The topic of "opening a radio up" that was designed before the 60m frequency allocation is periodically discussed. The usual questions are "how to open the radio up for transmit" and "will operating on 60m damage the radio"? Virtually every radio made in the past 15-20 years has used a broadband solid state final and contains a microprocessor which defines what frequencies the radio is allowed to transmit on. It is very hard to make a broadband final that can operate over the HF frequency range but not operate on 60m. However this doesn't mean that 60m operation is safe for your radio. All radios need output filtering to reduce harmonics that are caused by non-linearities in the final stage. With solid state finals also came switchable fixed output filters which allow the radio to meet the regulatory requirements. Therefore radios contain several low pass filter sections that eliminate these undesired harmonics.
Generally operating on 60m with a radio that is using the 80m low pass filter will be problematic compared with a radio that uses the 40m low pass filter. Please note that a radio that uses a 40m filter for 60m may not meet the attenuation requirements for harmonics but that is a different issue and may be solved by using a single band antenna or an external low pass or band pass topology tuner.
The following data pertains to a variety of Icom radios but is applicable to all radios that have general coverage receivers. Since the low pass filter sections of the transmitter are also used for receive it is easy to determine what filter your radio will use on 60m. Set the radio to have some broad frequency step size (or spin the dial more), turn the volume down, and start tuning the radio at about 3.9MHz. At some frequency before 7.0MHz you will hear a relay click inside the radio indicating that a different low pass filter section has been switched in. If this frequency is below 5.3MHz. operation on 60m is possible without transmitter damage. However if the click occurs above 5.3Mhz. you may have heat problems and may damage the output stage of the radio. The measurements below are for several typical Icom radios.
Icom 765: (click at 4.0MHz and 8.0MHz) - Should be OK on 60m
80m LPF (2.00MHz - 3.999MHz)
40m LPF(4.00MHz - 7.999MHz)
Switch from 80m LPF to 40m LPF at 4.00MHz
Icom 746PRO: (click at 5.0MHz and 8.0MHz) - Should be OK on 60m
80m LPF (2.00MHz - 4.999MHz)
40m LPF(5.00MHz - 7.999MHz)
Switch from 80m LPF to 40m LPF at 5.00MHz
Icom 756PRO: (click at 6.0MHz and 8.0MHz) - Should NOT be OK on 60m
80m LPF (2.00MHz - 5.999MHz)
40m LPF(6.00MHz - 7.999MHz)
Switch from 80m LPF to 40m LPF at 6.00MHz
Icom 756: (click at 6.0MHz and 8.0MHz) - Should NOT be OK on 60m
80m LPF (2.00MHz - 5.999MHz)
40m LPF(6.00MHz - 7.999MHz)
Switch from 80m LPF to 40m LPF at 6.00MHz
Icom 756PRO2: (click at 6.0MHz and 8.0MHz) - Should NOT be OK on 60m
80m LPF (2.00MHz - 5.999MHz)
40m LPF(6.00MHz - 7.999MHz)
Switch from 80m LPF to 40m LPF at 6.00MHz
Icom 781: (click at 4.0MHz and 8.0MHz) - Should be OK on 60m
80m LPF (2.00MHz - 3.999MHz)
40m LPF(4.00MHz - 7.999MHz)
Switch from 80m LPF to 40m LPF at 4.00MHz
*information provided by Mike, KD9RC Nov. 2008
Icom 746 original: (click at 5.0MHz and 8.0MHz) - Should be OK on 60m
80m LPF (2.00MHz - 4.999MHz)
40m LPF(5.00MHz - 7.999MHz)
Switch from 80m LPF to 40m LPF at 5.00MHz
*information and testing provided by Mac, K2GKK Dec. 2008
From the above table it would appear that the Icom 765, 781, 746PRO, and
the original 746 would work well on 60m (using the 40m LPF) while the 756,
756PRO, and the 756PRO2 (using the 80m LPF) would not. In fact
measurements on the 746PRO and the 756PRO show exactly this
(with the radios operated into a dummy load).
Icom 746PRO:
3.99MHz, 100w, DC current = 17A
4.99MHz, 100w, DC current = 22A
5.00MHz, 100w, DC current = 16.5A
5.36MHz, 100w, DC current = 16.5A
7.00MHz, 100w, DC current = 18.5A
Notice how the radio draws 13.6V*(22A-16.5A)=74.8W more at 4.99Mhz
than 5.00MHz. This power is all HEAT!
Icom 756PRO:
3.99MHz, 100w, DC current = 17.5A
4.99MHz, 100w, DC current = 18A
5.36MHz, 50w, DC current = 20A (max. power out 50w)
5.99MHz, 30w, DC current = 23A (max. power out 30w)
6.00MHz, 100w, DC current = 15.5A
7.00MHz, 100w, DC current = 18.5A
Icom 765:
Could not easily measure DC current due to the internal DC power supply but it would produce 100w on 5.36MHz.
A lowpass filter designed for 80m will probably have significant loss and a high SWR for the transmitter finals to work into at 5.3MHz. Note that the radio SWR meter will still show 1:1 since the meter is located at the radio output. The conclusion is that the 765 and the 746PRO easily operate on 60m (which they do) while the 756, 756PRO, and 756PRO2 are not well suited to operation on 60m. Both my 746PRO and my 765 produce 100W at 5.36MHz easily without heating up while my 756PRO can only muster 50W at 5.36MHz and does heat up significantly more than it does on the other ham bands. Your mileage may vary but this is the primary explanation why some radios work better on 60m than others do.
