Decibel DB-4379

[WRKC935]

I have a combiner that I am needing to replace the cables between the output cans and the combiner spider.  Currently have a loss figure WAY above what the manufacture rated the system at for the spread I have.  Freqs are GMRS, spacing is 725, 675 and 600 so .050Mhz and .075Mhz.  Combiner is rated to .025 but the losses go up a lot when it's that tight.

Any big brains out there that know how the harnesses in these work that can help a brother out?

 

[Radioguy7268]

If you've got a 5 port "star" (4 transmitters feeding into 1 antenna) - then the 4 harness legs coming from each tuned transmitter cavity would typically be an odd wavelength ( 1/4, 3/4) of the frequency you're using. 

I question the spacing you're using, as typically if you get that tight on frequency (less than 50 KHz) spacing with a combiner, you will need a hybrid/ferrite combiner - and 10 dB of loss is typical of those. 250 KHz would be more typical of the absolute minimum spacing for a bandpass cavity design with a dual stage isolator for each channel, not 25 KHz.

http://www.repeater-builder.com/antenna/db/pdfs/db-about-combiners.pdf

[WRKC935]

22 hours ago, Radioguy7268 said:

If you've got a 5 port "star" (4 transmitters feeding into 1 antenna) - then the 4 harness legs coming from each tuned transmitter cavity would typically be an odd wavelength ( 1/4, 3/4) of the frequency you're using. 

I question the spacing you're using, as typically if you get that tight on frequency (less than 50 KHz) spacing with a combiner, you will need a hybrid/ferrite combiner - and 10 dB of loss is typical of those. 250 KHz would be more typical of the absolute minimum spacing for a bandpass cavity design with a dual stage isolator for each channel, not 25 KHz.

http://www.repeater-builder.com/antenna/db/pdfs/db-about-combiners.pdf

Well, I have these same combiners in other commercial sites running at 25Kc spacing and they were bought that way.  Not questioning what you are saying.  Just know through research and experience that this model will support 25Kc and not have this level of loss. 

Gonna try rebuilding the cabling with the odd multiple of a 1/4 wave to the star with the frequencies now in play.    If that doesn't work, the unit is actually a DB-4379-8.  Meaning original design was 8 transmitters on one antenna.  I have the unit split into two 4 port units.  If I need more spacing I will put 725 and 600 in this one and put 675 in the other one on a different transmit antenna. 

It doesn't sound like the ham frequency should be a issue at all though?

 

[WRKC935]

Still fighting with this dumb thing.  And it's really doing odd things. 

725 with 40 in gets me 2 watts out

675 with 40 in gets me 10 watts out

600 is with 40 in is 20 watts.

So I decided to disconnect the 600 and 675 and see if it was interacting.  It is... I get no output from 725 with 675 disconnected.  But it has no effect on 600.

Gonna run 600 and 725 in one combiner and 675 in the other combiner and see what that does. 

I did make new cables from the cans to the star but that had little effect. 

Gonna play with putting ham stuff in the other ports but I need to get a second transmit antenna in the air so I can split these stupid combiners.    I do love radio,,, it's so much fun.

 

[Radioguy7268]

Since you're still trying to do the impossible - it's not surprising that you're having trouble. Pretty good bet that 1 or more of the isolators is burned up.

There's a max amount of reflected power that an isolator will handle. Think of what you're doing when you place another transmitter right next door on a frequency too close for the design spec. Think what happens once that 3rd transmitter keys up... it's adding to the problem, and it's not that much further away.

Loss on a typical bandpass single cavity/dual isolator combiner setup should be 1.5 dB per channel.

 

 

[WRKC935]

Well I was playing with it tonight.  The cable length of the spiders is SUPER critical. 

I do hate tuning by cable length.  I need to find my male and female N barrels.  Found a few and was able to move it around some and got some of teh power back. 

Had 18 watts out with 40 in at one point on one channel, but the other channel was 1 watt out.  That's not gonna work obviously.  I did pull 675 out of the mix in that set of cans and was able to get something resembling reasonable output of 8 watts.  While that sounds like a silly low output, I was able to hear the thing from 60 miles out with 2 watts of output, so it's not as bad as it looks.    Will be setting down this weekend with it, retuning the isolators and building new cables.  Considering using sueperflex hardline for the cables as it's WAY easier to put together and test and then trip if needed as opposed to RG-214 that has to have the braid brushed out and the center pin resoldered every time it's cut.  Only issue with the hardline is the length measurement for the actual connectors.  Will cut long and creep up on the right length.  A drill powered prep tool is your friend here.  Decibel and dB Spectra both used hardline on some of their 800 combiners so it's not something that hasn't been done before.  I know that 125Khz is gonna be really close spacing.  But I have the same unit with 452.2375, 452.3625, 454.475, 454.075, 454.175.  It works,,, lossy but not as bad as mine right now. 

And if that doesn't work there are is a two port hybrid sitting on the floor next to it that will work.. but it's only a two port so I would still need two antenna's to get all three GMRS repeaters on the air. 

 

I would be elated with 15 watts out.  If I get to 20 watts, I will seriously need to consider receive sites. 

