On Short Waves

Radio Record, Volume II, Issue 32, 22 February 1929, Page 32

On Short Waves

Some Peculiar "Freaks" QNH of the most fascinating things about short-wave work is that you occasionally stumble on something which seems to be contrary to all the rules, and gives you a most interesting time before you get to the bottom of it, says a writer in an exchange. Even if you never quite fathom the reason for the particular snag you have struck, you generally glean some useful practical information which will probably come in very handy in the future. Now, one of these peculiar little difficulties which causes great bewilderment when it is experienced for the first time is 2 form of hand-capacity effect of a very strange nature. Ordinary hand-capacity effects, of course, are fairly simple to understand, and not very difficult to .reduce to almost harmless proportions.

They generally appear at two points: (a) on the tuning condenser, where a slight change in tuning occurs when the hand is placed on or removed from the dial, and (b) on the reaction condenser, where it may happen that on getting the set on the verge of oscillation and then taking the hand away it

will either break into oscillation or subside to a point considerably below the most sensitive "just on the edge" state, so that signals disappear altogether. Simple Remedies. SIMPLE remedies, such as taking care to put the moving vanes to the "earth" side of the filament, the use of a good vernier dial so designed that the fingers do not approach very near to the body of the condenser, the choice of.a good circuit, and so on, will do a great deal to keep these troubles down, but it is far otherwise with another form of hand-capacity effect, which is so freakish as to be quite startling the first time you experience it. What happens when this is present is that, you discover that there is quite a bad capacity effect between your hand and any earthed part of the circuit ! For example, if you adjust reaction so that the set is just oscillating and then touch the earth terminal oscillation ceases, while as the hand approaches the terminal, or any other earthed point, such as the L.T. leads, the earthed side of the tuning condenser, and soon, there is a notice able change in tuning,

hi he ie ian There appear to be two main causes for this peculiar state of affairs, the first being the use of. too long an earth lead, ‘which is most likely to cause trouble on the shorter wave-band (2035 metres). Where you suspect this is the trouble, therefore, try a "capacity" earth, consisting of a few yards of wire lying on the floor, and any

other type of aiternative earta (sas or water pipe, ete.) which may be available. Try, also, connecting the earth to various different points instead of to the "HH" terminal, e.g., to the negative of the L.T. battery to the moving vanes of the tuning condenser, or to the negative terminal of the detector valve socket. (Connect to only one of these points at a time, of course.) The other possible cause is the presence of H.F. currents in the ’phones, and where this is the trouble you will generally find that if you adjust the set to the very verge of oscillation and then grip the ’phone cords tightly in the fingers it will immediately oscillate strongly. This is a rather more difficult trouble

to get rid of, but one or more of the following dodges will usually effect a cure :-- Put a condenser of .001 mfd. across the ’phone terminals, try a different HLF. choke (a No. 50 plug-in coil is usually quite good), reverse the leads to LS. and OS. of the LF. transformer if one is used (not desirable if it can be avoided), bind the ’phone cords round with No. 24 or 22 D.C.C. wire for about a foot of their length and connect one end of the binding wire to earth.