This antenna is a dipole, each leg approximately 60 ft long (overall length ~ 120ft), fed with open wire feeder (ladder line). Photo 1 shows the ladder line (constructed from plastic garden plant staples garden centre stuff is usually good at withstanding the outdoor conditions!), and the feed point insulator. A short length of 12 guage wire is used to take the wire tension and support the ladderline on each side of the insulator. The ladder line spacing, around 3 inches in this case, and its characteristic impedance, approximately 500 ohms here, is not important. (Spacing determines the maximum power handling capability (volts) and wire thickness (current)).
The ladder line enters the house and is connected directly to the transmatch by two lengths of URM 67 coax stripped of its outer sheath and braid (photo 2). These two 2 ft lengths take the feedline through two 15mm speed fit Polyethylene pipes through the wall and into the living room. The impedance at the point of connection to the Transmatch was measured with an MFJ 269 antenna analyser on all amateur bands (inc the three 1979 WARC bands) from 160m to 6m. The impedance R +/- jX ohms varied considerably, as expected, and ranged from 15 -j202 at 7.03 MHz to 500 + j888 ohms on 10.114 MHz.
Note the MFJ antenna analyser only gives the magnitude of the reactance, the sign must be determined by varying the frequency about the measurement frequency and noting whether the reacance increases or decreases. If by increasing the frequency the reactance decreases, the sign of the reactance will be -ve (i.e. capacitive).
The Transmatch, a Palstar AT-2K, is a 'T' match (C-L-C) which transforms the presented impedance to 50 + j0 ohms on all bands inc 6m. Note that because of the wide impedance transformations some very high intermediate impedances - and consequent high voltages - are generated in the transmatch. Even at a 100w transmit power many kV is possible and wide spaced high voltage capacitors are required. The Palstar variable capacitors are rated to 4.5kV and the switch wafers to 3kV.
The maximum power handling capacity of the transmatch will be achieved when the maximum values of capacitance (highest number on the INPUT and OUTPUT Palstar controls) is used, resulting in minimum voltage across each C, to achieve a 1:1 match.
With the system as described above, the maximum CW power on 80m (capacitor settings around mid value ~ 50) before arcing (flash over) in the Palstar ATU, was measured at around 900 watts, using a Tentec Titan 425 driver.
Note the MFJ antenna analyser only gives the magnitude of the reactance, the sign must be determined by varying the frequency about the measurement frequency and noting whether the reacance increases or decreases. If by increasing the frequency the reactance decreases, the sign of the reactance will be -ve (i.e. capacitive).
The Transmatch, a Palstar AT-2K, is a 'T' match (C-L-C) which transforms the presented impedance to 50 + j0 ohms on all bands inc 6m. Note that because of the wide impedance transformations some very high intermediate impedances - and consequent high voltages - are generated in the transmatch. Even at a 100w transmit power many kV is possible and wide spaced high voltage capacitors are required. The Palstar variable capacitors are rated to 4.5kV and the switch wafers to 3kV.
The maximum power handling capacity of the transmatch will be achieved when the maximum values of capacitance (highest number on the INPUT and OUTPUT Palstar controls) is used, resulting in minimum voltage across each C, to achieve a 1:1 match.
With the system as described above, the maximum CW power on 80m (capacitor settings around mid value ~ 50) before arcing (flash over) in the Palstar ATU, was measured at around 900 watts, using a Tentec Titan 425 driver.
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