253 replies to this topic

[alanh]

James,
Try it for yourself

Jrp,
Glass is an insulator and the polarisation is rotated 90 degrees.
What does phase shift have to do with it. This only occurs if the speed and direction changes such as when it enters a more dense material such as glass. If the wave is reflected from the front surface therein no change in speed, just direction.

AlanH

[James T Kirk]

alanh, on Feb 8 2011, 02:33 PM, said:

James,
Try it for yourself

Jrp,
Glass is an insulator and the polarisation is rotated 90 degrees.
What does phase shift have to do with it. This only occurs if the speed and direction changes such as when it enters a more dense material such as glass. If the wave is reflected from the front surface therein no change in speed, just direction.

AlanH

AlanH
I need not, I understand what has occurred and clearly described to others how to replicate it. It was just your poor grasp of the possible variables that created your confusion and misunderstanding of what was happening.

Jrp001
Can I suggest just looking at the statements made such as "Glass is an insulator and the polarisation is rotated 90 degrees" and just do a little checking if you are unsure yourself.
Respond to AlanH as you see fit however I suggest you be sure to watch out for the inevitable point of diminishing returns.

James

[jrp001]

alanh, on Feb 8 2011, 02:33 PM, said:

James,
Try it for yourself

Jrp,
Glass is an insulator and the polarisation is rotated 90 degrees.
What does phase shift have to do with it. This only occurs if the speed and direction changes such as when it enters a more dense material such as glass. If the wave is reflected from the front surface therein no change in speed, just direction.

AlanH

alanh you said there was a polarisation shift from horizontal to vertical from the reflection off a flat brick wall in other post.

I said there would be a phase shift but no polarisation change.

What is your wall made of glass now.

Edited by jrp001, 08 February 2011 - 07:06 PM.

[alanh]

Jrp,
I don't know why you are worried about phase changes. The reason why I mentioned glass, is because it is not conductive and you maintained were talking about the need for conductive surfaces.

AlanH

[James T Kirk]

jrp001, on Feb 7 2011, 03:22 PM, said:

alan your link explains polarisation of unpolarised light & how it is done.(It's splittting the polarisations not shifting them)

You were talking about polarisation shift from horizontal to vertical of a polarised signal, from a reflection off a brick wall.

No i don't think this is true

I'm not saying it can't happen, say off a ruggerd mountain but not off a flat brick wall.

Phase shift by an amount depending on the conductivity of the reflector, but not polarisation shift

Jrp001

Your understanding of what happens when a signal strikes some flat face and reflects, the importance of the resulting change in phase and unchanged polarity and subsequent questioning of AlanH is quite sound.

James

[jrp001]

alanh, on Feb 8 2011, 07:14 PM, said:

Jrp,
I don't know why you are worried about phase changes. The reason why I mentioned glass, is because it is not conductive and you maintained were talking about the need for conductive surfaces.

AlanH

Alanh my 2nd post was a reply to your brick wall theory & the polarisation shift of a horizontal polarised signal.

The only shift would be phase not polarisation off your brick wall.

I will be sure to do a print out of that and call it.........ALANH'S BRICK WALL THEORY

I will make sure i file it right up there next to my.....RADIO COMMUNICATIONS & ANTENNA ARRL HANDBOOKS.............NOT.

gumby...........mmmmmmmmm maybe we would get more sense out of gumby

[mtv]

jrp001, on Feb 9 2011, 08:39 PM, said:

gumby...........mmmmmmmmm maybe we would get more sense out of gumby

Quite likely.... Gumby is flexible.  

[MLXXX]

James T Kirk, on Feb 7 2011, 08:37 PM, said:

Alanh

I believe you may have found the levels you describe but not remotely for the reasons you describe.
You have deceived yourself by forgetting antenna basics and typically not being sufficiently self critical or investigative before going to print.

What you describe has happened very simply and would have occurred if the brick wall was there or not.

