Steve R., Eversharp, et al Ferrite antenna? - FARMERIK


Rik
 

I read articles in the files section here but they are way beyond my understanding. I want to have a 47.5 uH Ferrite antenna made for me locally. It should be as directional and 'sensitive' as possible.

My thoughts are two 95 uH coils widely spaced on the ferrite. Not sure if contra wound  would is better or if they should be wound in the same 'direction'. Connected in parallel for 47.5 uH .

I am thinking of the 12 inch Amidon core, but it may not be much better than the 7.5 inch ferrite they sell.

I plan to use the highest count Litz wire I can find reasonably priced. I want Litz for the stronger signals they get compared to ordinary wire loops of the same diameter.

Any comments welcome- FARMERIK


Paul S. in CT
 

Farmeric:

Some mutual friends from Crystal Radio Days past may be of use. As for a more intellegible "theory" Ben Tongue's website bentongue dot com and Dave Schmarder's (n2ds) website are a valuable rescource. I do have questions... why are you settling upon 45-50 uH? Standard coils are 250-400uH, and division by two is 125-200uH. Are you thinking of a balun transformer to boost inductance to these levels?

Paul S. in CT


Steve Ratzlaff
 

Hi Farmerik,
The following is just my . It sounds like you are hoping to build a non-resonant (untuned) antenna with a very small inductance, using a ferrite rod. (47.5 uH would require a very large tuning capacitor, if you were planning to make it a tuned loopstick--normally inductance in the range of 285 uH is used for the AMBCB.) Your idea would require experimentation to figure out how many turns on the loopstick would be needed--it won't take many turns since the normal -61 (125 permeability) ferrite rod magnifies the turns a lot to give a big increae in inductance. I suggest you take your LC meter and start winding turns and do your own experimentation, since whoever you get to build the final version would have to do the same unless you told him how many turns, spaced such and such a distance from the ends of the rod, whether you wanted two coils, etc. Lots of things to figure out. And the size of the Litz probably makes little difference since using big Litz gives the improvement primarily for a tuned loopstick, not an untuned one. I doubt that use of contra winding would give any improvement over regular since again contra winding is for tuned circuits.
Normally the shortest ferrite rod gives the deepest nulls, but that's counterproductive for best sensitivity where the longest rod works for that.
So, I suggest you dive in and start doing your own experimentation and figure out for yourself what would work to best meet your objectives. I have no idea where to start for number of turns, etc.--that's all where I would have to start, too, if I were involved in such a theoretical project.
Good luck!
Steve

On 10/3/2015 3:59 PM, farmerik@... [ultralightdx] wrote:
 

I read articles in the files section here but they are way beyond my understanding. I want to have a 47.5 uH Ferrite antenna made for me locally. It should be as directional and 'sensitive' as possible.

My thoughts are two 95 uH coils widely spaced on the ferrite. Not sure if contra wound  would is better or if they should be wound in the same 'direction'. Connected in parallel for 47.5 uH .

I am thinking of the 12 inch Amidon core, but it may not be much better than the 7.5 inch ferrite they sell.

I plan to use the highest count Litz wire I can find reasonably priced. I want Litz for the stronger signals they get compared to ordinary wire loops of the same diameter.

Any comments welcome- FARMERIK




Rik
 

My early 1950's tube Fisher AM 80 tuner was made for a 47.5 uH loop antenna  they supplied for the back of the cabinet. It connects in series with a shielded coil inside the tuner.

I had a 48.5 uH Litz air wound antenna I had made to experiment with. My radio repairman cut it back to match the Fisher loop antenna input. It works VERY well, but I'd like to try ferrite and a larger air core loop too.

-FARMERIK


Rik
 

I thought there might be a calculator for winding ferrite loops as there is for winding air core loops.

Can I use Litz instead of ordinary wire for winding an air core loop and still have the loop calculator be correct?

-FARMERIK


Steve Ratzlaff
 

Yes, the air loop calculators such as the very good one of Reg Edwards will work fine with Litz.
One can space the turns closer using Litz due to less turn to turn capacitance, if desired.
Steve

On 10/4/2015 1:35 PM, farmerik@... [ultralightdx] wrote:
 

I thought there might be a calculator for winding ferrite loops as there is for winding air core loops.

Can I use Litz instead of ordinary wire for winding an air core loop and still have the loop calculator be correct?



Rik
 

Thanks. Yes I still have the LC meter, but less use of my hands and poorer eye sight.

I tried to upload photos of the tuner and Litz loop here, but it would not go.

Here is some discussion about it by Pete who does my repairs-

Antique Radio Forums • View topic - AM loop antenna for Fisher AM-80 or 80-T

 

-FARMERIK


Steve Ratzlaff
 

Thanks, good info there, especially the link to the Fisher manual. Too bad the parts list doesn't give details such as the turns ratio for the AM input transformer and the tuning cap values for AM. It appears the external loop inductance is being stepped up by the input transformer and then resonated at the transformer secondary. The transformer could be most any turns ratio though integral turns ratios (2,3,4,5, etc.) don't yield inductances at the secondary that one would like, so they probably use non-integral turns ratio for the transformer.

Remember that for the a ferrite rod, inductance is max with the coil in the center and decreases as the coil moves toward either end. Since you indicate Peter is your repairman, it sounds like you should just pay him to do all the experimenting to try this idea (he or someone mentions that idea).
I doubt many turns would be needed to get 47 uH on a ferrite rod with 125 permeability (-61 mix, normal AMBC ferrite rod type). That might mean it would turn out to have poorer sensitivity than a much larger air loop.

(Photos need to be uploaded into the Photo section of the Files.)

Steve

On 10/4/2015 2:42 PM, farmerik@... [ultralightdx] wrote:
 

Thanks. Yes I still have the LC meter, but less use of my hands and poorer eye sight.

I tried to upload photos of the tuner and Litz loop here, but it would not go.

Here is some discussion about it by Pete who does my repairs-

Antique Radio Forums • View topic - AM loop antenna for Fisher AM-80 or 80-T

 

-FARMERIK



Rik
 

As I was just looking on eBay I found a photo of a Fisher AM loop in this listing for the AM FM 80 T version-

Fisher 80-T Tuner with wood case and manual

 


The 80 T only has two AM selectivity settings, not three like the AM 80. All three are well chosen.

I have ordered the Amidon 12 inch ferrite. Still looking at Litz. I think # 46 wire is right for AM BCB, still have to choose how many conductors.

For the air core loop I plan to have my 4 foot LW loop cut down to 38 inches which is supposed to be 47.5 uH with 4 turns 3/4 inch apart. I'll have it set up so it could have part of another turn for final matching.

If I want to use either resonated to a regular radio  antenna input,  according to the loop calculator I used, my cap with a 6:1 reduction would tune them from about 1200 Kc to 7.4 Mc. Then the Litz high Q would work.

Yes, I know the Fisher is definitely not a ULR, but here are the most experienced ferrite and air core loop makers I know.

Thanks - FARMERIK