nuckolls.bob(at)aeroelect
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 Posted: Mon Jan 24, 2011 6:03 pm Post subject: Comm Antenna & SWR Reading: Additional Info |
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At 03:24 PM 1/24/2011, you wrote:
Bob, et al,
Here is some additional information:
To Bob's inquiry "It would be interesting to do an ohmmeter test from
center pin on the antenna's connector and ground. If is shows a
'short', I suspect that some sort of matching system is installed.",
my ohmmeter showed a complete open between the center pin and the
shell of the BNC connector on the antenna base.
Okay, that's not a guarantee that there's
no networking components in the base, the
matching system could be capacitively
coupled . . . but interesting info.
2. The antenna base is installed on the exterior surface of the
composite fuselage and the ground plane is on the inside
surface. They are about 7/16" of an inch apart and, as I mentioned
before, electrically bonded by the four stainless machine screws that
mount the antenna to the airplane. I haven't researched whether the
antenna must be mounted directly on the ground plane -- if so, that
will be an issue since some composite surgery will be required that
I'd really rather avoid.
Not necessarily right ON the ground plane but
if your antenna mounting screws put a "crush"
on anything other than metal, their long
term integrity as electrical conductors is
suspect. I think you mentioned 10-32 attach
hardware. I'd fabricate some spacers with 10-32
clearance holes and just shy of structure thickness
for length. The goal is to have the spacers mate
up with the base of the antenna and the lower
surface of the ground plane. The majority of attach
bolt tension goes to maintaining crush on
the spacer. Your looking for PRESSURE on the
ends of the spacers, they don't need to be
real big in diameter, say 3/8"?
Of course the ends of the spacers and the
surfaces they contact should be bright at
bolt-up time. Use washer's under the nuts on
top of the ground plane to get a flattening
of the ground plane to the top of the
spacer. Torque the hardware to 80% of
limits.
3. I didn't see any adjustment on the antenna itself.
And typically, no TSO/DO qualified antenna would
have any adjustments . . .
Based on the responses, it looks like I've got some more
tweaking/testing to do. Specifically:
Re-test the antennas (including the cat's whisker) and their feed
lines separately, and then together after I've got the feed lines cut
to final length and with their final BNC connectors in place.
I have never seen an antenna "go bad" . . . they're just
too simple and quite robust. If they're not obviously
broken, the probability is that it's okay.
Coax feed lines with properly installed connectors are
high probability performers too. Given what we know
of your installation right now, ground plane bonding
offers the most exciting hypothesis for the measurements
you observed.
The RG-400 feed line from the Comm 1 antenna to the Garmin 430W will
be about 10' long. I can vary (i.e., lengthen) that length if it
will help performance. Is there a rule of thumb in how long to make
antenna feed lines for optimum performance?
Short as practical but allow some service loops if it's
necessary to dismount a black box and withdraw it some
distance before you can access connectors. This goes for
other harnesses as well. Don't agonize over an 'extra' foot
or two of coax at either end.
When re-running the tests, would it make any difference to use a more
powerful transmitter than my little hand-held? Or should I stick
with what I used originally to eliminate one more variable that might
affect the follow-on data?
The Bird 43 is the gold-standard for test equipment
we unwashed can afford. Although with a 5w slug,
you ARE way down on the best linearity curve for
the detector diode. More power MIGHT be helpful.
And yes, measure the SWR at the end of the coax
that attaches to your transceiver.
Bob . . .
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