Jeff Liebermann wrote:

There are calculators that will predict the isolation.
......
"Minimum Tx and Rx frequency difference: VHF 3.5Mhz"
That's not going to work with 0.6MHz spacing on the 2m ham band.

This is what a proper (Phelps-Dodge) 2m duplexer looks like:
http://www.LearnByDestroying.com/k6bj/K6BJ%20Repeater/slides/2m%20duplexer2.html
Note the much large size cavities.

Thanks, that's exactly what I needed to know.

Geoff.


--
Geoffrey S. Mendelson N3OWJ/4X1GM
Making your enemy reliant on software you support is the best revenge.

Jeff Liebermann wrote:
Take two radios, two j-poles and a lot of coax and make a repeater.

Retch.


Why? Don't you make experimental stuff? I did not say it was a permanent
repeater, it's actually a test bed for some repeater/internet linking
systems we are trying out.

Budget is zero, but I can swing a roll of rg-6 and maybe a $60 duplexer
if I don't buy anything else (radio/computer related) that month.

Plugging into:
http://awapps.commscope.com/products/bsa/_calculators/qhisolation.asp
I get about 30dB isolation (assuming 2dBi antenna gains).
The synthesizer noise belching from your xmitter is maybe -60dB down
from the +43dBm xmit carrier. That puts the noise level at -17dBm at
the receive antenna. Your receiver sensitivity is probably -106dBm,
which means you need about 90dB of isolation. The antenna spacing
will provide 30dB of that. You cavities are suppose to provide the
remaining 60dB of isolation. That's not going to happen with tiny
mobile duplexer cavities.

OK, thanks.


Hint1: You'll be close with vertical antenna isolation.

Explain, do you mean just the distance if they are both vertical, or
the distance AND one is vertical and the other horizontal?

Hint2: Some radios have quite a bit of synthesizer noise, much of
which will be on your repeater receive frequency. This is why some
repeaters still use crystal oscillators instead of synthesizers.

Also almost irrelevant. The radios wll be what I have, HTX-202s or what
I can scrounge (TBD). I'd love to get my hands on 2 channel Maxtracs
or similar radios out of taxis, but the people sitting on piles of them are
much more interesting in refurbishing and enhancing them than selling them
cheaply or giving them away.

I'd be very happy with ones that receive only or have blown finals that I
can get 2w or so out of.


One will be used to receive a signal, the other to relay it. They both
will be somewhere on the 144-146mHz band, with the output being a few watts
with a max of 20.

Somewhere? Duplexers are usually bench tuned to some specific
frequency. It's not a trivial exercise and requires some expenditures
in time and equipment. You can't easily move in frequency.

I know, but I don't have a "pair" yet. I'm not even sure there are any
available or a test one. So why be specific when I don't have that information
yet, and why wait for it to ask a technical question, which will have
the same answer no matter which frequencies I use?

You can't improve things by simply adding more cavities. All you'll
do is add more loss:
Bigger cavities = higher Q and therefore closer frequency spacing.
More cavities = deeper notch and therefore more isolation.

And that's the answer I need.

Thanks,

Geoff.

--
Geoffrey S. Mendelson N3OWJ/4X1GM
Making your enemy reliant on software you support is the best revenge.

On 8/30/2011 6:01 AM, Rob wrote:
Geoffrey S. wrote:
Rob wrote:

No. There are two sides, one of them for the high frequency and the
other for the low frequency. The filter response is such that it passes
on one frequency and notches on the other.

Ok thanks. Note that they are tunable anywhere in the 136-17? range.

Assuming the pass side is set to pass 145.600 (the input) and the notch
side is set to notch out 145.000 (the output) and they are connected in
series, would that work?

It is not a pass side and a notch side. Both sides are pass/notch.

But on one side the pass freq is above the notch freq, and on the
other side the pass freq is below the notch freq.

On the Chinese duplexers you have found, the Q is probably not high
enough to tune the pass and notch to frequencies 600 kHz apart.

one might be able to cascade two of them, but Rob's point about Q is
well taken.

