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RADARS Linear project

HF 800W Linear, 1.8 - 30MHz A medium power HF linear
Measuring 14" x 12" x 10.5"
Photos to follow shortly
The HF Linear

Circuit diagram
JPG (20k)  or PDF (250k)
 


This is designed to cover all bands from 1.8MHz to 30Mhz.  Using a pair of Gi-7B it should give 800W output (continuous) for about 50W drive.
Front View
Front view showing the SWR meter and the tune/load controls. 
rear View

Rear view showing RF input/output and heater/mains/HT-ve connections and lead for HT+.
Mains is used to power the blower and is on when the heaters are on.
 

Anode connection
Internal view showing mounting arrangements.  The anode connection is made from a thin brass strip and the grid ring turned from brass (G4VVT). 

The PA output Pi section consists of 2 variable Cs (20 - 120pF and a second (home made) 50 - 450pF switched in for the lower bands by a simply modified 12v power relay).
The PA coil is in two section.
8.5t x 2" dia (3/16th Cu tube) - 3" long
tapped at 4t (28MHz), 6t (18, 21, 24MHz) and 8.5t (10, 14MHz)
30t x 2" dia (12swg Cu wire) - 5" long
tapped at 10t (7MHz), 18t (3.5MHz) and 30t (1.8MHz).
The anode choke (200t, 24swg 5" long on 1" dia PTFE tube stock) is mounted directly onto a 3300pF 10kV doorknob capacitor using a small turned brass ferule.

Grid Compartment

The grid compartment.  The main fan blows into the sealed grid compartment, and exits through holes around the grid bases to flow through the PA chimneys. The heater/cathode connections  are again made from thin brass strip. The heater choke is wound on ferrite rod (from a scrap transistor radio, bifilar 28t 10swg).

The input circuit consists of a 100ohm non-inductive 50Watt resistor (Farnell - about £3) directly across the input. The grid load is also about 100ohm under normal operating conditions so the input VSWR should be quite good.
Some SWR variations are expected depending on drive levels but the SWR should remain within limits for a typical 100W HF transistor rig (eg IC735)

Power meter / Bridge.  Click for Circuit Diagram
A close-up of the SWR bridge from a design in the RSGB handbook.
Circuit Diagram.