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 Post subject: Re: How many turns are needed for an antenna coupling coil?
PostPosted: Sep Sun 06, 2020 12:39 am 

Joined: Nov Mon 06, 2017 2:35 pm
Posts: 126
Location: Texas, U.S.A.
Alexander Schure, Superheterodyne Converters and I-F Amplifiers, John F. Rider Publisher Inc., 1963, pages 89 - 91 ... Schure.pdf

R. C. Koch, U.S. patent 2,880,312, filed 2/2/1955, Transistor Oscillator-Mixer with Received and Local Oscillations Applied Between Emitter and Base

What szoftveres wrote, but the coefficient of coupling from antenna coil to input coil (antenna coupling coil) may be taken into account.


 Post subject: Re: How many turns are needed for an antenna coupling coil?
PostPosted: Sep Mon 14, 2020 2:18 pm 

Joined: Jan Thu 16, 2020 12:29 am
Posts: 768
Don't you hate it when people suggest you just have to try it and see, could there be a better insight ?

Energy taken from the small secondary coil, due to the mixer-osc transistor's base current, or an RF amplifier's base current, damps the main resonant circuit, lowers its Q and broadens the bandwidth.

So your question is how many turns on the small winding is enough to give the required signal strength, but without lowering the Q and reducing the selectivity below an acceptable level ?

(of course this problem only applies to the loading due to the base input resistance of the BJT, this is why some designs prefer fets or tubes)

Generally there is a rule of thumb for an medium wave AM radio with a bjt's base circuit loading the coupling coil, the maximum loading on the small coil (from the transistor's input resistance) is best kept above N^2 x 150 ohms, where N is the number of turns on the coupling coil. (it is affected by the total large coil turns which will be lower with a ferrite rod of higher permeability, but I'm giving you a useful rule of thumb here).

This limits the loading so the selectivity of the main tuned circuit is maintained with only a minor deterioration for most MW band ferrite rods main tuning coils.

So for a 1 turn coupling coil, you don't want to load it with less than 150 ohms, and say a 4 turn coil, not less than 16 x 150 =2400 ohms. (of course you can verify this with your own transistor radios with a simple experiment with a turn of wire added around the rod over the main winding and a 150R load)

The transistor's input resistance is very roughly its hfe x re, where re is 25 divided by the collector current in milliamps.

So lets say for argument's sake that the collector current is 0.5mA and the transistor's hfe is around 70, then the transistor's input resistance is roughly around 3500 Ohms, so solving for 3500 = N^2 x150, N = root (3500/150) = 4.8 turns, rounded to the nearest turn, that 5 turns for the coupling coil.

Now of course you could go for 6 turns, 7, 8, 9 etc, but with each step you will reduce the selectivity of the main tuned circuit but have more gain.

(Generally the coupling coil on the rod, due to its location is fairly tightly magnetically coupled to the main winding, the rule of thumb above takes this into account).

I could have mentioned, the amount of loading tolerated depends on whether the set is a Superhet or a TRF. In the TRF case, to get any reasonable selectivity it requires little loading on the RF tuned circuits. You can get away with more loading and a broader response on the ferrite rod coil in a typical superhet transistor radio, because most of the selectivity is provided by the IF amplifier.

To give you an example, this receiver had a 6 turn coupling coil into a 3k input impedance and selectivity of the tuned circuit is maintained fairly close to the unloaded state, it has to be as its a one tuned stage TRF:

In any case my advice is to use between 5 and 10 turns for your application, you could settle on lucky number 7.

 Post subject: Re: How many turns are needed for an antenna coupling coil?
PostPosted: Sep Thu 17, 2020 7:53 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 4481
Location: Perrysburg, OH, 43551 U.S.A.
Here's a circuit from the SC-13 RCA Transistor Manual (1967) that explains a circuit similar to the one Rich posted:
RCA Transistor Manual AM Rcvr Ckt Pt1.jpg
RCA Transistor Manual AM Rcvr Ckt Pt1.jpg [ 255.4 KiB | Viewed 95 times ]

RCA Transistor Manual AM Rcvr Ckt Pt2.jpg
RCA Transistor Manual AM Rcvr Ckt Pt2.jpg [ 499.43 KiB | Viewed 95 times ]

I have outlined and highlighted the portion of the circuit and its description responsible for generating the local oscillator signal. The loop antenna step-down winding connected to the base of the mixer/oscillator is not involved with generating the LO signal. It's major purpose is to match the impedance at the base of the transistor.

“Never attribute to malice that which can be adequately explained by stupidity.”
― R. A. Heinlein

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