New 8.33 radios sound rubbish

Peter wrote:

It isn’t so much the loss (yes RG400 is a GHz cable) but the leakage through the shield of a crappy coax which feeds back all over the place.

But the leakage (especially at the low power levels used by a VHF COM radio) is absolutely trivial even with RG58 (the stuff I have has a foil screen under the braid).

If the coax shield is radiating, this is usually because of a poor antenna installation, and will happen even with the most expensive, thickly shielded coax. RF in the cockpit is PROB99 caused by the shield radiating, and not by leakage.

I’m using a 430 with KMA20 (!) and Sigtronics intercom. No noticeable difference in 8.33 vs 25Khz audio quality – maybe i need new headsets (or hearing!).

No 8.33 problems when I switched from KX155 to GNC 255 with Garmin audio. The same with GTN650 / GNC225 combo on another aircraft. The user configurable sidetone took a bit of work but we got there in the end.

As Peter says it’s I’d most likely that skimping on the wire quality and not paying attention to correct grounding of the shields is likely to be the cause of most of the problems.

RG58 (with a PVC sheath) has been banned for years in certified installations because it doesn’t meet the flammability requirements, not just that the screening capability is poor. I don’t recall seeing any installations in recent years where an installer has been cheap and used it.

It isn’t so much the loss (yes RG400 is a GHz cable) but the leakage through the shield of a crappy coax which feeds back all over the place. See e.g. here for one result of using a crappy coax (by Socata) and if this wipes out VHF comms it isn’t going to be helpful anywhere else either The leaked RF is probably gonna get rectified by various nonlinear semiconductors and mess up the audio. This is why I never believe statements like “wifi cannot interfere with avionics because it is 2.4GHz while ILS is only 1xx / 3xx MHz and any harmonics will be above 2.4GHz”.

I’ve just stripped a few bits I had lying around:

Unshielded intercom wiring is utter negligence. All RF, composite NAV, analog NAV signals (e.g. +GS -GS) and all audio must be shielded cable.

I can tell you for sure it must be a duff installation if you’re getting poor audio/deaf radio with a Garmin GTR225. I have one and it is WAY better than the Narco it replaces – better audio quality, much better Tx and Rx performance with the same antenna. No echo. The receiver is most definitely not deaf (from Cumbria I can hear aircraft calling London Info when crossing the French/UK FIR boundary)

Peter wrote:

Also one has to use good quality coax, like RG400, which most don’t like using because it is expensive

RG400 is not really necessary for VHF, especially the short runs that light aircraft have. RG58 is perfectly adequate, unless you’ve got a very long run. A 3m run of RG58 will have a loss of less than 0.5dB on VHF (unless you have a badly matched antenna, but then the solution is to fix the antenna). Also your installer probably doesn’t have a crimp tool for anything other than RG58 :-)

We had the same replacement as Aircraft 1 above
The Trig is quieter, but has better quality audio, and there is more interference (from the strobe light) but this is caused by the intercom having unshielded cables so I switch it off when flying alone. Generally the radio is an improvement, and the extra functionality from the Trig is a bonus. If all else fails try fitting ferrites?

You need to get the installer back and get him to fix it. He may be minting it due to the amount of 8.33 work but this is not acceptable. The bigger problem I see is:

• why do it badly in the first place (a lack of knowledge on how to run shielded cables, perhaps)
• why release the aircraft to service in that condition

8.33 has more than ample audio bandwidth. The transmitter is the same. The receiver and only when set to an 8.33 channel has a reduced bandwidth but it is still wider than normal aviation audio. I have a KX165A/8.33 which is superb and sounds exactly the same as the KX155A above it.

BTW it is not necessary for the work to be done by an engineer qualified to work on certified aircraft. You just need one of these to sign the job off. The avionics scene is full of people with no formal qualifications (some good, some bad, some in between, some are very good wiremen but with no understanding of the principles – just like much of the “145” avionics scene really ) and they work under the approval of the hangar owner in whose hangar they are working.

I don’t know much about specific radio installations (been doing electronics since the mid 1960s though) but I do know that the wiring needs to be done very carefully, with the shields grounded in the right place, and not connected to the airframe all over the place because that feeds back airframe currents. Also one has to use good quality coax, like RG400, which most don’t like using because it is expensive and since most customers don’t know this, it makes an installer using it appear to be uncompetitive.

The audio is excellent (much better than the Narco COM850 it replaced) on my Garmin GTR225. I don’t have an audio panel, I use the GTR225’s built-in intercom.