Re: KX155A/KX165A with lower GPS interference in the RF output?
Brian Whatcott
> Hi All,
>
> I have a case of that well known interference on the 13th harmonic of
> 121.15 121.175 121.20 messing up GPS....
> Any tips welcome...
>
> x----------x
This is right off the top, with no guarantees.
If a radio interferes on transmit via the cable or antenna,
a 5pF cap across the radio's output coax connector might
possibly help.
[Xc of 5pF at 150MHz X 13th harmonic about 20 ohms ]
A usable 5 pF cap could be made out of a two or three turn
twist of two insulated wires.
If a radio interferes while on receive, a fine copper mesh,
even an aluminum foil wrap round the radio to check
for signal leakage might be a useful test.
All in all, this note is probably worth at most,
what you paid for it..... You need somebody who has been
there, done that. It goes without saying: if the radio
is for other than experimental use, you need a pukka
av radio repair outfit to look at it.
Brian W
Bill
ELT
by disconnecting the ELT antenna and rechecking.
You might try putting a 50 ohm dummy load at the antenna connection.
If that's OK, you know it isn't a problem in the tray or coaxes over
there.
Also, disconnect the control cable to the ELT.
Bill Hale BPPP Instructor K0QA
On Apr 16, 6:05 am, Peter <nos...@nospam9876.com> wrote:
> Hi All,
>
> I have a case of that well known interference on the 13th harmonic of
> 121.15 121.175 121.20 messing up GPS.
>
> The standard TED filters don't do much, probably because I cannot fit
> them precisely on the back of the radios but there is a ~ 2ft length
> of coax cable which is inaccessible (without seriously major
> dismantling).
>
> Is there any known version, or a modification, of these radios which
> has reduced GPS interference? I wonder if any homebuilders have done
> an internal mod on one of these radios? There isn't enough room inside
> to fit the GPS filter though. Maybe there are some conductive gaskets
> one can fit?
>
> There is a lot of stuff on google, indicating that the KX155 (not "A")
> was terrible but the "A" version is supposed to be a lot better.
>
> Unfortunately my COM1 radio is directly below the KLN94 GPS. The COM2
> radio generates much less interference, as one might possibly expect.
>
> Modifying the KLN94 GPS is not really an option since the interference
> also wipes out any other GPS in the aircraft...
>
> Any tips welcome...
>
> x----------x
Brian Whatcott
> I have disconnected both antennas as close as I can to the radios
> (about 2ft away from the radios is the best I can do) and put 50 ohm
> terminators there (1 watt rated is the biggest I can easily get and I
> didn't TX for more than a few seconds) and the interference does NOT
> change.
>
> So the radiation is either from the actual radio, or from that 2ft
> long cable length. I cannot get to this part without $xxx's worth of
> labour at an avionics shop.
>
> Next thing I will try is some copper tape (conductive adhesive
> version) from Chomerics and stick it over the various case joints of
> the radio(s).
1)
You say "the interference does not change."
Does this mean "does not change transmitting or not"
or
" does not change as between
transmitting via the antenna, OR transmitting into a dummy load" ??
2)
Does the radio extract forward, with a fixing screw accessed from the
front panel??
Brian W
Brian Whatcott
....
>> " does not change as between
>> transmitting via the antenna, OR transmitting into a dummy load" ??
>
>
> One gets the same GPS interference even if driving a 50 ohm
> terminator.
>
>> 2)
>> Does the radio extract forward, with a fixing screw accessed from the
>> front panel??
>
>
You COULD possibly rig a power and ground lead, plus a wire for transmit
control to the radio and put a dummy load right in the radio connector
while it is pulled forward. That would give you a better chance to check
for the sensitivity to distance between GPS and radio. The local dummy
would cut out the possibility of a leaky coax back connector too.
Brian W
Ben Jackson
> There is a lot of stuff on google, indicating that the KX155 (not "A")
> was terrible but the "A" version is supposed to be a lot better.
I have a KX155A that doesn't interfere with handheld GPSs. Have you
tried swapping the unit with another one to make sure yours isn't
particularly bad? Any avionics shop should have a spare KX155A
floating around. While you're at it, make sure the tray->radio
connection is solid.
--
Ben Jackson AD7GD
<b...@ben.com>
http://www.ben.com/
o...@uakron.edu
lashup?
This is one of those times you need to find a spectrum analyser.
Where is your GPS antenna in relation to the radio? Is it on a
ground plane?
Is this a fiberglass aircraft?
Have you tried a bandpass filter? I'm assuming the radio was designed
to ARINC specs, so is the leakage from the coax or the power/control
lines to the radio?
Steve
Tauno Voipio
> o...@uakron.edu wrote
>
>> Your in a metal framed radio mount, correct? Not some polymer based
>> lashup?
>
>
>> This is one of those times you need to find a spectrum analyser.
>
>
>> Where is your GPS antenna in relation to the radio? Is it on a
>> ground plane?
>
> antennae. The third (a Garmin 496) has its external antenna on top of
> the instrument panel.
>
>> Is this a fiberglass aircraft?
>
> believe.
>
>> Have you tried a bandpass filter? I'm assuming the radio was designed
>> to ARINC specs, so is the leakage from the coax or the power/control
>> lines to the radio?
>
>
> As I wrote earlier, the nearest I can get them (without extensive
> dismantling) to the radio RF outputs is about 2ft. It is probably that
> 2ft cable that is radiating. It is likely RG58 and not RG400.
Check the grounding of the COM antenna bases. If the grounds are
bad, the feed line shields work as counterpoises, wreaking all
kinds of havoc.