AlanH, you appear once again to have become caught up in an assumption of your own making.  My quick survey of material on the web relating to radar reflections reveals that:

• a flat surface or sphere will reflect a linearly polarized wave with the same polarization as received
  
• reflections will reverse the sense of circular polarization.

See for example: http://www.rfcafe.co...olarization.htm and http://www.ccrs.nrca...apter5/01_e.php

There are many references on the web to phase changes when radio waves reflect off hard surfaces. And there are many references to polarization changes when radio waves interact with the ionosphere. But there are no references I have found, in a quick survey, to a flat surface giving the effect you have claimed (for orthogonally incident radiation).

In the circumstances, will you either:
{a} supply a link to a webpage that supports your claim that horizontally polarised VHF waves when reflected back towards the transmitter by a brick wall become instead vertically polarised VHF waves ; or,
{b} retract the claim?

Edited by MLXXX, 09 February 2011 - 10:23 PM.

[M'bozo]

With the advent of DTTB, it has been possible to achieve rather good results utilising reflections from surrounding features, particularly when the direct path is almost completely obstructed.

For the ones done where success has been achieved, apart from some lumpiness in the multiplex, I have not observed a polarisation change in the reflected signal.

Therefore, for me, this myth is busted*!

Edited by M'bozo, 10 February 2011 - 03:29 PM.

[alanh]

MLXXX,
As I said try it for yourself. A pair of polaroid glasses will produce the same effect on light.

Alanh

[MLXXX]

alanh, on Feb 10 2011, 12:34 AM, said:

MLXXX,
As I said try it for yourself.

Did you do the experiment without the wall, in line with James T Kirk's observation?

What I'm looking for is confirmation of the theory you espoused of rotated polarisation, not confirmation of the measurements you took of signal level.

Quote

A pair of polaroid glasses will produce the same effect on light.

You could test what happens when linearly (not circularly) polarised projected light is reflected off a mirror (or silvered projection screen) back towards the projector.

I think you have been misled by reading about what happens when light is reflected at a shallow angle off a mirror, or what happens with radio waves being bent by the ionosphere.

Your experiment involved the plane of the wall being at approximately 90 degrees to the direction of arrival of the radio waves, i.e. a 90 degree angle of incidence, not a shallow angle of incidence.

Edited by MLXXX, 10 February 2011 - 01:03 AM.

[alanh]

MLXXX,
I was only mentioning shallow angles because of the critical angle is required for light. This is because the thickness of glass allows the light to be bent going through the front surrace and te out of the other side of the glass before it hits the mercury coated surface. This is not the case with the RF case. Its the same as if the you used a front silvered mirror.

I did not use a very acute angle.

AlanH

[James T Kirk]

If the angle wasn't very acute, acute meaning between 0-90 degrees, do you consider the angle obtuse?
(NOTE TO SELF: I resisted, yay!! how's that for personal discipline, I didn't use the other meaning of obtuse)

James

Edited by James T Kirk, 10 February 2011 - 08:40 AM.

[jrp001]

Keep digging that hole alanh

[Jeffry]

Can I revert to something mentioned some time ago here. Masthead amplifiers.

I don't want to revise the debate on screened/unscreened, internal filters, gain etc. but just to make an observation.

I have compared many brands and models of amplifiers in recent months and confirmed the bleeding obvious; the better (lower) the noise figure, the better the end result.
GME Kingray MDA series amplifiers I have found to have superior noise figure to any other amplifier I have compared them with {and that includes the Hills LNDA, which is not as good}. Sure, they are lower gain than most but, if you really, really need more signal level then any cheap in-line amplifier will give you that. Main disadvantage where I am is that most installations are seperate B3 and B4+ antennas and MDA doesn't have an internal diplexer. I have found that a good, screened, F connector diplexer is fine with negligible affect on overall signal quality and not very expensive in the overall scheme of things.

Jeff

[mtv]

Jeffry, on Jul 19 2011, 03:43 PM, said:

Can I revert to something mentioned some time ago here. Masthead amplifiers.

