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VK2TV > C15 09.03.08 00:13l 105 Lines 4822 Bytes #999 (0) @ WW
BID : 35401_VK2TV
Read: GUEST
Subj: Re:A Weighty Problem
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Sent: 080308/2200Z @:VK2TV.#MNC.NSW.AUS.OC #:33799 [Kempsey, QF68JX] $:35401_VK
From: VK2TV@VK2TV.#MNC.NSW.AUS.OC
To : C15@WW
# Generated by: TstHWin v2.21b - Registered to VK2TV
# On : 3/9/2008 8:59:30 A
# UTC: 3/8/2008 21:59:30
>From: VK2XB@VK2XB.#RIV.NSW.AUS.OC
>To : C15@WW
>
>GerDay Ray & Folks,
>
>Indeed I have become more ...................... rotund in my advancement
>of years, Ray. More rotund but not THAT rotund!!!!!!!!!!
I'm glad we sorted that out!
Lots of snip to get back to a solution.
>I am thinking that (excuse the colloquialism) donkey tubing ** may be the
>go. I am just not sure that said flexi tubing will work as well to
>sheild as would the copper tube. Any ideas as to where I could source
>said flexible tubing in Oz? Additionally, Bill seems to think that half
>the job was as a result of using RG213. Again, cost isn't such a
>problem, but RG213 would be awkward to route, so all you techo's out
>there that work with radio installs for a living, what has the best
>sheilding amongst the thinner 50 Ohm cables available in Oz.
There used to be available a flexible, metal electrical conduit but I
haven't seen it in years but I imagine it would still be available for high
temperature installations. You could try electrical wholesalers - Cetnaj,
Lawrence & Hanson, etc.
As for coax, the quality (that probably should read quantity) of the shield
varies considerably from one brand of cable to the next. If shield quality
was important to me I would go for a well-respected and well-known cable
manufacturer. Beldon comes to mind but there others. You could use double
shielded cable - RG214 being the double shilded equivalent of RG213. Be
prepared to spend a LOT of money. Both braids and the inner conductor are
silver plated. In the RG58 style of coax there is a double shielded
equivalent - RG223.
I would be more than amazingly astonished if radiated noise was being
picked up by the coax, regardless of which route it takes to get from the
radio to the antenna. In my opinion that is another of those myths that
technically inclined people shouldn't believe. With 30 years of
(professionally) installing two-way radios in all manner of vehicles, from
small cars to prime movers, I've never even considered pickup by the coax
to be responsible for noise. If the coax shielding is that bad it has no
place in a radio installation. I've always solved interference by other
means.
If you want to see if it's the coax picking up the noise, and not the
antenna, remove the antenna and see if the noise goes away. It really is
that simple.
(If you want to track down the source of noise, use a length of coax from
the radio, with a DC blocking cap (1,000 to 10,000pF) as a probe and start
touching things.) The blocking cap is only to prevent your radio getting and
12/24V surprises!
If the antenna is doing its job, i.e. receiving signals, and you have a
signal (noise) you don't want to receive, eliminate that noise at its
source. That may be as simple as a capacitor or a choke (ferrite device) or
it may be as complex as some serious shielding of the source.
In larger 4WD's and trucks, there is usually a severe lack of bonding,
which facilitates radiation from individual components. The engine, drive
train and exhaust system is mounted on rubber blocks with plenty of
opportunity for radiation. The cabin is also mounted on rubber blocks.
Bonding around rubber mounting points can make a huge difference to
radiated noise levels.
Consider; if the radio is mounted to the metal cabin, and the antenna is
mounted to the bullbar which is attached to the chasis and, if those
components are not deliberately bonded together, the antenna coax becomes
the (lengthy) bonding wire and it is quite feasible for noise voltage to
appear on the braid and be coupled to the inner conductor. Think hum from
ground loops in audio equipment.
Deliberate bonding of the chassis to the body, as close to the antenna as
possible, would help to reduce such voltages by keeping the voltage between
one end of the coax, and the other, as close to zero as possible. The
bonding braid needs to be reasonably heavy to minimise inductance. Remember,
we're not looking for DC bonding, but RF bonding, and we don't need any
unintentional chokes. Battery strap braid is ideal, but I've also used the
outer braid of RG213 style coax. Crimp or solder it into suitable lugs.
In a heavy vehicle, especially, crimping is prefered because solder ceates
a stress point where the solder stops. Don't overlook bonding around the
hinges on a tilt-cab vehicle.
>I await with baited breath for the responses. I expect to receive baits
>in return!!!!!!!!!!!!!!
><Grin>
Ian, How long is the coax run from the antenna to the radio? I don't
believe it will solve your problem but I have some RG223 here that you can
have, if I have enough for your purpose.
Cheers ... Ray vk2tv
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