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pwmac(at)sisna.com
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 Posted: Mon Jan 24, 2011 11:09 am Post subject: Comm Antenna & SWR Reading |
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I am not Bob , but I use a forward/reflected SWR meter. Its a low
cost unit that did not have serious badmouthing from the Ham guys.
MFJ-862 SWR/Wattmeter. It has a 30 and 300w max range setting
PaulW
=========
At 08:14 AM 1/24/2011, Noel Loveys wrote:
| Quote: |
Bob what did you use to measure the forward and reverse power? A bird or
just a regular SWR meter?
Noel
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Float Flyr
Joined: 19 Jul 2006
Posts: 2704
Location: Campbellton, Newfoundland
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| Posted: Tue Jan 25, 2011 9:43 am Post subject: Comm Antenna & SWR Reading |
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To accurately measure VHF frequencies the meter has to have fast acting
diodes. Usually meters designed for HF (under 30 mHz) are not fast enough
gating to accurately read swr for higher frequencies. To be sure take your
meter to someone who has a bird or similar meter and see if the calibration
is close.
A technician at a local phone company or amateur who works on repeaters may
be a good place to start.
Noel
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Noel Loveys
Kitfox III-A
Aerocet 1100 Floats |
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nuckolls.bob(at)aeroelect
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| Posted: Tue Jan 25, 2011 11:42 am Post subject: Comm Antenna & SWR Reading |
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At 12:38 PM 1/25/2011, you wrote:
| Quote: |
To accurately measure VHF frequencies the meter has to have fast acting
diodes. Usually meters designed for HF (under 30 mHz) are not fast enough
gating to accurately read swr for higher frequencies. To be sure take your
meter to someone who has a bird or similar meter and see if the calibration
is close.
A technician at a local phone company or amateur who works on repeaters may
be a good place to start.
|
Certainly the characteristics of detector diodes become
more critical as the frequency goes up. Criticality is
also increased as the magnitude of exciting signal goes
down. We touched on this when questioning whether or not
the .5 watt excitation was going to yield good measurements
on a 5 watt full scale instrument.
The rule of thumb for even the Bird instruments is to
select a signal source and element that produces forward
readings in the upper half of scale travel (greater than
2W on a 5W element).
All of the DIY antenna analyzer projects routinely
call out specialty detector diodes line the 1N5711
(point contact Schottky). It's equally critical
to build the bridge from the right kind of resistors
and very tight geometry. There's a lot of reference
data out there for folks interested in DIY antenna
measurements.
Bob . . .
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nuckolls.bob(at)aeroelect
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| Posted: Tue Jan 25, 2011 11:58 am Post subject: Comm Antenna & SWR Reading |
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Oops! Re-posted with the proper subject line to keep it
attached to the proper thread.
| Quote: | As for the ground plane assuming your ground bond is good to your aluminium
and the centre conductor is intact you may want to try bonding the aluminium
to the aircraft ground system with a separate wire.
|
Wires use to connect RF ground systems to any other ground
system are never useful for controlling antenna system
performance.
Bob . . .
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Float Flyr
Joined: 19 Jul 2006
Posts: 2704
Location: Campbellton, Newfoundland
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| Posted: Tue Jan 25, 2011 12:44 pm Post subject: Comm Antenna & SWR Reading |
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Thanks for filling in the gaps Bob. What I was getting at was trying to
remove any errors that may have been caused by the meters.
Noel
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Noel Loveys
Kitfox III-A
Aerocet 1100 Floats |
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nuckolls.bob(at)aeroelect
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| Posted: Tue Jan 25, 2011 1:05 pm Post subject: Comm Antenna & SWR Reading |
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As for the ground plane assuming your ground bond is good to your aluminium
and the centre conductor is intact you may want to try bonding the aluminium
to the aircraft ground system with a separate wire.
Wires use to connect RF ground systems to any other ground
system are never useful for controlling antenna system
performance.
------------
The bond I was referring to would be to the same system. Doing that could
nullify the effects of a bad connection to the braid at the antenna end.
You lost me. This is a composite aircraft with an
aluminum sheet on the inside surface for a ground
plane and approx 7/16" worth of composite structure
between the ground plane and the base of the antenna.
I presume there's a centrally located clearance hole
in the composite material for the coax connector.
He described 4 threaded fasteners holding this 'sandwich'
together.
To be sure, a low resistance (micro-ohms), high integrity
(gas tight) connections between the base and the ground plane
are called for. Having "soft" material in the makeup
of this sandwich is counter productive to achieving that
goal. Hence the suggestion for metallic spacers through
the structure to take the mate-up tension loads in the
screws and provide low-inductance jumpers (4 in parallel)
between the antenna base and the ground plane.
"Ground system" is generally interpreted to mean the DC
power ground. On this aircraft, it may well be a firewall
single point ground with lots of wires returning to it.