It depends on whether the problem is the "depth of rejection" or the
"steepness of skirt". A single resonant unit has a narrow spike on top
of gentle shoulders. Say one section has Q of 1000 (i.e. the 3dB
bandwidth is 100kHz for 100MHz center frequency), but the rejection at 1
MHz away is only 20dB. I could cascade 3 sections, tuned exactly the
same, and I'd have 9dB loss at 100kHz, and 60dB rejection at 1 MHz away.
The 3dB bandwidth might be around 30kHz, or, you could stagger the
tuning slightly to get a little broader flat top, but keep the 60dB
(i.e. moving the center frequency of one cavity by 10kHz won't change
the attenuation at 101 MHz)

The design challenge is that you need to suppress the transmit signal
(at, say, 50W, +47dBm) low enough so that the front end of the receiver
can handle it without blocking (say, you want it down around -50dBm).
With our not so hot cavities described above, you'd need to stack up 5
sections at 20dB per section to get the 100dB suppression. Well, that's
not so great, because you probably now have a bunch of additional loss
in the receive path, AND you've got a real tuning chore on your hands to
make sure that all of them are tuned appropriately to get the required
bandwidth.

Enter the idea of a notch. Let's say our example is 101 MHz Tx and 100
MHz Rx. Rather than rely on the "far away" response of a resonator to
suppress the transmit, I can put a 101 MHz notch filter on the input to
my receiver. If I can get 30dB/section rejection, then 3 or 4 sections
will knock the Tx power down low enough to not block the receiver's
front end.

But wait, there's more... that Tx isn't a narrow spike. It has phase
noise sidebands that go out fairly far. Unfortunately, the run of the
mill transmitter might only suppress "off channel spurious and noise"
by, say, -50dBc (and a rig designed to operate half duplex is probably
worse). That means that your 50W (+47dBm) transmitter is putting out
-3dBm *at the receive frequency*, so no amount of filtering on the
receive path will help. Again, enter the notch filter.. you put a 100
MHz notch on the output of the transmitter.

Now.. as Rob points out, the Q might be kind of low. If the Q is, say,
100, then that 100MHz filter is 1MHz wide, and by the time you cascade
enough to get the BW down, the loss will be huge.


As you can see, this whole duplexer/diplexer/multicoupler design thing
can get pretty complex, and there's not usually a simple cookbook answer.

On 8/30/2011 8:29 AM, Geoffrey S. Mendelson wrote:
Allodoxaphobia wrote:

We still don't have a clear description of what the OP is trying to
accomplish.

Take two radios, two j-poles and a lot of coax and make a repeater.
Obviously I will need some sort of controller in between but that's
irrelevant.

I want to place one J-pole (or similar antenna) at one corner of my garden,
and another at the other one (10 meters apart).


This will actually help a bit

The free space attenuation for 10 meters at 144MHz is
32.4+20*log10(0.01)+20*log10(144) 32.4-40 +43.2 = about 36dB.

If your Tx is 20W (+33dBm), then the receiver is going to see about
-3dBm at the Tx frequency. If you want to get the Tx signal down to
around -40dBm or so, you only need 30-40 dB of attenuation at the Tx freq

Then there's the Tx phase noise to worry about. Figure it's 60dB down,
so +33-60-36 = about -63dBm.. probably enough to completely mask any
received signals (what's your sensitivity.. -130dBM?) so you need a
notch with 70dB rejection on the output of your Tx.

You're 600kHz away, implying you need a Q better than a few hundred.
If you just tune up some of those cans for that, and the Q is too low,
then you'll be attenuating your transmitter.


Any reason why you're using a split of 600kHz? Why not go to a
non-standard split to make life easier. Pick two frequencies 3 MHz apart
(assuming you can get them coordinated, which is more a political than a
technical issue)

One will be used to receive a signal, the other to relay it. They both
will be somewhere on the 144-146mHz band, with the output being a few watts
with a max of 20.

Besides the distance, I was looking for a cheap way of not having
the transmitted signal block the receiver. The duplexer in question
is rated at 75dB isolation with a 3.5mHz split, I can only have .6 mHz.

What I was wondering is that since at a .6mHz split, the isolation will
be a lot less, can I somehow combine the two sides to make a better
filter?

Thanks,

Geoff.

On Tue, 30 Aug 2011 16:29:03 +0000 (UTC), "Geoffrey S. Mendelson"
wrote:

Jeff Liebermann wrote:
Take two radios, two j-poles and a lot of coax and make a repeater.

Retch.

Why? Don't you make experimental stuff?

I don't like J-pole antennas. I've also built too many repeaters in
the past to know that they can't just be thrown together from
available parts. One of my friends learned that expensive lesson with
his 2m repeater. He started with two ham transceivers, went to two
Maxtrac radios, and finally to a GE MSTR II. Duplexers went from 4
cavity junk, to a 6 cavity monster.

