On Sun, 5 Jan 2014 00:01:47 -0500, "Ralph Mowery"
wrote:
I understand the idea of using 'perfect' items in electronics, then going
for more exect calculations if needed.
I like to design perfect antennas and circuits as a sanity check to
see if it can be done. Then, I throw in the losses and see what
happens. It's also a crude form of sensitivity analysis, which tells
me which parameters are most important.
The Wilkinson combiner is possiable for relative narrow frequencies. Not
sure if building one out of descrete components or full size transmission
lines would be broad enough for the whole FM band either. Wild guess it
would be about the same if just two pieces of transmission line of the
correct impedance and length were used.
Good guess. Discrete or coaxial performance (loss, isolation, and
bandwidth) are about the same. You're also correct that it wouldn't
cover the entire FM band. I could do it with a single stage Wilkinson
combiner by lowering the Q of the components. However, that will
increase the losses, which is not a great idea.
Much better is to use a multi-stage Wilkinson combiner:
http://www.microwaves101.com/encyclopedia/wilkinson_multistage.cfm
It's a common stripline technique. You probably recognize the general
pattern:
http://www.eee.bham.ac.uk/yatesac/Web%20PDF%27s/Test%20Gear/Wideband%20Wilkinson%20Coupler_1-2%20GHz_Layout.pdf
http://www.eee.bham.ac.uk/yatesac/Web%20Pages/Wideband%20Wilkinson%20Splitter%20&%20Combiner.htm
Isn't the Wilkinson combiner just two pieces of transmission line (or
simulated with components) with a resistor across two of the ports to
absorbe the diffeance if the loads/sources are not ballanced ?
The resistor is NOT to provide a load in case of an imbalance. It's
to provide an impedance match for a 180 degree out of phase path
between input/output ports. A signal that tries to go between the two
input/output ports has two paths along which it can go. One is down
one 1/4 wave coax, and up the other 1/4 wave coax, resulting in a 180
degree phase shift. The other is through the resistor with a 0 degree
phase shift. If everything is roughly impedance matched, the signals
through the two paths cancel, resulting in very good isolation between
ports.
As the subject is combining, I have not looked into the losses of splitting,
but it would be 6 db for the simple resistor designs not counting the minor
losses. That would be 3 db for the ports and 3 db lost in the resistors.
Yep, that's correct.
http://www.microwaves101.com/encyclopedia/resistive_splitters.cfm
To combind signals you would get the loss of the resistors of 3 db and a
fraction of other loss.
Yep, that's correct.
I am using a HP 8924C for a test set. It has just about everything you can
think of for a service monitor. Calibrated from 30 to 1000 MHz but usuable
uncalibrated to about a half of a mhz.
http://www.amtronix.com/hp8924c60.htm
http://axfp.org/god-bless-the-hp-8924c-a-tale-and-tutorial-of-the-service-monitor/
Nice. I'm into opening a museum of antique test equipment:
http://802.11junk.com/jeffl/pics/home/slides/test-equip-mess.html
Yes, phasing harnesses on antennas are not totally loseless, but will be
mainly whatever the loss of the coax is between the elements.
Nope. A phasing harness is much like the Wilkinson combiner without
the balancing resistor. Isolation between antennas would be nice, but
kinda futile with the antennas that close. Like the Wilkinson
combiner, the cables are odd multiples of 1/4 wave electrical. Like
the Wilkinson, such phasing harnesses have a limited bandwidth, where
losses increase the further away one gets from resonance. In other
words, you can't supply a single number for the losses in a phasing
harness. What's needed are numbers for the losses at resonance and at
band edges.
At this time, I still don't know if a Wilkinson combiner or phasing
harness will have sufficient bandwidth to cover the FM broadcast band.
That's 20 Mhz bandwidth at 100 Mhz or Q=5. I don't think that's
possible. To make my life more difficult, it's not possible to easily
model coax cables using NEC2. I've been using a mythical 50 ohm open
wire line, which can be modeled.
I would recommend either a messy multistage Wilkinson power
splitter/combiner, or go the broadband route with a common CATV/FM
power splitter/combiner.
If were the origional poster and there were not too many transmitters near
me, I would try a good preamp first. Mast mounted if possiable as it is for
receive only.
Preamps are a mixed blessing. With a good antenna, they can pickup
signals at impressive distances. However, they can also overload
miserably if there is a nearby transmitter on a nearby frequency. The
directionality of a Yagi is a big help, but if the nearby transmitter
is too close, the amplifier will overload, desensitize, belch
intermod, or otherwise cause problems. At best, the tower mounted amp
should be used only to compensate for coax losses. Any more gain than
that reduces the dynamic range of the system. Therefore, if the coax
cables is fairly short, and the cable is low loss, I wouldn't bother
with an amplifier. If the coax cable run is long and/or the coax is
junk, a tower mounted amp might be worth trying.
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060
http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558