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1. 03:04 AM - Re: coax (Brian Lloyd)
2. 03:10 AM - Re: coax (Brian Lloyd)
Message 1
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--> Avionics-List message posted by: Brian Lloyd <brianl@lloyd.com>
On Jan 4, 2005, at 1:11 AM, Ron Davis wrote:
> --> Avionics-List message posted by: "Ron Davis" <l39parts@hotmail.com>
>
> I was working under the following three assumptions:
>
> 1) People capable of designing a sophisticated device like a Garmin
> 530
> would have the knowledge, skills, and facilities to determine what
> cable is
> appropriate.
Surprisingly, this often isn't the case. An electrical engineer capable
of designing a GPS receiver probably has little or no experience with
various types of wire in the field, their longevity, and their failure
modes. In fact, the engineer probably had nothing to do with the
specification of coax in the manual and probably had nothing more to
say about it than when the tech pubs guy came by and said something
like, "hey Ralph, I have to put some sort of coax spec in here and
George said any 50 ohm coax would do. Is that right?" I would bet that
Ralph then said something like, "yeah, sure," and that was the end of
it. No more thought given.
> 2) Firms selling GPS equipment would have a vested interest in it
> working
> well so the customers would be happy and they would write the
> installation
> manual toward that goal.
Yes, you would think so, wouldn't you. Having worked in a number of
high-tech companies building bleeding-edge products I can attest that
it often never crosses anyone's mind.
> 3) Manufacturers err, when they err at all, toward the expensive,
> unnecessary side of the equation. Use only Mopar oil filters for your
> new
> Chrysler, etc...
That usually comes from the marketing department, not engineering,
since the Mopar filters are probably OEM'd from Fram anyway.
> PVC-insulated wire would be cheaper, but the manual calls for Tefzel.
> I use
> shielded wire where they specify it, and unshielded where they don't
> specify
> shielded. I guess you could use shielded for all of the wires if it
> makes
> you sleep better.
Nope. Sometimes unshielded is better than shielded. But that isn't the
point. My point was that RG-400 is to RG-58A/U what tefzel is to PVC
wire. If you feel you need the qualities of tefzel in your airplane you
should apply that to your coax as well.
> Adequate is good enough, and better than adequate is good enough, and
> costs
> more.
And my experience with various types of coax in various harsh
environments leads me to believe that RG-58A/U low-loss foam dielectric
coax is no more suitable for an aircraft environment than is PVC wire.
When there is a good replacement, RG-400 in this case, and that
replacement is not that much more expensive, it seem to me to be
prudent to use it.
Lastly, there is one fact that is indisputable: RG-400 coax will still
be working and meeting its specifications long after RG-58A/U will have
failed to do so. It only remains for you to decide if it matters.
After all, it is your airplane to do with what you please.
Oh, and I think this horse is dead.
Brian Lloyd 6501 Red Hook Plaza
brianl@lloyd.com Suite 201
+1.340.998.9447 St. Thomas, VI 00802
I fly because it releases my mind from the tyranny of petty things . . .
Antoine de Saint-Exupry
Message 2
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--> Avionics-List message posted by: Brian Lloyd <brianl@lloyd.com>
On Jan 4, 2005, at 1:25 AM, Ron Davis wrote:
> --> Avionics-List message posted by: "Ron Davis" <l39parts@hotmail.com>
>
> The Belden spec sheet for RG58A/U (Belden part #8219) shows its loss
> at 18.1
> dB per 100' at 1 GHz. Curiously, Belden doesn't list an attenuation
> for
> RG400 above 400MHz.
And another manufacturer did give attenuation specs for RG-400 at 1 GHz
and above. That the marketing doc doesn't list it does not mean it is
not suitable.
There are two clear points from my previous post:
1. the attenuation specs for RG-400 and so-called low-loss RG-58 are
essentially the same;
2. the attenuation specs for RG-400 are substantially superior to
solid-dielectric RG-58.
These facts are verifiable.
Brian Lloyd 6501 Red Hook Plaza
brianl@lloyd.com Suite 201
+1.340.998.9447 St. Thomas, VI 00802
I fly because it releases my mind from the tyranny of petty things . . .
Antoine de Saint-Exupry
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