nuckolls.bob(at)aeroelect
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 Posted: Wed Feb 02, 2011 6:15 pm Post subject: When is a 'ground' not a 'ground' |
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At 09:58 PM 1/30/2011, you wrote:
Bob N., et al,
I have run a bunch of additional tests of my antennas in various
configurations to see what effect they had on VSWR readings. The raw
data and VSWR results are pretty variable (to my eyes) and not very
encouraging.
But my immediate issue is the one that Bob N. focused on shortly
after my original post -- the quality of the ground between my
antenna base and my ground plane. At the time I did the original
tests, it tested out at ~0.1 Ohms on my digital VOM.
But I dug out the four-wire ohmmeter that I build years ago based on
Bob N's directions in the AeroElectric Connection and soldered in a
fresh "D Cell" battery. The meters (especially the voltmeter)
fluctuated quite rapidly and over quite a range so I'm not sure my
data are accurate (although it did come close to the calculated
resistance of a hunk of 18 AWG wire that I used to check the
four-wire ohmmeter for accuracy).
While based on solid physics, THAT test tool was a
hurry-up jury-rig that helped me make some good
measurements in the field with locally acquired
materials. It had no constant current generator
and therefore taxed the operator to visually integrate
voltage/current readings to deuce resistance value
with something like 10% accuracy. The test current
was not REAL stable. Further, using 'shorted'
Adapter I'm offering from the website. I mention
this because one should approach low-ohms measurements
with some degree of caution. Be aware that SOME
devices may not tolerate so high a test current.
While exploring 'hardware' it's not a concern but
be cautious with the measurement of electro-whizzies.
As best I can tell, the resistance between my antenna mounting screws
and the ground plane are in the 250 milli-Ohm range. This seems to
be way too high. Is it? I believe you said that resistance should
be in the very low milli-Ohm or even micro-Ohm range.
Yes . . . and forgive me, I need to interject another
simple but significant idea into this discussion.
Using the DC ohmmeter to evaluate the metallic
connection between two components is a legacy
standard examination of the joints between ground
straps and the airframe. These investigations are
concerned with predicting performance of a high
current ground like a cranking battery or a bond-
strap for lightning protection.
The DC measurement offers SOME insight for grounding
antennas when the base of the antenna is brought
into forceful proximity with the belly skins
and structure of an airplane. But as soon
as we insert a gap between the base and ground plane,
the reliability of a DC resistance begins to fail
for predicting performance of the joint at very high
radio frequencies.
For example, We could set the comm antenna up on a
22" tall pedestal of copper tube. The DC resistance
of that tube may still be in the low milliohms but
because its LENGTH is exactly 1/4-wave at 126 Mhz,
the ground IMPEDANCE at the top of pedestal would
be THOUSANDS of ohms.
This is why I made the statement earlier this
week that the "quality" of your antenna grounding
was more of an "art" borne of experience and
command of simple-ideas than a matter of measuring
milliohms from one point to another.
I've got one of your newer low resistance testers on order and as
soon as I get it, I'll measure the resistance again.
I'll probably just have to pull the ground plane and re-do the
mechanical mounting again and hope I get a lower resistance.
As stated above, a HIGH DC resistance reading in a
stack up of metallic components is reason for further
investigation, a LOW reading is not necessarily
golden at radio frequencies.
Would it make sense to get some conductive epoxy and bond a ground
strap from the antenna base to the ground plane? I know soldering
copper to aluminum doesn't work and corrodes almost
immediately. I've seen some silver-filled epoxies rated as low as
0.001 Ohm-cm but that stuff is expensive. Could I solder a small
ground strap to the antenna's metal base without damaging the
antenna? I've got room in the hole for the BNC connector to run a
small ground strap up from the antenna base to a ring terminal that I
could clamp under one of the mounting screws. Would conductive gel
under the hardware help?
Yeaaahhh . . . maybe . . . but . . . As a
a practical matter, the TC aircraft industry
has learned to minimize process sensitive
materials that involve pastes, goops, glues, goos
and extraordinary technician's skills.
In addition to the demands of skill and process
to maximize performance, these materials tend
to have a poor service life. They're not even
close to both performance and longevity of a
(1) a single piece of metal or (2)
multiple pieces welded together or (3) pieces
bolted together with enough force to achieve and
maintain gas-tightness between the pieces in
spite the variables of temperature, moisture,
flexure and time.
Since I've got a glass skin between the antenna base and the washer
stack that leads to the ground plane, I didn't "brighten" the
underside of the ground plane -- just the top under each of the four
machine screws. I'm thinking if I brightened up both sides of the
ground plane, there might be some improvement because of the
additional contact between the hardware stack, the machine screws and
the underside of the ground plane.
Okay, if it were my airplane . . .
I'd ditch the aluminum ground plane. Get a piece
of brass sheet, .032 to .040 thick and 1" larger
than extremes of antenna base on all four sides.
Drill #10 close clearance holes and BNC clearance
holes to match antenna base and exactly centered on
the sheet.
Cut 4 spacers from brass. 3/8 to 1/2-inch diameter and
drilled for #10 close clearance. Cut and drill
these on a lathe. By definition, the end of a cylinder
faced in a lathe is perfectly perpendicular to
the axis. Also, the thru holes will be exactly
centered. Make them 1/32" longer than the distance
it takes to come through the thickness of the
composite belly material.
Use 63/37 solder to 'tin' both ends of the spacers
and the area around the bottom side of the brass plate.
Wipe the 'tinning' off with a dry rag while still
hot . . . you don't want any visible 'thickness'
of solder. Bolt the spacers to the plate and solder
to the plate. Tighten the bolts while the solder is
molten to make sure spacers are flat to the plate.
You need just enough solder to form a small fillet
around the spacer.
When you're done, you'll have a one-piece of metal
that can be installed in the airplane. The four
spacer holes need to be enlarged to about 1/8"
larger radius than the spacer diameter. Put some
10-32 screws in the holes, nuts against the plate.
Tape over the bottoms of the four spacer holes in
the belly skin. Fill the holes about 1/2 with
catalyzed resin. Paint the underside of the plate
with catalyzed resin. Press the assembly into the
final resting place and fixture with sand bags,
shot bags, etc.
When the resin has set up, remove tape and temporary
screws. Clean up any resin that would interfere
with seating the antenna on the ends of the
spacers. Make sure the tinned ends of the spacers
are clean.
Fabricate at least 4 but no more than 8 'radials'
from copper foil. 3/4" to 1-1/2" wide and 21" long.
these can be multiple strips spliced together by
soldering to get desired total length. Lay these
out in as uniformly circular pattern around antenna.
Length can curve up the sides and 'hop' over
ribs, stringers, stiffeners, etc. Solder end of
radials to the brass sheet using 63/37 electronic
solder. Heavy duty iron will flow these joints
one at a time and with minutes between joints to
keep from heating the whole thing up and putting
resin at risk. Put strips of glas and resin over
the radials to keep them in place and protect from
damage.
Brighten under-surface of antenna base before
installing antenna using 10-32 grade 8 or stainless
cap head screws. Screw heads against antenna base;
MS20365 or equal metallic lock nuts over washers against
the plate. Caulk around base of antenna with RTV.
There are no doubt other ways to get an RF-SOLID
installation but this one is based on materials
and tools I have. I know too that the processes
and configuration would produce an installation
with robustness and performance difficult to
improve upon. A DC ohmmeter test of this installation
would be superfluous.
Bob . . .
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