DHC OTA Members Meet: Kanata and Area - Page 4 - Canadian TV, Computing and Home Theatre Forums

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post #46 of 316 (permalink) Old 2011-11-22, 02:02 PM
 
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Nothing in ORLEANS then?

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post #47 of 316 (permalink) Old 2011-11-22, 04:12 PM
 
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Looks like there's going to be a pretty good turnout! Probably wouldn't fit in my livingroom this time!

Looking forward to seeing everyone.
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post #48 of 316 (permalink) Old 2011-11-22, 04:26 PM
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Feel free to organize one lima20.

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post #49 of 316 (permalink) Old 2011-11-23, 07:53 AM
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Thanks. The meeting last night was great.

First the discussion of difficulties of some in certain areas receiveing Global on CH 6. Possible reasons why.

Then:
The presentation by DXer (? I think) on NOISE really great and very relevent for reception.

(as mentioned last night, having a good noise margin is essential, esp. for the new Digital signals, need a good noise margin, and also for weaker far off signals - if you are trying to receive very weak, far off signals)

How the various sources of noise in your system add up - and what a good low noise a pre-amp can do for you.

And then ... GerryB 's self made low noise wide band VHF - UHF pre-amps - Awsome. With PHEMT (?) very low noise ( approx 0.6 dB noise) wide band RF amp.

I just reviewed the Forum thread now, on GerryB 's pre-amp. That is awsome work.

[ Just from memory, the Research Communications, very low noise pre-amps, from the U.K. - I have an R.C. pre-amp in service in Kingston - I think they mention that they use a similar device, a "PHEMT device" to achive very low noise factor for their preamps - similar noise values I think, around 0.6 db ]


anyway ... thanks. great meeting.
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post #50 of 316 (permalink) Old 2011-11-23, 09:08 AM
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Agreed! Very good meeting! It was great putting faces to names and I agree that DXer's presentation on noise was very good (filled in some holes in my understanding). Could you post your presentation notes somewhere?

Hind sight being 20-20 we should have had the presentation on noise first as it may have enhanced the discussion on reception of Global since we seem to agree that noise (though impulse not thermal) is such a major component of peoples reception issues.

Thank you very much DXer for organizing this meeting. Already looking forward to the next one.

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post #51 of 316 (permalink) Old 2011-11-23, 09:16 AM
 
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Wish i could have been there. Interested in seeing the notes, too
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post #52 of 316 (permalink) Old 2011-11-23, 09:47 AM
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About possible problems some may be having receiving Global Ottawa on ch 6.

It was mentioned at the meeting that Global Ottawa transmits in CIRCULAR polarization.

( Comment: Right hand or Left hand Circular polarization (?), electrical convention - is an important detail - if trying to receive a given signal with a circular polarized antenna. Your antenna design must match the polarization of the signal you are trying to receive - Circular Right, or Circular Left ... and you must get it correct based on how signal is transmitted, and the convention used. )

Then there was some side discussion about trying to take advantage of that ... circular polarization ... to try and improve Global reception.

Well .... I have been reading on the internet about Circular Polarization. Very interesting.

And that may indeed be a possibility.

Lots of interesting information on the internet about Circular Polarization - found just by searching.

But here is one, I found, some very practical information and ideas:

About CIRCULAR Polarization, and Antennas:

http://eb4dka.laserenadigital.com/Am...larization.pdf

So, I pose a question:

Could reception of Global 6 in Ottawa be improved by the use of a properly designed antenna to receive CIRCULAR polarization ?

(as hinted at ... mentioned ...in the meeting ?)
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post #53 of 316 (permalink) Old 2011-11-23, 10:04 AM
 
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This note might be relevant to circular polarization...

On my Subwoofer's troubleshooting page it's clear it generates FM noise. Furthermore, they not only mention to use Quad shield RG6 cable but also to "not use a folded dipole antenna" for FM.

model: Axiom EP-350
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post #54 of 316 (permalink) Old 2011-11-23, 11:20 AM
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flavoie, interesting (your name and issue came up in the meeting a couple times last night BTW ).

I would ask Axiom to be sure, but I would say that your problem is the sub-woofer radiating RF noise. In that case a power-line filter will not have any effect. The other option of course is that it radiates power-line noise and your household wiring is acting like an antenna and transmitting RF noise.

Regarding the use of Quad-Shield, I would say this is likely only necessary for cable that is physically close to the sub-woofer (even if it is inside the wall so check the route of your cable runs).

As for them not recommending the use of a folded dipole, I would assume what they really mean is don't use an indoor antenna close to the sub-woofer since most indoor FM antennas are folded dipoles and they have a reasonably wide beamwidth. Although a Yagi contains a folded dipole, I don't think that would count as it has a much narrower beamwidth.

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post #55 of 316 (permalink) Old 2011-11-23, 11:25 AM
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Roger, I wouldn't make any assumptions about an indoor directional antenna being able to "avoid" RF interference generated indoors, especially in a house with Aluminum siding, or other RF reflective sources. The RF noise from the subwoofer can be reflected all over the place, and enter the antenna from unexpected directions.
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post #56 of 316 (permalink) Old 2011-11-24, 09:27 AM
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We're going a little off topic ... but anyway ...

Sorry - But I'm going to be a little tough on the product / the sub-woofer.

This sub-woofer has AC power to it? And its own internal circuity (amplifier / and power supply) inside? Is that correct? [ so indeed ...it could generate this sort of interference - ex. switch mode power supply inside? ]

IF it is generating enough local RF noise and interference itself, to affect nearby FM or TV reception - that is not good.

My opinion ... the (correct) solution should be to stop the offending product (the subwoofer?) from generating or emitting that excessive noise or RF interference - in the first place.

