High quality Masthead Preamp with noise figure below 1.5dB, type RP4SM. I have recently created a spreadsheet to determine the optimum amount of gain for a preamp.
I now see from the spreadsheet that anything over 15dB adds very little to improve the noise figure and on many VHF radios the S meter is 3dB per S point, (not 6dB per S point) so a 21dB preamp increases the meter reading by 7 S points. From now on my preamps will have the gain set at between 12 & 15dB. Even that might increase the S meter reading by 4-5 S points but will reduce mixer overload and achieve the required improvement in S/N ratio. The RP4SM has RF sensed switching with through power capability up to 125W continuous.Through SWR including the Station Box is usually below 1.3:1. This version of Preamp has been in continuous production for over 30 years. It has a monostable switching circuit with fast attack of about 25milliseconds and with a hang time of typically 500mS so it doesn't drop out when using SSB between words or syllables, only when you pause for breath. RF sensitivity for reliable switching is about 500mW at the Masthead Unit.
Operates from your own external 13.5V bench supply via the Station Box that combines the DC and RF. Consumption is 45mA on receive and 15mA when on transmit.The Station Box is supplied with a 75cm red/black lead to connect to the bench supply. The rectangular box shown in the pictures now usually is supplied as a 50mm square diecast box. The relay in the Masthead Unit is ON when in receive mode and disengaged in transmit or when power is removed by switching off the Station Box. So it is fail safe and has inputs and outputs protected against static and EMC pulses. The DC is fed via the coax feedline and there is both reverse supply protection and forward over-voltage protection. Connectors are SO239 unless otherwise requested. Connection to the antenna is provided by an RG58CU or URM76 coax tail 1.5 metres long which passes into the Masthead box via a compression gland on the underside of the box. The Masthead unit is bolted to a stove enameled aluminium fixing plate with tabs with holes top and bottom for screw mounting to the antenna mast. Also supplied are two 250mm releasable tie-wraps that can secure the tabs to the support mast. This item is in the category "Mobile Phones & Communication\Radio Communication Equipment\Ham/Amateur Radio\Ham Radio Transceivers". The seller is "spectrumcomms" and is located in this country: GB. This item can be shipped worldwide.