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FL760 , transmissions are 1/5

echostar7

Active Member
I flew today and I was told that I was broken and barely readable on the radio but I could receive very clearly. I tried two different headsets and there was no change in my transmissions clarity. What can be the cause? Only new thing I can think of is I added the GDL-82, can that interfere somehow with the com, or just a coincidence?
 
The fact that you can receive just fine. But, transmit is "1/5", makes me think its more than likely VSWR problem. When was the last time that radio transmitted good? And, did it ever work ok after you installed the GDL 82?

The GDL 82 has nothing to do with your com radio. But, is it possible you disconnected the coax from the com antenna and forgot to hook it up? Or, maybe the coax connecter was damaged while installing the GDL 82?
 
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I asked for a radio check today and it was loud and clear to whoever responded at the field. It may be a different story though for artcc miles away, I'll inspect the coax connections for damage, thanks.
 
I asked for a radio check today and it was loud and clear to whoever responded at the field. It may be a different story though for artcc miles away, I'll inspect the coax connections for damage, thanks.

Another thing too: on some radios its easy to get the box a little off when reinstalling it in the rack. Especially if you're not very familiar with how the latching mechanism works.

If you had your com radio out during the GDL-82 install, you might just try re-racking it to make sure it is properly seated.
 
Agree with Ken. On receive, nearly anything (coat hangar) works for an antenna. Transmit is a lot fussier. Specifically, look for good connections where the coax shield is attached to the connector, and where the connector attaches to ground. Both at the transmitter but especially at the antenna end. May be something as simple as corrosion between the connector and ground.
 
Being well versed in the reciprocity rules of radio communication, I am puzzled by the differentiation being applied to transmitting and receiving here. Transmitter protection circuitry kicking in?

I offer that in military comm, the RX and Tx are often not in the same place. I wonder if the FAA might sometimes have similar non co-located facilities for their Rx, TX. That could explain the anomaly. Just a guess.

I did make an aircraft VHF transmitter half a century ago from 2n708 computer transistors and it sounded pretty good with even the proverbial coat hanger attached. Crummy VSWR alone should not effect audio quality other than lower the effective output. Normally one SHOULD expect a little more solid link TO the airplane because the ground station has an edge in power output.
Ron
 
I flew today and I was told that I was broken and barely readable on the radio but I could receive very clearly. I tried two different headsets and there was no change in my transmissions clarity. What can be the cause? Only new thing I can think of is I added the GDL-82, can that interfere somehow with the com, or just a coincidence?

What was the weather like that day. We lost all our coms after flying around/through a storm cell. Once the weather got better and everything dried out the comms have worked great. Several people mentions it was likely static build up. When I returned home we checked/rebedded all the antennas.
Jon
 
Crummy VSWR alone should not effect audio quality other than lower the effective output.
Ron

Crummy SWR doesn?t just mean less power to the antenna. It also means significant power flowing back into the transmitter, where it creates all sorts of trouble.
 
Crummy SWR doesn?t just mean less power to the antenna. It also means significant power flowing back into the transmitter, where it creates all sorts of trouble.
As a guy who has designed a lot of circuitry, waded around in waveguides, magetrons and other esoteric hardware for a long time, I think I've got the subject down pretty well. It has been my experience that vswr is rarely related to "bad transmitter audio", could be, of course. But it would be a rare instance.

A more likely cause, and one I ran into often, was off frequency receivers and/or transmitters. Channels are very tight now-a-days. A particularly vexing combination to troubleshoot is a tx leaning one way and the rx the other way. Both stations work ok except the combination, and often only in one direction because one unit is more bandwidth tolerant. This result is consistent with the OP's description.

To the OP: If this get to be more than that single instance, I would check the transmitter frequency. All later aircraft radios use synthesizers. So, freq tolerance checks are simpler. Checking one freq checks them all.
 
As a guy who has designed a lot of circuitry, waded around in waveguides, magetrons and other esoteric hardware for a long time, I think I've got the subject down pretty well. It has been my experience that vswr is rarely related to "bad transmitter audio", could be, of course. But it would be a rare instance.
Allow me to comment as designer of a number of aviation band radios and other RF products.

AM transmitters can be severely affected by bad VSWR. Reflected energy can be significant and interfere directly with the modulation circuit using a number of different pathways depending on phase relationships and modulation schemes. The most common effect however is creating a audio feedback path via grounds which can include your audio wiring. This can lead to severe distortion, howling or in mild cases something that sounds echoey like "bathroom sound". Bad VSWR can, via grounds, cause trouble completely outside of the radio itself - via the audio paths.

Bad VSWR can actually destroy your transmitter - that's how bad it can get. Some radios have built in protection for this and can reduce or disable the transmitter if this is detected (this is not a 100% solution due to lag but much better than nothing).

A more likely cause, and one I ran into often, was off frequency receivers and/or transmitters. Channels are very tight now-a-days. A particularly vexing combination to troubleshoot is a tx leaning one way and the rx the other way. Both stations work ok except the combination, and often only in one direction because one unit is more bandwidth tolerant. This result is consistent with the OP's description.

To the OP: If this get to be more than that single instance, I would check the transmitter frequency. All later aircraft radios use synthesizers. So, freq tolerance checks are simpler. Checking one freq checks them all.

This is highly unlikely in my own experience - older radios with 25Khz channel spacing tend to be around +/-20ppm accurate while newer radios with 8.33Khz spacing have to be better than +/-5ppm. In reality - most will be around +/-1ppm. It is no longer much of an issue to produce a radio that will perform to what once was expensive laboratory standards. Temperature compensated active crystal oscillators are now the norm and low cost - thanks in no small part to mobile phones.

I have never had a (older) radio on the bench that deviated more than 8ppm or so after decades of use - and that's pretty decent going.

AM receivers are also quite tolerant of deviation (both side bands contain all of the audio information) - you have about +/-12Khz on each side of center frequency available with 25Khz spacing where the receiver will work just fine. So the combined, worst case tolerance to make it work is about 50ppm !!! (100ppm per side, assuming the other is spot on).

If your TX audio is bad, there are four major causes: The first, as mentioned is VSWR or simply bad audio wiring or antenna much too close to headsets etc.
The second affects radios that use "drain" or "collector" modulation using a relatively simple modulator connected directly to the power supply. This type of modulator transposes whatever rubbish you may have on your radios power supply directly onto the transmitted signal - and usually wide band as well (so that splatters into adjoining channels). While a fail in FCC terms, this is not revealed by FCC test methods as a clean power supply is always used. Many older radios do it this way.
The third cause is a damaged transmitter - usually the final output stage transistor - power check tells the story here.
The fourth cause is low supply voltage during TX - usually caused by a bad contact somewhere (corrosion), unsuitable wiring, tired battery etc. In these cases RX is usually just fine (very low power draw) but the moment you press that PTT a few amps are needed to keep the transmitter happy. If the supply cannot provide that the voltage drops, possibly below minimum the transmitter needs to operate properly.

No matter what radio you may have - if your transmit is iffy, your first step is to measure TX Power, VSWR (or SWR) or use a dedicated antenna tester.

Rainier
CEO MGL Avionics
 
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