I concur with Jim's comments on loss. I view one should always look at
the component which contributes the greatest loss in the system, that
is most often the feedline. One should strive to improve the condition
by using a line type of less loss. {i.e. better quality feed line} and
eliminate excessive feed line lengths. Factors influencing line
losses are; type of line, age of the line, length of line, frequency and
reflected power. Manufactures produce charts and tables showing loss of
a specific type of line taking into consideration of length and
frequency. These numbers are for new or known good line and not likely
"hamfest" bargain line.
A second component which often contributes significant loss is the all
famous "antenna tuner". Recent tests show some tuners, under some
load conditions can contribute up to 25% of added loss or more. And
while at the same time, the loss in the feedline remains the same.
This brings me to the point where I view many hams obsess over SWR
values. Unless the transmitter is folding back power, as many
un-necessarily do, then the use of the ATU may benefit making the
transmitter happy, but at the sacrifice of added loss to the system. To
that end, there are several brands and models of radios, past and
present, that do not fold back power with reasonable SWR values. I find
it not at all uncommon to operate with a 3:1 SWR without issues. In
this configuration, adding the ATU makes the SWR to the radio look
better, but adds loss induced by the tuner and does not change the loss
in the feed line.
There is an interesting compilation of data on various match boxes i.e.
ATU's and their performance found on the following link. The compiled
information and data is from various sources and presented in XLS format.
http://www.dj0ip.de/antenna-matchboxes/matchbox-shoot-out/
73
Bob, K4TAX
On 2/11/2018 10:25 PM, Jim Brown wrote:
Thanks for this excellent post, Alan! IMO, "mismatch loss" is a
figment of the imagination of those who never get outside the lab.
Transmission lines was one of my favorite EE courses, and I never
heard of it until I heard it referenced in online discussions a few
years ago. And the purported losses in connectors are an urban legend
with almost basis in fact. Several years ago, W8JI poetically observed
that if the 1 dB loss falsely attributed to UHF connectors was true,
each would be burning 35 W carrying a legal limit signal, and be
starting fires!
Keeping track of losses in systems is, of course, a great thing. Our
FD group runs (and has won several times) FD 1A QRP Battery. Every
piece of coax in our station is low loss RG8 or RG11.
73, Jim K9YC
On 2/11/2018 8:03 PM, Alan wrote:
Hi Al,
Yes, but don't forget that the connector "loss" is a mismatch loss,
not absorptive power loss. In other words, it affects the SWR
slightly but does not actually absorb any power.
If you are using any kind of antenna tuner and tuning for 1:1 SWR,
mismatch "losses" have no effect. Even if you aren't doing that, the
antenna is probably not a perfect 50-ohm resistive load anyway, so
the connectors' mismatches are about as likely to make the SWR better
as worse, depending on the phase and magnitude.
But the general point is sound. Power loss is even more important
for QRP than for QRO even though the number of watts of loss is
less. When the other station can barely hear you, every dB counts!
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