Bob and towertalkians:
Return Loss is an engineering term, not a Cable TV term, and is applicable to
any
impedance system. It
is very often used in 50 Ohm systems for a variety of measurements. Return Loss
is
another way of expressing VSWR, as Bob correctly stated. Return loss is used
by AEA in a very typical engineering way to more accurately measure the value
of the match in a cabling system. (Not only TV cable, but any coaxial cabling
system).
I use the AEA analyser in the Return Loss mode to measure how well a 1/4
wave matching stub is cut to the frequency/band I am interested in. A very
good match might be shown by a 50 dB return loss. Since this value is more than
40 dB, the VSWR might be shown as nearly 1:1 but a return loss of 40 dB
might show the same VSWR because such a low VSWR might not be measurable
as VSWR but would be measurable as Return Loss. The difference is NOT
a subtle one. 10 dB is a significant value to be measured and Return Loss
allows
you to measure it, VSWR doesn't. That's why AEA incorporates it in their
measurement tool. Much like all engineering measurements, there
are different measurement parameters for different purposes.
BTW, the way I use it is to attach a "T" connector to the AEA, connect a 50 OHM
dummy load to one termianl of the "T" and the to-be-cut 1/4 wave open-end stub
to
the other
end of the "T". On the band of interest, the AEA would measure close to zero
(since the stub appears as a short)
Return Loss which is hard to measure. On even multiples of the band of
interest,
the Return Loss would
measure high (say 50 to 70 dB) at resonance which is easy to measure. This is
true since at even multiples,
the open stub would appear as a open at the "T" and the impedance seen would be
closer and closer to a perfect match (50 Ohms) as you cut the cable closer to
the
band/frequency of interest. This is a very easy way to cut your 1/4 wave
matching
stubs. (Start with
a cable that is a 'lil bit longer than you'll need using the calculations in the
handbook.) This works with
70 Ohm cables as well, since the impedance of the cable doesn't matter if you're
trying to cut a 1/4 wave
section of line and the only thing that matters is the electrical length of the
line. This procedure also
eliminates any calculation error due to manufacturing distortion in velocity
factor in the line under test. (As opposed
to calculating the electrical length with velocity factor, cable length, etc.)
Sorry to be so long-winded on this...got carried away..
Bill, N3RR
Bob Wanderer wrote:
> Same difference. Return loss is the amount, in dB, that the
> reflected signal (power) is down from the forward or incident wave.
> The ARRL Handbook has a converstion nomograph. However,
> it is obvious that a 1:1 VSWR would be a very high value of
> return loss (>40 or 50 dB), and a short or very high
> SWR would yield a RL close to 0dB.
>
> I'm surprised that AEA is using RL as that is usually used in the 75 Ohm world
> of cable television, whilst SWR is the 50 Ohm world of RF communications.
> Maybe I should introduce Towertalkians to SRL (structural return loss), which
> is used to qualify hardline coaxial cables?
>
> 73,
> Bob AA0CY
> 20 years in the technical side of cable television
>
> ----------
> From: Tom Champlin[SMTP:w0hh@ix.netcom.com]
> Sent: Monday, September 28, 1998 7:10 PM
> To: towertalk
> Subject: [TowerTalk] return loss
>
> Gentlemen,
> Saturday I received my AEA CIA-HF, sn#0073 ver.1.3. Since then I have
> been trying to familiarize myself with it. I'm getting "return loss"
> figures in DB. Would someone be kind enough to tell me about this as I
> only have a vague idea about it.
> Thank you,
> Tom, W0HH
>
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