On Sat, 11 Jan 2014 00:01:57 +0000, Ian Jackson
wrote:

In message , Jeff Liebermann
writes
On Thu, 9 Jan 2014 21:08:11 -0800, "Sal" salmonella@food
poisoning.org wrote:

Another experiment I ran (back around 1975) was to take 100 feet of cable
and measure the loss, then repeat the measurement using a different 100 feet
made from ten different pieces. Yup, the loss was about 3 dB more,
indicative of an average 0.3 dB loss per joint, neatly within the range you
specified.

0.3dB per connector at what frequency?

This is more fun:
http://802.11junk.com/jeffl/antennas/connector-loss/index.html
Just take every connector that you can find, put them in series, and
measure the loss. In this case, it was done at 2.4Ghz and 450MHz. End
to end loss at 2.4GHz was 2dB for about 25 adapters or about 0.08dB
per adapter. At 250MHz, the loss was about 0.2dB or 0.008dB per

Oops. That should be 450 Mhz, not 250 MHz.

adapter.

I've done similar demonstrations using two wattmeters at the local
radio club meeting. The results are typically that the adapter string
has the same loss as an equivalent length of small coax cable. I had
a surplus of BNC T connectors, so a strung about 50 of them in series
and obtained similar results.

Bottom line: Connectors and adapters aren't as evil as the data
sheets and literature suggest.

I've always assumed that the loss measured through connectors and
adapters was mainly
(a) because they have unavoidable length (ie not a lot), and
(b) because the impedance match through them is less than perfect (ie
not a lot).

As I misunderstand it, below about 1GHz, most of the loss is ohmic,
which are the surface and contact resistance of the connections and
conductors. Above 1GHz, the dielectric losses begin to become
significant. Extra points of dissimilar metals and bad construction.
Except for the PL-259/SO-239 and phono connectors, most of the
connectors are fairly close to 50 ohms.

The ohmic contact resistance may also be a tiny tad higher than the same
length of coax (even less).

Yep. It's more than a tiny tad higher. For example, at 2.4 GHz,
LMR240 has an attenuation of 12.6 dB/100ft. Each coax adapter is
maybe an inch long, resulting in:
12.6 dB/100ft = 0.126 dB/ft = 0.01 dB/inch
which is 8 times less than the 0.08dB/adapter that the measurements
show. However, there's plenty of room for measurement error here. I
suspect that if quality connectors were used, such as SMA, the numbers
could come out closer to a similar wire gauge coax cable.


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