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Author Topic: Radio Reception while transmitting  (Read 16443 times)

pilotart

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Radio Reception while transmitting
« on: January 23, 2010, 10:10:08 AM »

I have finished all the archived forum topics from 1998 to present.

Just one minor point that I can supply information on (to confirm that Ric's understanding was correct):
Quote
========================================================================
Date:         Fri, 27 Apr 2007 12:38:34
From:         Ric Gillespie
Subject:      Re: Betty's notebook

> From Hue Miller
>
>> From Ric
>> As far as we can tell from the available photos, the transmitter and
>> receiver in NR16020 were completely independent systems.  The
>> transmitter had the capability of using the same antenna as the
>> receiver but there is nothing connected to that terminal.  The lead
>> from the transmitter in the cabin goes to the dorsal vee antenna.
>> The lead from the belly wire antenna goes into the bottom of the
>> fuselage under the copilot's seat where the receiver is mounted.

>>
>>> If you accept that AE was listening at the same time the
>>> transmitter was
>>> on, you have no choice but to accept that there was a second
>>> receiver
>>> onboard.
>>
>> If it works for a second receiver it seems to me that it would work
>> for an independent single receiver.

>
> Well, it's time for me to dig out the schematics again. Every other aircraft
> had some way to ground the receiver antenna and switch off the receiver
> power, but AE's ship did not?

I'm a bit surprised that you've checked every other aircraft.

> When you have a wire antenna for receiver in proximity to a transmit
> antenna, this receiver wire picks up a LOT of voltage when the
> transmitter
> is on.

How close is "proximity."  The Electra's transmit antenna was
ballpark 8 feet from the receiving antenna.


> The circuit in the receiver steps this up - even without the amplifiers
> working in it - to hundreds of volts. The solution is a relay contact that
> grounds the receiver input circuit when the transmitter is on. This control
> wiring is controlled from the transmitter. If you lack this feature, you risk
> damaging the receiver, and you're treated to a blast in the earphones
> that will be like a slap to your ears. Also, how would the "transmit sidetone"
> work ( this is a little bit of your speech into transmitter fed into
> earphones, so you can hear what's being sent out - explanation for some of the
> readers ), work with the receiver being blasted by the transmitter?

I think we need to figure out how much "blasting" there would
actually be.  Perhaps Bob Brandenburg could offer an opinion.


> There's no avoiding this fact: AE did NOT receive anything when her
> radio transmitter was ON.

I think I'd like a second opinion.

> It doesn't matter if in the receiver - transmitter pair, the receiver is in
> the same box in your car's CB, or it's an Army receiver in a dugout a
> quarter mile away from the transmitter: the receiver antenna is grounded,
> audio is removed during transmit.

So "proximity" is within a quarter mile?  No receiver is going to be
able to receive if there's a transmitter functioning on the same
frequency within a quarter mile?
I fly a Conquest with the factory Collins Radio package, the only nav/com modification has been replacement of Collins Com 1 and Nav 1 with a Garmin 530PW Nav/Com.

Two Audio Panels and two microphones are installed and either left or right Audio Panel's can feed their audio from either com, but the left and right microphones/mike jacks and control wheel ptt's are isolated to their Audio Panels.  Each Audio Panel can use any com for selecting its mike to transmit.

As a 'single-pilot' I will often have my headset listening to both coms, and the boom mike for com 1 while the second Audio Panel has its mike set for com 2. 
This way I can monitor and speak to Air Traffic Control on com 1 while using the com 2's hand mike to call arinc or unicom.  This transmission will block reception from com 2 but will not effect com 1.

The antenna for com 1 is on top of the cabin while com 2's antenna is below the cabin, transmitting on one will not affect the other (unless they were tuned to the same frequency and then it would be received by the other com).

This is not to say that there can be any comparison between the 1936-7 Electra (no Audio Panels needed) installation and my 1985-'05 avionics package, but to say that you can transmit with one radio without interfering with reception on a second receiver in the same aircraft.  This had held true using two Collins Com's prior to the the Garmin 530PW install as well and the 'split' between LF/MW/HF simultaneous frequency's would usually be greater than frequency's within today's VHF range.