 

[Radioguy7268]

As everybody told you in the other forum, the spacing was too close.

As for the cable length, yes you need to be fairly precise, and your measurements need to take the connector length into consideration. I can tell you that TX/RX Bird used to stock cables in 1/8" increments, and they would sell them to you at reasonably inflated pricing, but to me it was still cheaper than the time spent to make your own. Not sure if they still do that.

There's a reason I know how close they'll go. I was down this road with VHF combiners 20 years back, trying to make existing stuff work at spacing under spec. Don't try to jam 100 watts into the combiner, don't expect miracles, and pay attention to the size of the dummy loads you're dumping the isolators into. You will need something rated for more than 5 or 10 watts.

[WRKC935]

You know,,,, the obvious... sometimes ain't so obvious.

Frequency spacing is my problem.  Too close.  For commercial, and even ham radio (density of used repeater pairs) you have to work with what you have.

GMRS.... NOTHING is assigned, nothing is fixed use.  If you want to put a repeater up.  You put it up.  If you're a nice guy, you work with the co-channel users and either host them or assist them with reprogramming their stuff,  but it's not difficult to work out a mutually agreeable situation with regards to repeater frequency pairs. 

462.725 and 462.550 will fit in the combiner... and run 600 on another combiner.... problem solved

Epiphany in the shower this morning.

[tweiss3]

Glad you figured it out. I was going to say, give the manufacturer a call. I know of one tower that was having issues, ended up needing a cable set made by the manufacturer as length was critical to the 1/32 of an inch, and it worked as designed then.

[Radioguy7268]

Make sure to terminate any un-used port on the combiner star if you remove the cable and/or isolator from use. I think you're going to find that one or more of the isolators you were using will no longer tune. Measured forward loss through any specific dual isolator when measured on its own should be less than 1 dB when properly tuned.

EMR has a ton of white papers online concerning combiners/filters/isolators, antennas, and duplexers. Lots of good info there, even if I never liked their square cans.

EMR Corp tech papers

I'd suggest reading "The care & Feeding of the RF Isolator" on that link if you haven't already. It will set you straight with what's going on.

EMR also did a writeup on how to Field Tune a Dual Isolator:

Field Tuning a Dual Isolator

[JohnE]

7 hours ago, WRKC935 said:

You know,,,, the obvious... sometimes ain't so obvious.

Frequency spacing is my problem.  Too close.  For commercial, and even ham radio (density of used repeater pairs) you have to work with what you have.

GMRS.... NOTHING is assigned, nothing is fixed use.  If you want to put a repeater up.  You put it up.  If you're a nice guy, you work with the co-channel users and either host them or assist them with reprogramming their stuff,  but it's not difficult to work out a mutually agreeable situation with regards to repeater frequency pairs. 

462.725 and 462.550 will fit in the combiner... and run 600 on another combiner.... problem solved

Epiphany in the shower this morning.

 that is going to be tight. typical spacing is 250kC's unless you go to a hybrid combiner that has a typical loss of 6-9dB
550 and 725 still miss the mark by 75kC's
good luck, JE

[WRKC935]

OK, so a final update on the path I went....

 

I did have some 3 dB high power directional couplers that were in band for GMRS frequencies. 

I connected the dual isolators to that 3 dB coupler and terminated one of the coupler outputs to a 100 watt 50 ohm load.  The other output was then connected to the can in the combiner.  Effectively I built a hybrid combiner.    Tuning it is a bit sketch as I have to tune it between the two frequencies.  But I am getting 3 times to power out of it I was before.  Mind you that's only 12 watts per frequency, but it's progress.  I am gonna check to see what the repeaters are set to power wise and if they are under 50 watts, I will be turning them up to that level.  I expect a minimum of 3 dB of loss in this configuration but that's WAY better than 10 or 20 dB that I was seeing.  

I have the new antenna in the air and the cables in the building.  I need to finish terminating the lines in the building and land them on the PolyPhasers.  Then cut and run new lines to the combiners from there.  Have the materials, just need the time to get it done. 

Planning on flying a DB420 to replace the DB-408 receive antenna as well. 

Once that is all complete I will be working on redoing the interface for the 600 repeater to the Midwest link.   I have been having audio issues on the transmit site, but i think that was more a subscriber issue.... will know more once I get the repeater on the other combiner and into an antenna. 

Once all that is done, I will be spinning up a ham repeater on one of the combiners.  Should be analog at first.  May look at doing a P25 mixed mode with it.

 

[WRKC935]

Final post on this, with results.

Got the antenna jumpers run today from teh ground bar to the combiners. 

725 and 675 are in the same combiner that is rebuilt as a hybrid.

725 is running 40 watts in and 14 out.

675 is running a bit more and getting 20 watts out

600 is in a different combiner, had to retune the isolators for that combiner.  Will also have 442.775 in it.

600 is also at 20 watts out with about 35 in so that combiner is working better than the other, but it's not in a hybrid configuration.

With 20 out the building and the line loss my realized gain on the antenna system is 6dB, so my ERP should be right around 80 watts. 

Plans are still in the works to swap the RX antenna from a 408 to a 420 but that's another project.

 

Thanks to all those that gave guidance with getting this all working.