Eric,
Why is a quarter-wave shorted stub impractical at 2m?
I have been told that a quarter-wave shorted stub would serve two purposes:
a) provide a dc path to ground for static caused by precip or wind, and also
b) substantially reduce the strength of a transmitter's second harmonic.
73 and aloha, Eric KH6CQ
--- On Mon, 7/27/09, Eric Lemmon <wb6...@verizon.net> wrote:
From: Eric Lemmon <wb6...@verizon.net>
Subject: RE: [Repeater-Builder] Sinclair Q202 duplexers (frankenstein series)
To: Repeater-Builder@yahoogroups.com
Date: Monday, July 27, 2009, 4:51 PM
Norm,
A Polyphaser does not put a DC ground on the center conductor of the
feedline- nor does any other inline arrestor of any brand, except a
quarter-wave shorted stub. But that is impractical at 2m. My point was
simply that a single bandpass cavity on either the TX or the RX side,
between the duplexer and the antenna tee, will put a DC ground on the
feedline at the transmitter end. Most antennas are DC grounded, but a lot
can happen to that feedline between the antenna and the duplexer. One 2m
repeater I have on a hilltop suffered a lot from wind-caused static
discharges (aka triboelectric charging) until I put a single bandpass cavity
on the receive side. My intent was to prevent desense from the adjacent FM
broadcast station, but the static elimination was a bonus.
Regarding the determination of high pass versus low pass, this is usually
determined by the design of the duplexer. Some designs are symmetrical,
while others are asymmetrical. In most cases, the loop coupling will be
different between the high side and the low side, so it is convenient to
simply follow the manufacturer' s settings, and their tuning instructions.
In the case of the Sinclair Q202-G, the loop assemblies are all identical,
and the notch tuning capacitors are the same as well, regardless of which
pass side they're on.
You're correct about bandpass duplexers being unsuitable for the 600 kHz
split at 2m. However, I have a 8" bandpass duplexer on a commercial
repeater that is using a 5.26 MHz split on VHF, and it works perfectly. I
specified it because of the antenna being the high point on the tower, and I
wanted DC ground at the duplexer for repeater protection.
73, Eric Lemmon WB6FLY
-----Original Message-----
From: Repeater-Builder@ yahoogroups. com
[mailto:Repeater-Builder@ yahoogroups. com] On Behalf Of NORM KNAPP
Sent: Monday, July 27, 2009 11:20 AM
To: Repeater-Builder@ yahoogroups. com
Subject: Re: [Repeater-Builder] Sinclair Q202 duplexers (frankenstein
series)
Again Eric
Thanks for the wisdom and information. I will digest this over the next
several days and when I can I am going to apply it.
I have what I feel is a very good service monitor with tracking
generator/spectrum analyzer. I have access to an Anritsu site master. It has
a return loss bridge built in (I think) and I will give the low pass cans
another go.
The thing that I am curious about is what determins what side of the pass
the notch will go? What makes a "can" a low pass "can" and not a high pass
"can"? Does the value of ths capacitor do this?
You mentioned the fact that BpBr duplexers don't have DC ground potential. I
do have polyphasers and grounding pretty well covered. The repeater is at an
old AT&T Long lines microwave site. Grounding is not a problem.
What were you saying about using bandpass only duplexers? I didn't think
they were sharp enough for 600khz split. How many cans would you need to
accomplish this and what are the advantages!
I recently aquired another set of sinclair duplexers. They have no model
info on them. They are high band VHF and are in the 154-158 range now. They
were connected to a 250watt micor repeater. They apear to be hybrid ring
type but they are small like 1/2 gallon milk carton sized and the harness
has exposed braid between the cans (cartons) and the "T's". They are mounted
on a 19" rack panel with a cover. The cover is missing. The rack panel has
the Sinclar tag and logo with "ERP" and the atom on it... Very strange.
Again, I thank you for the wisdom and advice.
73 de N5NPO
Norm
----- Original Message -----
From: Repeater-Builder@ yahoogroups. com
<mailto:Repeater- Builder%40yahoog roups.com>
<Repeater-Builder@ yahoogroups. com
<mailto:Repeater- Builder%40yahoog roups.com> >
To: Repeater-Builder@ yahoogroups. com
<mailto:Repeater- Builder%40yahoog roups.com>
<Repeater-Builder@ yahoogroups. com
<mailto:Repeater- Builder%40yahoog roups.com> >
Sent: Sun Jul 26 11:20:33 2009
Subject: RE: [Repeater-Builder] Sinclair Q202 duplexers (frankenstein
series)
Norm,
Thanks for the update on your "quest." The loops have two adjustments, and
they are not complementary. The degree of coupling, and the amount of
insertion loss, is adjusted by loosening the three screws that secure the
loop mounting plate, and turning the plate slightly to achieve the desired
insertion loss- which is generally around 0.5 to 0.8 dB per can. The
variable capacitor mounted in the loop plate is used to move the notch
closer to, or away from, the bandpass peak. Are you following the Q202G
tuning instructions shown on the RBTIP? Those instructions are here:
<www.repeater- builder.com/ sinclair/ ci-1069-Q- Series-Tuning. pdf>
Because the bandpass peak is so vague, the best way to tune it is to use a
network analyzer or a spectrum analyzer with a return-loss bridge. When
tuning for return loss, the image on the display is a very sharp notch which
is easy to get right on the money. Then, the analyzer is switched to
transmission loss to set the notch capacitor. Finally, the loop plate is
rotated to achieve an insertion loss of between 0.5 and 0.8 dB. If the
jumper cables between each pair of cans are the correct length, the
insertion losses should add exactly. I normally go through this routine at
least three times to get the tuning as good as I can.
