Re: [Repeater-Builder] Re: Dual receivers on one antenna for RX only site

[no6b]

At 3/10/2010 02:42, you wrote:

>On Mar 9, 2010, at 8:37 PM, n...@no6b.com wrote:
>
> > t most certainly does. Try random length cables from the cavities to the
> > T instead of 1/4 wavelength (like one local did several years ago) & watch
> > your sensitivity drop by over 20 dB if you're unlucky (as he was). That
> > mistake literally killed off a local radio club, as few of the members 
> were
> > able to use the repeater following the addition of the T & wrong cables.
>
>Thanks both Bob and Skipp for explaining that one odd-ball configuration 
>that would crush the receivers with random cable lengths that just happen 
>to hit the right "sweet spot" to do this.
>
>I suspect, that if someone saw a 20 dB loss while installing this setup, 
>they'd at least STOP and start asking questions -- maybe they wouldn't 
>"get it" that they'd hit this "perfect storm" combination -- maybe they'd 
>think they had some kind of receiver failure when it suddenly was "really 
>deaf" --  but I also doubt that *most* people would hit the problem.
>
>Would you agree with that assessment?  (Skipp's comment that if there's a 
>train wreck to be found, he'll be there... I know that feeling.)

In this case, the owner wrote the poor sensitivity off to site noise.  The 
club was based a good 30 miles from the repeater, but before it was 
transferred it worked just fine in the target area.  After the system was 
modified/deafened, an article was written in the club newsletter explaining 
how the repeater was too far away from the club's user base for HTs to work 
there.  Funny how after the repeater was sold off to yet another trustee, 
it suddenly began to receive well again.  That's when I found out what was 
done that made it so deaf in the interim.

Bob NO6B

Re: [Repeater-Builder] Re: Dual receivers on one antenna for RX only site

[no6b]

At 3/10/2010 11:16, you wrote:


>Actually, Gary, you are 180 degrees out. On a pass cavity, off frequency 
>signals see a very high impedence path, an open not a short. If your 
>version were true you could never use pass cans as a duplexer since both 
>sets of cans together would show a "short" to EVERYTHING.

I wish I had the VNA data from the pass cavities I measured several years 
ago when I built a 2-port UHF combiner using them, but they were measured 
while the pen plotter was connected to the VNA & before I wrote a program 
to convert the Citifile output from the VNA to Excel spreadsheets, so the 
data was only saved on paper & I have no idea where I would've stuffed the 
plots.

But my best recollection is that at the reference plane of the cavities 
(front surface of the female N or SO-239 connector, they looked fairly 
close to an open, but not quite - maybe 10 to 15 degrees off of an open, on 
the inductive side.

Bob NO6B

RE: [Repeater-Builder] Re: Dual receivers on one antenna for RX only site

[Jeff DePolo]

> When I say the notch presents a "short" it is not really a 
> short but a very low impedance of say a few ohms. But by 
> having the unwanted source impedance high rather than at 50 
> ohms it is much easier to pull the high impedance down with 
> the "few ohms" short circuit than it would be if we were 
> working at 50 ohms for the unwanted.
> 
> It works like a voltage divider between the two impedances. 
> The higher the source is (from previous cavity) to the short 
> the more loss there will be which is just what we are looking for.

Your use of the voltage divider description is probably the best way to
explain the effect, and shows why having the correct intra-cavity cable
length is important for getting the maximum rejection.  

Using exactly the "right" cable length between two cavity filters will give
somewhere around 5 or 6 dB of additional rejection (i.e. if the cavities
individually afforded 40 dB of isolation, when cascaded with the right cable
length you'll get about 85 dB total).  If you use exactly the "wrong" cable
length (i.e. if you're off by a quarter-wave), the combined isolation will
be LESS than that of the cavities individually; a good indication that you
have the "wrong" cable length is when you can't get the notches to overlay
at the same frequency (often the notches will look like they're "chasing
each other" on the VNA as you adjust them).  Between the "right" and "wrong"
cable lengths you'll end up with a net isolation somewhere between the two
extremes.  Assuming the cavities individually present a good match at the
pass frequency, varying the interconnecting cable length between two
cascaded filters will not affect the insertion loss or return loss.

In contrast, the cable length from the tee to the first cavity on each side
of the duplexer primarily affects only the insertion loss through the
duplexer, and the return loss from antenna to/from either Tx or Rx port,
unlike the cable between adjacent cavities which affects the isolation
(rejection) afforded by that half of the duplexer.