I would expect this capability to be common in any aircraft set up for two pilots (with two Audio Panels), to allow for simultaneous communications using com 1 and com 2 on different frequency's.
_____________________________________________________________________________

In the case of Electra #1055 with the one transmitter and the one seperate receiver, they would have logically been able to operate simultaneousely without interference on different frequencies.  As I understand the aircraft radio methods of that era, it was common to transmit to a ground station on a 'calling' frequency and receive a reply on their (different) assigned frequency. 

On that unfortunate morning of July 2nd, the ideal would have been to tune the receiver to Itasca's 425KHz CW/MCW LF Transmiter using Electra's Loop for direction (and a 90° heading change for a brief period would confirm the direction and supply a good distance estimate).  Using the transmitter on 3105KHz would have allowed two way communications without having to retune the receiver, just by turning the loop 90° out of null point.  Less ideal, but if you wanted to accomplish a DF Approach, you could transmit continuous 'A's on 500KHz and listen for Heading Corrections on Itasca's 3105KHz MCW Channel.

So close and yet so far away; for the lack of a Current IFR Ticket and perhaps an ATC/RDF Specialist in the Radio Room (and, of course that missing primary receiver antenna).

Art
Art Johnson
 
« Last Edit: March 01, 2012, 05:52:12 AM by pilotart »
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Ted G Campbell

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Re: Radio Reception while transmitting
« Reply #1 on: February 25, 2012, 06:59:55 PM »

For the navigation, radio and fuel consumption gurus out there.  Can we pin down an area where AE and FN may have been when AE transmitted “we must be on you but we can not see you”?

My suggestion is to work backward from the TIGHAR hypothesis that they ended up on Niku.

How?

1.   Draw a series of S5 signal strength circles around Howland Is. i.e. this gives us a probable point of entry into the intended landing destination.
2.   Draw a series of parallel 337-157 lines that are “visual distant apart at 1000 feet altitude”.  These lines will be drawn to the East and West of Niku (remember we are assuming that TIGHAR’s hypothesis is that they landed on Niku).
3.   Draw a line perpendicular to the 337-157 line that is “4 hr. max fuel consumption” North of Niku (see TIGHAR’s note in 2 above)
4.   Draw a series of lines perpendicular to the 337-157 lines suggested in 2 above i.e. at “1000 foot visual range” South and North of Howland.  Remember they did not see Howland.
5.   Draw a series of lines perpendicular to the 337-157 lines North and South of Baker Is.  Obviously they didn’t see Baker either or they would have known how to navigate between the two islands i.e. Baker and Howland.

Within these series of circles and parallel lines can we shade in an area of high probability of the flight path entry into the Howland Is. area?  If so does this lessen or strengthen either the “crash and sank” or the TIGHAR hypothesis?
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JNev

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Re: Radio Reception while transmitting
« Reply #2 on: April 03, 2012, 05:49:46 PM »

Maybe you've seen it, but if not, take a look - the Monte Carlo is the best way I know of -

I like your idea, Ted, but have no inkling of how it can approach the millions of permutations processed by the Monte Carlo analysis.

The Monte Carlo scatter brings that position with some fair probability (some of us believe) into an area somewhat around 150 - 200 miles SW of Howland if I am recalling how to read the chart correctly (my old eyes don't 'cipher the fine print so well). 

Of course even the mightly Monte Carlo depends on human-imposed constraints - from people who weren't on the flight.  But, those latter-day humans may have understood flight conditions about as well as those on the flight, given all things human - and what actually happened (no arrival at Howland) - which may mean Dr. Jacobson is no more lost than AE and FN were, and therefore very possibly on the mark as to where they were lost... and now I've lost myself.

But, how much faith one puts in the Monte Carlo therefore largely has to do with how much confidence they have in the assumed constraints.  Personally I think it's the best method of 'working backward' or trying to do a stubby-pencil flight recreation we have to-date.

What I find hard to argue with is the incredibly large number of permutations the computer grinds through to get to this - and that it doesn't attempt to 'pinpoint' anything, just present a workable scatter of probabilities: look at the 'cloud' and work the possibilities from there. 

