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nuckolls.bob(at)aeroelect
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 Posted: Thu Jul 23, 2015 8:03 am Post subject: VOR antenna question |
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At 06:47 AM 7/22/2015, you wrote:
Hi, Bob -
I have a 1962 Skylane, and I am not satisfied with its VOR/GS performance at all. Add to that the fact that the autopilot won't track worth a damn, and I'm wondering if the Nav receiver/antenna setup is at fault.
How steady is the CDI display when receiving VOR? Are
the CDI warning flags lifted? Could you hand-fly the
airplane accurately using the display? If so, then
it seems unlikely that quality of data from VOR Rx
to AP is probably not the issue. If the
CDI warning flags are not dropping, then your
tracking problem is probably centered elsewhere.
I've decided to do a little troubleshooting before I send the S-Tec 55x in for a major (expensive) overhaul. At the very least, I want to replace the VOR/GS coax and balun (The antenna is a horizontal dipole on the top of the vertical stabilizer, like many 182s).
The balun is a typical made-from-coax affair that has never been changed since the airplane was new in 1962, as far as I can tell. I'm sure it has drowned in water - or at least moisture - for a very long time. The dipole is then just connected directly to the balun.
My questions are these:
1. The first thing is to replace the long run of coax. What coax do you recommend? I know RG 58 will work (50 ohm), but I've heard there are much better, lower-loss cables for this application.
Yes. A much more robust and lower loss product
like RG141/142/400 is never a bad bet. There
are less expensive 50-ohm coaxes with better
performance than RG58 like the LMR195, same
size and connectors as RG58, similar outside
insulation but better shield construction and
lower loss insulation around the center
conductor . . . but a little fussier to work
with than the double-shield, Tef-like products
cited earlier. I'd go with 141/142/400 but the
195 is fine too.
2. From what I read, the only purpose of the balun is to keep RF off the braid on the coax; the balun's matching function really doesn't do much for performance.
Yes. "Keeping RF of the braid" is a transmitting
issue . . and yes, it takes laboratory grade
instrumentation and observation methods to
deduce any benefits for adding the balun to
a VOR antenna. All other things being equal,
the pilot would not be able to tell whether
or not a balun was installed.
Since this is a receive-only setup, do I even need to worry about a balun, or could I just connect my new coax (unbalanced) to the antenna and forget about it?
We built thousands of airplanes at Cessna in the 60's
and 70's that were wired that way . . . and VOR performance
of vacuum tube receivers was deemed adequate. And
again, all things being equal, the pilots would not
know if a balun was present or not.
3. If you think a balun is worth it, where can I find a decent 1:1 potted balun, so I don't have to worry about fabricating a new coax one?
If the airplane's factory-delivered configuration
included a balun, then there's always risk of running
afoul of the TC-rule-gods.
They're easy to build, especially from RF141/142/400
coaxes that will readily stand off the effects of
soldering.
http://tinyurl.com/yytxwd3
So it's kinda your choice. A fresh hunk of modern
coax simply tied to the VOR whiskers will perform
nicely. Whether or not this produces better VOR/AP
performance is problematic.
Bob . . . [quote][b]
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n744bh(at)bellsouth.net
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| Posted: Thu Jul 23, 2015 8:48 am Post subject: VOR antenna question |
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Give Kenny Poyner a call at 817-215-7637 (Cobham) and explain the situation to him. He is a whiz at the S-Tec a/p's and can be very helpful.
Bill
On Thursday, July 23, 2015 11:05 AM, "Robert L. Nuckolls, III" <nuckolls.bob(at)aeroelectric.com> wrote:
At 06:47 AM 7/22/2015, you wrote:
Hi, Bob -
I have a 1962 Skylane, and I am not satisfied with its VOR/GS performance at all. Add to that the fact that the autopilot won't track worth a damn, and I'm wondering if the Nav receiver/antenna setup is at fault.
