On Tue, 2 May 2017 07:01:32 -0700, Taxed and Spent
wrote:
On 5/2/2017 6:36 AM, Jeff Liebermann wrote:
Amazing. From the specs, it looks like there's a simple passive
splitter/combiner inside the box. $39 is quite a bit to pay for a $2
combiner.
it includes a bandpass filter, although apparently Channel Master wants
to keep that a secret!
from http://www.warrenelectronics.com/ant...Jointennas.htm
Thanks for the details on the device. As you've noticed, there is
much more going on behind the curtain.
Under the heading on the above page:
"(For blocking or passing a single channel)"
"JoinTennas are NON-RETURNABLE!
Limited to stock on hand - no back-orders!"
There seems to be more than one model. In the "Antenna Coupler
Typical Applications" on the right of the page, the 3 examples show
some of the part numbers to be:
Model 0578, Model 0585-2, Model 0576, and Model 0579
The multiple models, combined with the non-returnable ordering
suggests that this is a custom device, tuned to frequency. The chart
at the bottom of the page shows only a few UHF channels available.
UHF Channels Avail. Model*
17, 18, 19 0585-1
53, 54, 55, 57, 58 0585-2
which is odd as other model numbers are mentioned under "Antenna
Coupler Typical Applications". The limited channels may be only what
the dealer stocks for his local channels.
The filter must be rather wide as the warning suggests:
"Note: There is significant attenuation on either side of
the channel the JoinTenna is tuned for. We do not recommend
using a JoinTenna if you are trying to receive a channel
adjacent to your specified channel."
None of this appears on the product page, data sheet, or installation
manual:
https://www.channelmaster.com/JOINtenna_p/cm-0500.htm
I like the first users comments:
"Having a rotating antenna was not the solution because
with today's digital tuners, every time you rotate the
Antenna, you must rescan your channels."
I guess he doesn't know how to manually add channels.
The Channel Master JoinTenna is perfect for those situations when you
need to add a second antenna to pick up a broadcast station in another
direction but don't wish to use a single antenna and rotator. The
JoinTenna blocks all frequencies but the one it is tuned for,
eliminating the ghosting and reflection that can happen when you connect
two antennas together.
With digital TV, one does not see ghosting or reflections on the
screen. The modern term is "multipath".
http://www.hdtvprimer.com/ANTENNAS/glossaryG.html#multipath
http://www.hdtvprimer.com/ANTENNAS/siting.html
See section under "Skyline Multi-path".
If it blocks everything except the channel to which it's tuned, it
must contain a BPF (bandpass filter) which adds some loss. The -2dB
loss specified seems rather optimistic. The splitter/combiner
typically has -1dB loss. A single channel BPF would have somewhat
more than -1dB. My guess(tm) is the loss through the device is
somewhat more than -2dB. There might also be a corresponding notch
filter on the other port, also with some loss.
Seems like this device is a usable solution if:
1. One antenna is intended to only receive one distant station.
2. The signal strengths of most stations are strong to overcome the
losses.
I think it fair to suggest that Channel Master would not have gone
though all the trouble of installing filter(s) if there was no problem
with interaction between antennas using 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