And, personally, I believe Dr. Jacobson did a very reasonable job at establishing the necessary constraints, but YMMV as others differ on just what should be imposed for various reasons.  He is an extremely thorough, thoughtful and capable man - take a look at his databases... think of what went into putting all that together.

LTM -
- Jeff Neville

Former Member 3074R
 
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Gary LaPook

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Re: Radio Reception while transmitting
« Reply #3 on: April 03, 2012, 10:34:18 PM »

Maybe you've seen it, but if not, take a look - the Monte Carlo is the best way I know of -

The Monte Carlo scatter brings that position with some fair probability (some of us believe) into an area somewhat around 150 - 200 miles SW of Howland if I am recalling how to read the chart correctly (my old eyes don't 'cipher the fine print so well). 

But, how much faith one puts in the Monte Carlo therefore largely has to do with how much confidence they have in the assumed constraints. 
LTM -
I'm not going to repeat everything I have said about the Monte Carlo simulation except to remind you of two points. First, the simulation started with a completely unrealistic assumption that no navigation was done en route, that it was DR all the way. This is contrary to the reason for having a navigator aboard and contrary to Earhart's statement that "Noonan must have star sights."

Secondly, if the plane ended up in the area predicted by the simulation then flying on a 157° course from there would have caused the plane to miss Gardner my many miles to the west so the Monte Carlo does not support the Gardner hypothesis.

gl
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Gary LaPook

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Re: Radio Reception while transmitting
« Reply #4 on: April 05, 2012, 02:48:58 AM »


As to where the Monte Carlo placed the flight at the 1912 call, well, back to the 'blue lagoon'.  You yourself noted in a somewhat recent post that you finally did agree that the flight terminated about where the Monte Carlo says (I'll have to dig it out but I remember it well), but that there was no way the flight could have continued and found Gardner.  I also disagree with that.  Gardner is a large, bright affair compared to the likes of Howland or Baker and would be visible for many miles


O.K. put a number on it. From how many miles away do you believe that Gardner was visible based on the conditions that existed near that island on July 2, 1937.

gl
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Heath Smith

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Re: Radio Reception while transmitting
« Reply #5 on: April 05, 2012, 03:48:58 AM »


If they ended up 150-200 miles SW of Howland the skies to the East should have been completely clear to the horizon. FN knew the time of sunrise at Howland and should have been able to easily verify where they were longitudinally with only the sunrise and nothing else other than his chronometer.

While I commend the effort of creating the Monte Carlo Simulation, I agree with Gary that it completely negates the skills of FN on board the plane.

Now if it were only AE flying with an auto-pilot, the results would make sense.
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Martin X. Moleski, SJ

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Re: Radio Reception while transmitting
« Reply #6 on: April 05, 2012, 07:55:08 AM »

While I commend the effort of creating the Monte Carlo Simulation, I agree with Gary that it completely negates the skills of FN on board the plane.

All that the Monte Carlo simulation does is play with various interpretations of reported positions.  Since Amelia did not provide a time-stamp with her partial position reports, there is a broad path of possibilities from one position report to the next.  I imagine that Jacobson would not deny that Fred may have done more navigating other than what was reported by Amelia and logged by the Itasca, but he stuck to what we know from the logs.

People can make up new position "reports" based on what they think Fred could have, would have, or should have done, but they should clearly flag those imaginary "fixes" as imaginary.  Once we cast Fred in the role of our idealized (and imaginary) navigator, we can get him to go just about anywhere we want him to.  Gary makes his imaginary Fred conduct a search box in the vicinity of Howland that fails to find Howland, runs the plane out of gas, and leaves the remnants underwater.  TIGHAR's imaginary Fred searches to the southeast of Howland (running "south" on 157-337).  It may well be that having gone south for a time, they then set up a search box that led them to Gardner. 

If the plane is found underwater near Howland, that would make Gary's reconstruction of the flight more probable than TIGHAR's.

If remnants of the plane are found near Niku, that would make TIGHAR's reconstruction of the flight more probable.

All of the post-loss transmissions must be hoaxes or errors if Gary's reconstruction is correct.

