-400 Mast preamplifier 4m
SP-400 Mast preamplifier 4m
What is a mast-mounted preamplifier (Mast Pre-Amp)?
A mast Pre-Amp is an amplifier for the received signal that is attached directly to the mast near the antenna. This avoids cable losses and improves the signal-to-noise ratio. At the end of a long cable, directly in front of the receiver, such a preamplifier would also amplify the unavoidable inherent noise of the cable; directly at the top of the mast, very close to the antenna, this is not the case. Hence the name - mast preamplifier.
How is a mast preamplifier supplied with voltage?
A mast preamplifier requires a DC voltage to operate, usually around 12V, and the current consumption is relatively low. There are two possibilities for the voltage supply: either together with the signal on the coaxial cable, or via a separate line. If the supply is via the coaxial cable, it is also called phantom supply.
And where does the supply voltage come from?
This depends on the station. There are transceivers that are able to provide a supply voltage on the antenna socket, often this voltage can be switched off. If the transceiver does not offer this, there are feed switches (aka Bias-T, DC Injector). This is a T-piece that is looped into the coaxial line behind the transceiver and supplied with voltage. The Bias-T passes the current only in the direction of the antenna and mast Pre-Amp, but not to the radio. Another method is to use a sequencer. A sequencer is usually used when using a power amplifier and separate transmitter and receiver. The sequencer ensures the correct timing of the control signals as soon as the PTT is pressed. Since this also includes switching the mast preamplifier, a phantom power supply (DC injector) is usually also installed here.
How is the switching done during transmission?
A mast preamplifier always sits in the coaxial line, even when transmitting. To prevent damage to the sensitive transistor in the preamplifier, it must be bridged when transmitting. This is usually done with two coaxial relays.
The signal for switching comes either from the station or from a built-in HF vox, i.e. a circuit that detects whether a transmit signal is present. The HF vox is very convenient and works safely and quickly enough, but only up to a relatively low power level. If you want to use higher transmitting power, you have to use an external switching signal. This signal for switching is either a separate control line, or - much simpler - switching off the supply voltage when transmitting. In order to protect the transistor, this switching must take place a few milliseconds earlier, before the transmit signal is output. This time sequence is controlled by the sequencer: PTT on - Pre-Amp off - transmitter on. For transceivers with built-in remote power supply, this temporal sequence is also followed.
What makes a good preamplifier?
A mast preamplifier is essentially characterised by two parameters: the gain and the noise figure. The gain should be high enough to compensate for cable losses, but not too high so as not to overdrive the receiver. It is best if the amplification can be adjusted within certain limits, values of 6 to 20 dB are common. The noise figure describes the noise inevitably generated by the Pre-Amp itself and should be as low as possible. A value of 1dB or less is very good. There are also other measurements such as the IP3, the maximum permissible transmission power, etc., but the above are the most important..
It is also important to know what the preamplifier does when it is without power - is it then bypassed, so can I transmit even if no one is bothering to switch the mast pre amp? Most preamps are switched to "by-pass" without power, so they route the transmit signal past the sensitive amplifier.
Another feature of some mast preamplifiers is a built-in filter that limits the received and amplified signal to the band to be used. This avoids unwanted mixed products in the amplifier. Of course, a good Pre-Amp must be weatherproof.
The SP-400 from SSB Electronic
The SP-400 mast preamplifier is designed for the 4m band (70-71 MHz) and makes otherwise inaudible signals usable. Especially on bands with rapidly changing propagation conditions, a powerful preamplifier can noticeably extend the audibility of a DX signal - long enough to log a new country>.
Mounted directly on top of the antenna, it amplifies the received signal with an adjustable gain of 9-18 dB. At the same time, the inherent noise (noise figure) is only 0.7 dB. The SP-400 can be supplied with power either via the coaxial line to the station or via a separate line. The separate supply line is also connected to a coaxial socket, so inexpensive coaxial cable can be used, and at the same time the line is protected against irradiation.>.
The SP-400 mast preamplifier has an RF vox, so there is no need to control the switching. The max. permissible transmitting power in vox mode is 100 W. If you want to use higher power of up to 750 W PEP or 400 W continuous wave (CW, FT-8), a sequence control like the DCW-15 or similar must be used. The SP-400 requires an operating voltage of 12 to 14V with a current consumption of approx. 260 mA. Pole clamps are included and are designed for a maximum pole diameter of 58 mm..
|Frequency Range||70 - 71 MHz|
|Noise Figure (dB)||0.7|
|Insertion Loss (dB)||0.03 dB|
|Amplification (dB)||9 - 18 dB|
|Max. Power [W]||100 W|
|Impedance (Ω)||50 Ohm|
|Current Intake (A)||0.26|
|Supply Voltage||12 - 14 V|
|Max. Mast Diameter [mm]||58|
|Connector A||N Socket|
|Connector B||N Socket|