O.K. - they're trying to be helpful and suggest a possible solution ...
BUT ...for the product manufacturer(?) simply to suggest: "do not use a folded dipole antenna" i.e. do not use any nearby indoor antenna

or alternately, to suggest(?) "use a better sheilded coax" [ie. use better sheilded coax cable to an antenna outdoors / furthur away from *our* interference generating product]

... is not a great solution to the problem.

My opinion:
Their product should not generate or emanate the offending interference in the first place. (there are standards and regulations for this sort of stuff)

Maybe they needed different design(s) or better sheilding, in key areas, or around key circuits or components in the first place.

A possible experiment:
So then, I am wondering, would some sheilding help? Sheild around the sub-woofer cabinet - or inside the cabinet, sheild around the power supply and/or other circuity?

Have they neglected to sheild around some noise generating circuity in the sub-woofer?

The solution to this problem may not be so easy to figure out ... but at least you think you've found the offending product. Can always turn it off, unplug it, completely disconnect it, and not use it when it's causing the problem.

But there's also the chance it's radiating troubles for others nearby - and that's not so great ...

Yes - contact the manufacturer directly and explain the situation with their sub-woofer product - maybe they know more about the specific problem now, and since then, have developed a solution which may help you.

(sort of like a product re-call solution - never know)

Axiom is a Canadian company - yes? Build quality speakers - yes?
They should be very interested to help you out.
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post #57 of 316 (permalink) Old 2011-11-24, 09:56 AM
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mrvanwinkles. I totally agree with you. I am surprised that a company like Axiom would have this kind of problem with their products. My opinion of them has gone way, way down with this revalation.

If they do the right thing and fix the problem rather than suggest band-aids, it would go a long way to mending their image IMO.

Axiom's marketing department would likely be interested that their good name is being trashed by this issue.

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post #58 of 316 (permalink) Old 2011-11-25, 06:38 AM
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Well - these problems can be complex - and it's hard to know exactly what is really causing this particular trouble - just here on the forum - so we need to be really patient and understanding with all, including the manufacturer.

Could be just one of their many products that has an issue.

Could be something totally different - not the product at all.
(I've read: "Check to see if your AC power supply has a proper ground connection, and if live and neutral are correct.")

Could be just one unit with a trouble.

Who knows what could really be the cause of this (?)
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post #59 of 316 (permalink) Old 2011-11-26, 08:31 PM
 
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Great meeting last week. Enjoyed the discussion and the presentations.

One thing I want to add to the problem of the long cable between the antenna and preamp is that it isnt just the cable loss - which can be high at high frequencies - I think - maybe even more importantly - it is the impedance matching and SWR. This is why I say use a piece of cable between the antenna feedpoint and the preamp that is much less than the shortest wavelength - so just a few inches if possible.

The reason I say this is that those baluns are notoriously inaccurate - as we know. If the impedance match isnt perfect, the mismatch will cause more problems in a cable that is long compared to a cable that is short. The signal will reflect up and down the cable multiple times, multiplying the original coaxial losses on paper. Im not sure exactly what the math is, but I know that once you go a full ring around a smith chart, at a full wavelength, there can be dire consequences for mismatched feeds. Ultimately you would want to couple the preamp directly to the antenna with 0 inches of coax.

Also, beyond the baluns, the impedance of the antenna itself at any one channel is also impossible to perfectly pinpoint. So use as short a coax as possible to connect a preamp to an antenna
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post #60 of 316 (permalink) Old 2011-12-02, 09:55 AM
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GerryB,

That is a very interesting point you mention in the previous post.

Re: Try to keep line length between Antenna and pre-amp to a minimum.

Yes, makes sense.

There's two parts of THEORY I remember from school.

1. About RF transmission lines. Mismatches cause higher VSWR's on a transmision line, leading to signal reflections, and to standing waves on the line, and to larger MAX's and Min's in voltage along the length of the transmission line. Points of high and low voltage along the TL - transmission line lead to higher losses.

And also, less power is tranferred to the load at the other end of the line.

(and they mentioned and showed us that energy is consumed and stored along the length of a transmission line - so keeping that first, critical TL, between your antenna and your pre-amp, as short as possible must be a good advantage)

2. About getting the maximum signal down the line - towards your receiver.
To transfer maximum energy (and signal) to the load - the characteristic impeadance of the load must be the same as the line ( ie. they must be MATCHED).

Actually, they showed us, with complex numbers, vectors/phasors, impeadance, reactance and reluctance ... that the load impeadance should be the conjugate of the source impeadance. (same resistance, opposite reactance ... something like that).


These impeadances vary with frequency. The antenna, the pre-amp, the transmission line.

So your point makes sense to me - and has possibilities for improvements in system efficiency - i.e. getting fewer losses, and more signal down the line.

That means, as you mention - keep the line short between the antenna and the pre-amp.

But it also means that that the input stage of the pre-amplifier (the "LOAD" in this case) has to match as closely as possible the impeadance of the TL and the ANTENNA - across the frequency band you're working.

So this all suggests a PRE-AMP design that could mount right on the antenna, right at the terminals, practically right beside the driven element, and match it as closely as possible - over the band.

So now you're looking at a pre-amp that's custom built for a given antenna - mechanically (mounting and position - and designed so that it does not interfere with the signals around the elements of the antenna - since it it so close) and electrically (to match close as possible).

If this can be accomplished - you can get some great performance out of your antenna / pre-amp combo .

The same holds after the pre-amp, down the line to the receiver ... but I am guessing it becomes less important, because after the pre-amp, you have already amplified the signal up enough that those losses and in-efficiencies become less important. You still want a match here .. but the one with the antenna is most important.

Just some ideas and thoughts.

It's all theory that I remember from E-SKOOL ... but it all makes sense.
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