If any of the post-loss transmissions are authentic, then the most likely place for them to have originated is Niku.  There aren't a lot of other places for the plane to land, transmit, and not be found afterward other than Niku. 
LTM,

           Marty
           TIGHAR #2359A
 
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JNev

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Re: Radio Reception while transmitting
« Reply #7 on: April 05, 2012, 12:05:46 PM »

O.K. put a number on it. From how many miles away do you believe that Gardner was visible based on the conditions that existed near that island on July 2, 1937.

gl

I don't need to - all I need to do is ask a chap who has flown to ocean islands a number of times in his flying career -

Gary,

From how far away did you see Grand Bahama after you left the Florida east coast?  VFR?  Conditions?  Altitude?

---

Grand Bahama is bigger than Gardner - but Gardner was no small target compared to say, Howland.  It had (and has) a big bright lagoon nested within a ring of coral - and at the time had 90 foot trees in forestation along the northern shore, etc.

LTM -
- Jeff Neville

Former Member 3074R
 
« Last Edit: April 05, 2012, 12:31:56 PM by Jeff Neville »
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Martin X. Moleski, SJ

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Re: Radio Reception while transmitting
« Reply #8 on: April 05, 2012, 01:29:05 PM »

O.K. put a number on it. From how many miles away do you believe that Gardner was visible based on the conditions that existed near that island on July 2, 1937.

I don't have a number.

And I don't know what the conditions may have been near Gardner on that date.

It does not seem to me to be a big stretch to say that Niku should be easier to spot from the air than Howland.  Here is a comparison of Niku and Howland from the same eye-altitude, using Google Earth.  I took it from directly overhead each; I suppose it would be more interesting to do a view from the same altitude and distance, but I don't know how good a representation that would be of real-world optics.  The point I wanted to make, as others have, is that the lagoon seems to me to be something that would catch someone's eye.  The lagoon itself seems to me to be twice the area of Howland; then you've got all of the other features mentioned by others that help make Niku stand out besides.

LTM,

           Marty
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« Last Edit: April 05, 2012, 01:33:41 PM by Martin X. Moleski, SJ »
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John Ousterhout

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Re: Radio Reception while transmitting
« Reply #9 on: April 05, 2012, 10:33:17 PM »

Thanks for the clear photos Marty.  They do, indeed, help show the difference between the sizes and colors of the islands.  But, (you knew there was going to be one), a more valuable comparison would be the views from ~1000 feet altitude under scattered clouds from some different distances and directions.  I would like to know what each looks like from 15 miles, 10 miles and 5 miles.  Throw in a modest-sized ship belching a smoke trail, and early to mid-morning angled sunlight at Howland to complete the picture. 
I still have this uneasy feeling that Howland would be quite difficult to distinguish from a cloud shadow at just 5 miles.

When I've been unable to spot an unfamilar airfield at a distance of just a mile or two, I've used the radio to resolve the confusion.  Why didn't AE?
Cheers,
JohnO
 
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Thom Boughton

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Re: Radio Reception while transmitting
« Reply #10 on: April 05, 2012, 11:06:21 PM »

.....The point I wanted to make, as others have, is that the lagoon seems to me to be something that would catch someone's eye.  The lagoon itself seems to me to be twice the area of Howland; then you've got all of the other features mentioned by others that help make Niku stand out besides.........


Couple these pictures with the 2001 Tighar video "An Aerial Tour of Nikumaroro", where one can get a good feel for the truly limited size of the Niku lagoon....and you see just how tiny Howland is, and just how unlikely it was that it would have been found without DF.

Fascinating.


      .....TB
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Gary LaPook

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Re: Radio Reception while transmitting
« Reply #11 on: April 07, 2012, 11:49:47 PM »


[A]ll I need to do is ask a chap who has flown to ocean islands a number of times in his flying career -
Gary,
From how far away did you see Grand Bahama after you left the Florida east coast?  VFR?  Conditions?  Altitude?
---

Grand Bahama is bigger than Gardner - but Gardner was no small target compared to say, Howland.  It had (and has) a big bright lagoon nested within a ring of coral - and at the time had 90 foot trees in forestation along the northern shore, etc.

