Impedance Dilemma

[W1QWT]

This post doesn't pertain to AM directly but it is driving me crazy and you guys are smart.
I have a local repeater here in my house. I just replaced the transmitter which is an ICOM IC2200H
It appears to have a weird VSWR shutdown circuit.
When I replaced the existing IC2200H with the new one it did not put out any power.
I put it into a Bird 50 ohm Termaline dummy load and it worked fine, full 60 watts.
Then I put it back into the duplexer with a Bird model 43 Thruline in series and tuned the duplexer for
a VSWR of 1:1 and it put out full power. When I remove the Bird and go direct into the duplexer the
IC 2200 H doesn't put out any power as measured on the output watt meter feeding the antenna.(output of the duplexer)
I tried different lengths of cable between the transmitter and duplexer, thinking maybe it was transforming the impedance,  but it didn't help.
The darn transmitter only puts out power when a bird Thruline is installed in the line going to the duplexer from the transmitter!
Anybody got any ideas?
Regards
Q

[WD5JKO]

Q

   I'm just guessing, but I think you have an open or a short with the Bird removed. This could be a cabling issue where where the center pin connection has an issue, but not with the Bird? I had a similar issue with a PL259 where after using it for several years without trouble, I had an open all of a sudden. The center pin was perfectly centered  and yet not touching the SO-239. I presume you are using type 'N' connections?

   In a related story, at a job I once had, we were on a tough schedule to ship a product but suffered from a rare pulse that caused malfunction. Putting a 10X scope probe in there and it worked without failure, and we never saw the pulse. We shipped the device with a 10-meg resistor shunted by 47pf located where the scope probe was installed. So why not leave that Bird in there? 

Jim
WD5JKO

[W3GMS]

I am curious if the 2200 puts out power into the antenna itself.  In other words, bypass the duplexer and just run the rig into the antenna. 

When I tune the duplexer on my repeater, I usually do it for minimum loss at the pass frequency and then adjust the reject stubs for maximum rejection at the frequency your trying to reject.  I do everything into an good 50 ohm dummy load and then leave it alone!  I use Wacom, WP-641's.   

Also if the bird is not right at the output of the tranmitter you may think the SWR is good but in fact it may not be.  One handy device is to use an antenna analyzer right at the coax connector that goes to the output of the transmitter.  That will show what the transmitter really see's. 

I designed a system back in 1976 and repeaters can show some very weird problems that make you go nuts until one figures things out! 

Good luck!

Joe, W3GMS 

[Jim, W5JO]

Had a 450 Mc repeater one time that there was a small metal shaving from the outside shell of the antenna Tee on the duplexer that just nearly touched the center pin.  Not much but enough to destroy the signal pass and about the size of a human hair, hard to see.

[W1QWT]

Thanks for the advice.
Yes Jim I could just leave it in there as it is there now and working fine but I would like to 'figure it out'.
I think the two different cables I tried are ok (Not shorted or open) cause they worked fine with
the bird inline and the repeater had good coverage.
I have not tried an exact multiple of a 1/2 wavelength piece of cable but the old IC2200H worked fine with the ones I did try. The reason I replaced the old transmitter was because after 3 years use its high power section failed so it only put out 25 watts. I have added a fan to this new transmitter.
Joe that is a good idea about the antenna analyzer which I will try this weekend. I will also see if it works into the antenna by itself.
I will also make a piece of cable up that is an exact 1/2 wavelength and try that.
Regards
Q
W1QWT

[flintstone mop]

Did you say DUPLEXER??

You need a pretty sophisticated piece of equipment to adjust a duplexer!!!
We have a $15K Motorola service monitor with a DUPLEXER mode and you have to tune for max RF power out and be able to max the receive sensitivity without DESENSE of the recveiver while in the repeat mode. You are tansmitting and receiving at the same time in a repeater system. Those filters usually take us 2 hrs to set-up when changing freqs for a customer.
UNLESS you are rich and can afford separate antennas. TX on top of the tower and receive 25 feet separation directly under the TX. Assuming vertical gain sticks.
If you have an MFJ you can look at the TX antenna by itself and determine if it's any good.
You might have some bad connectors taking on water. 450mhz doesn't like water in the transmission line

Fred

[WA1GFZ]

did you allign the cavities with a pad on each port?
This is the easiest way to get it right.

[The Slab Bacon]

Did you say DUPLEXER??

