Recomedation frequency separation of two close by repeaters

[12dbsinad]

Yeah, not an issue for a single repeater, and not really an issue at a commercial site when dealing with professionals.
It becomes an issue at sites that are not well managed, or have hams running a junky system that should have been tossed out when Nixon was in office.
Good components and proper design will address this.

You kind of hit on something. These "professionals" you speak of are becoming rare. The true RF theory guys are retiring and getting replaced with IT guys who may be really good with the networking but have no clue RF wise. It's plug-n-play to them. It would probably shock you how many systems out there don't work as they should because of RF problems. This becomes apparent when you need tech support and need to talk to the guy at TX/RX that's been there 35 years.

If you Co-locate at any cell site they will require all of this and also a structural analysis. Also, if you're doing anything in very close proximity to an airport.

 

[mmckenna]

You kind of hit on something. These "professionals" you speak of are becoming rare. The true RF theory guys are retiring and getting replaced with IT guys who may be really good with the networking but have no clue RF wise.

Unfortunately, I'm under the larger IT organization. The director above me oversees our network operations as well as my group. She's a good director and knows what she doesn't know. Generally leaves me alone and lets me do my thing. Usually our conversations are her asking "how can I help".
But it took a long time. I've had directors refer to them as "CB's". I've had to have discussions about why I was investing money in a new radio system for our PD and why I couldn't use "WiFI and Cellular". Had network guys get pissed off at me when I pointed out that 911 location data was still required even though we were switching to VoIP. Had one network engineer that was absolutely furious with me because he felt I was taking money that was better spent on improving WiFi coverage. <seriously>
Funny thing is, they love my -48 volts and inverter plants when they need to keep those power hungry Cisco routers running.

Unfortunately most IT guys out there think they are a telecom/radio experts. I sometimes wonder what will happen when I retire, then I realize it doesn't matter.

Too many assume they can learn everything they need to know from the YouTubians, and that the FCC is just some sort of annoyance.

 

[tropiradio]

LMR UHF repeaters in the USA are 5MHz split. If this was a real issue, there would be no co-located UHF repeaters anywhere. Same on 700MHz, 800MHz, etc, anywhere there is a standard repeater offset. That would make trunked radio systems inoperable.

In other words, the term "full of poop" comes to mind.

You have to be -REALLY- careful when dealing with some of the ham radio crowd. The term "Just enough knowledge to be dangerous" is appropriate here. This is one of the reasons I don't participate on sites like QRZ, or other ham only oriented sites. My mind goes numb reading some of their posts.

In very general terms:
His example of a 600kHz AM broadcast station and a 2 meter repeater and all the mixing products can cause an issue, but how the hell this guy is extrapolating that out to impacting a commercial LMR repeater is beyond me. I can see "on paper" it could be an issue, but you'd have to be using some truly crappy components, coax, duplexers and a lot of other questionable stuff to cause this.

Yes I know the type. When I started with radios in the mid 70's it was repairing friend's CB radios which we used at the time, getting more channels, etc. Then 79 started as a tech for the Kenwood ham radio dealer for around 5 years and had a lot of contact with hams of course. Most of those guys are OK persons but once every so often one comes across one of them that always knows more than others and specially you, no matter what.

Anyway glad that this whole thing was a false alarm and it all comes down to experience, common sense, good RF practices, and no hocus-pocus BS which probably just was put out there to get some protagonist time.

And then there are the more modern know it better types that tell you that they don't need radios, because everything can be done with Watsap. I once spoke to a guy that works at a big tourist beach resort to offer them radio communications solutions for their day to day logistics and security and to get the message to someone in management. But the answer that came back from someone in upper management was that they are doing fine with Watsap and its much cheaper than radio. I would like to see them trying to place a general call when they have an emergency, or just send out a message to a work group and then everyone starts replying on their own. What a mess that must be...

 

[MTS2000des]

Too many assume they can learn everything they need to know from the YouTubians, and that the FCC is just some sort of annoyance.

Turd tube is full of misinformation, bad data, and "teknishuns" who cobble together LMR gear from parts, make a video, and all of the sudden they are SMEs. That's fine for amateurs/hobbyists, but when it comes to PROFESSIONAL radio that PROFESSIONALS rely on to work in PROFESSIONAL space, it needs to be done RIGHT. The failure to do so can have consequences well beyond a poor functioning system.