LTM -
It never occurred to me at the time to carefully note how far away islands were when I first spotted them so that I would have data for discussing the disappearance of Earhart forty years later. But I do have some clear memories relating to this. I checked my log books and found that the first time I left dry land behind was on February 16, 1971 when I had a total of only 62 hours in my logbook. I took off from Fort Lauderdale and flew over Bimini and continued on to Nassau. (Bimini is a closer match, size wise, to Gardner than is Grand Bahama.) It is 53 NM from Fort Lauderdale International to Bimini. Being a newbie I was nervous so did the calculation that newbies do and calculated my "wet footprint," the part of the flight that I would not be able to glide to land, either back to Florida or ahead to Bimini, if the engine of my Cherokee 140 quit. Most light planes have a glide ratio of about ten to one, meaning they will glide forward ten miles for every mile of descent. I planned to fly at 7,500 feet, which is one and a quarter nautical miles high, so the plane would glide 12.5 NM in either direction leaving only 28 NM in the middle, 14 minutes of flying time, that I couldn't make it to dry land. I carefully calculated the time to the mid-point so that I would know if I should turn around or glide ahead if the engine quit. I took off and climbed to 7,500 and nervously scanned ahead for the first sight of Bimini. The midpoint came and went and I was even more nervous, worrying if I was headed off course since I hadn't seen Bimini yet. I don't know exactly how many minutes I was past the midpoint before I was able to take a deep breath after finally seeing LAND ahead, but it was well past the midpoint so was only 20 or possibly  15 NM from Bimini. I flew that same route many times after that but I did not get worried because I did not expect to be able to see Bimini until past the midpoint and I never did. (Here in California, most pilots flying out to Catalina Island fly high enough so that they have no wet footprint since it is only 26 miles out to that island.)

Size of the island is not that important in spotting them, if they are bigger than a certain minimum size then other factors become the limiting condition. If the island is beyond the horizon from your altitude then you will never spot the land even if it is a continent. If the land is on your side of the horizon then the prevailing visibility becomes limiting. You could be standing in New Jersey on the west bank of the Hudson river and never see Manhattan island if the visibility is poor enough, such as in fog. In the Gardner area the visibility is less than 25 NM 85% of the time in July and less than 10 NM 35% of the time. So even a 3.9 NM long island with a pretty lagoon (as seen from space) will not be visible more than 25 NM away 85% of the time so there is only a 15% chance that Earhart could have spotted it further away than that. The visibility graph only goes up to a maximum of 25 NM but even if the visibility does exceed 25 NM it won't be by much since the line on the graph is going almost straight up where it hits the 25 NM line. Some islands can be seen farther away than the prevailing visibility but these islands, such as Hawaii and Pico in the Azores, are many thousands of feet tall, so tall that they extend well above the haze level, up into the clear air where visibility is unlimited. Gardner is a very low island. It is similar to landing when there is a thin layer of ground fog. Looking straight down on the runway from high above you don't even notice the ground fog but when you are about to touch down the whole world disappears as you are now looking forward, not down, through the layer of fog. For those who have not tried landing in this situation, the next time you have a clear night and a layer of ground fog develops look up and you can see the twinkling stars clearly but look horizontally and the street light at the end of the block has disappeared.

I have attached a photo of Bimini from about 10 NM west and it also has a large shallow lagoon like area between North and South Bimini which is larger than the lagoon on Gardner. Which is the easier thing to spot, the dry land or the shallow, lagoon-like water between? The visibility conditions shown are the norm, limited visibility below the haze level and clear above. I have attached three Google Earth images of Bimini, Gardner and Howland all at the same scale. Bimini is 5.5 NM accross, Gardner is 3.9 NM and Howland is 1.5 NM.

If you were looking for Gardner from a satellite then that lagoon looks easy to spot but from an airplane, not so much. So the space invaders from Mars are more likely to attack Gardner than Howland so Howland would be a safer place for you to hide in order to avoid having your brains sucked out by the Martians.

gl
« Last Edit: April 08, 2012, 06:27:57 PM by Gary LaPook »
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