You need a pretty sophisticated piece of equipment to adjust a duplexer!!!
We have a $15K Motorola service monitor with a DUPLEXER mode and you have to tune for max RF power out and be able to max the receive sensitivity without DESENSE of the recveiver while in the repeat mode. You are tansmitting and receiving at the same time in a repeater system. Those filters usually take us 2 hrs to set-up when changing freqs for a customer.

Phred,
         Dont kid yourself. Everyone (especially thise who havent done it) make a big ritualistic mysterious process out of tuning duplexers. I have done many of them. all that you really need is an accurate sig gen, and a calibrated receiver. I usually use an old P.O.S. Motorola 1100 synthisized signal generator, and a little jap vhf rig (for the desired band) and a 50 ohm dummy load.

Tune the pass rods for the strongest pass on the desired frequencies, tune the notch caps for the deepest nulls on the desired reject frequencies. And let it rip!! No mysteries or secret rituals here. You dont have to say the secret incantation or human sacrifices required. It doesnt take much more than a half hour to tune a 4-can duplexer.

You do know how your duplexer works , though, if you are making up cabling for them. Most use the interconnecting cables as tuned stubs for the reject frequencies. If you are remaking the intercabling you need to be on the money, but tuning one is easily done..

The amazing thing is transmitting and recieving at the same time on 1 piece of coass. Those damned little UHF electrons are getting to be too small for my old eyes to see running up and back on the wire   

[flintstone mop]

I agree somewhat, Frank
It's not all Step 1, 2 and 3 and away we go.
De-sensing of the receive is another problem from a mistuned, or tune for peak/Reject duplexer.
There's a big difference in a system that has a little over half of the transmit power comming out the antenna port and the receiver opening up and the TX repeating with a .2 microvolt RX level.

We've re-aligned those peaked duplexers, where almost rated power was coming out the antenna port and you needed 2 microvolts to open the receiver.
Those little UHF electrons need protection and guidance.
Phred

[The Slab Bacon]

Phred,
         Dont forget that bench tuning doesnt always get the "perfect" tune. But for the most part it does, or at least get you close enough to finish it up on site.

Sometimes the antenna and feedline will affect the final tuning, and you may need to tweak it up a little at the site. I've done many the way that I described in the last post and they worked just fine. The desense is usually caused by some of the transmit freq leaking by back to the receiver. A high SWR on the antenner will cause all kinds of strange things to happen with the duplexer.

Dont forget we didnt have mega buck test equipment, or the money to pay a 2-way shop to tune them for us. We made due with what we had on hand back then.
I felt like I had the world by the BA's when I came up with the jing to buy a used CE5 service monitor. (which I still have ) 

Still all in all, its not hard and there are no mysteries to tuning a duplexer, as long as the one doing the tuning knows what he is doing. But perfect on the bench is not always perfect in the field..........................

[flintstone mop]

We've been pretty lucky so far. A good installation with good connectors and a properly tuned antenna should be flat. 1.2:1 max. We usually look at the antenna and line to make sure it is almost perfect with a transmission line analyzer, then get into the duplexer, if we need to.

These are commercial installations UHF and higher and it's gotta be right.
I'm sure there are ham repeaters where they achieve the near perfect match. And the duplexer is very happy.
We take the system apart and check each aspect of it. It's proving out the transmission line with a dummy load at the top or replace the antenna till we get the prefect match.
6M (and even 10M) and up needs to be the best match possible. A lot more losses TX/RX involved at those freqs.

Another .02 centavos
Fred

[AB2EZ]

Based on the symptoms and observations that Q described in his initial post, I agree with Jim that the most likely problem is a connector. The Bird has very tight connectors on it... and it is likely that a plug that is making a good center conductor contact to the Bird is not making a good center conductor contact to whatever it plugs into when the Bird is not present.

If the Bird is measuring 1:1, then the SWR will be 1:1 if you move it closer to the transmitter or closer to the antenna (provided the cable between the Bird and the transmitter is okay ... i.e. behaving as a 50 ohm transmission line).

I have a problem with a DowKey relay (SO-239 connectors in that case) whose connectors a not tight. They do not make reliable center conductor contact to PL-259's. I rolled up some 1/4 inch wide strips of aluminum foil into 1/8 inch diameter cylinders... and stuffed one into each of the SO-239's with a small diameter rod (like a cannon ball being loaded into a front loading cannon). That fixed the problem.

Stu

[WA1GFZ]

Stu,
I go in and bend the 4 contacts in to make them tighter. My V2 CDC had a very loose connector from years of use. It is fine now. Dental pick works great.

[AB2EZ]

Frank

Yes, that sounds like another approach that I will have to try next time.

