Dimme wrote:
hear difference with my handheld when I set it on a 8.33 frequency
Because the difference is in the receiver part, no?
Yes. And the receiver should also be tuned to the same bandwidth as the transmitter. Also I understand that human voice has limited bandwidth. However, in an aircraft you have a lot of wideband noise that gets picked up by the microphone, although limited.
Anyway, I can speculate all I want but I need to do a couple of tests to see it myself :)
See e.g. here.
With amplitude modulation, the frequency deviation of the transmitted signal is just the spectrum of the audio signal, so it will be the same with a 25k or 8.33k radio.
The only difference is in the receiver, and a radio set to 8.33 mode should tighten up its receiver selectivity somewhat, but it doesn’t need to in reality, which is why nobody can tell if somebody is using a 25k radio on say 125.005. All the “.005” does is tell the receiving section to tighten up its bandwidth. It is a “fake” frequency setting.
I did some measurements and it looks like I might have been wrong after all.
I transmitted a white noise audio signal using the microphone of my handheld Yaesu FTA-550.
First I transmitted the audio signal using the 123.450 MHz 25 kHz frequency and then I transmitted the audio signal using the 123.455 kHz 8.33 kHz channel. As previously known, both channels are using the same carrier frequency of 123.450 MHz, so that should not change. What I wanted to measure was whether the transmitted bandwidth similar or not.
The result shows that in both cases only about 6 kHz of bandwidth was emitted, despite the fact that a noisy wideband audio source was selected. That leds me to believe that the Yaesu applies the same bandpass filter on the transmitted signal regardless of the type of channel being used.
The measurements where obviously done in a shielded anechoic chamber 
PS: Disregard the frequency shift of my spectrum analyzer, I didn’t bother to calibrate it as it doesn’t matter in this case.
Peter wrote:
The only difference is in the receiver, and a radio set to 8.33 mode should tighten up its receiver selectivity somewhat, but it doesn’t need to in reality,Yes it does need to in reality, because selectivity is specified in the ICAO standards and it is less for a 25 kHz channel.
Airborne_Again wrote:
Yes it does need to in reality,
Receiver selectivity :
6 dB bandwidth ±8.0 kHz minimum
60 db bandwidth ± 22.0 kHz maximum
when set to 8.33 channel
Receiver selectivity :
6 dB bandwidth ± 2.773 kHz minimum
60 db bandwidth ± 7.37 kHz maximum
It’s way more selective when set to *.005 channel.
Peter wrote:
because the audio is band-limited anyway
If you tune your radio to 118.000, the 118.0833 MHz signal will (almost) fall in the “6 dB bandwidth selectivity ±8.0 kHz minimum” of your receiver . So you will likely hear radio interference.
Now if you tune your radio to 118.005, you’re unlikely to receive interference as the 118.0833 communication will be rejected by the tighter selectivity of your receiver (60 db bandwidth ± 7.37 kHz maximum)
TQ KRT2 and Funke ZATR833S – are these any good for 8.33, and installation options?
I am looking at the options to the 8.33 date looming. I have just purchased a Grumman Cheetah and it has Mode S, but only 25khz radios. So none of the bundles available are needed. Cost is the primary factor, which leads me to two choices, the TQ KRT2 and Funke ZATR833S. Both available for just under £1000, so what is the catch? Can I install them and get them signed off, or will it need an engineer to do the whole job and what will the ball park figure be?
Or, as has been the club room chatter, can I just get a handheld and show that when compliance is requested? I called the CAA and was told to send an email to the tech team. The tech team so far haven’t responded and I would like to get the ball rolling on this one before December.
Any thoughts or ideas will be gratefully received.
Thanks Grumman.
[ post moved to existing 8.33 thread ]
As far as I know you can install the radio and get it signed off. There is also Trig TY91 a bit over 1000 pounds. It is pretty common and has a clear display.
