Ferrite antenna design in older portable radios


gary_thorburn <gwt@...>
 

I've accumulated portable radios dating from the 1950s to the present. I've noticed that in general, older radios were usually wound with the turns widely spaced.  I have an old 8-inch ferrite antenna with turns spread out nearly its entire length.  Modern radios typically have the turns closely spaced on a paper form positioned on the rod.  Anyone have ideas about what the difference would be?  I would expect the wider spaced turns would result in less capacitance in the coil, which I think would be a good thing.  But a closely wound coil on a paper form permits sliding the coil on the rod, positioning it for correct inductance. This permits some optimization at assembly time.  Incidentally, I have a working Philco portable, model B-631, from about 1954.  It has a 7-inch long, 17mm diameter ferrite, wound widely. (NOT an ultralight!)  A Philco ad for this radio claimed that it was the first commercially available portable using a ferrite antenna, which they called the "Magnecor".


Rik
 

A lot of European radios from my collection have unusual windings, and excellent performance, including radios with coils at both ends of the ferrite.
Some time ago I looked for internal  Online photos of PL-600 and PL-450 radios which are sharply directional. I could not see anything special about the windings on the ferrite.
I wish I knew!- FARMERIK


mediumwavedx
 

I've tried both ways of winding. If I remember correctly, I get better signal pickup with a winding that is the full length of the ferrite.
 
That said, I'd be curious to know the purity differences of the figure-8 pattern when the coil is short versus full length. Which is the better nuller?
 
Bill
SW Arizona
RADIO-TIMETRAVELLER
https://radio-timetraveller.blogspot.com


Rik
 

Were your widely spaced windings made with Litz?  I think wide spacing lowers self Cap. and so does using Litz.

It was awhile ago I looked at Online photos showing the winding on PL-450 and PL600 radio ferrite antennas. There was nothing remarkable I could see compared to other radios which are not especially directional. In my experience ferrite antennas are a bit more sharply directional than air core antennas which pick up off the open sides. Some pick up off the rounded edges which is sharper. Both my loops which do that are amplified, and at least one of the air wound Litz [wound on a removable 'peg'  form and sprayed to hold just the Litz wire together] designs I have picks up off the rounded edge, but I am not sure what that winding is called. That is too small for DXing.

I do have a 3 coil  contra wound with Litz form I use over a 2.125 X 16 inch stack of ferrite which is sharply directional for some unknown reason. I am working on a lighter more portable version, and plan to buy more lighter weight  toroids needed to get enough turns to reach my target uH range with this mix which is supposed to be higher Q at MW . Need a payday first.

-FARMERIK

There is mention of different windings and coverage in articles in the files of this group, which I have studied. On a fairly long ferrite rod, moving the coil from the center to either end cut the uH about in half. I am experimenting with ferrite just under each of three coils, and then varying the space between assemblies. Some changes are probably only measurable with test equipment, but optimal designs should be the best for DXing. One article mentioned increased Q when the ferrite was spaced under the center of the turns of Litz. There are many things to try.


Paul S. in CT
 

The older and later 'pocket radio' designs were both made with a mix of price and quality. Older designs had weaker transistors/tubes and needed a high quality ferrite coil winding. A pocket model of the 60's had better transistors and could be made physically smaller, the circuit would compensate for the smaller ferrite rod.

Nonetheless, both designs are not optimum, as the most recent studies have shown. I refer to Ben Tounge's (of Blonder-Tounge Radio/TV) Article #29 at his namesakes website. I also note that there were mathematical errors in popularly published catalogs of ferrite material.

So what is a near optimum design one might ask?