When you have to deal with OSHA, district attorneys, and civil litigation brought on by workplace incidents, you'll feel true pain. This business is not a one week course on throwing up a wi-fi access point bought off CDW.

 

[prcguy]

These types are also a disservice to their company or city/county/state organization and tax payers when equipment is not installed right and doesn't meet expectations or cover needed areas.

Turd tube is full of misinformation, bad data, and "teknishuns" who cobble together LMR gear from parts, make a video, and all of the sudden they are SMEs. That's fine for amateurs/hobbyists, but when it comes to PROFESSIONAL radio that PROFESSIONALS rely on to work in PROFESSIONAL space, it needs to be done RIGHT. The failure to do so can have consequences well beyond a poor functioning system.

When you have to deal with OSHA, district attorneys, and civil litigation brought on by workplace incidents, you'll feel true pain. This business is not a one week course on throwing up a wi-fi access point bought off CDW.

 

[tropiradio]

So I finally understood what that ham guy on RB was trying to say about mutual interference between two repeaters evenly spaced between them. The example bellow did not come from the same initial guy that was never able to explain himself clearly, or just wouldn't bother. But this other guy certainly cleared up things for me.

Basically the idea is that when portables are operating very close to the repeater, like in my scenario, their transmit frequency (which would of course be one of the repeater's RX) could also possibly become part of the mix to create IM if signals that might be strong enough, and the generated IM product would then potentially fall squarely on one of the receivers. One would just have to add one of the repeater RX to the IM calculation and there certainly seems to be a big potential problem.

His example was as follows:

Repeater 1
TX1 460.000
RX1 465.000

Repeater 2
TX2 460.700
RX2 465.700

3rd order IM formula with 3 frequencies: F(im)=a+b-c

RX1 465.000 + TX2 460.7 - TX1 460.000 = RX2 465.700

I had decided on going 100KHz separation between R-R and T-T of both systems (and 5MHz between R-T), but now being aware of the above possibility, not sure really how high the real world chances would be of this happening. I even tried to play a bit modifying one of the repeater frequency pairs, but it quickly because obvious that would not help at all. Anyway, I am not sure if the interaction of the two TX's of the repeaters, which would be much stronger signals, would even make it possible for a much weaker signal in comparison to the other two still be able to interact enough to create a resulting IM product strong enough that would be an exact match of one of the the receivers.

Any thoughts on the above?

BTW equipment is starting to arrive, so will put up some additional information Shortly on that on my next post.

 

[tropiradio]

Well as mentioned earlier equipment finally started to arrive. I got the RD982i, and the two Sinclair Q3220E factory adjusted duplexer's, the 1/2 hardline and connectors, rack enclosure, plus some other stuff, among them all the 2-foot RF interconnecting jumpers made of RG142. Still waiting on the Telewave antenna as factory suddenly out of the blue pushed back delivery to late this month after the order had already been confirmed and processed.

Besides the Sinclair duplexer I still wanted to use at least a single stage circulator on each repeater so I stared looking around and came across a listing on ebay of someone that was offering a complete dual stage UHF IM Panel made by Decibel from around the 90's. It has two DB4325-B 450-512MHz circulators, one DB4332-B 450-470MHz LP Filter, one 100W and one 300W Decibel loads. All neatly mounted on an open aluminum rack, and of course only SS or aluminum hardware to avoid affecting the circulators. Obviously will have to take the pieces and remount individually on each repeater. Seller was asking $175, but after a week of no takers went down to $150. I then had a couple of back and forth chit chats and then politely offered $100 to which he agreed. My shipping was $75. I think the two loads alone are worth more than what I paid so I think that was not a bad deal even if I end up not being able to use it for the current project.

I put the new repeater on the bench with the duplexer and one circulator, plus the LP filter at the output of the circulator and everything seems to be working fine. The circulator has about -0.4dB forward insertion loss, and about 23.8dB isolation in reverse. These circulators don't seem to be adjustable, but worked just fine even though I doubt that they where operating anywhere near where I am which would have been a big coincidence. They seem to be rather broadband, hence the no adjustment? I thought all circulators had adjustments, but on these nothing under the labels, no adjustment holes at all. Also checked the LP filter between 400-960MHz and it is almost flat up to about 475MHz after which it starts to drop so looks good to suppress any 2F or above products.

