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#11
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"scratchy" radio transmission
Cy Galley
Dec 31 2005, 10:30 pm show options Newsgroups: rec.aviation.homebuilt From: "Cy Galley" - Find messages by this author Date: Sun, 01 Jan 2006 03:30:55 GMT Local: Sat, Dec 31 2005 10:30 pm Subject: "scratchy" radio transmission Reply | Reply to Author | Forward | Print | Individual Message | Show original | Report Abuse I would try replacing the coax from Radio to antenna in ONE piece Both coaxes from both radios to both antennas? Odds seem kinda high that both are bad, don't you think? Jim |
#12
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"scratchy" radio transmission
Jim, do you have an audio mix panel, and if so does the mic input go
to it before splitting to the radios? Also, I read your original post pretty quickly and didn't notice the part where you said you were hearing the sidetone okay. That eliminates a lot of potential failure points. Another thought: Do you have an "intercom" that both mics and the PTTs connect to before the mic audio gets to the radios? If so, the intercom could be the culprit. There are so many ways that your aircraft could be wired up that its hard for me to think of all of them. You could probably get to the bottom of it all pretty quickly with an oscilloscope if you have one and know how to use it. RST, if both radios have the same static problem (with their different RF XMIT sections, T/R switches, and antennas), it is not very likely an RF problem. OTOH, it could be a DC voltage problem, as in the DC voltage supply to the radios is too low, causing the transmit power to be less than it should. Jim, have you tried both radios and verified that ATC hears them both as scratchy? No audio mix panel. Mic goes to the intercom then goes through a DPTT switch (to deliver Mic to either radio but not both at the same time and same with the PTT and audio output). When the intercom is turned off the mic input bypasses it straight to the radios and it is the 'radio' sidetone I hear clearly from both radios, not the intercom's. ATC verifies both radios sound the same. I used to suspect the landing gear getting in the way but as I said I have one antenna on the bottom and another on top. Since I'm always headed to the facility on callup I even had the whacky idea that the metal prop was getting in the way so I would turn the plane 90 degrees and call again. Needless to say, no difference. I am also leaning to checking the DC supply circuits again. Thanks. Jim |
#13
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"scratchy" radio transmission
Bought a homebuilt once. Both radio antennas had been installed the same
way - with the cardboard drilling pattern glued to the aluminum - then left there, and the antenna installed over the top of it. No ground plane. The SWR was OK (1.8:1), believe it or not. It worked out to 10 miles or so - then things started getting weak and scratchy. Both radios are mounted to aluminum rails bolted to fuselage longerons however each radio has an aditional grounding wire (from it's main plug) to fuselage as a precaution. The SWRs also check out the same as you discribe as does the performance. Power output does not seem to be the issue (again I never checked power output in the winter), just audio quality. Jim |
#14
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"scratchy" radio transmission
It is not the RADIO ground that was needed, it was the GROUND PLANE that is
needed on a 1/4 wave antenna. http://www.eecs.wsu.edu/~pedrow/classes/ee416/Spring_2004/share_pdfs/teals/April_4/Quarterwavemonopoles1.pdf#search='quarter%20dipole ' You must have the outer conductor of the coaxial cable connected either to a lot of metal at more or less 90 degrees to the vertical, or you have to get scientific with 3 or more tuned stubs. This has little to do with the radio's electrical ground. The link explains what's going on pretty well. You should be ok with an aluminum RV as long as the coax outer conductor to the skin reads a short on an ohmmeter. Both radios are mounted to aluminum rails bolted to fuselage longerons however each radio has an aditional grounding wire (from it's main plug) to fuselage as a precaution. The SWRs also check out the same as you discribe as does the performance. Jim |
#15
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"scratchy" radio transmission
That's not a good way to measure whether or not the coax is terminated
properly. I just inspected an antique where the antenna was properly mounted on a sufficiently big ground plane, but the coax braid wasn't even connected to the ground plane at the antenna end. You might just a well have run Romex as coax if that's the case. And, of course, there WAS continuity to ground as the coax WAS terminated at the radio end to the airframe. THe question was whether BOTH antennas or coax could be "bad". I can argue that if the fellow who made the plane didn't know how to assemble a connector properly the odds of him screwing up BOTH the coax cables in the same way are quite good. THere is no substitute for a good mechanical inspection of the coax connections at both the radio AND antenna ends. I didn't notice whether or not the OP said that he borrowed a known good headset, since that's the one thing that is common to both rigs. Jim You should be ok with an aluminum RV as long as the coax outer conductor to the skin reads a short on an ohmmeter. |
#16
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"scratchy" radio transmission
I didn't notice whether or not the OP said that he borrowed a known good
headset, since that's the one thing that is common to both rigs. Several headsets were tried with same results. Both these headsets worked fine in wife's C-172 (ATC reports crystal clear) One of the radios has a new store bought antenna on the belly with new BNC connectors that came on a "factory built" length of coax to the radio which came with an antenna "pigtail" already on the tray. This complete replacement setup seems to xmit no better than the "cheesey" antenna and antique radio it replaced and is reportedly no different than the 2nd radio/antenna setup. Jim |
#17
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"scratchy" radio transmission
Try this description one more time, please. I"m not understanding why a
factory built length of coax with BNC connectors can have pigtails. Jim wrote in message oups.com... One of the radios has a new store bought antenna on the belly with new BNC connectors that came on a "factory built" length of coax to the radio which came with an antenna "pigtail" already on the tray. |
#18
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"scratchy" radio transmission
Try this description one more time, please. I"m not understanding why a
factory built length of coax with BNC connectors can have pigtails. Jim (...radio which came with an antenna "pigtail" already on the tray) RADIO which came with an antenna "pigtail" already on the TRAY Thats "pigtail" on TRAY not on coax from antenna. And before I get chastised again I realize that the term "pigtail" may not be technically correct but I think most will get the picture here. Jim |
#19
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"scratchy" radio transmission
Not if the same person made the ends and ty-rapped them too tight.
wrote in message oups.com... Cy Galley Dec 31 2005, 10:30 pm show options Newsgroups: rec.aviation.homebuilt From: "Cy Galley" - Find messages by this author Date: Sun, 01 Jan 2006 03:30:55 GMT Local: Sat, Dec 31 2005 10:30 pm Subject: "scratchy" radio transmission Reply | Reply to Author | Forward | Print | Individual Message | Show original | Report Abuse I would try replacing the coax from Radio to antenna in ONE piece Both coaxes from both radios to both antennas? Odds seem kinda high that both are bad, don't you think? Jim |
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