I did not say it was a permanent
repeater, it's actually a test bed for some repeater/internet linking
systems we are trying out.

Sounds interesting. Plan on everything costing at least 4 times
what's predicted, and for things to take perhaps 4 times as long.
Unplanned repairs and "tweaks" should double the required time.

Budget is zero, but I can swing a roll of rg-6 and maybe a $60 duplexer
if I don't buy anything else (radio/computer related) that month.

Initial budget can be zero. Just plan on a replacement budget equal
to the price of a proper repeater.

How's this for an expensive mistake?
http://802.11junk.com/jeffl/K6BJ-MSF5000/
I've burned about $800 on this mess and it's still not on the air.
Wanna guess what went wrong? (Hint: it involves the internal
duplexer).

http://awapps.commscope.com/products/bsa/_calculators/qhisolation.asp

Hint1: You'll be close with vertical antenna isolation.

Explain, do you mean just the distance if they are both vertical, or
the distance AND one is vertical and the other horizontal?

Assuming vertically polarized monopole antennas, vertical isolation is
from tip of one antenna to the tip of the other antenna. Horizontal
isolation is just the horizontal spacing. If they're seperated both
vertically and horizontally, then there's a problem. You only need to
be offset horizontally slightly, for the antennas to couple. I ran
into this problem when I tried to put two vertically seperated
antennas on opposite sides of a tower. The isolation was better then
they were directly in line with each other, on the same side of the
tower. Incidentally, mounting one antenna upside down offers
additional distance and improved isolation.

Hint2: Some radios have quite a bit of synthesizer noise, much of
which will be on your repeater receive frequency. This is why some
repeaters still use crystal oscillators instead of synthesizers.

Also almost irrelevant. The radios wll be what I have, HTX-202s or what
I can scrounge (TBD).

Retch 2.0. Radio Shock HTX-202 handhelds tend to stick on the air
with low battery voltage. The receivers are also easily overloaded.
We have the remaining few HTX-202 radios used for APRS WX packet. I
would love to replace them, but nobody (including me) want's to spend
the time.

I'd love to get my hands on 2 channel Maxtracs
or similar radios out of taxis, but the people sitting on piles of them are
much more interesting in refurbishing and enhancing them than selling them
cheaply or giving them away.

Patience. On Jan 1, 2013, everything in the US is suppose to go
narrow band. There should be tons of cheap radios available on eBay
and from public safety outlets.

I'd be very happy with ones that receive only or have blown finals that I
can get 2w or so out of.

Make sure you get the ones with the 16 pin accessory connector. Many
of the Maxtrac's on eBay are actually cobbled together Radius or M120
radios, with odd boards and strange firmware. Caveat emptor.

I know, but I don't have a "pair" yet. I'm not even sure there are any
available or a test one. So why be specific when I don't have that information
yet, and why wait for it to ask a technical question, which will have
the same answer no matter which frequencies I use?

Careful. Some repeater frequencies are a problem. For example,
146.760 is on the 41st harmonic of the common 3.579545MHz clock osc
frequency. No problem at the repeater, but it will drive the users
nuts.

If co-located with other radios, be sure to do an intermodulation
study.
http://www3.telus.net/PassiveRF/
We found out too late that a mix of some of our transmit frequencies
lands on one of our inputs.

You can't improve things by simply adding more cavities. All you'll
do is add more loss:
Bigger cavities = higher Q and therefore closer frequency spacing.
More cavities = deeper notch and therefore more isolation.

And that's the answer I need.

You might find it easier to build a UHF repeater. With 5MHz spacing,
the cheap mobile duplexers will work with low power (25w) radios.

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

Jim Lux wrote:
Any reason why you're using a split of 600kHz? Why not go to a
non-standard split to make life easier. Pick two frequencies 3 MHz apart
(assuming you can get them coordinated, which is more a political than a
technical issue)

Don't forget that in Region 1, the 2M band is only 2 MHz wide, the
bottom 500 kHz is dedicated to SSB and the top 200 kHz is for satellite.

On 8/30/2011 11:14 AM, Rob wrote:
Jim wrote:
Any reason why you're using a split of 600kHz? Why not go to a
non-standard split to make life easier. Pick two frequencies 3 MHz apart
(assuming you can get them coordinated, which is more a political than a
technical issue)

Don't forget that in Region 1, the 2M band is only 2 MHz wide, the
bottom 500 kHz is dedicated to SSB and the top 200 kHz is for satellite.