I looked at some Q202G loops from a 2m duplexer, and they are plain copper-
not silver-plated. The standard VHF loop assembly has a 1/4" wide copper
strap bent into a rectangle that measures about 1-1/8" by 3-1/8", so if your
loops are much different in size, they may be unsuitable for 2m operation.
The notch tuning capacitor is a Johanson 5602, which is rated 1 to 30 pF,
and has a Q of greater than 800. I have to wonder if the added capacitors
you found were high-Q silver micas or ordinary ceramic capacitors. If the
latter, the cavity cannot be tuned properly. The Johanson capacitors are on
page 4 of this brochure:
<www.johansonmfg. com/pdf/Air- Capacitor. pdf>
One thing to keep in mind about BpBr duplexers- and not just those made by
Sinclair- is that the presence of the notch tuning capacitor means that
there is no DC ground anywhere on the feedline between the antenna and the
receiver. A high-voltage spike caused by a nearby lightning strike, or an
electrical system fault, can sail right into the receiver front end. That's
one good reason for having a true bandpass cavity- which has DC grounded
loops- somewhere in the RF chain. I have heard a few reports of notch
tuning capacitors that were destroyed by high-voltage arcing. This damage
would not be visible.
73, Eric Lemmon WB6FLY
-----Original Message-----
From: Repeater-Builder@ yahoogroups. com
<mailto:Repeater- Builder%40yahoog roups.com>
<mailto:Repeater- Builder%40yahoog roups.com>
[mailto:Repeater-Builder@ yahoogroups. com
<mailto:Repeater- Builder%40yahoog roups.com>
<mailto:Repeater- Builder%40yahoog roups.com> ] On Behalf Of NORM KNAPP
Sent: Sunday, July 26, 2009 3:58 AM
To: Repeater-Builder@ yahoogroups. com
<mailto:Repeater- Builder%40yahoog roups.com>
<mailto:Repeater- Builder%40yahoog roups.com>
Subject: [Repeater-Builder] Sinclair Q202 duplexers (frankenstein series)
Ok, it is official. I have a set of sinclair Q202 duplexers that didn't come
as a set. At least two of the cans have different serial numbers and
different factory tuned frequncies. DRAT! Ok, so here is the problem. With
my trusty Aeroflex 2945 (I think that is the model) I can easily get a -35dB
notch on each of the hi pass cans at 147.825 pass and 147.225 notch. No
problem with that side. Ok, the problem is on the low pass pair (mix-mached
I am sure). I can barely get a -30dB notch on each can even if I spin the
loops. So, just out of (morbid) curiocity, I pulled the loop out of one just
to show a fellow ham what was in there. Lo and behold there was a 12pF cap
soldered accoss the notch tune cap. Hmmmm... What would happen if I took
this cap off there since to get the -30dB notch the tuning cap is almost
screwed all the way out (min capacitance) . So I got out the soldering iron
and removed it. It worked, sort of. I was able to get a -35dB notch on my lo
pass (147.225 pass 147.825 notch) side, but at the expense of some pass loss
higher than the high pass side. What happend is it seemed to make the low
pass can into a high pass can. The notch went to the other side of the pass.
I spun the loop to get the best result and tuned the notch cap near its max
capacitance to get the notch where it should be. It worked barely and with a
bit more loss in the pass. I am thinkin I may need to go back and put a 6pF
cap accross the notch tune to get it where I want it, but I am not sure. On
the second can of the lo pass high notch side I tried to remove the cap, but
it didn't turn out the same. The loop inside it was not silver plated like
the previous can, it was just plain copper and appeared to be a bit longer.
When I removed the cap on this can, the notch went to the low side of the
pass and I couldn't get it to come back around. I will definitly have to try
a 6pF cap accross the notch tune cap on this one.
Now, I just replaced the harness with RG-214/U jumpers the length needed to
get 14" between centers of the "Tee's". The old ones were RG-142/U and were
also apparently the correct lenght as well. Changing the jumpers didn't seem
to affect the tuning ability of the cans. It was a waisted effort really,
but oh well, they have a really nice new harness on them, makes em look 10
years newer.
Any suggestions guys. It lookes like I may have a modified pair of cans that
are actually high pass instead of low pass. Should I play with the length of
the loop? What about changing the 12pF caps accross the notch tune caps
(parallel) to 6pF or so? Right now, they are working, but the high pass pair
look really good and the low pass pair look so-so to kinda oK. The repeater
is working very well, but I feel if I get the cans right it will be even
better.
Anu suggestions?
Call me if you like 251-234-0295.
73 de N5NPO
Norm
