Duplexers, Can style vs portable/mobile style, Q&A

[Logan5]

I have been running a repeater here in south Florida, I have been using an RFS compact duplexer the whole time, but it seems to be the source of my high SWR readings,  I acquired this unit with the Uniden repeater and had it tuned by a local HAM. so I purchased a Phelps Dodge 4 can style duplexer tuned to TX471 MHZ RX 474 MHZ , I chose based on technicians recommendation and  I have arranged to have it shipped directly to the technician for a professional tune. Now with questions,

Should I have going with a 6 can set?

are full can style better than the small portable duplexers?

some units use small cheap looking coax.

yet others use LMR400, any difference?

 

I know this is a small forum, but I like the people here, so ring in guys.

 

[JohnE]

the PD should out preform the smaller CW. full cans are always better if you have the room for it.

there are 2 duplexers in this pic. the one on top I assume is the type you have and the one in the middle is what your going to get less 2 cans.

http://i1105.photobucket.com/albums/h358/erscom/650mindy004.jpg

[Logan5]

Yes John, the one on top is my old one, and the one in the middle is what I am waiting to be tuned, what is the extra 2 can's? should I plan a future upgrade? I know there are notch cavity's and filter cavity's. your input is greatly appreciated.  Jim,.,,,  

[JohnE]

there are 4 and 6 can duplexers. its a matter of isolation and insertion loss.

my private 650 radio in previous post has 105dB of isolation between the ports.  the insertion loss is 0.8 on the RX side and 0.9 on the TX side. the smaller CW has ~75dB iso and ~1.5dB in losses.

the 4 can version should yield >80dB iso and ~1dB losses roughly. depending on what it is tuned on ask for the spec of the tune. isolation,insertion losses and vswr if it was done on a network analyzer. should look like the tune pics I posted for my 575 duplexer.

if it is done right it you should see a difference.

I forgot to address you SWR in my other post. what make you think it the duplexer? did you check power/swr w/o the duplexer and what is it vs w/it? also what meter are you using?

[quarterwave]

I have one of the Celwave mobile type, I run it with a 10-15 watt repeater, they don't tend to work well approaching 40+ watts. I run 3 cans of an old Motorola unit, forget who made them for M, but they were gray with the big black knobs....just about every motorola UHF repeater from the 80's on had one. 

 

I have used it for 20+ years...works great. I run 2 receive cans and 1 transmit. It's on my MTR2000 now, 50 watts. Used to use it with 2 M120's. 

 

I would use the bigger one, over the smaller one...under 25 watts the small ones are ok, but that's about it. They are called "mobile" duplexers. 

[JohnE]

the big square cans? those thing are a PIA, they had to be tuned in the field b/c they went out of tune so easily.

[Logan5]

there are 4 and 6 can duplexers. its a matter of isolation and insertion loss.

my private 650 radio in previous post has 105dB of isolation between the ports.  the insertion loss is 0.8 on the RX side and 0.9 on the TX side. the smaller CW has ~75dB iso and ~1.5dB in losses.

the 4 can version should yield >80dB iso and ~1dB losses roughly. depending on what it is tuned on ask for the spec of the tune. isolation,insertion losses and vswr if it was done on a network analyzer. should look like the tune pics I posted for my 575 duplexer.

if it is done right it you should see a difference.

I forgot to address you SWR in my other post. what make you think it the duplexer? did you check power/swr w/o the duplexer and what is it vs w/it? also what meter are you using?

the meter is the Mfj842, I plan to get a better digital unit, should make it a priority.

 

I plan to check the TX SWR w/o the duplexer tomorrow. will let you know how it goes.

[Logan5]

the big square cans? those thing are a PIA, they had to be tuned in the field b/c they went out of tune so easily.

yes, I have hear that they are non-plated copper elements, not sure if that is why they are PIA. I was looking at wacoms and PD's after I heard about the square ones.

[JohnE]

they de-tune very easily in transit. been there done that, I stay away from them. also shy away from Sinclairs.

older PD's , CW, TX/RX , and Wacoms IMO are better choices. just my experience.

[Logan5]

The one I purchased is an older PD. but I got an e-mail from the technician this morning and he said it tuned right up and preformed better than expected. here is the data sheet. 