I wonder if there is an "unmet market need"  for a PL-259 worn out socket adapter (let's keep this "clean", because this board caters to a general audience of readers)

For example, it would accept a PL-259 on one side, and it would have a spring-loaded, expandable center pin PL-259 on the other side.

We could probably sell these for (rough guess) $0.25 each, including postage and handling.  

Best regards
Stu

Stu

[W3GMS]

Fred,
Give me 15 minutes and I can tune at least the Wacom Duplexers!  Lots of practice since 1976!  The old Phelps Dodge ones I need 30 minutes.  RF generator and a good receiver is all it takes.  Lots of PAD's come in handy as well.  I am duplexing 250W with zero desense with a rx sensitivity of .16uv. 
Joe, W3GMS 

[WA1GFZ]

All about the port impedance

[W1QWT]

Give me 15 minutes and I can tune at least the Wacom Duplexers!  Lots of practice since 1976!  The old Phelps Dodge ones I need 30 minutes.  RF generator and a good receiver is all it takes.  Lots of PAD's come in handy as well.  I am duplexing 250W with zero desense with a rx sensitivity of .16uv.

Joe I agree with the tools needed to do the job as even years ago when I worked in the two way radio business that's all we used but I wonder about zero desence.
On your 250 watt system I wonder how many cans you have?
I found that it generally has to do with the math. Amount of rejection in dB vs system power.
My system has 4 Wacom cans. (2 RCV & 2 XMT) If I can get it to 2dB or less of desence then I am happy and the system
works fine. For a .16uv 20dB SINAD sensitivity 2dB only drops it to ~.2 dB (20dB SINAD).
That doesn't seem too bad to me but maybe I am too cheap to buy more cans. Besides more cans equals more insertion loss and I don't want that either in my little PW system.

Regards

Q

[W3GMS]

That is not bad at all but I have always fought hard to get rid of any desense!  Double shielded cables from the RX and TX port to the cans along with careful tuning of the transmitter with a good spectrum analyzer to get rid of any junk close into the carrier.  I also prefer the xtal transmitters as compared to ones with a PLL.  Starting with a very clean and well tuned transmitter really helps.  My current system uses one antenna at the 105' level and another antenna at the 80' level on a separate tower.  To duplex the 250W I split the cans and use one can on the rx port and 2 cans on the TX.  It really works great.  I have duplexed with zero desense using 2 cans on the TX and another 2 on the receiver with the output of each set of 2 tied together at the 100W level with no problem.  I love the extra margin running the 2 separate antennas and it buys one a lot of system margin.  It also seems less prone to guy wire noise and things like that.  Plus if one antenna craps out in the middle of winter then I just go back on a single stick and things continue on the air.  My transmitter is a homebrew job I designed and built back in 1975 and it has yet to die!  I normally use ARR preamps, but my current receiver is hot enough without it.   Fortunately we have a lot of AM'ers on the repeater otherwise I would pull the plug!  A lot of homebrewers so we have some interesting QSO's on 146.985.         

Regards,
Joe, W3GMS

[W1QWT]

Yeh the separate antenna system really helps alot. A few years back I built a 6 meter repeater with separate antennas and it worked great!
When I first put my 2 meter repeater on the air it was with homebrew duplexors. Years ago there was a ARRL book on repeaters that had plans for a 2 meter duplexor by W1GAN I believe. They worked fine for a number of years. Then I bought the Wacom ones.
Repeaters around here are getting quieter. I put IRLP on mine to liven it up and it helps a little.
It's cool to be able to talk to some guy leaving a pub in Ireland when I am on my way home from work.
Ofcourse even I am to blame for some of the quiet as I spend more time on 6 meter AM than I do on 2 meter FM.

I do use double shielded RG-214 or 1/2 inch heliax as interconnecting cables from xmtr and rcvr to the cans.


Regards
Q
W1QWT

[The Slab Bacon]

We also used to steal the helical resonator front end cavities from Motorola Motracs and add them to the existing front ends of GE receivers to help clean out some of the RF soup mix from the old paging transmitters that used to tear everything to hell here in Baltimore. It was pretty bad for a while.

The trick was to find the ones with the preamp stage built into them. IIRC, some of them had a preamp and some had the first mixer built into them. Its been a while since I messed with any of them. (I still have 20 or so motracs stashed behind the furnace)

I guess the time has come to dispose of the stash of 'tracs. (anyone want a few??)
I'll prolly pull the finals out of them and recycle the aluminum.

[WA1GFZ]

Frank,
Those are nice cavities. I've tuned a few. RF amps not bad either if you get a hot one. fc