04-17-2023, 10:36 AM
Hi PatMat
What model radio are you dealing with? Is it transformer powered? AM Broadcast only or shortwave? FM?
Radios that require an external antenna usually have a tuned circuit that includes an air core transformer "antenna coil". The primary connects to antenna mad ground. the secondary is tuned and connects to the grid of the 1st RF amp or 1st Detector (Convertor or Oscillator / Mixer).
If the radio has 2 wires for the antenna, one should be hooked up to an "Inverted L" (more later), and the other should be connected to a "ground" such as a cold water pipe. Connecting to the electrical supply ground may or may not inject a hum. If a separate ground rod is sunk, some recommend bonding it to the electrical system ground, some don't. It depends on local codes, ground moisture, etc.
If you can find the owner's manual for the radio, there will be recommendations for antenna and ground.
I believe that an "inverted L" antenna is directional for reception, with the strongest direction being opposite that where the lead -in wire is connected.
As to the length of wire, ideally, if it is end-fed, ideally, it should be "1/4 wave", in other words, the length of the wire should be 1/4 the length of the wave. Therefore, for the AM Broadcast Band (assuming 1000 KHz has a wavelength of 300m, the wire should be 75m or 225 ft. This is not practical in most places. Fortunately, for short wave bands, to have a 1/4 wave antenna is a lot easier to erect, and unless you are trying to get distance records for AM, a 75ft antenna will do very well. For local stations, even 25-50 ft will do well. many apartment dwellers routed an antenna around the perimeter of the ceiling of the room, hidden by moulding or a "picture rail".
Standing waves affect the efficiency of the antenna. Attached below is a treatise from RCA regarding the best lengths for antennas depending on the band or frequency:
As to a grounded or polarized plug, you definitely need the schematic of the radio. If transformer powered, ensure that the switch is on the "hot" side (narrow blade) and the unswitched side on the neutral side (wide blade). Ensure that the any fuse is also on the hot side. Filter caps should be across the transformer primary after the switch. If there is only 1 cap between the transformer and the chassis, it is safer if that cap is on the neutral side. Ensure that you are using "Safety Caps"
If the radio is a "hot chassis" type without a power transformer, then some rewiring needs to be done. For some reason, possibly doing with minimizing hum, the power switch on AC-DC radios was always placed at the "neutral" side of the radio. These same radios had either a 0.05 - 0.1 uF cap and 10K resistor connecting the chassis to the neutral sidefor RF coupling, or the neutral side was directly connected to the chassis. If the switch was on and the plug was connected so the chassis was on the neutral leg, then there would be no shock WITH THE SWITCH ON. However, with the switch off, the chassis now becomes dangerously hot due to the flow of current from the hot leg through the now cold, low resistance tube filaments. The following rewiring makes the radio safer (This procedure is not possible if a working "Cordohm" power cord will remain in use):
Hope this helps.
What model radio are you dealing with? Is it transformer powered? AM Broadcast only or shortwave? FM?
Radios that require an external antenna usually have a tuned circuit that includes an air core transformer "antenna coil". The primary connects to antenna mad ground. the secondary is tuned and connects to the grid of the 1st RF amp or 1st Detector (Convertor or Oscillator / Mixer).
If the radio has 2 wires for the antenna, one should be hooked up to an "Inverted L" (more later), and the other should be connected to a "ground" such as a cold water pipe. Connecting to the electrical supply ground may or may not inject a hum. If a separate ground rod is sunk, some recommend bonding it to the electrical system ground, some don't. It depends on local codes, ground moisture, etc.
If you can find the owner's manual for the radio, there will be recommendations for antenna and ground.
I believe that an "inverted L" antenna is directional for reception, with the strongest direction being opposite that where the lead -in wire is connected.
As to the length of wire, ideally, if it is end-fed, ideally, it should be "1/4 wave", in other words, the length of the wire should be 1/4 the length of the wave. Therefore, for the AM Broadcast Band (assuming 1000 KHz has a wavelength of 300m, the wire should be 75m or 225 ft. This is not practical in most places. Fortunately, for short wave bands, to have a 1/4 wave antenna is a lot easier to erect, and unless you are trying to get distance records for AM, a 75ft antenna will do very well. For local stations, even 25-50 ft will do well. many apartment dwellers routed an antenna around the perimeter of the ceiling of the room, hidden by moulding or a "picture rail".
Standing waves affect the efficiency of the antenna. Attached below is a treatise from RCA regarding the best lengths for antennas depending on the band or frequency:
As to a grounded or polarized plug, you definitely need the schematic of the radio. If transformer powered, ensure that the switch is on the "hot" side (narrow blade) and the unswitched side on the neutral side (wide blade). Ensure that the any fuse is also on the hot side. Filter caps should be across the transformer primary after the switch. If there is only 1 cap between the transformer and the chassis, it is safer if that cap is on the neutral side. Ensure that you are using "Safety Caps"
If the radio is a "hot chassis" type without a power transformer, then some rewiring needs to be done. For some reason, possibly doing with minimizing hum, the power switch on AC-DC radios was always placed at the "neutral" side of the radio. These same radios had either a 0.05 - 0.1 uF cap and 10K resistor connecting the chassis to the neutral sidefor RF coupling, or the neutral side was directly connected to the chassis. If the switch was on and the plug was connected so the chassis was on the neutral leg, then there would be no shock WITH THE SWITCH ON. However, with the switch off, the chassis now becomes dangerously hot due to the flow of current from the hot leg through the now cold, low resistance tube filaments. The following rewiring makes the radio safer (This procedure is not possible if a working "Cordohm" power cord will remain in use):
- Disconnect the wires from the power switch. Note that often that the ground side of the volume control is connected to the load side of the switch also.
- Put the switch on the "hot" side (input to the rectifier plate and heater).
- Connect the "neutral" wire of the power cord (wide blade) to the load connection and volume control low side connection removed from the switch this usually connects to the common B- and low side of the 1st AF tube's heater. This also connects to the aforementioned capacitor that couples the chassis.
- If the radio uses a "Cordohm" (sometimes referred to as a "curtain burner") where a tube heater dropping resistor is contained in the power cord, this feature must be substituted by either a resistor or suitable AC electric capacitor.
Hope this helps.
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Best Regards,
MrFixr55
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