In this instance, no potential benefits to be gained
by adding conductors between power ground and anything
associated with this antenna installation.
Bob . . .
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Float Flyr
Joined: 19 Jul 2006
Posts: 2704
Location: Campbellton, Newfoundland
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| Posted: Tue Jan 25, 2011 7:55 pm Post subject: Comm Antenna & SWR Reading |
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Bob:
I think we are talking about slightly different things here.
I suspect there is no direct connection between the braid of the coax
(ground) and the ground plane. Most of these antennas are designed to be
mounted on aluminium skins. I am thinking that the ground plane is totally
afloat and not connected in any way to the braid. Of course this is easy to
check by simply ringing between the ground plane and the exterior of the
BNC.
Noel
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Noel Loveys
Kitfox III-A
Aerocet 1100 Floats |
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nuckolls.bob(at)aeroelect
Guest
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| Posted: Wed Jan 26, 2011 8:24 am Post subject: Comm Antenna & SWR Reading |
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At 10:51 PM 1/25/2011, you wrote:
| Quote: |
Bob:
I think we are talking about slightly different things here.
I suspect there is no direct connection between the braid of the coax
(ground) and the ground plane. Most of these antennas are designed to be
mounted on aluminium skins. I am thinking that the ground plane is totally
afloat and not connected in any way to the braid. Of course this is easy to
check by simply ringing between the ground plane and the exterior of the
BNC.
|
The braid MUST come to a very low impedance connection
to the ground plane else the coax becomes 'un-terminated'.
In fact, the high SWR readings Bob cited may have been
the result of poor contact between the antenna base
and the ground plane.
The ideal ground plane behaves as an infinite number of
elements that might otherwise be a center fed dipole
oriented in a plane at right angles to the antenna.
Consider the dipole with coax center conductor to one
element, shield to the other. Yeah, balanced antenna
and unbalanced feedline . . . not the best we know how
to do.
Now bend the shield-side element 90 degrees to center-
conductor element. Very lopsided and not quite so balanced
antenna . . . with the 'bent' element NOT contributing to
a useful radiation pattern. Start adding elements on
the shield side. Once you get to 4 elements, 90 degrees
apart, the current in each element is 1/16th and you have
4 resonant elements in parallel giving 1/4th feedpoint
impedance compared to a single element. Tendency of the
'radials' to radiate is much suppressed and the lion's
share of energy is conducted to the center-conductor element.
Now increase the number of radials to 1 bazillion and
you have a solid surface, not unlike the skin of an
airplane . . . or the sheet of aluminum inside the
composite shell of Bob's airplane.
But for the antenna to perform at its best, the
ideal radiation resistance of the ground plane
is zero (lots in parallel). Further, the electrical
connection from shield ground to ground
plane is as close to zero ohms as we can
make it.
Bob . . .
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Float Flyr
Joined: 19 Jul 2006
Posts: 2704
Location: Campbellton, Newfoundland
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| Posted: Wed Jan 26, 2011 1:30 pm Post subject: Comm Antenna & SWR Reading |
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Now... I think I'm on your page!
Most of the fancy high speed antennae I've seen have been made specifically
made to mount on metal planes. Antennae for composite planes generally have
the ground plane built in to them. The base of the antenna makes the
connection for the ground plane while the mounting screws are simply that, a
method to physically mount the antenna to the plane.... not necessarily to
provide a path for the ground plane.
BTW I do admire your explanations.
Noel
The braid MUST come to a very low impedance connection
to the ground plane else the coax becomes 'un-terminated'.
In fact, the high SWR readings Bob cited may have been
the result of poor contact between the antenna base
and the ground plane.
The ideal ground plane behaves as an infinite number of
elements that might otherwise be a center fed dipole
oriented in a plane at right angles to the antenna.
Consider the dipole with coax center conductor to one
element, shield to the other. Yeah, balanced antenna
and unbalanced feedline . . . not the best we know how
to do.
Now bend the shield-side element 90 degrees to center-
conductor element. Very lopsided and not quite so balanced
antenna . . . with the 'bent' element NOT contributing to
a useful radiation pattern. Start adding elements on
the shield side. Once you get to 4 elements, 90 degrees
apart, the current in each element is 1/16th and you have
4 resonant elements in parallel giving 1/4th feedpoint
impedance compared to a single element. Tendency of the
'radials' to radiate is much suppressed and the lion's
share of energy is conducted to the center-conductor element.
Now increase the number of radials to 1 bazillion and
you have a solid surface, not unlike the skin of an
airplane . . . or the sheet of aluminum inside the
composite shell of Bob's airplane.
But for the antenna to perform at its best, the
ideal radiation resistance of the ground plane
is zero (lots in parallel). Further, the electrical
connection from shield ground to ground
plane is as close to zero ohms as we can
make it.
Bob . . .
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_________________
Noel Loveys
Kitfox III-A
Aerocet 1100 Floats |
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