How steady is the CDI display when receiving VOR? Are
the CDI warning flags lifted? Could you hand-fly the
airplane accurately using the display? If so, then
it seems unlikely that quality of data from VOR Rx
to AP is probably not the issue. If the
CDI warning flags are not dropping, then your
tracking problem is probably centered elsewhere.
I've decided to do a little troubleshooting before I send the S-Tec 55x in for a major (expensive) overhaul. At the very least, I want to replace the VOR/GS coax and balun (The antenna is a horizontal dipole on the top of the vertical stabilizer, like many 182s).
The balun is a typical made-from-coax affair that has never been changed since the airplane was new in 1962, as far as I can tell. I'm sure it has drowned in water - or at least moisture - for a very long time. The dipole is then just connected directly to the balun.
My questions are these:
1. The first thing is to replace the long run of coax. What coax do you recommend? I know RG 58 will work (50 ohm), but I've heard there are much better, lower-loss cables for this application.
Yes. A much more robust and lower loss product
like RG141/142/400 is never a bad bet. There
are less expensive 50-ohm coaxes with better
performance than RG58 like the LMR195, same
size and connectors as RG58, similar outside
insulation but better shield construction and
lower loss insulation around the center
conductor . . . but a little fussier to work
with than the double-shield, Tef-like products
cited earlier. I'd go with 141/142/400 but the
195 is fine too.
2. From what I read, the only purpose of the balun is to keep RF off the braid on the coax; the balun's matching function really doesn't do much for performance.
Yes. "Keeping RF of the braid" is a transmitting
issue . . and yes, it takes laboratory grade
instrumentation and observation methods to
deduce any benefits for adding the balun to
a VOR antenna. All other things being equal,
the pilot would not be able to tell whether
or not a balun was installed.
Since this is a receive-only setup, do I even need to worry about a balun, or could I just connect my new coax (unbalanced) to the antenna and forget about it?
We built thousands of airplanes at Cessna in the 60's
and 70's that were wired that way . . . and VOR performance
of vacuum tube receivers was deemed adequate. And
again, all things being equal, the pilots would not
know if a balun was present or not.
3. If you think a balun is worth it, where can I find a decent 1:1 potted balun, so I don't have to worry about fabricating a new coax one?
If the airplane's factory-delivered configuration
included a balun, then there's always risk of running
afoul of the TC-rule-gods.
They're easy to build, especially from RF141/142/400
coaxes that will readily stand off the effects of
soldering.
http://tinyurl.com/yytxwd3
So it's kinda your choice. A fresh hunk of modern
coax simply tied to the VOR whiskers will perform
nicely. Whether or not this produces better VOR/AP
performance is problematic.
Bob . . . | Quote: |
ist" target="_blank" rel="nofollow">http://www.matronics.com/Navigator?AeroElectric-List
el="nofollow">http://forums.matronics.com
_blank" rel="nofollow">http://www.matronics.com/contribution
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nuckolls.bob(at)aeroelect
Guest
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| Posted: Thu Jul 23, 2015 1:56 pm Post subject: VOR antenna question |
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At 11:46 AM 7/23/2015, you wrote:
| Quote: | Give Kenny Poyner a call at 817-215-7637 (Cobham) and explain the situation to him. He is a whiz at the S-Tec a/p's and can be very helpful.
Bill |
Thanks for the heads-up Bill, I've forwarded the suggestion
to Jim . . .
| Quote: | On Thursday, July 23, 2015 11:05 AM, "Robert L. Nuckolls, III" <nuckolls.bob(at)aeroelectric.com> wrote:
At 06:47 AM 7/22/2015, you wrote:
Hi, Bob -
I have a 1962 Skylane, and I am not satisfied with its VOR/GS performance at all. Add to that the fact that the autopilot won't track worth a damn, and I'm wondering if the Nav receiver/antenna setup is at fault. |
Bob . . . [quote][b]
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