Question about antenna coils on AM tube radios

[ct92404]

Hi,

When I restored my Emerson model 503 tube radio, I had to make a rear cover and antenna coil for it. It uses a flat coil of wire that would go on the inside of the cover...which is what I'm assuming most small antique AM radios would have used. I think it's called a "loop" antenna? I tried experimenting with several different coils I made, and I noticed that the longer the length of wire and the more turns, the better the reception. But I had a question...

Does inductance play a role with this, or is it simply the length of the wire? If I were to have a larger coil with a longer length of wire and even more turns, would that improve the reception even more? Or could the inductance of the coil affect the tuning and you get to a point where you have diminishing returns? I'm assuming that since the tuning knob is a variable capacitor connected with the antenna coil, then resonance must be a factor and you could have an antenna that is too long?

I guess that's really my question...can you have an antenna that's too long?
Right now the radio seems to be working fine, but I was just curious if I could improve it at all...pick up farther stations, get a better reception, etc.

I have some experience with electrical circuits, but I'm very new to trying to work on radios. And this was the first time I ever completely fixed an antique radio like this.

- Chris

[joebog1]

To get "further" stations use an outside antenna.
A long piece of wire as high as you can get it. Sort of
As a kid I used to have a VERY large gumtree at the back fence, my room was 40 metres away. I used climb as high as I could ( maybe 15 metres) and tie a piece of rope to the tree, this was my "insulator" the rope was maybe 5 feet long to get me out of the "leaves". I tied my aerial wire to the rope and proceeded to add another piece of rope to the eaves of the house. The other end of the aerial wire was tied to that. The remaining tail ended up going through the window to my radio. Was freezing in winter !!!!!
Anyway, for AM radio within reason there is not really too long!.
Inside the radio will be an aerial coil that is used for the actual tuning of the stations, and the "big coil of wire" that you made was a substitute "long wire"antenna.

The actual reception of radio waves is a fascinating subject in itself, and I would direct you to the local town library. The book to obtain is the ARRL handbook. ARRL = The American Radio Relay League.
They have been publishing handbooks yearly, or nearly yearly since their inception, which is way back farther than I can even remember.
It will give you a very solid grounding in all things to do with radio, AND many other things if you get a modern version. I would suggest a copy from say 1955 to 1960.

This is the publication by the American "hams" ( TERRIBLE word for amateur radio operators ). They will walk you through the whole subject from basic theory of elements to very sophisticated radio apparatus, of both receivers AND transmitters, power supplies, antennas for both reception and transmission, antenna tuners, etc etc.

With all the best for the festive season
Joe

p.s. as you clean snow from the footpath Im drinking beer!! Its almost 40C degrees here today, and STILL NO RAIN.
jb

[G8HQP Dave]

In many cases the coil of wire will be part of the first tuned circuit in the radio, and then its inductance has to be correct. As you added wire you were probably approaching the correct value.

In any case, yes you can have an antenna which is too long. How long is too long depends on a number of factors, including frequency and the input stage details of the radio receiver.

[peter_sol]

Looking at the circuit diagram of your radio |(schematic)
The frame aerial (antenna) is the tuned circuit so yes for maximum performance it would be best correct.

[Hartley118]

A reasonably refined front end MW/LW tuned circuit will assume a specific aerial capacitance as part of its tuning and will feature an 'aerial trimmer' to be adjusted to suit the aerial installation. Too long an aerial could have excessive capacitance that could be untunable within the range of the trimmer.

Domestic sets of 1930-40s vintage generally assumed a long wire of 50-foot plus. On the other hand, early car radios assumed a 2 or 3 foot whip plus a bit of coax feeder. They have a high input impedance and the setting of the aerial trimmer can be pretty critical for good reception. A long wire would certainly be unhelpful to that setup!

Most of today's car radios of course use a ridiculously short antenna, but which is carefully matched to its wideband head amplifier, giving results that would have been unbelievable in the 1930s!

Martin

[Radio Wrangler]

Joe, we have plenty of water in Scotland. If you can ship us a bit of your surplus sunshine, we could ship you some water.

The ARRL handbook is very carefully written to work for a wide range of people. It has to be welcoming and encouraging to new beginners and planned so that they pick up more and more on subsequent re-reading of material. It has to go far enough to be stimulating for seasoned practitioners, and it has to be carefully right so that real experts can't find any loose threads. As a kid I got a lot from an old 1956 edition lent to me for a few weeks (I copied out bits and memorised the rest) So in the early 1990s when I was asked to write a chapter for it it was an opportunity to repay a big debt. I think I did it OK, the material went out in the 1995 edition and is still there in the 2016 edition.

[Silicon]

Hello Chris. I found a schematic for the Emerson 503 online.

It seems that the loop antenna (internal frame aerial) is also acting as a coupling transformer.

It probably has many turns of wire to pick up your AM Broadcast band (Medium Wave) but it may also include a few turns of wire within the winding/bundle that are connected to an aerial and earth terminal.

The signal in an external long wire antenna (aerial) would be transferred into the main loop winding to increase the signal strength.
If you get the connections wrong the signals may oppose each other and reduce the signal strength.

As this radio is an AC/DC set it is important for safety reasons to isolate the external connections from your mains supply. The original loop antenna did that.

Internal loop antenna are very convenient and often very effective.

If the owner of another Emerson 503 could count the number and size of the windings of the loop antenna you would have most of the information you need.

[ct92404]

Thanks for the help guys!

Joe, I just have to say, you were crazy when you were a kid! Lol...it sounds to me like you would have just made a lightning rod! And yeah, we are in winter here (well, technically in a few days). But I live in southern California, so no snow here. We did just get some rain though, which is good because we're in the middle of a drought. About maybe 3 weeks ago, we had a downright SCARY thunderstorm here...pouring rain and some VERY close lightning strikes! I'm glad I didn't have any wires up in any trees nearby! Haha!

I had a feeling that the inductance of the antenna coil may have a role, because with the shorter coils I made at first, only the higher frequency stations would come in. I suppose that, conversely, if I made the coil too long with too many turns, only the lower frequencies would come in and higher frequency signals would drop out? Is that right?

- Chris

[Herald1360]

Originally Posted by ct92404

I had a feeling that the inductance of the antenna coil may have a role, because with the shorter coils I made at first, only the higher frequency stations would come in. I suppose that, conversely, if I made the coil too long with too many turns, only the lower frequencies would come in and higher frequency signals would drop out? Is that right?


- Chris

Yes. The antenna coil is the inductance in the input tuned circuit for the set. If you can measure the max/min capacity of the tuning gang acoss it, then the required inductance is easily calculated.

If the set design is good, it should work out at a convenient whole number of turns with enough spare adjustment range on any trimmers to align everything nicely