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HF antenna issues

 
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l.p(at)talk21.com
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PostPosted: Mon Nov 19, 2012 2:23 pm    Post subject: HF antenna issues Reply with quote

Bob and all.
I'm trying to insulate an aircraft's wiring from the rfi/emi effects of an HF antenna in a fibreglass Long-EZs wing. A counterpoise to the antenna will lie in the opposite wing.

I had thought of using a grounded (?) copper conduit for all the wing wiring ... fuel tank capacitance gauge/strobe and navigation lighting/vhf nav-com cables, but with an output of 100W I am worried that the in-wing HF antenna will set up a field that will still cause issues.
There is still the case of using a common ground for the existing wiring and the new radio causing problems.

Any thoughts on a suitable insulator or solution to this?

Many thanks.
Patrick Elliott.
G-LGEZ
[quote][b]


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nuckolls.bob(at)aeroelect
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PostPosted: Tue Nov 20, 2012 3:35 pm    Post subject: HF antenna issues Reply with quote

At 04:22 PM 11/19/2012, you wrote:
Quote:
Bob and all.
I'm trying to insulate an aircraft's wiring from the rfi/emi effects
of an HF antenna in a fibreglass Long-EZs wing. A counterpoise to
the antenna will lie in the opposite wing.

I had thought of using a grounded (?) copper conduit for all the
wing wiring ... fuel tank capacitance gauge/strobe and navigation
lighting/vhf nav-com cables, but with an output of 100W I am worried
that the in-wing HF antenna will set up a field that will still cause issues.

There's a VERY high probability for that condition.
Keeping HF out of the electro-whizzies on a metal
airplane can be a challenge. There's no hard-and-
fast rules for an installation that's not first
researched in the lab and then vetted over a series
of successful field installations.

The question to be asked first is, so it interferes
with SOME things. Does this interference create a
hazard? So gauges don't read right while your talking
but are okay when you're listening.

A/P servos trying to turn the airplane upside down
are problematic. You'd attack that problem with shielding
and filtering local to the actuator.

Quote:
There is still the case of using a common ground for the existing
wiring and the new radio causing problems.

Any thoughts on a suitable insulator or solution to this?

It's not an issue of 'grounding' but of the potential
susceptibility of all the victims to stresses they
were never qualified for. Conduits and special grounds
are of limited if any value. Suggest you get it all installed
and then attack each critical condition one-at-a-time.
Bob . . .


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toaster73(at)embarqmail.c
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PostPosted: Tue Nov 20, 2012 4:54 pm    Post subject: HF antenna issues Reply with quote

This is somewhat timely. Here is my setup...
RV-10
Garmin stack from top to bottom: G430, SL30, GTX327, GMA340, trutrak A/P
To the left is 2 GRT HX screens with GRT EIS underneath and switches all
along the bottom of panel.
I have only recently installed the SL30 - wiring was all there just didn't
have the unit in the tray yet.
SL30 uses Archer wingtip antenna, 430 uses bent whip on belly.
RG400 runs from units to antennas

I was in the cockpit with engine off learning the new SL30. I had the entire
panel powered up. I noticed while transmitting over the SL30 my fuel
pressure would rise from zero to about 50 psi if I held the push to talk. I
tried the 430 today and it also responds but about half as bad and I had
never noticed it before, it had never caused the FP to exceed limits so I
did not notice it in flight.
I have flown with the SL30 and when transmitting it sends the FP up enough
to exceed the limit (35) which annoyingly gets my attention.

The FP sender wires pass near the SL30 antenna wire so today I separated
them but that only helped a little, or did it??. When I unhook the antenna
wire from the SL30 pigtail and transmit there is no effect - when I use the
copilot push to talk I get the same effects.

The FP sender gets power from the EIS 12V output not direct ship power,
grounds at forest of tabs and third wire inputs to EIS.

I am doing the conditional after a year of flying and have put off some
gripes, so I have planned to spend some time on things

Any ideas on how to limit this interference?

Thanks
Chris Lucas
RV-10
N919AR

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l.p(at)talk21.com
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PostPosted: Wed Nov 21, 2012 8:16 am    Post subject: HF antenna issues Reply with quote

Bob.
Many thanks for your reply, informative as always.

You're right of course ... if some gauges don't work while transmitting then that's not a real problem. And it's highly unlikely that I'll be transmitting on vhf and hf at the same time.

But my concern is that while transmitting on hf that the induced field may cause some more long-term effects in these electrical systems.
And also as the low end hf frequencies are getting closer to microwaves, I'm a little worried that the foam may get very hot and lose it's structural properties.
Am I worrying unnecessarily?

Either way, I shall do as you say ... install and test.
Many thanks again.
Patrick Elliott.
G-LGEZ
[quote][b]


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dlj04(at)josephson.com
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PostPosted: Wed Nov 21, 2012 10:26 am    Post subject: HF antenna issues Reply with quote

As Bob mentions, it is very likely that your HF installation will create
disturbances in other aircraft systems. Every transmission will be a
test, it's your responsibility to explore all frequencies you will be
using against all aircraft systems -- it's a large matrix. Changes of a
few kHz may cause huge differences in behavior as you excite one or
another tuned conductive element. Having designed and debugged a few of
these in light twins, I can offer some comments. First of all, you will
not be able to "shield" everything because the shields cannot enclose
the entire victim system. Near-field RF energy will couple into
everything conductive, and every "shield" will be some kind of antenna
for RF. Second, behavior will be very different at different operating
frequencies. You may find no disturbance at all at some frequencies and
significant disruption at others. There is no such thing as a "common
ground" here because all wires will have significant reactance at the
frequencies in use.

