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 Sensitivity and Tracking of AM radios
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 Return to top of page · Post #: 1 · Written at 9:22:26 AM on 8 May 2022.
Fred Lever's Gravatar
 Location: Toongabbie, NSW
 Member since 19 November 2015
 Member #: 1828
 Postcount: 1018

While I am still recovering from Covid I did some thinking and a bit of workshopping to do with aerials, and coils and what they actually do, the relation between those and the oscillator coil and the IF product.
That followed on to the subject of tracking. That also led to the IF coils and the pass band response.

Covid , by the way, is something you never want to get!
We were fortunate to not need hospitalisation both with advanced heart lung disease, cancer and all the normal afflictions that 80 year old survivors have! After the initial lung/breathing/head symptoms subside there are re-occurring episodes of loss of balance/headaches cough/sneeze/nausea and an over riding loss of energy. A bit like recovering from major surgery. Still persisting at the 5 week mark. Great!

Anyway, I had a go at trying to firm up in my mind what the AM superhet coil thing is all about and how to get a measure on it with my crude lot of instruments. There is nothing like actually prodding and measuring things to sharpen up your grip on how something works. Even if you go off in a wrong direction that still helps as you find out what does not work or is stupid.

I made notes as I was playing on the bench and came up with this essay which is a bit open ended and rambling. To you guys who actually know more about the subject, I am doing my best to get up to speed!

AM Sensitivity Testing


 Return to top of page · Post #: 2 · Written at 8:32:09 PM on 8 May 2022.
DangerousDave's Gravatar
 Location: Toongabbie, VIC
 Member since 1 September 2020
 Member #: 2438
 Postcount: 34

Looking forward to the content.

 Return to top of page · Post #: 3 · Written at 5:18:12 AM on 10 May 2022.
Brad's avatar
 Location: Greenwich, NSW
 Member since 15 November 2005
 Member #: 1
 Postcount: 6758

Document uploaded.

A valve a day keeps the transistor away...

 Return to top of page · Post #: 4 · Written at 8:02:15 PM on 10 May 2022.
BringBackTheValve's Gravatar
 Location: Linton, VIC
 Member since 30 December 2016
 Member #: 2028
 Postcount: 337


What a fantastic article. Theory + real world radio physics + fun. Many hours of serious study here, and very timely for my project.

I haven't gone past 1st quarter but already have two comments.

1. Scroogle??? Never heard of it but word describes perfectly, and as a bonus, spellcheck seems happy.

2. I often thought along the same lines as you about antenna coils, i.e. Z-matching plus step-up transformer. However, now that I read your notes I also wonder if there is another purpose, and the first thing that comes to mind is a step-down transformer for traffic going the other way. In other words, very low RF emissions from the oscillator radiating away (and annoying the guys next door or even next block)

 Return to top of page · Post #: 5 · Written at 9:03:09 PM on 10 May 2022.
Fred Lever's Gravatar
 Location: Toongabbie, NSW
 Member since 19 November 2015
 Member #: 1828
 Postcount: 1018

Hi BBTV, and everybody, antennas always seemed to make perfect sense to me for power use, that is when used with a transmitter.

Then you "match" the antenna impedance with the power valve/s impedance with a transformer (coil) to get maximum power transfer in a resonant system. That I can handle as a power engineer thinking in terms of Kw's.

But with a receiving antenna that is not resonant ( I think) and does not handle power (i think) i'm not sure just what the transformer/coil actually does in all of its requirements. When you read the many texts available on antennas they invariably are biased towards radio transmitting or start waffling about propagation of radio waves. All I am trying to get a handle on is what the receiving coil should do without going too far into mathematics.

Maybe in this essay I am already there and sure dont mind anybody commenting on that, any discussion is good and BBTV I never thought of traffic going the other way. Hmmmmmm.

Fire away all.


 Return to top of page · Post #: 6 · Written at 7:33:20 AM on 11 May 2022.
DangerousDave's Gravatar
 Location: Toongabbie, VIC
 Member since 1 September 2020
 Member #: 2438
 Postcount: 34

Terrific article Fred. You must have the sharpest tuned KL going around.

