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 Post subject: How many turns are needed for an antenna coupling coil?
PostPosted: Aug Thu 20, 2020 7:07 pm 
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Ok, I have decided to try my hand at winding my own ferrite antenna. I have found enough information on the antenna winding, such as matching it to the air capacitor & what UH values I should be obtaining. I am unsure though as to how many turns or UH'S I need for the coupling/ oscillator coil. Is there a simple ratio like 1:10 for instance?? I plan on purchasing 100/46 litz wire for this project, unless somebody has a better suggestion. Thanks!
Bill


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 Post subject: Re: How many turns are needed for an antenna coupling coil?
PostPosted: Aug Sat 22, 2020 3:54 am 
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I think you will have to experiment. I would try about 10 turns, but that's a guess.

Or buy a DIY kit:

https://www.ebay.com/itm/Superheterodyn ... SwVIVfNju9

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I just assumed you meant a transistor radio.

Rich

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 Post subject: Re: How many turns are needed for an antenna coupling coil?
PostPosted: Aug Sat 22, 2020 5:16 am 
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In Rich's pic above, L1 is the antenna coil, connected across the tuning cap, and L2 is the osc coil, with one side connected to the base of the converter Transistor, other eventually to circuit ground. If a tube set, one side to the control grid.

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 Post subject: Re: How many turns are needed for an antenna coupling coil?
PostPosted: Aug Sat 22, 2020 2:08 pm 
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Yes, I should have been more clear, this is for a transistor radio. Actually started when one of my kits came with an open antenna winding. I substituted an antenna from a scrap radio, and it worked pretty good! Now I just want to make a more precise antenna for the next kit. One schematic shows 100 & 10. I assume this is windings, because the rest is in Chinese! :roll:


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 Post subject: Re: How many turns are needed for an antenna coupling coil?
PostPosted: Aug Sat 22, 2020 7:07 pm 
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khill wrote:
Yes, I should have been more clear, this is for a transistor radio. Actually started when one of my kits came with an open antenna winding. I substituted an antenna from a scrap radio, and it worked pretty good! Now I just want to make a more precise antenna for the next kit. One schematic shows 100 & 10. I assume this is windings, because the rest is in Chinese! :roll:

Well, you could always learn Chinese... :wink:

The 100/10 is pretty close to the pictured 90/8. You can always experiment with the antenna winding's, starting with 100, and unwinding a turn or two at a time, then checking across the band to see if you have raised or lowered the gain.

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 Post subject: Re: How many turns are needed for an antenna coupling coil?
PostPosted: Aug Sun 23, 2020 7:07 am 
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Quote:
n Rich's pic above, L1 is the antenna coil, connected across the tuning cap, and L2 is the osc coil,


Isn't the OSC coil the one in the can with a red core?

Image

Don't need to learn Chinese: https://www.usefulcomponents.com/main_c ... erhet.html

Rich


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 Post subject: Re: How many turns are needed for an antenna coupling coil?
PostPosted: Aug Sun 23, 2020 2:59 pm 
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Rich, the red screwed can is really an oscillator transformer. The smaller winding on the ferrite antenna is an oscillator coupler which sends out the signal from the oscillator circuit. Somebody like fifties would be able to give you a more detailed & correct answer to that.
If you liked building the blue kit, you might enjoy the HX108-2, which is a seven transistor, and a much better performer.


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 Post subject: Re: How many turns are needed for an antenna coupling coil?
PostPosted: Aug Sun 23, 2020 5:20 pm 
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The smaller winding on the ferrite antenna is an oscillator coupler which sends out the signal from the oscillator circuit.

This is an unusual configuration for a MW radio. It would be interesting to see a schematic.

Ted


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 Post subject: Re: How many turns are needed for an antenna coupling coil?
PostPosted: Aug Sun 23, 2020 5:21 pm 
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Location: Seattle, WA 98122 (from Hungary)
It's really something you can calculate / design, as long as you have some information about the radio that you're building.

I'll give you an example:

These Chinese radios use the 680uH / 137pF frontend combo. The antenna tuner has to have some Q in order to pick up distant stations, and to filter out the image (+/- 910kHz), but not too high: tracking error will occur between the oscillator and antenna tuning, a too high Q would reduce the sensitivity when the oscillator was not exactly 455kHz away. A good Q value is around 50 at 1MHz (+/- 50kHz).

The antenna tuner at 1MHz (680 uH coil + capacitor tuned to 37 pF) has an impedance of around 2.1 kohm at Q=1. This translates to 105 kohm at Q=50.

This 105 kohm needs to be matched to the input of your first transistor stage (autodyne mixer, RF amp, etc..)

Let's assume your first transistor (hfe=100) stage runs at 1mA bias current and has its emitter resistor bypassed. The emitter impedance is 26ohm (Utemp(26mV) / Ie(1mA)), The base impedance is 2.6kohm (Re(26ohm) * hfe(100)).

So you have a 2.6kohm input impedance (assuming your biasing resistor values are high enough, or connected to the "lower end" of the secondary coil) that you have to match to your 105 kohm. From now on, the problem is simply reduced to an impedance matching transformer design, where the primary (105 kohm) is the antenna tuner and the secondary (2.6 kohm) is the coupling coil.

I'm not sure how many turns there are on the ferrite rod in these red Chinese sets, but assuming 120 and the above impedance ratio (105 : 2.6) , the secondary needs to have 18 turns. And the more current goes through the first transistor stage, the less turns the secondary should have, in order to match to the antenna tuner.

