On Sat, 4 Jan 2014 17:40:20 -0500, "Ralph Mowery"
wrote:
I agree that stacking antennas works, the problem I have is the type of
combiner that is used.
Well, one could always use an active combiner. For just the FM band,
that's quite easy.
The 30% was from another web site and I assume it was from someone at the
Winegard factory. Not sure why he would say 30% instead of db.
This may help explain the problem:
http://www.journalistunits.com
It doesn't include most electronic units of measure, but I think you
see the problem.
Even so 30
% is nowhere near the 3.5 db listed in the ad. From the url you gave, the
spec is for 3.5 db which is around what I would think it could be if simple
resistors were used.
Nope. A resistive combiner/splitter is -6dB.
http://www.microwaves101.com/encyclopedia/resistive_splitters.cfm
"Resistive power dividers are easy to understand, can be made
very compact, and are naturally wideband, working down to
zero frequency (DC). Their down side is that a two-way resistive
splitter suffers 10xlog(1/2) or 3.0103 dB of real resistive loss,
as opposed to a lossless splitter like a hybrid. Accounting for
3.0103 dB real loss and 3.0103 dB power split, the net power
transfer loss you will observe from input to one of two outputs
is 6.0206 dB for a two-way resistive splitter, so they are often
called 6 dB splitters. Dig?"
That combiner seems to be made not for stacking antennas for more gain, but
to combine several antennas either pointed at differant directions or so a
single feedline could be used for a TV and FM antenna or where you hae
seperate antennas on the same mast for UHF and VHF.
When one combines two different band antennas, the usual method is a
diplexer. Since the receiver only sees one antenna on each band, the
impedance is constant. A low pass filter can also be made very low
loss if you don't care much about rolloff and ripple. However, if
we're down to the point where small fractions of a dB produce a
noticeable difference, I suspect that additional gain (tower mounted
pre-amp) or less loss (better coax cable) will be more important.
Something like this:
http://www.solidsignal.com/pview.asp?p=uvsj
0.5 dB insertion loss. Oh well.
As the specs is for a 3.5 db loss, I assume that is if you hook up two
antennas to it, the antennas will have a gain of 3 db at the most, then you
go to the combiner and loose 3.5 db for an overall loss of .5 db.
Please re-read what I wrote. From each of the (input) ports to the
receiver port (output), there is only 0.5dB of loss. If two antennas
provide an additional 3dB of gain, and each port gobbles 0.5dB, then
the combined gain is 2dB.
That is where I don't see stacking two antennas and using that combiner for
more signal strength.
Would you rather make the yagi twice as long? Once we get to very
large antennas, 3dB of additional gain can easily become a mechanical
challenge.
I do agree that to get 3 db of gain from the antenna it would need to be
about twice as long. I did not look up to antenna to see that it was about
10 feet long already. A 20 foot long antenna would be large, but so would
two antennas 10 feet long and seperated by around 5 feet.
Note that FM broadcast stations with directional antennas use various
vertically mounted antennas, not Yagis. They're interested in
survivability as well as gain and pattern. A 20ft long antenna is
possible, but I don't think anyone wants to climb the tower and drop
the antenna to fix a broken element. That's much easier with a side
mounted barbeque grill type antenna, stacked dipoles, crossed dipoles,
horizontal loops, etc.
Maybe not too bad as I have several antennas on booms that are close to 15
feet long stacked about 5 feet apart. Not the best, but it was what I could
do for what I had to work with. You can see them on my QRZ.com page under
KU4PT.
You probably don't have overweight birds sitting on your yagi
elements. Yes, it can be made to work but it's so much easier and
neater to do it with a combiner.
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060
http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558