1.) Keep the ferrite bar at least 10 times its diameter for nulling properties.
2.) Try to keep the coil winding between 1/2 and 1/3 of the ferrite bar length.
3.) Space the coil winding 3x the wire diameter (in typical designs its usually 2x the diameter). For example #30 gauge wire of 0.01" diameter is wound at 0.03" instead of the typical 0.02". This removes some "proximity effect" especially when many turns are involved.
4.) The wire diameter itself has to be minimized to reduce the "Skin Effect". This seems counter-intuitive because thin wire has more resistance per foot (meter). But, thats DC resistance... we are looking to reduce AC (AM radio signal) resistance. Such AC resistance gets larger when very little current flows in the center of the wire: the smaller diameter wire "saturates" the wire better, with more AM radio signal current flowing in the center of the wire.
5.) Because of #3 and #4 above, a single wire of small gauge (say #30 or better yet #32) can be wound upon the ferrite bar. However, the best choice is still Litz wire based upon #46 gauge strands. Its more expensive, and you do get what you pay for. I will also say that one would be surprised at how good a single #32 wire (0.008") spaced at 0.024" is.
6.) Using a single wire, one can wind directly on the ferrite bar (most of this 'magnet wire' is insulated), but most prefer using a layer of heat-shrink tubing over the ferrite bar.

So, in recap, the older radios got the wide spacing right, and the newer ones got the small gauge wire right, but neither got both right. One thing not mentioned in most commercial radio designs is the actual inductance of this ferrite bar antenna. I do know that some designs are intentionally high up to 700uH for Sony's in the 60's-70's. Modern DSP designs need 350-500uH, and can use short bar lengths (re: small wire diameter).

Regards
Paul S. in CT FN31nl


mediumwavedx
 

Paul-
Great information, thanks for it. I'd love to paste these tips of yours in a blog post of mine if you wouldn't mind, giving proper credit to you of course. The additional information is invaluable for ferrite loops.
 
Rik-
No, I didn't use litz wire, just some 24 gauge I had on hand.
 
Bill
SW Arizona
RADIO-TIMETRAVELLER
https://radio-timetraveller.blogspot.com


Rik
 

Thanks Paul- Since you are relatively local, I thought I'd mention the locations of three stations I used to be able to completely separate on 560 AM. WHYN is quite strong here from Springfield MA. Portland ME is also a 5KW station I can separate any evening on the PL-450 & PL 600. With the three contra coil sleeve winding tuned on a PL 360, I could also hear a station over looking the Hudson river from NY. I have not heard that in a long time, and don't know if it is still on the air, but you must be much closer to that one than Portland ME. Have you heard it from your location recently?

With Bilateral patterns on a loop, I would expect the difference between Portland ME and some what upstate NY would be closer in 'direction' as I rotate the 3 coil from here.

On 1250 I got WARE in MA, which is pretty close but weak at night,  central PA and Central NY as well as a NJ on the 3 coil, but I forget which radio, and I have not done that recently.

Possibly the 'rules' will change going from one solid ferrite to a stack of toroids, and yet again to a FSL.  Enough toroids to stack 10 X as wide as diameter is probably out of my budget. Also there probably are more factors than  10:1length: diameter making a difference, but I am glad to hear of anything giving sharp directional patterns as it does boost the DX log.

My wild guess is the contra wound outer coils cancel the sides of the 'figure of 8' pattern leaving only its center. - FARMERIK


Paul S. in CT
 

mediumwavedx: Sure no problem if credited.

Regards
Paul S. in CT FN31nl


Paul S. in CT
 

Farmerik: Usually WHYN, CFOS, Cuba, and Maine here on 560. There was a station from Catskill, NY, but IIRC that was 540. I used to hear that one in the 60's-70's.

On 1250...Just Pittsburgh, and NJ.
A good check for you is on 1260... I have stuff from all over the compass. Boston, Erie PA, DC, Cleveland, New Brunswick, Syracuse, and VA.
On the low end, try 550.

Regards
Paul S. in CT FN31nl


Rik
 

Thanks Paul. I'll see what I can ID on 1260.

In all your experiments, have you found any way the narrow pick up patterns for choosing multiple stations per MW freq? -FARMERIK