--
Tauno Voipio, (CPL,ME-IFR, Avionics engineer)
tauno voipio at iki fi
Brian Whatcott
>>> Is this a fiberglass aircraft?
>>
>> No, all metal. However the roof is composite, with ground planes I
>> believe.
>>
>>> Have you tried a bandpass filter? I'm assuming the radio was designed
>>> to ARINC specs, so is the leakage from the coax or the power/control
>>> lines to the radio?
>>
>> Yes, tried the TED GPS filters.
>>
>> As I wrote earlier, the nearest I can get them (without extensive
>> dismantling) to the radio RF outputs is about 2ft. It is probably that
>> 2ft cable that is radiating. It is likely RG58 and not RG400.
>
>
> Check the grounding of the COM antenna bases. If the grounds are
> bad, the feed line shields work as counterpoises, wreaking all
> kinds of havoc.
>
I see where Tauno is coming from - but a contra indication is that a
dummy load 2 ft out from the radio does not mute the hash.
Anyway. The RST approach to stopping the mismatch wave traveling back
over the outside of the coax, is a couple or three ferrite doughnuts
rated for the VHF frequency - near the antenna base.
Come to think of it though - that kind of stopper wouldn't stop high
harmonics - so for that situation a couple of turns in the coax feed
taped together might have the desired effect. Worth a try anyway.
Brian W
o...@uakron.edu
I'd look at the reflected power from the antennas. Then look at the
crimped connectors. Next up is the grade of braid, 70% braid or
cheaper can really mess you up. These antennas probably need some form
of counterpoise in the form of a metal sheet at least a half wave in
dimensions. Ferrite on the Coax, as mentioned. And any corrosion,
anywhere, especially junctions between different metals and any
aluminum that is painted or doesn't have lockwashers to pierce the
oxide. Open grounding straps. Look at conducted radiation at control
lines, IF feeds to the nav gear, and while I doubt it, bypass the
power lines.
I'd take a long hard look at anything that is a fraction or multiple
of a wavelength and metal.
Usually something is broken some where metal.
Sometimes its a poorly tuned final.
Can you rig the system on a bench?
What are the manufacturers recommended solutions for the issues?
Also is this that bastard, slide in, half BNC like connector that
is on the back of my friend's aircraft radios? Ie loose as goosecrap
and depending on the frame to lock it in? I hate that design with a
passion.
I've been thinking of your lack of test gear. I've got two ideas. A
gps band pass filter from Digikey and a schottkey RF diode, and/or a
copper pipe filter, Harry Lythal style, the idea being if you have a
selective sniffer, you can find the leak, but since GPS is so weak to
begin with, I don't know if you can see the harmonic without a MAR or
ERA as a preamp. Harry has one for 1296, so it can probably be scaled
for 1.5 ghz. Diode detector circuits are all over the web
http://web.telia.com/~u85920178/
I'm not a PE, a A&E or A&P, nor do I play one on TV. So take my advice
with a grain of salt. The lawyer I cannot afford made me say
that. :-)
Steve.
Brian Whatcott
> The fact that terminating 2ft after the radios does nothing, means the
> emissions are pretty local to the radios.
Well: the penny finally dropped, when I asked the question: "Why is
there a 2 ft stretch of coax?"
And I am guessing that the answer is that you have several boxes strung
off that antenna coax.
Do you?
If you do, it's a problem
Brian W
JosephKK
wrote:
>
>Brian Whatcott <bet...@sbcglobal.net> wrote
>
>>Peter wrote:
>>
>>> I have disconnected both antennas as close as I can to the radios
>>> (about 2ft away from the radios is the best I can do) and put 50 ohm
>>> terminators there (1 watt rated is the biggest I can easily get and I
>>> didn't TX for more than a few seconds) and the interference does NOT
>>> change.
>>>
>>> So the radiation is either from the actual radio, or from that 2ft
>>> long cable length. I cannot get to this part without $xxx's worth of
>>> labour at an avionics shop.
>>>
>>> Next thing I will try is some copper tape (conductive adhesive
>>> version) from Chomerics and stick it over the various case joints of
>>> the radio(s).
>>
>>1)
>>You say "the interference does not change."
>>Does this mean "does not change transmitting or not"
>>or
>>" does not change as between
>>transmitting via the antenna, OR transmitting into a dummy load" ??
>
>The latter.
>
>One gets the same GPS interference even if driving a 50 ohm
>terminator.
>
>>2)
>>Does the radio extract forward, with a fixing screw accessed from the
>>front panel??
>
>Yes.
>
>Unfortunately there isn't a lot I can do behind it because the way the
>avionics stack frame is assembled, one cannot extract the RF
>connector.
>
>However, I've noticed that the FAA standard I should be working to is
>AC20-138A - not AC20-138 was updated in 2003!
>
>138A does not have the GPS/VHF interference test in the flight test,
>so I can get on with it anyway.
>
>It does seem odd to me that the FAA took out that test. Perhaps they
>had a reason; EASA does not have it either and they gold plate
>absolutely everything.
Sounds like all hell is out for noon if they retroactively decertify
one or more radios.
.
Brian Whatcott
> stack is constructed.
Hmmm...I think that means you cannot visualize the coax route up close
and personal. I am guessing - maybe guessing wild - but I imagine an
antenna, a coax run, a connector close up to the stack leading to one or
two coax tees and 2 or three radios all looking at the same antenna.
If you could run a straight coax shot from ONE KX155 radio to ONE
antenna and recheck, you might just find the gps intereference gone, I'm
thinking. But you might need to pull the radio, and thread a fresh coax
to the antenna for testing this idea. This would be a coax mismatch issue...
Brian W