As done some time ago for the 137 Kc band, here is a brief description of the equipment that I have realized to operate on the new medium-wave amateur band, (472 to 479 Kc). The antenna is still the same used on 137 Kc that has been retuned, eliminating the intermediate coil situated outdoor and reducing drastically the inductance of the loading coil.
The efficiency of such an antenna is of course very low, although I think that the loss around 34 or 35 dB estimated on 137 Kc in respect to an isotropic radiator, on this new band will be reduced under 30 dB; I'm attempting to evaluate these loss, as soon as I get a reliable result it will be inserted here.
As regards the equipment I've thougt to use my homebrew transceiver for the 137Kc band, buildig a small transverter able to convert the 472 band into the 137 one, equipped with two quartzs so covering the 7 Kc of the band, being the 137Kc transceiver planned to cover only 3.5 Kc. The transceiver has been a bit modified, inserting a switchable mixer in the circuit of the display, this way, summing the frequency of the transceiver with the frequency of the quartz running in the transverter, the display will show the actual frequency. A new power amplifier has been also built, using two IRFP450, able to output up to 100 Watts if supplied whit 24 Volts or even more using higher supply voltage.
The picture below (left to right) shows the 137 RTX, the small transverter and the power amplifier.
The RTX is well detailed in the pages of my 137Kc activity, see details the small transverter is almost conventional, using two LM 1496 as mixers and small IF transformers as band filters, (see the layout at right)
The picture below shows the layout of the power amplifier; at first glance it may appears a bit complicated, in fact it must be able to give its power with an input of only 50 millivolts. That has given some problems in decupling of the various stages to avoid self oscillation. I've used the mosfets in analogical mode, without need of a dedicated IC like TC4426CPA or similar, the efficiency is very good, over 60% and the wave-shape pratically sinusoidal with harmonics lower than 40 dB, measured over a resistive load. The antenna Q will do the rest.
To increase the overall performance, several wires were arranged horizontally on the roof, covering an area of around 30 square meters. These wires as well as the other metallic structures in the vicinity of the antenna, were connected to the cold side of the circuit, together with the ground lead, so acting like a counterpoise. In this way, the current induced in these structures, returns directly to the circuit, without dissipate power in the walls.On this new band, being the lenght of the ground lead (about 26 meters)a meaningful part of the wavelenght, I'm considering the advantage to cut this connection, to avoid that the counterpoise can subtract a part of current to the ground lead that, being situated outdoor,even if relatively close to the building , contributes to the radiation.
To achieve better results, the same loop antenna built for the 137 khz is used for receiving (see pictures); its output, opportunely mixed with signals coming from vertical antenna, generates a "Cardioid" effect, useful for reduction of noise, always present on this band.
At last, to further increase the power, I've tought to use my 400 watts 137 Kc amplifier whose tank circuit has been modified accordingly (see layout below).Band-switching is accomplished with SW4 and SW5. Being the input circuit aperiodic it can be fed directly from the above-mentioned 100 watts amplifier. More details about the 400 watts amplifier are in the 137 page see details .
The efficiency of this equipment, even with limitations due to the fact that the station lies in the town, that is strong noise level and impossibility to erect a good antenna, I think can be better in respect to the 137Kc band,considering the greather efficiency of the antenna, due to the better ratio between its dimensions and the new wavelenght. At the moment, being just starting the tests, not many contacts obviously have been made. Activity detail.To everybody who whishes further informations about the activity on this band, I suggest visiting the site Long_Wawe_Radio
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