The Length of a Ferrite AM Antenna Doesn't Tell the Whole Story

Hugo uses his tech expertise to come up with some provocative questions regarding ferrite antennas:

I've bough an old Realistic Radio (I think SW-100 model) in Ebay a few weeks ago and after testing it to see how good it was on AM I've found it had a quite mediocre AM sensitivity and because it cost me so cheap (10 dls) I began to disassemble it but it occured to me to change the 4" ferrite for a new 15" just for fun (using the same coil) and with this new ferrite the radio performed like a real DX champion ala GE Superadio 2, after having fun I changed the ferrite with a  piece 6" of a broken 10" ferrite (which I've bought from an Ucranian Ebay seller and days ago I broke it and I kind of glued the biggest parts survivors), anyway to my surprise the radio STILL was very sensitive in AM like a DX champion but obviously a little less than before... 
 
So I was wondering Why a little change in size from the old 4" stock to the broken piece of 6" could make so biiig difference in reception...??  Anyway I changed again the ferrite (third change) but this time with a smaller piece of 3" from the same original ferrite (Now the ferrite is smaller than the original) and to my surprise the radio was as sensitive as it was with the old 4" stock ferrite....
 
I was scratching my head for a while trying to understand why a a broken and glued piece of ferrite of 3" make the radio  as sensitive as the original 4"....  and the only one explanation I have is that the Chinese factory used ferrite of very low quality.... What do you think of this ?

PD I left my radio with the broken piece of 6" and it performs on a par with my best AM radios....not bad for a Realistic one....
 
Regards
Hugo Ali
 

 
 

Comments

[Keith Beesley]

I have 3 or 4 of those old Radio Shack multi-band sets. Some models had AM, FM, and shortwave; some had AM, FM, police, weather, etc. My best performing one is the worst-looking--it has some holes in the cabinet!

All have at least good reception and sound, although not in the same class as the vintage Sonys, Panasonics, Zeniths, et al.

[Carlos]

Witness also the well-reputed prodigious AM Dxing capacity of the Sony ICF-S10MK2.

[Radio Russ]

I just got a AM-only Panasonic R-1551 on eBay with a large (6") good quality ferrite bar. It's blowing away almost everything else I have for AM performance. Also, the Tecsun R-308 is right up there with it but not quite.