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1-3 of 3 messages
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Moved the radio, S went from S9 to S3-4...
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by VE3NEAR on October 28, 2009
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Quick question, i just moved my radio from the garage, where from 213 was running into my GP-15.
The antenna is at about 30 feet and the cable was about another 30 feet.
I got a box at a local HamFest of RG-58(Belden 450ft for 20$-i couldn't refuse it!!!) and moved the rig to the basement where i live. The length of the cable is about 80-100ft. A local repeater that i listen to went from S9 to S3-4. Would the loss of that coax at this lenght contribute to that?
I have LMR-400 sitting at my garage and plan on drilling another hole and pulling the cable through the concrete to where my rig is. Also i am planning on moving the antenna to about 30 feet away from my FT-847.
I guess i just answered my question about the loss, and the lenght of the cable.
Would RG-58 be ok to run it to the antenna for 30 feet? instead drilling another hole for the LMR?
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RE: Moved the radio, S went from S9 to S3-4...
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by WB6BYU on October 28, 2009
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VK1OD's transmission line loss calculator is a handy way
to check your line losses:
http://www.vk1od.net/calc/tl/tllc.php
though simpler methods will work as well in this situation.
It shows that the loss of RG-58A coax at 2m is about
6dB per 100'. So you'll lose half your power (on both
transmit and receive) just in a 50' piece.
Contrary to popular myth, S-meters are not reliably
6dB per S-unit, especially those on FM receivers. Most
of the mobile rigs I've measured that have bargraph meters
have about a 12dB range from the first bar turning on
to the last one. So a 6dB loss would be about half your
meter scale, which sounds about right for the numbers
you quoted. (In practice the loss also depends on the
age and physical condition of the coax, the manufacturer,
dielectric material, etc.)
How long of a piece you can use depends on how much
loss you can afford. Generally I try to limit RG-58 to
25' on 2m, though even 50' probably is acceptable if it gets
your antenna up higher and in the clear. In many cases
repeaters are strong enough that even 6dB of loss is
quite tolerable, but if you are trying to work more
distant / weak ones you may have better results with
lower loss cable. (RG-213 will have about 2.5dB loss
in 100', while LMR-400 has about 1.5dB.)
There is no problem mixing cable types, beyond the
increase in loss due to each individual length. If you
have to splice in a piece of RG-58 to fit through a
hole that is too small for something larger, that will
have lower loss than making the whole length out of
the thinner coax. (Presuming the connectors are
installed properly, of course.) You might try the
RG-8X varieties: the losses are lower (under 4dB/100')
but they are only a bit thicker than RG-58.
But RG-58 is fine for a lot of HF work, especially on
the lower bands: on 40m the loss is just over 1dB / 100',
and even lower on 80 and 160m.
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RE: Moved the radio, S went from S9 to S3-4...
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by WB2WIK on October 28, 2009
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As Dale said, "S" meters don't mean much on FM.
Every rig I own (many), when used on FM, has S-meter readings that are nearly meaningless. I can change signal strength by 2 dB and see 2-3 "S" units worth of difference on the meter.
This is because FM is a non-linear mode, using non-linear amplifiers (in both receivers and transmitters) and a non-linear detection system.
My FT-3000M indicates "S1" at 0.1 uV and well over "S9" at 1 uV. That's only a true 20 dB change for more than 8 S units indicated. The same meter goes to "full pin" (full scale, as high as it can go) with about a 3 uV signal.
Obviously, this is a huge "S meter" change for a very small signal strength change, but it's pretty typical of meters on FM receivers.
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