--- In Repeater-Builder@yahoogroups.com, "skipp025" <[EMAIL PROTECTED]> wrote:
"I lifted the below text from the yahoo rfamplifiers group page,
posted this last week by my friend Dave. It's right on the money.
http://groups.yahoo.com/group/rfamplifiers/
[paste]
If you are able to measure the SWR of a coax connection at VHF, then
you've installed a connector wrong. I used an HP 8510C network
analyzer to measure a whole bunch of UHF to N, UHF to BNC and so
forth, adapters, all stacked on top of each other to a foot long, and
the lowest frequency where I measured anything of concern was at 2
GIGAhertz, where the SWR rose to 1.1:1. Yes, I know, amateur radio
lore is full of the claim that UHF connectors are awful. They are, but
only if you compare then to BNC and N, which are suitable for 10GHz
and above. Assembled correctly, a UHF connector is invisible below 1GHz.
[end]
________________________________________________________________________
I hope nobody is buying into this BS.
Connector and adapter mis-match and loss issues are hardly "amateur
radio lore"
RF manufacturers and instrumentation companies have stressed this for
years - justifiably.
It's easy to verify if you have access to instrumentation. Just out
of curiosity I decided to run a couple of tests.
When you measure a device for SWR you are actually measuring return
loss. You can do this on a network analyzer or with a spectrum
analyzer and tracking generator plus a signal separation device such
as a return loss bridge or directional coupler.
I decided to try both instruments and compare the results. You first
calibrate the measurement setup with precision components from your
cal kit [e.g. Agilent 85032B]. The critical component is a precision
50 ohm load. With a good bridge or coupler you should be able to
measure a return loss of 40 dB or greater [equivalent to an SWR or
1.02:1]
Normally your instrument test ports will be type N although some
instruments have APC-7 test ports. Your cal kit should match the test
ports.
I started with a full 2-port cal of the VNA and a normalization of the
Spectrum analyzer. My reference load did measure as expected.
The next step is to "insert" the device under test. In this case we
are interested in looking at a UHF connector. Obviously, since the
instrument is type N, we have to use adapters. Just out of curiosity,
I decided to compare two pairs of adapters. First an NF-NF mated to
an NM-NM. [UG-57B/U and UG-29B/U] This created an "insertable" device.
These are "standard" adapters as opposed to "precision" components
and as expected, there was some degradation of the return loss
measurement.
I then compared this to two other adapters, NM to UHF-F and UHF-M to
NF [UG-146/U and UG-83/U], again creating an "insertable device" This
pair has just a single UHF male to female junction and results in a
significant decrease in return loss [i.e. increase in SWR.]
I also tried inserting an 90 degree UHF adapter [UG-646/U] in line
between the UG146/U and the UG83/U which resulted in further
[significant] return loss degradation.
I measured several sets of these parts and the results were not only
consistent from part to part but generally match within a few tenths
of a dB between the VNA and the spectrum analyzer. I always like to
see comparable results from two significantly different instrument
setups - it confirms your thought process and results.
Results at: 150 MHz, 450 MHz, 900 MHz
NM-NF adapters:
[VNA]: 36 dB, 35 dB, 31 dB
[SA/TG] 37 dB, 35.7 dB, 30 dB
N-UHF + UHF-N adapters:
[VNA]: 26 dB, 16.5 dB, 11.4 dB
[SA/TG]: 23.6 dB, 17.2 dB, 11.5 dB
N-UHF + UHF 90 + UHF-N
[VNA]: 20.2 dB, 12.03 dB, 9.3 dB
[SA/TG]: 21.6 DB, 11.9 dB, 9.1 dB
Now just for reference, a return loss of 11 dB would be equivalent to
an SWR of 1.785 : 1 [ONE UHF junction @ 900 MHz] and a return loss of
17 dB would be an SWR of 1.329 : 1 [@ 450 MHz]
UHF connectors "invisible" below a gigahertz??? Hardly.
Also - someone asked about the 90 degree N fitting at 900 MHz so I
tested a few of those. That's easy because being M on one side and F
on the other, it is inherently "insertable"
The round kind [UG-27A/U] averaged return loss of 22 dB. The square
ones[UG-27D/U] are better at 27 dB [1.094 : 1 SWR]
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