I don't want to revise the debate on screened/unscreened, internal filters, gain etc. but just to make an observation.

I have compared many brands and models of amplifiers in recent months and confirmed the bleeding obvious; the better (lower) the noise figure, the better the end result.
GME Kingray MDA series amplifiers I have found to have superior noise figure to any other amplifier I have compared them with {and that includes the Hills LNDA, which is not as good}. Sure, they are lower gain than most but, if you really, really need more signal level then any cheap in-line amplifier will give you that. Main disadvantage where I am is that most installations are seperate B3 and B4+ antennas and MDA doesn't have an internal diplexer. I have found that a good, screened, F connector diplexer is fine with negligible affect on overall signal quality and not very expensive in the overall scheme of things.

Jeff

Jeff,

I agree.... and realistically, the output level of the MDA series is usually more than sufficient in most situations.

[alanh]

MTV,
GME Kingray MDA20H contains a 174 MHz (channel 6) high pass filter and will remove a lot if potential interference from impulse noise, radio transmitters and analog TV transmitters below channel 6.
MDA20U contains a 520 MHz (channel 27) high pass filter, which will remove any high powered VHF and 400 MHz band two way radio transmissions.

To compare like with like, you would have to compare an MDA20L (which contains a 44 MHz (channel 0) high pass filter, with other wideband masthead amplifiers of a similar gain to determine the effect of a low noise figure.

I agree that low noise figure is advantageous, because it minimises internally generated noise, but it has no effect on noise sources coming in from the antenna.

Compare MDA20H to the following amplifiers fed via an FL3BPMH set to only V_h and U into MHW24FSE with each of the following in turnMHW24FS
This is to see what is more important. Try in the Penrith region.

AlanH

[mtv]

alanh, on Jul 20 2011, 01:42 AM, said:

MTV,
GME Kingray MDA20H contains a 174 MHz (channel 6) high pass filter and will remove a lot if potential interference from impulse noise, radio transmitters and analog TV transmitters below channel 6.
MDA20U contains a 520 MHz (channel 27) high pass filter, which will remove any high powered VHF and 400 MHz band two way radio transmissions.

To compare like with like, you would have to compare an MDA20L (which contains a 44 MHz (channel 0) high pass filter, with other wideband masthead amplifiers of a similar gain to determine the effect of a low noise figure.

I agree that low noise figure is advantageous, because it minimises internally generated noise, but it has no effect on noise sources coming in from the antenna.

Compare MDA20H to the following amplifiers fed via an FL3BPMH set to only V_h and U into MHW24FSE with each of the following in turnMHW24FS
This is to see what is more important. Try in the Penrith region.

AlanH

I am fully aware of the various models in the MDA series and bands they are designed for.

[James T Kirk]

mtv, on Jul 20 2011, 03:22 AM, said:

I am fully aware of the various models in the MDA series and bands they are designed for.

Hi MTV
Can you imagine alanh while visiting his GP?

alanh
"Doctor, my backside is sore when I sit, particularly when I go through the motions"

Doctor
"I suspect you may have hemorrhoids"

alanh
"Doctor, Hemorrhoids are painful, swollen veins in the lower portion of the rectum or anus.
This condition is very common, especially during pregnancy and after childbirth. Hemorrhoids result from increased pressure in the veins of the anus. The pressure causes the veins to bulge and expand, making them painful, particularly when you are sitting.
The most common cause is straining during bowel movements. Hemorrhoids may result from constipation, sitting for long periods of time, and anal infections. In some cases they may be caused by other diseases, such as liver cirrhosis.
Internal hemorrhoids occur just inside the anus, at the beginning of the rectum. External hemorrhoids occur at the anal opening and may hang outside the anus.

