On Fri, 14 Feb 2014 06:38:54 -0800, dave
wrote:

On 02/13/2014 06:07 PM, Jeff Liebermann wrote:
On Thu, 13 Feb 2014 17:41:28 -0800, Bennett


Not good enough. If you just crammed a 68 ohm resistor into the
F-connector jack, you probably have a fairly good approximation of an
antenna. It's going to radiate junk, and pickup more junk (ingres).
If you insist on doing it like that, wrap the resistor with some
insulating tape, and then cover it with aluminum foil.

Do you have any idea what the wavelength is at 600MHz? It's half a meter
(Meters times megaHertz equals 300). That makes the length of a quarter
wave is 1/8 meter or about 5". A tiny resistor is not going to radiate
efficiently enough.

Whether it radiates efficiently or inefficiently, it still radiates.
Assuming a 1/2 watt resistor, with one lead wrapped around the
grounding screw, and the other shoved into the F-connector center
wire, there will be about 1/4" of exposed wire between the center pin
and the 60 ohm resistor. That's not much, can probably be tolerated,
and will be sufficient to slightly increase base line noise level in
the area. I would hate to think what might happen if EVERYONE decided
to use leaky terminators.[1]

I like to play ham radio (VHF/UHF) in my vehicle when driving. I
constantly hear the digital hash as I drive repeatedly by specific
locations. At first, I assumed that these were leaks from the
overhead cable plant. Nope. When I started banging on doors, I found
some really creative wiring and connector crimping. Most were caused
by distribution amps with unterminated cables, where the coax cable
becomes somewhat of an antenna. I also found either a defective drop
or tap on the pole (Comcast didn't tell me what they found, but they
did fix it).

Obviously, a leaky cable is a much longer length of wire than 1/4" of
exposed center conductor. Yet, after the gross leaks were fixed, I
still could still hear the test tones on the borrowed Trilithic
something leakage detector. I had to install terminators on literally
everything to reduce it to zero. Admittedly, I was working well below
the FCC Part 76.605 leakage limits:
http://transition.fcc.gov/mb/engineering/605.html
http://www.cablefax.com/tech/sections/columns/broadband/Broadband-Signal-Leakage-in-an-All-Digital-Network_33882.html
http://www.cablefax.com/tech/deployment/bestpractices/A-Leak-Is-A-Leak&hellip%20Or-Is-It_52272.html
but I needed to get the radiated junk down to a level where I could
use my radios in the neighborhood. Overkill yes, but necessary.

It's also a problem in the other direction, which is called ingress.
Nearby transmitters, such as mine, can easily get into the cable
system, and interfere with reception. Once it gets in, there's no
easy way to get rid of the signal. The best approach is proper
shielding. It isn't just my transmitters, but broadcast stations,
intermod with cordless phones, RF remotes, wireless security cameras,
and other RF devices. Mostly, they cause problems with cable modems,
which use the higher RF frequencies, but in extreme cases, can affect
TV reception. It's also difficult to see with a spectrum analyzer:
http://www.jdsu.com/ProductLiterature/DSAM-D3-Spectrum-Analyzer-App-Note.pdf
Can you see it? Spoiler:
http://blogs.jdsu.com/hfc/Lists/Photos/dd6cc9ceb7e6a11e1006e1ad654edd7f.jpg

Mo
"Characterizing Signal Leakage from an All-Digital Cable Network"
http://www.incospec.com/resources/webinars/files/Characterizing%20Signal%20Leakage%20from%20an%20Al l-Digital%20Cable%20NetworkMS.pdf

If the "splitter" is a hybrid the unterminated port
will cause higher attenuation through the splitter. Maybe a single dB.
Not important when receiving.

Without a termination, the splitter begins to resonate with the coax
cable which can be made to radiate. I'm not sure of the unterminated
losses, but I suspect it's more than 1dB. I'll try it on the bench
when I have time (mostly to clean off the accumulated junk from the
bench).
http://802.11junk.com/jeffl/pics/home/slides/test-equip-mess.html

You'd be better off getting a DC block. The splitter loses at least 3 dB.

A DC block won't do anything useful except block DC. I think you
might have meant a straight through barrel connector.



[1] Incidentally, you might be amused to look at my study of monopole
antenna gains versus length.
http://802.11junk.com/jeffl/antennas/Monopole/index.html
The title includes the wavelength. For example, 0_750 is 3/4
wavelength long and 5_250 is 5.25 wavelengths long. If you look at
the antenna gains, you'll notice that the gain doesn't really drop
very much for shortened antennas. From the antenna patterns:
Wavelength Gain dBi
0.250 5.19
0.125 4.85
0.050 4.75
That's a 0.45dB gain loss in an antenna that's 1/5th the length of a
1/4 wave radiator. That might explain why I'm concerned about even
small amounts of exposed center conductor.


--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558