_____
From: Repeater-Builder@yahoogroups.com
[mailto:[EMAIL PROTECTED] On Behalf Of Jesse Lloyd
Sent: Monday, August 27, 2007 9:39 PM
To: Repeater-Builder@yahoogroups.com
Subject: Re: [Repeater-Builder] Duplexers
If your coax is the same impedance as your transmitter, but different than
your load, can it still be a transformer though? Is it possible to
transform a load that isn't 50 ohms to 50 ohms using 50 ohm coax?
Yes it always acts as a transformer when the load impedance is not the same
as the coax impedance.
You can not transform any impedance to 50 ohms with a 50 ohm cable. You can
transform to something above or below the 50 ohm cable impedance.
The reason changing the length of the coax to a transmitter helps sometimes,
even though the transformation of impedance is not to 50 ohms, is that the
transmitter may see an impedance that it is happier with than what the
original transformed impedance was.
Yes your right VSWR is the ratio between Vmax and Vmin, node and anodes, of
the interference pattern caused by standing waves. Even still there is a
point where the voltage is at a minimum on the line. What happens if that
point is at the transmitters output... does it help keep the heat down in
the transmitter due to high SWR?
It doesn't matter where the min and max are on the line. The same amount of
reflected power will be seen at any point. Reflected power does NOT get back
into the transmitter. It gets re-reflected back towards the antenna when it
reaches the transmitter circuits.
If you have a 100 watt transmitter with 10 watts reflected from the load
your wattmeter will read 110 watts forward and 10 watts reflected. The extra
10 watts forward power comes from the 10 watts that is reflected from the
load and re-reflected at the transmitter. The re-reflected power adds to the
original 100 watts forward power for a total of 110 watts forward power. All
of the 100 watts eventually gets radiated by the antenna. This is of course
disregarding any line loss which would lower the reflected power indication
by the amount of line loss. Line loss would also claim a portion of the
re-reflected power too.
If you have two watt meters and an antenna matching device you can put one
wattmeter between the transmitter and the matching device and tune it for
minimum reflected power on the first meter. Then with a second meter between
the tuner and the mismatched load you can see the second wattmeter that is
reading the reflected power. The second wattmeter will have a higher forward
power reading than the first due to the added re-reflected power.
With a mismatched load the transmitter may run hotter because it is under or
overloaded due to the non 50 ohm load that it is seeing but it is not
dissipating any of the reflected power. Many solid state transmitters are
sensitive to reactive loads and may draw more current because of this.
73
Gary K4FMX
On 8/27/07, Gary Schafer <[EMAIL PROTECTED]> wrote:
VSWR (voltage standing wave ratio) will be the same at any point on a
transmission line. The imaginary standing wave does not move as the forward
and reflected power does. The voltage standing wave ratio is the ratio of
the forward voltage to the reflected voltage at a given point on the line.
As you move up or down the line the forward voltage will change and so will
the reflected voltage but the ratio or difference between the two will work
out to the same value. Thus the term "standing wave". The wave appears to
stand still on the line as it oscillates up and down in a sin wave manor.
As Jeff has said the impedance shown to the transmitter will be different
with different lengths of transmission line only if the load is not a
perfect 50 ohms assuming a 50 ohm line. With a load that does not match the
line the line operates as an impedance transformer. Think about what a
quarter wave length line looks like with a short on one end. It transforms
that short to a high impedance or open at the other end. If one end is open
the other end will look like a short to the transmitter.
With a load impedance that is not 50 ohms what is seen at the transmitter is
something between an open and a short depending on how far from 50 ohms the
load is. In other words the load impedance gets "transformed" to something
else.
73
Gary K4FMX
_____
From: Repeater-Builder@yahoogroups.com
[mailto:[EMAIL PROTECTED] On Behalf Of Jesse Lloyd
Sent: Monday, August 27, 2007 11:48 AM
To: Repeater-Builder@yahoogroups.com
Subject: Re: [Repeater-Builder] Duplexers
The length of coax doesn't effect impedance. Trimming the coax effects what
is read on the VSWR meter because what is actually happening is that there
is an interference pattern created when you have a mismatch on the end of
feedline. This pattern is sinusoidal and changes in voltage and current
along the line, in 1/2 wave periods. You will find max voltage peaks and
min voltage peaks. Also current will go up and down too. When you are
using a VSWR meter you are measuring voltage, if you move the meter to a
different spot on the cable, the voltage is different, therefor it gives you
a different reading.
Now if you put a voltage null at your transmitter, what would happen?
Normally with high SWR your transmitter will get hot because its dissipating
the reflected power into its heatsink. If you put it at a voltage null, I
would suspect that the SWR would not get dissipated by the transmitter as
much as if you put it at a voltage peak. The standing waves are still
there, there is still a mismatch, you will get the same power out, but its
just not going to hurt your transmitter as much because of the heat.
The only time coax length makes a difference to power out is if your using
it in a matching stub, or a matching section ie. if you take 1/4 wave of 75
ohm cable put it on the end of 50 ohm cable you will get a match with a
112.5 ohm load.
You make an interesting point though, why does the cabling of duplexer's
need to be a certain length. I would suspect that its because they are
looped and make an inductor. This then is part of the LC filtering, and
changing the length effects L. But I could be wrong on that.
Jesse
On 8/27/07, R. K. Brumback <[EMAIL PROTECTED]> wrote:
I have heard this point argued for years. "Does trimming the coax affect the
SWR?" If the length of coax has an affect on impedance, then how could it
not affect power out? We strive to maintain 50 ohms at the tail of all
devices to match the end load. GE puts matching networks in their Mastr
II's. I have taken a MFJ-259 and soldered a PL259 only at one end and then
started trimming the coax down and watched the impedance change
significantly with each cut. Duplexers come with precise lengths of cabling.
I have heard that trimming coax only fools the meter. Not being an engineer
with millions worth of equipment I can only make a SWAG (scientific wild ass
guess) as to whether coax length makes a difference in power out.
Randy
W4CPT
-----Original Message-----
From: Repeater-Builder@yahoogroups.com [mailto:[EMAIL PROTECTED]
ups.com] On Behalf Of Jeff DePolo
Sent: Saturday, August 25, 2007 12:30 AM
To: Repeater-Builder@yahoogroups.com
Subject: RE: [Repeater-Builder] Duplexers
> When you put the Bird between the TX and the duplexer, you
> have changed the
> length of the jumper cable, which upset the tuning.
Adding a wattmeter or any other length of cable between the transmitter and
the duplexer Tx input port has no effect on the tuning of the duplexer. It
may change the load Z the transmitter sees, which may make the transmitter
happier (or sadder) depending on the resulting Z, but in no way does it
alter the tuning of the duplexer itself.
Adding or removing cable lengths between the transmitter and duplexer also
does not change the VSWR as seen by the transmitter (minimal cable loss
effects notwithstanding).
--- Jeff
-----------------------------------------
Jeff DePolo - [EMAIL PROTECTED] <mailto:jd1%40broadsci.com>
Broadcast Sciences LLC, Valley Forge PA
v: 610.917.3000
f: 610.917.3030
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