A while back I posted a blurb that demonstrates the effects of using the
"wrong" cable length between cascaded filters.  I don't know if that made it
to the web site?

--- Jeff WN3A

RE: [Repeater-Builder] Re: Dual receivers on one antenna for RX only site

[Gary Schafer]

Well yes the T is sort of a magical device that makes the OTHER SIDE of the
T disappear electrically. Actually it is not the T itself that does the job
(that is just where IT happens) but it is the quarter wave length cables
that perform the magic!  

 

Without the quarter wave length cables between the T and each set of
cavities the duplexer would not work! That is what provides the 50 ohm
isolation between tx and rx cans so the feed line still sees 50 ohms.

The quarter wave cable effectively "disconnects" the transmitter from the
feed line at the T (at the receive frequency).

The quarter wave cable on the receive side of the T effectively disconnects
the receive side from the feed line (at the transmit frequency). 

Without doing this each would load the other down and there would not be 50
ohms at the antenna port of the T.

 

Once you are on the other side of the T (the antenna port) the feed line
length has no effect on the duplexer operation. All that the quarter wave
lines do on the duplexer side of the T are to give isolation to the opposite
side (tx-rx) so each does not short out the feed line.

 

A similar thing happens between can cables in a duplexer but rather than
using them for isolation they are used to enhance the notch of each can by
presenting a high impedance at each cans T from the previous cavity. Working
with a high impedance is easier to notch out than a low impedance.

 

The notch in the first cavity presents a short (low impedance) at the
unwanted  frequency and 50 ohms at the wanted frequency. By coupling the
next cavity with a quarter wave length cable (at the unwanted frequency)
that short is transformed to a quite high impedance at the next cavity while
at the same time the wanted signal being at 50 ohms is passed to the next
cavity where it sees 50 ohms and goes on its way unatenuated. But we are
left with the high impedance at the unwanted frequency that was transformed
by the quarter wave cable. The second cavity notch is also tuned to the
unwanted frequency which it pulls down to a short (low impedance) to give
further attenuation.

 

When I say the notch presents a "short" it is not really a short but a very
low impedance of say a few ohms. But by having the unwanted source impedance
high rather than at 50 ohms it is much easier to pull the high impedance
down with the "few ohms" short circuit than it would be if we were working
at 50 ohms for the unwanted.

It works like a voltage divider between the two impedances. The higher the
source is (from previous cavity) to the short the more loss there will be
which is just what we are looking for.

 

In the case of the quarter wave cable to the T on the output of the duplexer
we want to transform the low impedance up to a very high impedance at the T
so that it does not load the circuit at that point on that frequency.

 

73


Gary  K4FMX

 

 

  _  

From: Repeater-Builder@yahoogroups.com
[mailto:repeater-buil...@yahoogroups.com] On Behalf Of Dan Hancock
Sent: Wednesday, March 10, 2010 12:50 PM
To: repeater builders
Subject: [Repeater-Builder] Re: Dual receivers on one antenna for RX only
site

 







One thing was missed regarding cable lengths. The loops in the cans are part
of the equation for figuring the 1/4 wave length. I've seen that discussed
here many times in postings related to inter-cable lengths on duplexers. But
the 1/4 wave length issue only applies to the inter-cabling between the
cans.
It is my understanding that the antenna to duplexer lengths are irrelevant
since the T connector and the rest of the feedline are all part of the
equation. It's not like the T is some magical device that makes the rest of
the feedline disappear electrically. The only time length might be a problem
is if the entire feedline happens to be a resonant length. If by some chance
that happens, then changing the jumper a couple of inches will clear that.

Dan N8DJP

Posted by: "n...@no6b.com
 "
n...@no6b.com 
no6b
Date: Tue Mar 9, 2010 8:29 pm ((PST))

At 3/9/2010 20:12, you wrote:


>OK, question...
>
>If you put a cable which is 1/4-wavelength at VHF between the T and the 
>UHF cavity, it's 3/4-wavelength at UHF. Since any odd multiple of a 
>quarter wavelength will invert the impedance, what will this really 
>accomplish on the UHF cavity side?