[JohnE]

looks good

[Logan5]

The new duplexer came today, very nice, very heavy. unfortunately I have to wait for proper cables to hook it up. I checked thru my adapters, but no way to rig it. so I figure Friday or Saturday.   

[JohnE]

pic's? depending on how old it is should be N or SO239.

[Logan5]

Ok, I got the PD duplexer installed yesterday, when I do my SWR test, I get 11watts forward and 0.5 reflected. in fact the reflected needle does not really move at all. Is this normal? with the previous mobile duplexer, I was getting 11 watts forward and 2.5 reflected. Someone please help me understand these readings. SWR readings confuse me. 

[PastorGary]

Logan - With all variables the same, except for the duplexers, this means that the newest duplexer is operating more efficiently than the previous one and that whomever set up the second one knew exactly what they were doing.  That's an excellent setup now, whereas the first one was outputting much less RF to the antenna than the new one.

[Logan5]

Thank you PG. I am only just now understanding the SWR test readings. you can count on more questions, as soon as I figure out what my questions are. 

[PastorGary]

Logan - in very basic real-world terms, put yourself in a water park where they have a wave generating system in a pool. As that first wave is generated and travels the length of the pool, there isn't much of anything in it's way to modify it or slow it down. When that wave hits the vertical wall at the far end of the pool, most of it spashes all over the place into the air, but a small portion of it heads back the way that it came - running into any fresh waves. This creates a nulling of part of the fresh waves. 

 

Now think of a transmission system and antenna, duplexer or in-line fittings that has some type of imbalance within. The first frequency wave from the transmitter gets to the first imbalance point and part of that wave runs into the imbalance and starts heading back the way that it came, nulling parts of the fresh waves. The FORWARD SWR is the amount of power being sent down the feedline which includes the losses or nulls from wave portions heading back up the line from imbalances.  The REVERSE or REFLECTED SWR is the amount of power in a ratio that is being wasted by those imbalances. If a duplexer is set up correctly, it will not throw much power back up the line at all.  But we have to also remember that there are LOSSES that sometimes do not affect SWR.  Each connector has losses even though they are designed to have the same impedance as the feed line.

 

Again, this is really basic data and there is much more to it than this in some cases.

[Logan5]

now it all makes sense. I allowed my misunderstanding of the use of meter to alter my learning how it really works. never thought of it as a wave pool. as basic as it is, now I have some more learning to do. but I get it. lol  I must assume my old mobile duplexer is "not" tuned properly, since my SWR was 2.5 into 11watt's or it is a bad duplexer to begin with. I had planned to reuse it but now I'm not so sure. I am still using 2 segments of LMR400 with a N male to N female splice in the middle. "no barrel" I wish to replace it with LDF4-50a, but current e-bag prices are not good. I have seen better deals in the past. I also have not added the stress loop as Jeremy mentioned. I figured I will have to do top maintenance eventually, I'll do it all when I raise it the final 10 feet. all and all range is not as good since I got my SWR readings down. I will check in from the airport on Tuesday, will let you know how that goes. Thanks and Best Regards....Jim...

[JohnE]

as a general rule 10% of forward power is the reflected limit. meaning if you have 10W reflected on 100W forward I would consider that to be a bad SWR.

you had 2.5W on 11W that is over 25%. that is NG for the radio and will cause damage. now you have 0.5 on 11 witch is ~5%, its OK but would like to see it a little lower. I personally like to see 1% if it is at all possible.

did you measure the SWR after the duplexer? most of the time unless there is somthing radiclay wrong w/duplexer the SWR should be a pass through, meaning you should see the same SWR on the input and output.

how much 1/2" hard line were you looking for?

[Shriekmau]

Logan - On your SWR meter there is forward power, reflected power, and SWR. Your forward and reflected lines cross at the top. The red toward the bottom is your SWR or Standing Wave Ratio. A perfect impedance match is 1:1. From what I saw on your previous setup you had a 2.5:1 SWR. Anything over 2:1 can cause damage to your radio, it will burn out the finals in the transmitter.

 

It sounds like you are getting 1.5:1 SWR now. This is acceptable, but like John, I don't like to see it above 1.2:1 if possible. Check your antenna by connecting it directly to your SWR meter, then from your SWR meter to your radio. You might be surprised to find that the SWR on the antenna is off. Even new antennas must sometimes be tuned.