Hello All,
I have a DeForest Crosley 405D radio. Canadian built. The Antenna coil assembly is somewhat damaged and the previous owner had bypassed it and put the antenna directly on to the tuning condenser.
Here is the schematic of the model 405D:


Detail of Antenna coil:


Can anyone explain the theory behind the open ended coil between the primary and secondary?

Here are some pics of the actual coil:




Inside the coil former is a bobbin, I believe this to be the primary.
I believe the secondary with tap is the dark winding on the exterior.
I believe the open ended coil is the green wire on the exterior.
As I said the coil was damaged with broken connections and broken fixings.
I am not even sure that the schematic is drawn properly.

Here is a Philco 51 schematic which has a similar circuit an i think is drawn correctly.


As I said I dont understand the theory involved with this type of antenna coil arrangement, so any info would be welcome.
Regards.
David.

At low frequencies it's more a capacitor than a coil. Eventually it would become a self-resonant tuned circuit, but whether that point is within the set's tuning range, I couldn't say, or just what its effect might be.

It appears to be an attempt to increase the low frequency range of the primary by having extra stray inductance as well as capacitance, I cant say if its very efficient.

Yours and the Philco are drawn correct. The reason for the tap on the secondary is to improve the impedance match to an apparently low impedance tube since the top of that winding is the highest impedance of that circuit. Its rather common throughout the tube era especially at SW and VHF frequencies and also used with SS.

Carl

Hello Alan/Carl,
Thanks for your replies. It seems like the design is something of the time and tries to enhance some area of performance.
Again thanks for your time.
David.

David,

What is commonly called an antenna coil is more properly termed an RF transformer. Generally these have two windings. One or both might have a resonating capacitor. Each winding with a resonating capacitor is tuned to the frequency of the desired radio station. The two windings of the transformer are coupled inductively, and may have capacitive coupling as well.

These RF transformers are functionally similar to IF transformers, but operate at the frequency of the station you are tuning. They provide some selectivity, and couple the RF signal from one side to the other.

The schematics you posted show two RF transformers, each with a tuned secondary winding.

The first RF transformer has its primary connected to the antenna terminal. The primary winding is untuned. There is an extra winding, with only one connection, that forms a capacitor with the secondary. This provides some capacitive coupling between primary and secondary. The secondary of this transformer is tuned by one gang of the main tuning capacitor. RF is taken from a tap on the secondary to feed the primary of the second RF transformer.

The second RF transformer also has an untuned primary, while the secondary winding is tuned by another gang of the main tuning capacitor. This winding is connected to the first tube in the radio.

The extra tuned circuit increases the selectivity of the radio at the station's frequency. This will increase the image rejection of the receiver, and also help reduce interference from strong signals that might otherwise cause interference due to intermodulation in the RF or mixer stages of the radio.

The increased image rejection is particularly important in this set, with the low, 175kHz IF.

So this is a good feature to have in your radio. It may actually work better than the more common approach where an RF tube is located between the first and second tuned RF circuits. More of any interfering signal is stripped away before amplification and possible distortion occurs. There is some loss of the desired signal, too, but the AM broadcast band is so noisy that any usable signals can still be amplified without adding too much noise from the radio itself.

As to repairing your set, the critical thing about the first RF transformer, or antenna coil, is to get the tuned circuit to track with the rest of the radio. It looks as if you should be able to repair this coil. You may have to add or remove turns to get proper tracking over the band. A few turns plus or minus on the other windings will not matter so much.

Ted

They might be trying to maintain a constant input impedance on the antenna terminal
no matter where that volume control is set to.

HelloTed / Steve Dow,
Thanks for your replies.

Ted you did a very good description of the input circuitry. The factory describes it as an Autodyne detector with preselection.
Now I quote you Ted.

Ok now that we have established the foregoing what does it actualy achieve? or have you answered it with the statement?

Regards.
David.

David,

No, I didn't really answer your question about why the capacitive coupling was added to the RF transformer. I doubt if it had to do with tracking.

The capacitive coupling should increase the coupling between the primary and secondary. It will increase it more at higher frequencies. So it should increase the gain through the transformer circuit at the high end of the band versus the low. I presume that this resulted in more uniform sensitivity across the broadcast band. This would matter more on a set like this that has no AVC.

The designer could have just moved the primary and secondary windings closer together to get more coupling, if only a narrow band of frequencies were of concern.

Ted

Looks like a pile of complications on a heap of wire that someone way back when thought was neat. This design would be difficult to understand without some detail lab tests or reviewing the engineering design calculations if indeed they did exist at one time. Regardless...I would like to see a competitive engineer analysis of this coil if anyone has the time and smarts to do it.
Thanks
Hank

This type of arrangement is called "complex coupling," because it can behave capacitively or inductively depending on frequency. The schematic shows the center winding as not connected on one end because that is how it is assembled. But there is also capacitance from the winding to the chassis or coil sheild. For the purposes of explanation, one can imagine a capacitor to ground from the open end of the winding.

At low frequencies, the reactance of the phantom capacitor is high and that of the open-ended coil is low, so the transformer behaves as if it is capacitively coupled. Whether the capacitive coupling aids or opposes the normal mutual coupling from primary to secondary depends on which way the open coil is wound relative to the others. At high frequencies, the coil has more reactance than the capacitor, and the transformer acts as though it is inductively coupled. At frequencies inbetween, where the open coil and phantom capacitance are in resonance, only the normal coupling from primary to secondary takes place. So the coefficient of coupling from primary to secondary varies with frequency.

Now when one changes the coefficient of coupling in an RF transformer, it affects the sharpness of tuning as well as the amount of signal that gets coupled. So by using this method, low frequency stations can occupy the same amount of space on the tuning dial as the high frequency ones, instead of crowding all the high frequency stations at the top of the dial. Given the nature of ground wave propagation, stations at the low end of the band also tend to come in louder than those at the top, especially during the day. So this is a method of leveling things out across the dial, something which radio purchasers looked for as a sign of quality. In some radios, similar effects were obtained with tuning capacitors that had specially shaped plates, or with variable inductors that were mechanically linked to the tuning capacitors.

For further reading, see Edward H. Loftin and S. Young White, Proc. I.R.E., vol 14, p. 605, October 1926, "Combined Electromagnetic and Electrostatic Coupling, and Some Uses of the Combination." See also "Radio Engineering," Frederick E. Terman, McGraw Hill, New York, 1932, p. 79-80.

Hello Chris,
You just scraped in on time with that explanation, I was about to sign off on this particular topic.
It seems that complex coupling deserves a complex explanation. I think this should put this to rest for now, of course I will have to read this about 114 times for it to sink in properly.
The interest in this circuit was generated when I noticed the RF (Antenna coil) was off its mounts inside the can and the set antenna was wired to the tuning condenser.
The damage to the coil and mount was caused by the antenna wire being connected directly to the coil terminal. You can imagine any stress on the antenna wire would eventualy do in the coil (all this was legitimate factory stuff). On disassembly of the can and coil I found a break in the the coil in question, thats what spurred me to try and find out what this extra winding was all about. I think the whole coil is operational now but won't know for a while as this particular chassis has a large number of failures and missing parts that need to be sorted.
Anyway Thanks Chris/Ted et al for your time.