Doesn't matter at UHF, since the cavity "looks" like (hopefully something 
close to) 50 + j0 ohms @ UHF, so the cable length has no effect (other than 
plain ol' cable loss) @ UHF.  At VHF, the short at the UHF cavity connector 
(I'll take Gary's word that it looks like a short off-resonance, though to 
be sure you'd want to put the can on a VNA to get the actual phase angle at 
the connector) needs to be transformed to an open at the T so it has no 
effect & VHF.  The short-to-open transformation @ VHF is accomplished with 
a 1/4 wavelength of coa

RE: [Repeater-Builder] Re: Dual receivers on one antenna for RX only site-cable length

[Gary Schafer]

Good point. Yes loop length needs to be considered. Usually the velocity
factor of the loop is that of air so it needs to be calculated seperatly
from the cable and added.

Probably easiest to ask the cavity manufacturer. 

73
Gary  K4FMX

> -Original Message-
> From: Repeater-Builder@yahoogroups.com [mailto:Repeater-
> buil...@yahoogroups.com] On Behalf Of larynl2
> Sent: Wednesday, March 10, 2010 12:54 PM
> To: Repeater-Builder@yahoogroups.com
> Subject: [Repeater-Builder] Re: Dual receivers on one antenna for RX
> only site-cable length
> 
> 
> In all of the discussion on cable lengths between a T and cavities to
> split to receivers, I'm wondering if the loop length inside of each
> cavity is to be included in cable lengths.  It seems it always is
> included when calculating cavity interconnect cables on a duplexer, for
> example, but has not been mentioned in this thread.
> 
> If loop length IS to be included, what is the assumed velocity factor of
> a cavity loop?
> 
> Laryn K8TVZ
> 
> 
> 
> 
> 
> 
> 
> Yahoo! Groups Links
> 
> 
> 

RE: [Repeater-Builder] Re: Dual receivers on one antenna for RX only site

[Gary Schafer]

e
receive end.
You end up with the same kind of loss that you get on a transmitter due to
high vswr.

73

Gary  K4FMX




> -Original Message-
> From: Repeater-Builder@yahoogroups.com [mailto:Repeater-
> buil...@yahoogroups.com] On Behalf Of Nate Duehr
> Sent: Wednesday, March 10, 2010 5:42 AM
> To: Repeater-Builder@yahoogroups.com
> Subject: Re: [Repeater-Builder] Re: Dual receivers on one antenna for RX
> only site
> 
> 
> On Mar 9, 2010, at 8:37 PM, n...@no6b.com wrote:
> 
> > t most certainly does. Try random length cables from the cavities to
> the
> > T instead of 1/4 wavelength (like one local did several years ago) &
> watch
> > your sensitivity drop by over 20 dB if you're unlucky (as he was).
> That
> > mistake literally killed off a local radio club, as few of the members
> were
> > able to use the repeater following the addition of the T & wrong
> cables.
> 
> Thanks both Bob and Skipp for explaining that one odd-ball configuration
> that would crush the receivers with random cable lengths that just
> happen to hit the right "sweet spot" to do this.
> 
> I suspect, that if someone saw a 20 dB loss while installing this setup,
> they'd at least STOP and start asking questions -- maybe they wouldn't
> "get it" that they'd hit this "perfect storm" combination -- maybe
> they'd think they had some kind of receiver failure when it suddenly was
> "really deaf" --  but I also doubt that *most* people would hit the
> problem.
> 
> Would you agree with that assessment?  (Skipp's comment that if there's
> a train wreck to be found, he'll be there... I know that feeling.)
> 
> I guess what I'm saying here in a round-about way is... random cable
> lengths really shouldn't be that much of an issue in a setup like this,
> but yeah... agreed... once in a while it'll bite you like an alligator
> (had to get that elephant/alligator theme in here, just one more time!
> GRIN!)...
> 
> I've seen lots of people get away with it.
> 
> As far as the 3dB lost in a true broadband splitter -- also true, of
> course, Bob -- at most of the sites where we have to share a receive
> antenna with multiple rigs, the site measured noise-floor is so high the
> 3dB doesn't have much of an impact... just keeping the local crud out of
> the receivers is difficult enough -- sometimes that 3dB loss helps,
> instead of hinders, so to speak.  :-)
> 
> I guess we should all probably also mention the evils of not terminating
> all the unused ports on a multi-splitter with 50 Ohm loads, too... if
> we're going to get this picky, right?  ;-)
> 
> --
> Nate Duehr, WY0X
> n...@natetech.com
> 
> facebook.com/denverpilot
> twitter.com/denverpilot
> 
> 
> 

Re: [Repeater-Builder] Re: Dual receivers on one antenna for RX only site

[Nate Duehr]

On Mar 9, 2010, at 8:37 PM, n...@no6b.com wrote:

> t most certainly does. Try random length cables from the cavities to the 
> T instead of 1/4 wavelength (like one local did several years ago) & watch 
> your sensitivity drop by over 20 dB if you're unlucky (as he was). That 
> mistake literally killed off a local radio club, as few of the members were 
> able to use the repeater following the addition of the T & wrong cables.

