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1. 10:12 PM - Re: coax (Ron Davis)
2. 10:32 PM - Re: coax (Ron Davis)
Message 1
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--> 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.
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.
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...
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.
Adequate is good enough, and better than adequate is good enough, and costs
more.
----- Original Message -----
From: "Brian Lloyd" <brianl@lloyd.com>
Subject: Re: Avionics-List: coax
> --> Avionics-List message posted by: Brian Lloyd <brianl@lloyd.com>
>
>
> On Dec 29, 2004, at 12:10 AM, Ron Davis wrote:
>
> > --> Avionics-List message posted by: "Ron Davis" <l39parts@hotmail.com>
> >
> > This is a response to the posting that said to use RG 400 for the GPS
> > antenna. Garmin's manual says to use RG 58A/U.
>
> What Garmin puts in their manual is not my problem. I know the
> difference between various types of coax and I would never consider
> using run-of-the-mill RG-58 for GPS no matter what the manufacturer
> says.
>
> Let me put it another way. I am going to spend $100,000 on the airplane
> of my dreams with $20,000 in avionics and I am going to let a $20
> difference in the cost of coax determine what I put in? Heck, if you
> care that much I urge you to wire your OBAM airplane with PVC wire
> instead of Tefzel because it is much cheaper and it will carry
> electrons just as well.
>
> Wire is cheap, especially if you go looking for where you can get it at
> a reasonable price. Get the good stuff and then don't worry about it.
> If a $20 difference in price is going to break you, you should not be
> playing with an airplane because you are not going to have the money to
> maintain it properly.
>
> 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: "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.
----- Original Message -----
From: "Brian Lloyd" <brianl@lloyd.com>
Subject: Re: Avionics-List: coax
> --> Avionics-List message posted by: Brian Lloyd <brianl@lloyd.com>
>
>
> On Dec 28, 2004, at 1:04 AM, Garth Shearing wrote:
>
> > --> Avionics-List message posted by: "Garth Shearing"
> > <garth@islandnet.com>
> >
> > I'm going to go against the flow on this one.
> >
> > The data I have shows the RF loss in RG-58 and RG-400 is about the
> > same.
>
> Not quite. The loss in RG-400 is about the same as low-loss RG-58 with
> foam dielectric.
>
> Loss per 100M at 1000 MHz
>
> 78.6dB for RG-58 with solid dielectric
>
> 51.8dB for RG-58 with foam dielectric
>
> 52 dB for RG-400
>
> This is a *big* difference.
>
> If you would consider using plain-old RG-58 then you should consider
> using LMR-195 microwave cable. It is functionally the same as RG-58 but
> lower loss than either RG-58 or RG-400. If you are contemplating using
> RG-8 (transponder or DME) then consider using LMR-400.
>
> > RG-58 uses a solid polyethylene dielectric and the RG-400 uses a solid
> > teflon which provides operating temperatures up to 200 degrees C.
>
> The teflon construction makes it pretty impervious to just about
> everything, heat included.
>
> > RG-58 can
> > only go to 80 degrees C. So RG-58 is just fine for comm, nav, GPS, and
> > transponder. RG-58 is quite limited in terms of its ability to handle
> > continuous high power at high frequencies because the dielectric can
> > heat up
> > to 80 degrees C easily. This is not a problem with up to 200 watt
> > transponders because they are only transmitting a small part of the
> > time.
>
> Dielectric heating in aircraft applications just aren't a problem.
>
> > RG-58 can be used up to around 3 GHz, much higher than the 1.2 and 1.5
> > GHz
> > frequencies of GPS. Same goes for the connectors. Unless the cable
> > is in
> > the engine compartment, go with the RG-58. Way cheaper, easier to
> > work with
> > and weighs half as much as RG-400. I have used some teflon wire in my
> > engine compartment, so I do think it has its uses.
>
> You can use just about any coax at any frequency if you can accept the
> loss. (Actually, there is an upper limit for coax of larger diameter
> because at some higher frequency it stops acting as coax and starts
> acting as waveguide.)
>
> > I don't understand the moisture problem. There are millions of
> > installations out there working just fine. The only moisture problems
> > we
> > had occurred in outdoor cables where the cable ends were installed
> > without
> > drip loops and no shrink boots or tape wraps on the installed
> > connectors.
>
> Foam-dielectric coax tends to be hygroscopic, i.e. it absorbs moisture
> from the atmosphere on its own. It is interesting to measure loss in
> the coax with time. I used to use low-loss foam cables in my ham
> station until I discovered just how badly they degraded in just a year
> or two. I thought that a good coax seal to waterproof the exposed ends
> would solve the problem and it just doesn't. Mil-spec double-shielded
> RG-58 using silver-plated braid and center conductor does not degrade
> appreciably with time so that is what I use for comm and nav runs in
> aircraft. Still, RG-400 has lower loss and is even more resistant to
> the environment and an airplane is a very hostile environment.
>
> > We used to drill a small hole in the bottom of the drip loop to let any
> > moisture drain out, but this would not be needed in a typical aircraft
> > installation.
>
> You drill a hole in the coax? That is more likely to let water in than
> help it get out.
>
> > If you want lower losses or higher power, you have to go to larger
> > diameter,
> > which means heavier cable and connectors. You can also choose a cable
> > with
> > a foam or air dielectric. I think these choices are overkill given the
> > short lengths of cable normally required.
>
> In general I agree with you, at least for aircraft. You never have long
> runs. In the case of DME and transponder, your runs are usually not
> more than about 2M so almost anything will work. Still, an airplane is
> a harsh environment and coax does degrade with time. Good stuff
> degrades less (mil-spec RG-58) or almost not at all (RG-400).
>
> Consider that this radio installation will be serving someone about 20
> years from now. I repaneled my Comanche in 1985. The only change I have
> made since then was to replace the IFR LORAN with an IFR GPS last year.
> I am still using the same old comm and nav coax, something I should
> probably remedy and certainly will if I redo the avionics again.
>
> 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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