It's certainly a subject worthy of further exploration. I always try to think of them as just a RF transformer and therefore having the same properties and limitations of a transformer with inductive coupling. Never giving it much thought and with my loose understanding, I just took it for the most part that they were designed to have a low frequency resonance with a roll off below and capacitive coupling to assist the high end of the band. A generalised thought I know that doesn't always apply.

I did study advanced mathematics and calculus, but that was a long time ago and without exercising it daily, it's just a distant memory now.

 Return to top of page · Post #: 7 · Written at 10:06:14 PM on 15 May 2022.
Robbbert's avatar
 Location: Hill Top, NSW
 Member since 18 September 2015
 Member #: 1801
 Postcount: 1643

I don't have the instruments that Fred has, but I wanted to see what happened if I tuned a radio to the IF of exactly 455 kHz.

So I chose 2 radios from the to-be-scrapped pile. Each radio was tuned to the internal radio station here, currently using 1386. This gives a local oscillator frequency of 1841. I tuned my comms radio to 1841 and adjusted the radio being worked on to that exact frequency.

1. Kriesler 11-81 - this is scrapped because of the broken case and low sensitivity.. The local osc was barely audible on the comms radio due to the huge amount of hash in the area. I monitored the AGC voltage and carefully adjusted both IF transformers for maximum AGC. Then I tuned off my station to another nearby weak station and adjusted the aerial trimmer for max sensitivity. I'm not sure if it made much difference in the end. I probably should have looked at why the local osc was so weak - the 6AN7 or its components should have been checked, but I didn't think of it at the time. I did try swapping out the Radiotron 6N8 with a Philips Miniwatt one - not much difference, but I needed to go through the entire retuning procedure again.

2. HMV B-15 - this is scrapped because the case is wrecked and other hardware issues. The local osc was much stronger, more like I'd expect. The 6BE6 tended to drift a bit, with sudden small jumps now and then. The same adjustments as above were done. The result was the low end of the band was practically dead, so I undid the tape on the ferrite and slid the coil a few mm to peak up the sensitivity. Then back to the aerial trimmer, and back and forth a few times to get good sensitivity across the band. A new piece of tape finished the job. This radio is lots more sensitive than the 11-81. Again, I don't know if all this proved anything, but I suppose it was good practice.

 Return to top of page · Post #: 8 · Written at 7:29:06 AM on 16 May 2022.
Fred Lever's Gravatar
 Location: Toongabbie, NSW
 Member since 19 November 2015
 Member #: 1828
 Postcount: 1018

Good work Rob.
The more you adjust different radios the more you see what happens and what does what.
The point is: a difference of 455Kc plus/minus a bit is to be maintained between the tuning of the aerial coil and the oscillator coil over the whole tuning range.

Using test instruments to measure the difference is the logical thing, but not possible without the instruments.
So what you did is the next best thing to read 455.
If you can use the comms set to read 3 stations, low, middle, and high, on the dial you can get a rough idea how the coils are tracking by how much the 455 changes.
If the 455 stays within 5Kc or so then the IF coils will pass that ok.
If not you then have to re-set cores/trimmers/padders to trim the deviation, still with the pointer on the dial lining up with the station markings!


 Return to top of page · Post #: 9 · Written at 5:01:54 PM on 17 May 2022.
Fred Lever's Gravatar
 Location: Toongabbie, NSW
 Member since 19 November 2015
 Member #: 1828
 Postcount: 1018

Moving to the next subject when dealing with RF low voltage signals was having a meter that can measure micro-volts
That means building an amplifier that will boost an AM frequency RF signal from micro to milli volts, so I can use my hobby CRO's as the measuring devices.
Yess I know, just order one from China at $5 plus postage?
No, we are in valve land at the Lever Laboratories and it will be made by me with TV type valves!
So I set sail on defining what I think is required to do the job in terms of performance and how I think it shall be done hardware wise.
Besides that, I have no experience in doing this so "what could possibly go wrong?".!!!

I have padded out a beginning which I will get Brad to attach to this post.
Fell free to comment on any thing in the story.
I listen to everything anybody says, sometimes golden nuggets are in casual comments and if I make a blooper its nice to know about it.

Cheers, Fred.

Brad when I tried to first post this, the system hung me out to dry with a 500 error. Not happy.
good thing I cntrl c cntrl v.

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