Armstrong Jubilee Mark 2 tuner/amp - general advice

[Mr 1936]

Hi

To answer Cathoderay's question about the internal aerial, looking at the photo it appears to show a shunt inductor in parallel with the shortened dipole. This is most likely to tune out the capacitive reactance resulting from the shortening. I don't have time to calculate the source resistance after tuning, but it will be higher than the 75 ohms of an unshortened halfwave dipole. The whole setup is balanced.

Some shortened dipoles have distributed inductance by coiling the arms. The idea is the same, to eliminate (or at least reduce) the mismatch loss caused by shortening.

A Balun converts a balanced transmission line to unbalanced (or vice versa). Depending on the construction, it may also perform an impedance transformation, but this is a secondary function.

This theorising is all very well, but I'd be inclined to just connect to the standard unbalanced input and see how well it works.

[cathoderay57]

Thanks Mr 1936. I think I'll do as you suggest and experiment by making up a similar inductor and trying reception with and without the coil. For those who might be interested, the coil (as fitted to a Bush VHF61) has 22 turns and the feeder is connected to 6 turns from each end i.e. the feeder has a shunt coil of 10 turns across it with an additional 6 turns at each end connected to the dipole wires. My experiment will be a subjective listening test so don't expect any clever maths or signal strength measurements! The magic eye is disconnected on FM so I can't use that as a measure. Jerry

[cathoderay57]

I've attached an image of the replica coil. FM signal strength seems marginally stronger with the coil than without it and so I'll leave it in. With the coil fitted the aerial seems more directional since standing right next to the set reduces the signal whereas without the coil this effect is barely noticeable. Cheers, Jerry