philco: Loop replaces aerial

Fellow radiophiles:

Old radios often require an inconvenient long wire antenna for operation in the AM band. The Philco 39-116 is one such radio.

This top-of-the-line 1939 set can be tuned on the AM band plus 2 short wave bands with the slide-rule dial, or by a number of AM-band-only presets, that are selected remotely by the Mystery Dial remote control.

My friend Ron Roscoe owns one of these sets. The "WRON" preset you see lit up is for his Ramsey Kit solid state home transmitter. The other presets should be recognizable by Boston, Massachusetts  USA residents.

Ron also has a problem with a lot of light dimmer noise in his house. Long wire antennas are particularly susceptible to this kind of electric field interference.

Loop antennas reject the electric field pretty well, while deriving the signal from the magnetic field. So Ron and I set about adapting his set for reception with an external 1920's loop antenna which he already owned.

Alternatively, a new loop could have been hidden inside the spacious console.

The problem with using loop antennas is that they must be tuned with a capacitor to realize the full sensitivity of the antenna. Conneting the loop directly to the untunned primary of the loosely coupled antenna coil produced poor sensitivity.

The input RF coil is meant to be very loosely coupled to the tuned RF secondary to avoid detuning the secondary. Usually, the primary has a large number of turns that is placed a fair distance from the secondary. In this case the distance is nearly 2 inches (5cm).

The problem remains that the loop is not tuned with a capacitor, therefor it can't capture much magnetic field signal.

We decided to tune the loop antenna by disconnecting the AM band input coil and use the RF tuning capacitors already in the radio to tune the loop. This uses the variable RF capacitor (10) in local slide-rule tuning mode, and the 8 preset tuning capacitors for remote tuning.

We measured the secondary of the RF transformer (1) that we disconnected at terminal 3, and found 240uH of inductance.

In an extraordinary case of good luck, Ron's loop antenna also had 240uH of inductance, so it needed no additional turns of wire to increase the inductance, or to have turns removed or moved appart to reduce inductance for a good RF alignment.

If the inductance of the loop were too small, an external series inductor could have been used to add to 240uH. This additional inductor should have been an adustable one for ease of RF alignment.

The total inductance of series inductors is the sum of the individual inductances. The calculation for the net inductance of parallel inductors is the same as parallel resistors. i.e.: a 300uH inductor in parallel with a 2.5mH inductor has a net inductance of 1/(1/300uH+1/2500uH)=268uH.

Adapting the existing circuit to the external loop

The switching scheme between slide-rule tuning and preset tuning uses the RF preamp stage only for slide rule tuning. But this did not pose a problem because the original AM band antenna coil (1) that the loop replaces, was used in both modes.

click to enlarge

The colored lines mark the connectivity of the antenna coil secondary and for the new external loop in slide rule and preset tuning.

Internal connection of loop

We added a short 4 terminal strip to anchor the loop connections and to secure the disconnected terminal 3 of the disabled AM band RF antenna coil.

Click to enlarge the photo.

The two black and white wires lead to the loop antenna.

The anchoring tab of the strip was soldered directly to the terminal on the band switch wafer where terminal 3 of coil 1 was originally soldered. The white wire is on this tab.

Now terminal 3 of coil 1 is soldered to a spare lug on the strip.

The black wire was soldered to another lug an the tab and jumpered to where terminal 4 of coil (1) remained connected.

After the loop was connected, Ron added some wooden spacers between the black and white loop antenna wires to keep a stable separation between the two wires. Variation of this separation would affect alignemt at the top  of the AM band, where the tuning capacitance is smallest.

Alignment

The set can be aligned following the usual procedure, with the difference that the adjustment of the input coil is replaced with the adjustment of the loop antenna inductance, as described above.

The alignment is easiest for the presets because there is no compromise of maintaining good alignment over the entire AM band.

Do the slide rule alignment first, then the presets. The low end of the slide rule AM band should be aligned by varying the loop antenna inductance, while the high end of the slide rule AM band dial should be aligned with trimmer capacitor (10A). This high end aligment should be done with the loop and wires in the final operating position to account for parasitic capacitance properly.

Further comments

We tried 300 Ohm TV/FM twin lead to connect the loop, instead of the black and white wires, but the the capacitance of the twin lead was a few tens of pF and made it impossible to align the High end of the AM band.

Using thick wires (AWG<20) that are no more than 4 feet (1.3m) long helps preserve the Q, or sensitivity and selectivity, of the loop.

If you are a loop expert and can make a loop with a Q of 500 or more, it will be difficult to keep the loop in good alignment with the relatively loose tracking of the padder capacitor arrangement of the local oscillator. A Q up to 100 should be very sensitive and give good alignment performance.

Besides loop sensitivity, Q establishes the reception bandwidth. A Q of 100 at 1MHz means an RF bandwidth of 10kHz, or an audio bandwidth of 5kHz.

One definition of Q is the ratio between the inductive reactance of the loop and the equivalent series resistance of the loop, at a particular frequency.

Q=XL/Rseries

Q=2*pi*Fresonance*L/Rseries

This means that a tuned loop with a Q=100 is roughly 100 times more sensitive at resonance than the same loop without resonance. One way to understand this proportion, is that resonance reduces the impedance of the loop to the magnetic field from the inductive reactance plus the equivalent series resistance, to just the equivalent series resistance.

The high impedance/inductance of the original RF antenna coil (1) makes it suitable to connect directly to an externnally tuned loop, even up to the 240uH of Ron's loop. But Ron was not about to sacrifice the wonder of automatic control from 1939 with a manually tuned loop with an external tuning capacitor!

Note that the loop only works for the AM band. The two SW bands still take their signal from the original antenna terminals.

Regards,

-Joe

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Ron Roscoe has kindly provided dimensions for his 1920s loop antenna, that we adapted for use with his Philco World's Fair 39-116RX radio.

Thanks, Ron.

-Joe

Click to enlarge

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