First off, you need to take into consideration the "uncertainties"
of the measurement device. IOW, what is the accuracy of the
AEA device. Most devices which measure loss are, at best,
plus or minus 0.5dB and 0.75~1 dB is a more likely scenario.
This also assumes that it has been properly calibrated within
the past year.
Secondly, cable attenuation figures are usually worst case unless
specified as nominal. After 21 years in the cable television industry
and now working for an amplifier manufacturer where I measure
the cable between the amplifiers to the Nth degree with a network
analyzer, the actual loss works out to be closer to nominal (but higher
than that value) than to worst case. As an example, a particular
manufacturer specifies his cable to be 2.04dB/100' nominal and
2.16dB/100' worst case at 750MHz. I measure 2.07. Close enough.
However, every reel will be different and if you really want to tear
your hair out, any 100' segment will be slightly different. In essence,
the loss through the connectors may be a bit more than you figured.
You may want to contact PL259 manufacturers for loss specifications.
My network analyzers are 75-Ohm so I couldn't do the measurement
for you, but anyone with a good 50-Ohm network analyzer or spectrum
analyzer with a tracking generator could do the job. When measuring
connectors like that, you usually wind up having two going through a
female barrel and assuming the contribution of all three devices as
being pretty much equal. Don't forget to normalize out the interconnecting
cables.
Have fun.
Bob AA0CY
----------
From: Dick Green[SMTP:dick.green@valley.net]
Sent: Saturday, January 16, 1999 9:43 AM
To: Tower
Subject: [TowerTalk] Connector and switch losses
Can anyone tell me the approximate loss for a PL-259 connector at 28 MHz?
Also, does anyone know the approximate loss at 28 MHz for antenna switching
relays like the ones used in the Ameritron RCV-8?
Right now, I have 8 connectors between the amp and each of my antennas,
which includes junctions at a couple of Polyphaser lightning protectors and
a remote antenna switch. The switch adds one set of relay contacts. My AEA
CIA HF analyst shows total loss for the 260' cable run, including connectors
and the antenna switch, as 2.2 dB at 28.250 MHz, which is only .2 dB higher
than manufacturer's spec of 2.0 dB for a cable of that length (it's Times
Microwave LMR400UF and I got their loss figure from the attenuation
calculator on their web page.) That seems pretty reasonable, and would
indicate something on the order of .025 dB loss per junction (averaging the
connectors and relay contacts.)
I ask because I'm planning to install more PL-259's and more relays for
automatic two-radio switching and bandpass filtering, and am trying to
anticipate the additional losses. With some rearrangement of the existing
configuration, the basic switching adds 2 connectors, bandpass filtering
adds 2 connectors and switching for amp sharing adds 3 more connectors, for
a total of 15. (The latter is an experiment: sharing an 87A between two
radios with automatic switching, the idea being to take advantage of the 87A
automatic QSY.) The switches will add four more sets of relay contacts
(replacing the Ameritron with TopTen A/B and 1-of-6 switches.) If the above
loss-per-junction figure is right, I'm looking at an increased loss of about
.3 dB. Sound right?
73, Dick, WC1M
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