The most common HF radio systems for light aircraft for many years was
(may still be) Sunair. Their library of legacy product manuals is online
at www.sunairhf.com. Check out their older manuals for antenna tuning
units for good hints on how to do this. Try all the frequency bands
you'll be using, to see what disturbance is created to which systems.

You will be trying a new approach using the driven element in one wing
and the "counterpoise" in the other. Frankly this sounds like trouble,
especially in a fiberglass airplane with fuel tanks in the same wing.
There is little difference between something you call a counterpoise,
and for instance the nav light wiring, they all return to the ground
point of the antenna tuner. You will have high voltages at some points
on the driven element, and lots of opportunity for discharge through the
fiberglass in the vicinity of the tank. There can also be high voltage
nodes on the "counterpoise" and on every other conductive part of the
aircraft, because *all* parts of the aircraft will be part of the
antenna. In a Navajo I worked on, we had arcing around one of the flap
hinges at certain frequencies.

At the very least be sure there is a low inductance conductive path
completely enclosing the fuel tank (copper tape strips for instance)
that is bonded to the antenna tuner output (which will be the reference
ground point for the HF system.) At HF it doesn't have to be a solid
shield, but the openings in the mesh that you make need to be no more
than about 1/20 of a wavelength.


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nuckolls.bob(at)aeroelect
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PostPosted: Wed Nov 21, 2012 10:43 am    Post subject: HF antenna issues Reply with quote

At 10:15 AM 11/21/2012, you wrote:
Quote:
Bob.
Many thanks for your reply, informative as always.

You're right of course ... if some gauges don't work while
transmitting then that's not a real problem. And it's highly
unlikely that I'll be transmitting on vhf and hf at the same time.

But my concern is that while transmitting on hf that the induced
field may cause some more long-term effects in these electrical systems.

I've got no reason to believe this is likely.
It's VERY difficult to conduct significant energy
levels by radiation to cause damage to surrounding
components.

I remember one case at Boeing back about 1961 when
a B52 across the ramp from one I was working on radiated
the nose radar for a short period of time . . . that
took out the mixer diode in the radar receiver on the
airplane I was working on . . . or at least that was
the theory. The mixer diode was inop on a newly installed
TRX and the airplane opposite had the chin shield (harpex
lined) down for other maintenance issues. Never did really
find out for sure.

But that was 50KW pulses focused with narrow look-angle
antennas across a distance of perhaps 100 yards.

Quote:
And also as the low end hf frequencies are getting closer to
microwaves, I'm a little worried that the foam may get very hot and
lose it's structural properties.
Am I worrying unnecessarily?

Yeah. 99.99% of system integration grief is
limited to effects of one system degrading
performance of another system. The majority
of those don't present a hazard except for
interference with runway approach radios,
autopilots and electronic controlled fuel
injection/ignition systems.

It's probably unlikely that you'll be yakking
it up on 20m while on an coupled approach to a field
at minimums.
Bob . . .


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nuckolls.bob(at)aeroelect
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PostPosted: Wed Nov 21, 2012 11:21 am    Post subject: HF antenna issues Reply with quote

Quote:
You will be trying a new approach using the driven element in one wing and the "counterpoise" in the other. Frankly this sounds like trouble, especially in a fiberglass airplane with fuel tanks in the same wing. There is little difference between something you call a counterpoise, and for instance the nav light wiring, they all return to the ground point of the antenna tuner. You will have high voltages at some points on the driven element, and lots of opportunity for discharge through the fiberglass in the vicinity of the tank. There can also be high voltage nodes on the "counterpoise" and on every other conductive part of the aircraft, because *all* parts of the aircraft will be part of the antenna. In a Navajo I worked on, we had arcing around one of the flap hinges at certain frequencies.

D.L.'s misgivings stand on 80 years experience
and experiments to optimize HF performance on
airplanes while minimizing deleterious effects
to systems on board. A contemporary example presented
to your's truly about 8 years ago where an HF transceiver
installed on a Hawker 800 presented mysterious, inconsistent,
wildly variable problems that ranged from interference with
fuel level readings to total shutdown of all engine driven
power sources (including APU) while transmitting.

[img]cid:.0[/img]

The problem was 'mysterious' because this same configuration
of transmitter and receiver had been installed years gone
by with a tiny fraction of the problems.

The difference was a new antenna concept that abandoned
the older low current, high voltage wire antennas. Owners
begrudgingly tolerated those ugly wires so as to enjoy over-water
H.F. performance on their airplanes.

Needless to say, ALL were willing to pay killobux to have
an antenna integrated into the aircraft's structure. This
called for insulating the leading edge structure of the
vertical fin, grounding it to structure at the top and
exciting it with a very agile antenna tuner at the bottom.

Then the problems started. Bottom line was that aircraft
structure coated with primer and assembled with rivets
was never designed to be part of an antenna ground system.
Airplane to airplane variations produced an constellation
of effects that seldom repeated. Radiation levels in the
hell-hole were often high (200+ volts per meter) and
varied widely in terms of position within the hell-hole
and the excitation frequency.

I got to spend a week in the delivery center just before
xmas 2005 trying to keep the generators on-line on three
airplanes otherwise slated for delivery (before end of
year).

Discovered that legacy generator controllers were never
qualified to work in this new interference environment that
had not existed before the antenna change.

Managed to craft "special connectors"

[img]cid:.1[/img]

From RadioShack capacitors and Walmart epoxy that kept
huge levels of hell-hole RF from getting into the generator
controllers.

Didn't fix the original problem (which was never going to
be fixed) but it did get those airplanes flown away before
Dec 31.

I mention this to illustrate what appears to be an arcane
sifting of cause/effect to deduce the effective band-aid.
You won't know what the problems are until you test and
a really elegant solution may be unattainable.


Bob . . .


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