The Icom IC-24AT is truly a remarkable unit. Having it's genesis in the IC-R1,
Icom's extremely wide band Hand Held Receiver, it is capable of receiving a wide
range of frequencies, as well as being able to transmit on short excursions,
outside of the amateur two meter and 440 MHZ. bands.
Cross-band repeater
The IC-24AT can also function as a Mini Repeater transmitting on the UHF band
while simultaneously receiving on the VHF band, or vice versa. This procedure
requires both a small hardware as well as a software modification, and is
performed as follows;
- Locate diode D14. It is located adjacent to the location for diode D13 (see
above procedure). Clip or remove it.
- To enable Mini Repeater operation
- Push and hold the "Function" key and then the "C" key.
- Push and hold the "Function" key and then the "5" key and the "D"
key.
- To cancel the Mini Repeater operation, push and hold the "Function" key
and then push the "D" key.
Out of band receive
The more current models can be modified to receive out of band, by a simple
software (not necessary to physically open the transceiver) procedure as follows;
- Turn the power switch off.
- While simultaneously holding the "light" button, the "B" button, and the "#"
button, turn the radio on.
- All elements of the display will light up.
- Continue holding the buttons until the display goes out. It may be helpful to
have a buddy give you an extra hand during this procedure....
- Your unit will now receive out of band.
The older models of this transceiver, require the removal of diode D13, before
performing the above procedure. D13 is on the main logic board and is a discrete
diode near the center of the board. It is the diode on the left of the two on the
board when the radio is open and held with the controls away from you.
Keyboard entry of 100MHz and 10 MHz digit
The "stock" transceiver, was also not designed, to provide full direct keyboard
entry of the operating frequency. This can also be modified by one or both of the
two procedures below;
- To directly key in the 10Mhz. frequency, simultaneously hold the "light"
button and the "2" button, while turning the radio on.
- To directly key in the 100Mhz. frequency, simultaneously hold the "light"
button and the "3" button, while turning the radio on.
MARS / CAP transmit
The modification of the unit to enable it to transmit out of the amateur bands is a
fairly simple procedure. It is however, complicated by the fact that the
construction technique used in this transceiver is "Surface Mounted Technology."
If you have no experience dealing with this technology, you may well be better
advised to defer this procedure to someone who has. It is extremely small and
delicate, and is best done with specialized equipment. Also, because the unit is
not FCC type accepted for all the frequencies on which it can then transmit, it
is not legal for you to use it, despite the fact you may be licensed to transmit
on those frequencies. This information is provided only for those who are licensed
for and want to use the unit on MARS and CAP frequencies, where there are no
type acceptance restrictions. The procedure is as follows:
- Order a couple of DA204u diodes from Icom or your Icom dealer. These
are a special three terminal device with two diode in one package. They
are very very small, about half the size of one of the key buttons on the
touch tone pad.
- Locate and remove the diode device in position D8.
- Locate the vacant position of diode D7 and attach one of the DA204u
diodes at that location.
- Locate the vacant position of diode D12 and attach the other DA204u diode
at that position.
- If diode D13 (procedure above) has not been removed, remove it. The unit
will now transmit out of the amateur bands.
Expanded frequency receive performance
Following the frequency expansion modification (as specified by Icom), a sample
IC-24 was tested for tuning range and input sensitivity for an S9 signal.
While the sensitivity measurements should not be directly compared with the
manufacturer's specifications (usually measured reference to a signal-to-noise
quieting ratio), this "quick and dirty" data should give you a good idea of the
sensitivity to a medium-strength signal.
- The sensitivity from 138 - 150 MHz was 1 uV. From 150 - 240, the
sensitivity decreased almost linearly from 1 uV to 1 mV (with input voltage
plotted logarithmically).
- The sensitivity increased almost linearly for both these ranges (with input
voltage plotted logarithmically).
- The sensitivity plot looked like a "U" with the bottom of the "U"
corresponding to the range 440 - 450 MHz (1 uV). The sensitivity then got
worse below 440 and above 450.
The image rejection for frequencies outside 135-165 MHz, 400-500 MHz, and 800-950 MHz is poor. Tuning ranges on VHF below 135 MHz and above 165 MHz actually receive image signals between 135 and 165 MHz stronger than signals on the indicated frequency.
Normally the VCO frequency is above the signal frequency:
- fo = fvco - if (VHF < 200MHz, UHF < 800MHz)
- fo = (2 * fvco) - if (VHF > 200MHz, UHF > 800MHz)
- where:
- fo = actual frequency
- fvco = vco operating frequency
- if = first if = 30.875 MHz.
The image frequency is:
- fo = fvco + if (VHF < 200MHz, UHF < 800MHz)
- fo = (2 * fvco) + if (VHF > 200MHz, UHF > 800MHz)
- For example:
- To receive on 162.550:
- 162.550 = fvco - 30.875
- fvco = 193.425
- The image will be fvco + if:
- 162.550 = fvco + 30.875
- fvco = 131.765
The actual display frequency will continue to be shown as if the vco was above the operating frequency:
- fdisplay = fvco - if (VHF < 200MHz, UHF < 800MHz)
- fdisplay = (2 * fvco) - if (VHF > 200MHz, UHF > 800MHz)
- For the image case above:
- fdisplay = 131.765 - 30.875
- fdisplay = 100.8
Image frequency:
- fimage = fdisplay + or - (2 * if)
- fimage = fdisplay + or - 61.75