Doctor
"I am fully aware of the various symptomsand indications of hemorroids and the recommended treatments."

alanh
"Doctor, regarding treatments, over-the-counter corticosteroid creams can reduce pain and swelling. Hemorrhoid creams with lidocaine can reduce pain. Witch hazel (applied with cotton swabs) can reduce itching. Other steps for anal itching include, wear cotton undergarments, avoid toilet tissue with perfumes or colors, try not to scratch the area, sitz baths can help you to feel better. Sit in warm water for 10 to 15 minutes. Stool softeners help reduce straining and constipation.
For cases that don't respond to home treatments, a surgeon or gastroenterologist can apply heat treatment, called infrared coagulation, to shrink internal hemorrhoids. This may help avoid surgery. Surgery that may be done to treat hemorrhoids includes rubber band ligation or surgical hemorrhoidectomy. These procedures are generally used for patients with severe pain or bleeding who have not responded to other therapy.

Doctor
"sighs and eagerly awaits his next patient who he knows has a horribly offensive smell and needs a boil lanced"

James

[mtv]

James,

That's certainly one scenario, however, I would be more inclined to think alanh would insist what condition he had at the outset, rather than allow the doctor to make his own qualified diagnosis and then refuse to accept the doctor's diagnosis if it differed even slightly from his own diagnosis.

All rather amusing when you consider a common treatment for a painful a-h, is.... Preparation 'H'.

If only there were an e-version we could apply.  

[alanh]

MTV,
I assumed you know about the versions of MDA, I was really on about the noise figure. The end result is that what ever the noise figure is it will make the C/N ratio worse by this figure when it gets to the receiver. If the C/N at the wall plate is within a few dB of the AS1367 specification it can get you over the line.

AlanH

[alanh]

MTV,
I assumed you know about the versions of MDA, I was really on about the noise figure. The end result is that what ever the noise figure is, it will make the C/N ratio worse by this figure when it gets to the receiver. This also applies to the MER. The only variation on this is the Noise Figure of the receiver can change if the receiver gets a weak signal and it will increase its gain then the noise may increase. If the C/N at the wall plate is within a few dB of the AS1367 specification it can get you over the line.

AlanH

[mtv]

alanh, on Jul 20 2011, 07:37 PM, said:

MTV,
I assumed you know about the versions of MDA, I was really on about the noise figure. The end result is that what ever the noise figure is, it will make the C/N ratio worse by this figure when it gets to the receiver. This also applies to the MER. The only variation on this is the Noise Figure of the receiver can change if the receiver gets a weak signal and it will increase its gain then the noise may increase. If the C/N at the wall plate is within a few dB of the AS1367 specification it can get you over the line.

AlanH

Once again, I am fully aware of how it all works.

[MLXXX]

I note that alanh's posts #221 and #222 duplicated each other originally. [In the time I've taken to type this, alanh has done an edit of the second post, but suppressed the appearance of the "edited" tag, as is his wont.]

Perhaps it should be suggested to the designers of the software used by this site to incorporate an "Are you sure?" question, when a user is resubmitting an identical post within a few minutes.

In an on-line banking context, you can pay a bill twice if you're not careful!

[James T Kirk]

alanh, on Jul 20 2011, 07:37 PM, said:

MTV,
I assumed you know about the versions of MDA, I was really on about the noise figure. The end result is that what ever the noise figure is, it will make the C/N ratio worse by this figure when it gets to the receiver. This also applies to the MER. The only variation on this is the Noise Figure of the receiver can change if the receiver gets a weak signal and it will increase its gain then the noise may increase. If the C/N at the wall plate is within a few dB of the AS1367 specification it can get you over the line.

AlanH

Here in lies another problem with alanh, he never catches on that people with greater knowledge and experience are not seeking his input.

In this uncontrollable zeal to say more, as is typical, he searches for justification and just gets it so wrong again.

(eg) "The end result is that what ever the noise figure is, it will make the C/N ratio worse by this figure when it gets to the receiver." (This is simply wrong and the C/N degradation is level dependent)
(eg) "This also applies to the MER." (This is also wrong for the same reasons)

Alanh, just stop all this and we can stop showing you up because we won't need to protect the reader.

James