Generally operating on 60m with a radio that is using the 80m low pass filter will be problematic compared with a radio that uses the 40m low pass filter. Please note that a radio that uses a 40m filter for 60m may not meet the attenuation requirements for harmonics but that is a different issue and may be solved by using a single band antenna or an external low pass or band pass topology tuner.
The following data pertains to a variety of Icom radios but is applicable to all radios that have general coverage receivers. Since the low pass filter sections of the transmitter are also used for receive it is easy to determine what filter your radio will use on 60m. Set the radio to have some broad frequency step size (or spin the dial more), turn the volume down, and start tuning the radio at about 3.9MHz. At some frequency before 7.0MHz you will hear a relay click inside the radio indicating that a different low pass filter section has been switched in. If this frequency is below 5.3MHz. operation on 60m is possible without transmitter damage. However if the click occurs above 5.3Mhz. you may have heat problems and may damage the output stage of the radio. The measurements below are for several typical Icom radios.
Icom 765: (click at 4.0MHz and 8.0MHz) - Should be OK on 60m
80m LPF (2.00MHz - 3.999MHz)
40m LPF(4.00MHz - 7.999MHz)
Switch from 80m LPF to 40m LPF at 4.00MHz
Icom 746PRO: (click at 5.0MHz and 8.0MHz) - Should be OK on 60m
80m LPF (2.00MHz - 4.999MHz)
40m LPF(5.00MHz - 7.999MHz)
Switch from 80m LPF to 40m LPF at 5.00MHz
Icom 756PRO: (click at 6.0MHz and 8.0MHz) - Should NOT be OK on 60m
80m LPF (2.00MHz - 5.999MHz)
40m LPF(6.00MHz - 7.999MHz)
Switch from 80m LPF to 40m LPF at 6.00MHz
Icom 756: (click at 6.0MHz and 8.0MHz) - Should NOT be OK on 60m
80m LPF (2.00MHz - 5.999MHz)
40m LPF(6.00MHz - 7.999MHz)
Switch from 80m LPF to 40m LPF at 6.00MHz
Icom 756PRO2: (click at 6.0MHz and 8.0MHz) - Should NOT be OK on 60m
80m LPF (2.00MHz - 5.999MHz)
40m LPF(6.00MHz - 7.999MHz)
Switch from 80m LPF to 40m LPF at 6.00MHz
Icom 781: (click at 4.0MHz and 8.0MHz) - Should be OK on 60m
80m LPF (2.00MHz - 3.999MHz)
40m LPF(4.00MHz - 7.999MHz)
Switch from 80m LPF to 40m LPF at 4.00MHz
*information provided by Mike, KD9RC Nov. 2008
Icom 746 original: (click at 5.0MHz and 8.0MHz) - Should be OK on 60m
80m LPF (2.00MHz - 4.999MHz)
40m LPF(5.00MHz - 7.999MHz)
Switch from 80m LPF to 40m LPF at 5.00MHz
*information and testing provided by Mac, K2GKK Dec. 2008
From the above table it would appear that the Icom 765, 781, 746PRO, and
the original 746 would work well on 60m (using the 40m LPF) while the 756,
756PRO, and the 756PRO2 (using the 80m LPF) would not. In fact
measurements on the 746PRO and the 756PRO show exactly this
(with the radios operated into a dummy load).
Icom 746PRO:
3.99MHz, 100w, DC current = 17A
4.99MHz, 100w, DC current = 22A
5.00MHz, 100w, DC current = 16.5A
5.36MHz, 100w, DC current = 16.5A
7.00MHz, 100w, DC current = 18.5A
Notice how the radio draws 13.6V*(22A-16.5A)=74.8W more at 4.99Mhz
than 5.00MHz. This power is all HEAT!
Icom 756PRO:
3.99MHz, 100w, DC current = 17.5A
4.99MHz, 100w, DC current = 18A
5.36MHz, 50w, DC current = 20A (max. power out 50w)
5.99MHz, 30w, DC current = 23A (max. power out 30w)
6.00MHz, 100w, DC current = 15.5A
7.00MHz, 100w, DC current = 18.5A
Icom 765:
Could not easily measure DC current due to the internal DC power supply but it would produce 100w on 5.36MHz.
A lowpass filter designed for 80m will probably have significant loss and a high SWR for the transmitter finals to work into at 5.3MHz. Note that the radio SWR meter will still show 1:1 since the meter is located at the radio output. The conclusion is that the 765 and the 746PRO easily operate on 60m (which they do) while the 756, 756PRO, and 756PRO2 are not well suited to operation on 60m. Both my 746PRO and my 765 produce 100W at 5.36MHz easily without heating up while my 756PRO can only muster 50W at 5.36MHz and does heat up significantly more than it does on the other ham bands. Your mileage may vary but this is the primary explanation why some radios work better on 60m than others do.
Page content last updated Dec. 29, 2008
See the W0QE Home page if you wish to contact me
Copyright © 2008 Larry Benko, W0QE
Copyright © 2008 Larry Benko, W0QE
