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ceengland7(at)gmail.com
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 Posted: Thu Sep 19, 2019 9:29 am Post subject: Grounding radio antenna and transponder antenna |
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On 9/18/2019 10:06 PM, Argonaut36 wrote:
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Thanks to All for the interesting comments and suggestions.
In this post I would like to address the comments of Charlie on noise in the cockpit. I need a little more time to write a second post on the antenna solutions that have been proposed for testing (coax and external copper foil). I may have a couple of questions.
Yes, the problem is not just range. It is also noise. ATC says that my transmissions are weak and with a lot of static noise, but still acceptable. As I wrote in an earlier post, that is not the case if I go to Class B or C airspace and I need to call from far out.
My impression is that I receive better than I transmit. The noise level in the cockpit is very high and when I press the push-to-talk button, the static noise that I hear in the sidetone is louder than my voice.
I use what is considered a very good noise canceling microphone that I periodically replace. My understanding is that the noise wears out the microphone. I have also flown with a Bose headset; transmissions were better, but only marginally.
First, your experience with small-airport vs B/C airspace might not be
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purely range. Have you tried small airport comm from the same distance,
and over similar (as in flat) terrain as the B/C comm? And do the B/C
comms have the same problems, once you get as close to them as you
typically are to the small airports? This might help determine whether
you truly have a range issue. But regardless, don't forget 'human
factors'. The small airport guys are typically not very busy, and can be
more 'focused' on listening. They're possibly used to dealing with
poorer quality than the big guys, since small airport fliers are often
not as dependent on perfectly performing equipment. The big guys, on the
other hand, are often incredibly busy, and if they have the slightest
problem with readability, may just tell you you're unreadable and to go
away. They're also unlikely to have the audio skillset to give a useful
technical description of what they're actually hearing.
OK, on to some random *generalizations*:
'Normal' noise levels in the mic's designed-for environment should not
wear out a mic. I've got headsets that are older than my >25 year flying
experience, almost all in homebuilts, that still have good mics in them.
Not saying that extreme noise can't damage a mic, but wearing it out
shouldn't be much of an issue.
Range can obviously affect transmission quality, especially with AM
(what we use for a/c comm). But background noise can often make more
difference than carrier strength (range). Others are much more qualified
to discuss the RF side of things, but I may be able to offer some long
distance help with the audio side of things.
Intelligibility is closely tied to 'signal-to-noise ratio'. We usually
hear (pardon the on-the-nose pun) the term used in electronics, where
the signal (audio) is measured as a ratio to the level of 'noise', which
is all the background hiss, crackling, etc that exists in all
electronics. But for practical, real-world applications, the entire
system must be considered. Ex: in a concert hall, the signal is the
singer's voice and the noise is everything else; audience conversations,
squeaking chairs, foot falls, even reverberations of the singer's own
voice bouncing around in the room. An extreme (though somewhat twisted)
example would be 'feedback' when the PA system's output gets back into
the mic and is re-amplified.
Now, how do we improve S/N in the concert hall? Turning up the volume
will make the singer easier to hear, and help mask hall noise, but
eventually the PA will get loud enough to cause feedback and then no one
will hear the 'signal'. So what to do? One thing is to lower the noise
level. Padded seats, carpet, noise absorbing panels on the ceiling &
walls, and even more important, telling your neighbor to save their
conversation till the concert is over. But one of the big drivers of
poor S/N is the singer him/her-self. If the handheld mic is allowed to
point out into the audience, feedback is more likely. If the singer cups
their hands around the mesh area of the mic, it kills the mic's
directional selectivity. If they sing into the side of the mic instead
of the end (assuming a typical handheld mic), or they hold the mic too
far from their lips, or sing too softly, then *their vocal volume is a
smaller percentage of the total sound the mic hears*. If the singer
messes up in the above ways, the sound guy's only way to make them
louder is to turn up the gain, but the mic hears everything, so noise
gets amplified along with signal.
The 'singer' issues are very similar to issues in the cockpit. Headset
mics are directional mics. Not a terribly common problem, but with some
headsets (particularly the in-ear models), it's easy to get the back
side of the mic facing your lips. This will give preference to noise
over voice. If the mic isn't so close to the pilot's lips it's almost
touching them (left or right edge of your mouth tends work best, to
minimize popping from breath), then like the singer, the 'hall noise'
will be a bigger percentage of what the mic hears. If the pilot speaks
softly, then 'hall noise' will be a bigger percentage of what the mic
hears. If the 'front' of the mic isn't pointed directly at the pilot's
lips... Are we getting the idea yet? If there's another mic in the a/c
that's plugged into the system, it will often go 'live' whenever the PTT
is pushed along with the pilot's mic. If that happens....
ETC ETC
Now, what to do that's 'fixing' stuff, instead of just 'technique'?
Reducing the ambient noise level in a tube/rag a/c is very difficult
unless we're willing to accept a lot of extra weight. But there are some
'tweaks' to the system that will likely help. One thing will sound
counter-intuitive, but does work (I just went through it with my
neighbor, who's flying the prototype One Design unlimited acro a/c).
First, check with the mfgr of your headset, to see if there is a gain
adjustment *on the microphone* of the headset. Many electret condenser
mics have a tiny gain adjustment screw in the body of the mic. If yours
has one, turn the gain *down* as low as you can get it and still have
your voice transmitted. The reason: Those mics actually have a tiny
preamplifier built into them. If the total volume (voice plus noise) is
loud enough, it can drive the preamp into distortion. When that happens,
we get fuzzy, garbled sound as the peak volume is clipped off.
Next, find the mic gain or mic volume adjustment on your comm radio, and
turn *it* down as low as you can go, and still get your voice out there.
Even if you have to speak 'with authority' into the mic. Remember the
'too soft' singer? If you speak louder, your voice is a larger
percentage of what the mic hears. Now obviously, you need enough audio
to properly modulate the RF carrier, but you'll likely be surprised at
how much you can turn these setting down and still hear yourself. As
long as you sound ok in your sidetone, you're likely going to be ok with
modulation. (RF guys feel free to step in here.)
The last thing returns to technique, in the form of 'compensation' for
a/c deficiencies. When comm is critical, pull the power back. This was
the last step in getting the One Design mentioned above into the 'clear
communications' category. By reducing the throttle setting, the cockpit
noise goes down significantly, improving signal to noise ratio. (Could
this be a factor in small vs large airport operations, where you're
likely at reduced power close to the small airport but still at cruise
power when far from the large airports?)
Sorry for such a long 'epistle', but hopefully there will be some useful
info buried in there.
Charlie
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Argonaut36
Joined: 19 May 2019
Posts: 35
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| Posted: Fri Sep 20, 2019 5:47 am Post subject: Re: Grounding radio antenna and transponder antenna |
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Charlie,
I am carefully reviewing all the information that you provided and I am planning on following your suggestions.
Thank you very much for your help!
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