Thanks both Bob and Skipp for explaining that one odd-ball configuration that 
would crush the receivers with random cable lengths that just happen to hit the 
right "sweet spot" to do this.

I suspect, that if someone saw a 20 dB loss while installing this setup, they'd 
at least STOP and start asking questions -- maybe they wouldn't "get it" that 
they'd hit this "perfect storm" combination -- maybe they'd think they had some 
kind of receiver failure when it suddenly was "really deaf" --  but I also 
doubt that *most* people would hit the problem.

Would you agree with that assessment?  (Skipp's comment that if there's a train 
wreck to be found, he'll be there... I know that feeling.)

I guess what I'm saying here in a round-about way is... random cable lengths 
really shouldn't be that much of an issue in a setup like this, but yeah... 
agreed... once in a while it'll bite you like an alligator (had to get that 
elephant/alligator theme in here, just one more time!  GRIN!)...

I've seen lots of people get away with it.

As far as the 3dB lost in a true broadband splitter -- also true, of course, 
Bob -- at most of the sites where we have to share a receive antenna with 
multiple rigs, the site measured noise-floor is so high the 3dB doesn't have 
much of an impact... just keeping the local crud out of the receivers is 
difficult enough -- sometimes that 3dB loss helps, instead of hinders, so to 
speak.  :-)

I guess we should all probably also mention the evils of not terminating all 
the unused ports on a multi-splitter with 50 Ohm loads, too... if we're going 
to get this picky, right?  ;-)

--
Nate Duehr, WY0X
n...@natetech.com

facebook.com/denverpilot
twitter.com/denverpilot

Re: [Repeater-Builder] Re: Dual receivers on one antenna for RX only site

[wd8chl]

Ross Johnson wrote:
> Thanks for the reply’s everyone. That cleared it up for sure. I will go
> ahead and build the T to cavity cables to one electrical wave length for
> the other band. And is that ¼ wave plus velocity factor of cable? Which
> will be FSJ1. 

Actually, it's 1/4-wave times the velocity factor, sorta. If the VF is, 
say, 85%, then you multiply the 1/4-wave by .85.

Also, you can use any ODD multiple of a 1/4-wave: 3/4, 1-1/4, 1-3/4, 
etc. It'll be a real pain to try to connect to that 6" long(-ish) UHF 
1/4-wave cable!







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RE: [Repeater-Builder] Re: Dual receivers on one antenna for RX only site

[Ross Johnson]

Thanks for the reply’s everyone. That cleared it up for sure. I will go
ahead and build the T to cavity cables to one electrical wave length for
the other band. And is that ¼ wave plus velocity factor of cable? Which
will be FSJ1. 
 
Here is some more detail on the system. It will go in stages. The final
stage will be this remote receive setup with a UHF link on the bottom of
the tower to the transmitter site. Also toying with a VOIP link with UHF
as a failsafe. At this point the receivers are on separate antennas at
the top of the tower, with 2 bandpass Sinclair 1-150-1S7 cavity’s on the
VHF, and one big Wacom cavity on the UHF receiver. The remote TX site
hasn’t been installed yet (waiting to find a MSR2000 UHF RX board for
this divorced VHF TX site) so the transmitter is temporally at this site
also. There are two bandpass cavity’s DB4001’s on this Mastr II
transmitter with the antenna 40-50 feet down from the receive antenna.
Sensitivity is shocking good right now with this setup. Very little RX
loss, and very little desens. 
 
Will the receivers stay on one frequency as in a repeater 
receiver or do you need to move around each band a bit?
 
Yes they will stay put.
 
How much other RF is around? ... Does the site have a lot 
of transmitters and are any of the high power monsters as 
in the case of paging or broadcast?
 
None in these bands! :-) But wireless ISP I’ve found to be very noisy
allover the place there. 50Mhz and up!
 