Sure, that does make it harder. The OP is in region 2, though, where
there's 4 MHz theoretically available)

flame protective suit on And of course, a lot of the band plans are
basically gentlemen's agreements and have no force of law. As long as
you don't interfere with someone (challenging in some geographical
locations), you can pretty much do what you want.flame suit off

I wouldn't want to put input or output on 144.2, for instance.

But looking at the ARRL band plan, you could put your input up at
147.6-147.99 and put your output at 145.5-145.8 (Misc and experimental,
per ARRL).. not quite 3 MHz, but close, and a whole lot better than 600kHz.

Here in the Los Angeles area, either TASMA or the anti-TASMA folks would
probably round up people to have you tarred, feathered and run out of
town on a rail, no matter what you do. In any case, things move so
slowly that unless you were hideously inconsiderate and egregiously
interfering, you could probably run for a year or more before it would
get too nasty.

There is a 147.585/144.930 pair labeled for "portable repeater" in the
TASMA plan (max 72 hrs/month).. that's 2.5 MHz apart (to make filtering
easier). They also say 144.310-144.375 is for simplex unchannelized,
but I've not heard much on the air there..for an experiment, it would
probably work.

"Geoffrey S. Mendelson" wrote in message
...


snip


Geoff.


If your intent is to experiment with the cheap duplexer, what I have to
offer is irrelevant. However, if you aim to build a repeater, consider
getting your isolation over distance (much greater than 10m). Put the
transmitter and receiver a few km apart and link them using another band
authorized in your area (440?).

I encountered this in Key West Florida some years ago. Worked fine there.

"Sal"

Jim Lux wrote:
On 8/30/2011 11:14 AM, Rob wrote:
Jim wrote:
Any reason why you're using a split of 600kHz? Why not go to a
non-standard split to make life easier. Pick two frequencies 3 MHz apart
(assuming you can get them coordinated, which is more a political than a
technical issue)

Don't forget that in Region 1, the 2M band is only 2 MHz wide, the
bottom 500 kHz is dedicated to SSB and the top 200 kHz is for satellite.

Sure, that does make it harder. The OP is in region 2, though, where
there's 4 MHz theoretically available)

I think he is in Region 1. He posts with 2 callsigns, one is a 4X1
(Israel) and mentions:

but my 2m band is only 144-146
mHz, and the repeater portion is really just 145-146.

flame protective suit on And of course, a lot of the band plans are
basically gentlemen's agreements and have no force of law. As long as
you don't interfere with someone (challenging in some geographical
locations), you can pretty much do what you want.flame suit off

Sure, but to get a permit for unattended operation, at least over here,
you'll need to abide to the band plans.

(in fact in my country, Netherlands, it is even worse: the repeater
frequencies are not coordinated by the ham community itself, but by
the equivalent of the FCC. they work strictly by a set of rules
originally drafted by the amateur societies, in the days the bands
were still overcrowded. arbitrary figures were put in those rules
regarding things like minimal desirable distance between repeaters,
maximum height of antennas, maximum EIRP power, that were originally
just there to regulate the inflow of new repeater projects a bit, so
everyone would have a fair chance of running a repeater. if you were
a few km too close, had an antenna a bit high, or similar, it usually
wasn't a problem. but one day an amateur who got refused a license
went to court claiming that others had gotten a license while not being
within the rules, and as a reaction the authority now strictly follows
the rules and refuses every application that does not fully conform to
all the rules. of course, operating on one of the reserved repeater
channels is one of the rules.)

Sal wrote:

"Geoffrey S. Mendelson" wrote in message
...


snip


Geoff.


If your intent is to experiment with the cheap duplexer, what I have to
offer is irrelevant. However, if you aim to build a repeater, consider
getting your isolation over distance (much greater than 10m). Put the
transmitter and receiver a few km apart and link them using another band
authorized in your area (440?).

I encountered this in Key West Florida some years ago. Worked fine there.

It is done here on 10m, where the duplex offset is only 100 kHz and
a duplexer is physically very large.

However, what I hear from the repeater team is that one is in fact
building and maintaining 2 repeaters, doubling the chance of any faults
and problems. Setting up the 10m repeater was much more work than
everyone envisioned, and many had experience on 70cm etc.