Testing radio without long wire aerial

[mike1 ryan1]

What kind of signal can be fed into an old 20s radio aerial and earth connections to test it? I have a signal generator that I haven't ever used yet and would like to know what to set it to so I can check the operation of the radio before I go to the trouble of rigging up a proper aerial. Can it be directly coupled to the radio, or is there another way to feed in a signal. What level of signal etc do I use so as not to damage anything.
Thanks
Mike

[Mike. Watterson]

Very little!

A 20 turn loop on a shoe box or similar size plastic tub works OK for most old radios. Any shape.

I use about 15 turns of stiff wire about 10cm dia simply taped in a couple of places as a loop on a scope probe plugged into the signal generator to inject. Works well with all the battery valve and mains sets with a loop aerial as well as all sets with a ferrite rod. Start with coils touching and then separate a bit if you get a signal.

There is such a circuit as a "dummy aerial" for old radios and 50 Ohm or 75 Ohm signal generators. It's just an attenuator with capacitor isolation, as much to protect the signal generator.

The pair of loops ensures the signal generator is safe (a bigger risk than damaging the radio).
Note that cheap modern function generators or DDS generators have FAR too high a signal. Maybe x100 what is needed. A 10:1 scope probe and a coil solves that.

The wire type isn't important, I've used enamel, solid hook up wire, mains, speaker and internals of CAT5 computer cable. Even thin coax will work if you just use the outer.

[HamishBoxer]

Most radios will perform certainly as a guide line on just 20 feet of wire around the curtain rail etc.
If you have an earth great or use the radiator pipe ,if one is to hand.Mains earth at a push.

[paulsherwin]

You don't even need that for testing purposes if you have reasonably strong MW signals available. A couple of yards/metres of wire should produce something.

[Boater Sam]

My experience, a Pye A15 works well on 1 foot of wire in the aerial socket!

Shack aerial is 15 foot of wire in the ceiling dropping down to the bench, pulls in all I need even on short wave. If the set won't respond with that, it's faulty.

[ms660]

If it's a 1920's receiver I would recommend a reasonable length of antenna wire and a connection to earth, if you want to feed a signal in from a signal generator make sure the generator's output has no DC path via the receiver, eg: magnetic or capacitor coupled only.

Lawrence.

[mike1 ryan1]

This is a 20s or 30s era homebuild in an oak case. I do have an old signal generator not modern version I can't remember the make, I'll have to dig it out. If I remember it does have a dummy aerial. I'll take some pictures of it. So there's no physical connection between them just inductively connected through the wire loop and a short wire aerial on the radio then?
Mike

[Station X]

Just try it with the longest bit of wire you can manage. You've nothing to lose.

[mike1 ryan1]

I've got it in bits re-stuffing all the capacitors that need renewing at the moment but I'll try and get something rigged up when it's back together.
Thanks again for your help and advice,
Mike

[mike1 ryan1]

Hi,
Dug out signal generator when I got home from work. As I said I've never actually used it yet and guess I'll have to read up on it before using it. Was offered it some time ago by someone who had themselves inherited it and so couldn't tell me anything about it. Also have a frequency counter that I intended to use to test it, just never got round to it. It is something I'd like to know how to use as I have it.
Mike.

[g4aaw pete]

Hello Mike

Bill presented a copy of the manual for this generator here :-

https://vintage-radio.net/forum/showthread.php?t=35916

although this is the VHF version.

[PJL]

1920's sets will usually pick up something with say 10 metres of wire for an aerial but they can be tricky to operate. If it has a reaction control you will know it's working as it should oscillate all by itself!

[Mike. Watterson]

Few counters will work from the output connector designed to drive a radio, the level is too low. I added a BNC with a FET buffer before the attenuator on mine.

It's worth making a loop as they pick up VERY much less local electrical interference as a relatively short wire (< 1/8th wave) enhances pick up of E near field and the loop uses the M field.

I do use a wire above the curtain rail for SW on some mid 1950s to Early 1960s sets. Far too much noise for MW & LW. I have some decorative deep frames with MW loops and a switchable series 2.2mH coil (the 1/4W resistor body style) for LW. Sometimes called Paris aerials if there is also a tuning capacitor (one has, but only some sets suit that).
Above about 100 cm path ( 2 x (length + width)) the loop outperforms a ferrite rod on MW. The small 1941 to 1955 "Personal Sets" (only briefly in UK 1946-1948) have too small a loop. The Ever Ready N and N3, also Vidor CN420 had large loops so worked well. The K, T, Sky Queen / King / Prince / Lord had a decent loop with a LW loading coil. All basically the same chassis. I did one "pIcture frame" with tuner based on that.
The late 1920s giant "Suitcase" Portables have a massive air spaced loop in the lid that is part of the first RF triode or SG valve tuning. Naturally they work much better than any 2019 pocket "Sports" AM/FM set. Some even sound better too. About 1/2 the insides was batteries. The Lead Acid needed recharged less often than a smart phone and the HT pack might last months. Grid pack would last for shelf life.

So Batteries is the way to go on old battery sets. You can use multiple pairs of C size NIMH with diode splitters for charge and diode combiners for output of the "2V" cells. I use budget Alkaline AA cells in ten way holders as it works out cheaper than PP3 and allows all the "taps". Also 5 to 10 years shelf life. Replica box. As long as not connected while on charge, the LT via the diodes is close enough to a charged Lead Acid which is about 2.3V peak dropping to about 1.8V when the cell needs charged. I found Schottky diodes needed 1N5819, the 1N4007 was a bit too much of a drop. I did use the 1N4007 with series resistors per diode to recharge from a 5V phone charger. I fitted a fuse on output where the the acid filler plug/vent would have been and printed replica labels. In 1920s they were glass, but from 1929 plastic started to be used and then gel instead of liquid sulphuric acid from about 1931. Initially by using Silica, the stuff in the packets to keep things dry!

[mike1 ryan1]

Thanks for all the info, much appreciated.
Mike