Thanks again everyone! 
 
 
 
 
 
-Original Message-
From: Repeater-Builder@yahoogroups.com
[mailto:repeater-buil...@yahoogroups.com] On Behalf Of skipp025
Sent: Tuesday, March 09, 2010 4:29 PM
To: Repeater-Builder@yahoogroups.com
Subject: [Repeater-Builder] Re: Dual receivers on one antenna for RX
only site
 
  

> "Ross Johnson"  wrote:
> Hello to the group, 
> My name is Ross KC7RJK This is my first post. 

Hi Ross, 

My name is skipp and I'm a junkoholic... 

"hi skipp" 

and I %#*^& scuse me, lost my mind for a moment. 
Moving along 

> Most questions are answered from that amazing and up 
> to date web site! I thank you all involved very very 
> much for that. 

We don't play up the RB web site nearly enough... we also 
don't let Kevin, Scott or Mike run with scissors. 

> Well here's the question I've found little and conflicting 
> info on the web about. So feel free to point me the right 
> way here.

Simple, go west... better weather and less humidity. 

> Can a dualband antenna VHF/UHF for RX ONLY be fed to 
> two receivers one VHF, one UHF, without a quote "duplexer" 
> using a T instead? 

Of course, but it may not be the best situation. 

> Here's the idea. This is a remote RX site. The idea is 
> to run something like a beefed up X500 dualbander at tower 
> top, then 7/8 hardline 100 feet down to the receivers. 
> Both receivers will have one or two bandpass cavities
> inline before the T. Would a duplexer be necessary in 
> this case. Or could it be done with proper cable lengths 
> and a T?

Doesn't even need the special cable lengths but there is 
a reason for doing everything and here comes questions 101. 

Will the receivers stay on one frequency as in a repeater 
receiver or do you need to move around each band a bit? 

How much other RF is around? ... does the site have a lot 
of transmitters and are any of the high power monsters as 
in the case of paging or broadcast? 

If you don't have a lot of adjacent frequency operation 
going on there are two other options to consider. One is 
the Diamond or Comet type of band splitter, which actually 
would take the place of your T and be much better. 

Model CF-4160K 

http://www.universa

l-radio.com/CATALOG/hamantm/cduplex.html

And another very nice option would be the DCI dual band 
filter Model: DCI-146-444-DB. 

http://www.dci. 
ca/?Section=Products&SubSection=Amateur 

And you can use the plain T, a more traditional signal 
divider and various combination of band-pass cavity layouts. 

> Thanks for your time and for the probably obvious 
> answer I'm not sure of.
> Regards
> Ross KC7RJK 

be more worried when you feel sure of yourself. 

cheers, 
skipp 

Re: [Repeater-Builder] Re: Dual receivers on one antenna for RX only site

[no6b]

At 3/9/2010 16:29, you wrote:

> > Here's the idea. This is a remote RX site. The idea is
> > to run something like a beefed up X500 dualbander at tower
> > top, then 7/8 hardline 100 feet down to the receivers.
> > Both receivers will have one or two bandpass cavities
> > inline before the T. Would a duplexer be necessary in
> > this case. Or could it be done with proper cable lengths
> > and a T?
>
>Doesn't even need the special cable lengths

It most certainly does.  Try random length cables from the cavities to the 
T instead of 1/4 wavelength (like one local did several years ago) & watch 
your sensitivity drop by over 20 dB if you're unlucky (as he was).  That 
mistake literally killed off a local radio club, as few of the members were 
able to use the repeater following the addition of the T & wrong cables.

>but there is
>a reason for doing everything and here comes questions 101.
>
>Will the receivers stay on one frequency as in a repeater
>receiver or do you need to move around each band a bit?

If he's got bandpass cavities in front of the RXs already, they're very 
likely not frequency-agile.

>How much other RF is around?  ... does the site have a lot
>of transmitters and are any of the high power monsters as
>in the case of paging or broadcast?
>
>If you don't have a lot of adjacent frequency operation
>going on there are two other options to consider. One is
>the Diamond or Comet type of band splitter, which actually
>would take the place of your T and be much better.

That would be my choice, but if he's already got the cans, a pair of 1/4 
wavelength cables will be much cheaper.

I'd stay away from using a broadband isolated power divider (splitter), as 
you'll lose 3 dB in the split.  The frequency-splitting options lose 
virtually no signal.

Bob NO6B