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1-10 of 19 messages
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*#!^{/!*%! SWR
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by W8JJI on December 8, 2003
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How can the swr reading be different on 2 different radios but with the same coax and antenna system ??
As indicated by an external swr meter.
One radio gives a reading of about 1.5 (icom 746pro) and the other 2.0(yaesu FT897) ON THE SAME ANTENNA !!!??? What gives ?
Thanks!
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RE: *#!^{/!*%! SWR
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by W8JI on December 8, 2003
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There are only four causes.
1.It could be lack of linearity in the SWR meter and you are running different power with different radios. Some cross needle meters are bad about this.
2.It could be common mode currents on the feeder that actually changes the SWR when you change the ground path through the radio.
3.It could be very strong spurious signals on the output of the radio with higher SWR. This is unlikely but I had a FT757 that oscillated and gave all sorts of weird SWR results.
4.It could be April 1.
73 Tom
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RE: *#!^{/!*%! SWR
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by WB2WIK on December 8, 2003
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Tom's #3 would change if you try a dummy load instead of an antenna. A good dummy load (one that's good into the VHF-UHF region) will still measure 1.0 SWR even if the transmitter is loaded with spurious outputs. An antenna won't.
As for #4, it's a bit early. Or, maybe it's 8 months late!
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RE: *#!^{/!*%! SWR
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by AC5E on December 8, 2003
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Well, while Tom pretty well covers the subject I had a similar occurance when I first set my shack up for a rig shootout.
The setup was (and is) a jumper from the "test rig" and a jumper to the "standard" rig; both going to a 2 position antenna switched hooked up "backward" to select one rig or the other; a jumper to the Bird; and a jumper from the Bird to the main antenna switch. This allows very quick changes from one rig to the other, important when you want to actually compare two tranceivers.
I thoughlessly set it up with two different length jumpers between the 2 position switch and the rigs. So the indicated SWR was somewhat different at the two operating positions. A change to equal length jumpers "fixed" the problem.
Of course, it was not really a problem. While the actual SWR does not change anywhere on a transmission line, the apparent and indicated SWR changes from point to point on a transmission line.
Or as my professor said a half century ago, "If the SWR does not suit you adjust the length of the transmission line until it does." True in 1950, still true today.
73 Pete Allen AC5E
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RE: *#!^{/!*%! SWR
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by K5LXP on December 8, 2003
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This question was asked a couple weeks ago:
http://www.eham.net/forums/Elmers/51165
Mark K5LXP
Albuquerque, NM
k5lxp@arrl.net
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RE: *#!^{/!*%! SWR
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by KZ1X on December 8, 2003
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Which external SWR meter?
Do you have access to a Bird Thruline thermocouple-type wattmeter?
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RE: *#!^{/!*%! SWR
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by KA0MR on December 9, 2003
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The only SWR gauge that should be trusted is the Bird Wattmeter,if it's close accuracy your are striving for without resorting to a Hewlett-Packard Reflectometer for $ 25,000 dollar accuracy.
Bob
KA0MR
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RE: *#!^{/!*%! SWR
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by WB2WIK on December 9, 2003
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Actually, the classic Bird Thruline (like model 43) isn't such a hot "SWR bridge." It does have good directivity if the proper elements are used, but has the same full-scale power rating for both forward and reflected power, making it a very insensitive SWR bridge if the same element is used for both (by simply rotating it in the coupler).
Since the Bird 43 is rated 5% of full-scale accuracy:
If you have a 100W slug installed, and measure 100W forward, that should truly be somewhere between 95 and 105W.
Then, rotate the slug, and your accuracy is still 5% of full scale, which is +/- 5W.
If you read downscale (with any reasonable SWR you will be reading quite far downscale), and read, say, 5W reflected power, the Bird's accuracy is 5W +/- 5W. Which means its accuracy is +/- 100%. Pretty dismal for a "high accuracy" instrument.
The way to achieve better accuracy, of course, is to use two different "slugs," one to measure forward power and another, more sensitive one, to measure reflected power. If you anticipate having an SWR <2.0, the "reflected" slug should be about 10x more sensitive than the "forward" slug, e.g., use a 100W forward and 10W reflected -- this will achieve reasonable accuracy.
The advantage of the Bird Thruline design is not accuracy nor resolution; the advantages are that, by using easily swappable elements, a single instrument can cover from LF-MF through SHF at power levels from <1W to >5kW; and that the coupler is virtually lossless through UHF and SHF, so it can be kept "in line" not only for making measurements, but for daily operation, without impacting station performance. For a low-cost, portable instrument, that's quite a lot to offer.
But it's not a highly accurate device, especially if one uses a single element for forward and reverse power readings.
WB2WIK/6
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