Hope it helps.

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 Post subject: Re: How many turns are needed for an antenna coupling coil?
PostPosted: Aug Sun 23, 2020 9:39 pm 
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Usually Lurking wrote:
The smaller winding on the ferrite antenna is an oscillator coupler which sends out the signal from the oscillator circuit.

This is an unusual configuration for a MW radio. It would be interesting to see a schematic.

Ted

It's not unusual at all; most of them are built that way.

Here's a scan of part of the foldout from Practical Radio Servicing by Marcus and Levy. Please to note the smaller coil to the left of the antenna coil, for a typical Transistor receiver circuit.


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 Post subject: Re: How many turns are needed for an antenna coupling coil?
PostPosted: Aug Sun 23, 2020 9:44 pm 
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Rich, W3HWJ wrote:
Quote:
n Rich's pic above, L1 is the antenna coil, connected across the tuning cap, and L2 is the osc coil,


Isn't the OSC coil the one in the can with a red core?


Please note my partial schematic above; the osc winding over the antenna coil goes to the base of the collector for RF input, while it's emitter connects to the oscillator circuit. Two different functions.

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 Post subject: Re: How many turns are needed for an antenna coupling coil?
PostPosted: Aug Sun 23, 2020 10:12 pm 
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That is interesting Fifties. So basically the "loop antenna" is mixing the RF and the osc frequencies, so it is performing as both an antenna and mixer. Did I get that right?
I have never worked on a small AM transistor radio before.


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 Post subject: Re: How many turns are needed for an antenna coupling coil?
PostPosted: Aug Mon 24, 2020 12:05 am 
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You call it an "oscillator winding." I call it the RF input matched to the base of the first transistor. LO frequency is determined by red cored can, not the "oscillator winding." Semantics, I guess.

Rich


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 Post subject: Re: How many turns are needed for an antenna coupling coil?
PostPosted: Aug Mon 24, 2020 12:59 am 
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Rich, W3HWJ wrote:
You call it an "oscillator winding." I call it the RF input

But that could easily get confused with the actual RF input from the coil across the tuning cap. Yes, semantics.

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 Post subject: Re: How many turns are needed for an antenna coupling coil?
PostPosted: Aug Mon 24, 2020 5:01 am 
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The "actual" RF input is via the small winding on the main coil. It just steps down the impedance to match the first transistor base?

Rich


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 Post subject: Re: How many turns are needed for an antenna coupling coil?
PostPosted: Aug Mon 24, 2020 7:02 am 
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Scott wrote:
That is interesting Fifties. So basically the "loop antenna" is mixing the RF and the osc frequencies, so it is performing as both an antenna and mixer. Did I get that right?

No, neither did I. See below.

Rich, W3HWJ wrote:
The "actual" RF input is via the small winding on the main coil. It just steps down the impedance to match the first transistor base?

Rich


I've always gone by the understanding that the smaller coil was an osc winding, and the tank circuit formed by the larger coil across the tuning cap was the actual RF input, as normally found in tube radios with a connection to the control grid. But then in analyzing it here, how would that combination couple to the rest of the circuit? You are correct.

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 Post subject: Re: How many turns are needed for an antenna coupling coil?
PostPosted: Aug Mon 24, 2020 12:48 pm 
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Fifties, look at this video.

https://www.youtube.com/watch?v=qlx7KRSiPM4&t=106s

The circuit is different from the pic you posted, but yes, the small coil (at least in this circuit) is the oscillator feeding the stick and mixing the signals. In the circuit you posted, the small coil plays no part in the oscillator. So I guess the answer is "It depends on the circuit design".


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 Post subject: Re: How many turns are needed for an antenna coupling coil?
PostPosted: Aug Mon 24, 2020 6:57 pm 
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Quote:
I've always gone by the understanding that the smaller coil was an osc winding,


If the smaller winding is the Local Osc. winding and has no trimmer or variable capacitor across it, how does one adjust the LO frequency for alignment purposes?

Rich


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 Post subject: Re: How many turns are needed for an antenna coupling coil?
PostPosted: Aug Tue 25, 2020 12:21 am 
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Rich, W3HWJ wrote:
Quote:
I've always gone by the understanding that the smaller coil was an osc winding,


If the smaller winding is the Local Osc. winding and has no trimmer or variable capacitor across it, how does one adjust the LO frequency for alignment purposes?

Rich

As Scott illustrated above, it depends on the circuit. In the one I posted, no, not the osc coil.

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 Post subject: Re: How many turns are needed for an antenna coupling coil?
PostPosted: Aug Tue 25, 2020 2:20 am 
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Image

This is the typical circuit for a 1960s AM radio. The tuning cap is 2-sections... one for the antenna, the other for the local oscillator.

The oscillator is in an adjustable core "can."
The larger winding on the antenna rod is used to resonate at the desired station frequency. The LO operates 455 KHz. higher than the received frequency. The LO can be adjusted by moving the core in the can. The shorter winding on the antenna rod simply couples incoming RF into the base of the mixer. Empirically, you want just enough coupling to maximize signal strength without loading the resonant circuit too much.

When I built one of those Chinese kits, I couldn't get it to track until someone advised me to move the wire coil along the core to maximize signal input at the desired freq.

Rich


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