So all in all on the TX path I got -0.8dB for the duplexer HP, -0.4dB for the circulator, and -0.3dB for the LPF. Total insertion loss -1.5dB. Given I am planning to set the repeater to 20W (-43dBm), excluding feed line loss that should give me about 14.2W (+41.5dBm) at the antenna. I figure this would be enough for the portables to operate within the designated 1 mile of the repeater. Will have to test and adjust if necessary, but off the bat want to keep TX power as low as possible.

Now I'm still short of one LPF, but could not find any of the same anywhere, not used or new. Contacted dbSpectra which is still carrying the same DB4332-B filter, but they said lead time would be 6-8 weeks. It seems everything nowadays is the same, and the "in stock" just died. There is also a similar filter made by EMR Corp but I have not yet contacted them, not sure how good they are, but the name strangely seems to ring a bell in my mind from back in the 70s or 80s and more affiliated to 27MHz CB products. But I'm probably wrong on that.

So took the small DB filter apart, its very easy to open with 4 screws the back cover comes off. Its housed in one of those Pomona metal utility boxes with one NM bulkhead connector on one end, and a NF connector on the other. Inside the box it actually says Pomona. So there are 4 Johanson brand trimmers (these have absolutely no markings but its easy to recognize the piston type), and 3 short coils. I disconnected one of the coils so I was able to check one of the trimmers, which turned out has an adjustment range from close to zero to 10pf. And the coils are made out of #18 enameled wire, one has 3.5 turns, and the other two have 2 turns. All 3 have a 7/64" diameter.

I figured that it should be quite easy to duplicate this filter. But Pomona does no longer use anything but BNC connectors on their boxes, and they are smaller in size. So went to look on ebay and found the same box as the original only with PL male/female connectors. Then also separately ordered one NM and one NF chassis mount to swap out the PL connectors. Next found several sellers offering almost identical looking Johanson trimmers of the value I needed, two sellers where even offering them in 4-packs. So went with the one that looked more similar in size and got the ones supposedly rated at 150V. There where others same value rated at 250V but they where clearly physically longer than the originals. Lastly got a small spool of #18 enameled wire. So now waiting for the stuff to arrive and see if I can duplicate the filter, which seems quite doable.

Any comments and suggestions welcomed. Thanks.

 

[prcguy]

If you run dual isolators on each repeater you shouldn't experience any IMD problems with the example shown. A dual isolator will absorb upwards of 75dB of energy heading up the coax toward the transmitter which will prevent that from mixing in the transmitter and creating IMD. I've run a number of transmitter combiners with several transmitters fed to a single antenna and besides the modest isolation from the hybrid combiner, which by the way is seeing full power (100W) from each transmitter, the dual isolators are allowing all the transmitters to play nice together.

In a system with repeaters feeding duplexers and separate close spaced antennas you might see a couple of watts of energy picked up by one antenna and fed back towards another transmitter depending on frequency spacing and duplexer band pass characteristics. A dual isolator will knock that couple of watts down to the equivalent of a strong receive signal. For example, if 2 watts of energy has made it past a duplexer and is heading toward a transmitter, a dual isolator with 75dB of isolation will attenuate that to about -42dBm. That should be a realistic number if your repeaters are running 50 watts to 6dB gain antennas and your antennas are 3ft apart.

If you can place the same antennas 10ft apart you can get about 10dB more isolation lowering the level to about -52dBm and so on. In my opinion, design your system with whatever frequencies you need, keep antennas at a good distance apart, use a dual isolator on every transmitter and you should be fine.

So I finally understood what that ham guy on RB was trying to say about mutual interference between two repeaters evenly spaced between them. The example bellow did not come from the same initial guy that was never able to explain himself clearly, or just wouldn't bother. But this other guy certainly cleared up things for me.

Basically the idea is that when portables are operating very close to the repeater, like in my scenario, their transmit frequency (which would of course be one of the repeater's RX) could also possibly become part of the mix to create IM if signals that might be strong enough, and the generated IM product would then potentially fall squarely on one of the receivers. One would just have to add one of the repeater RX to the IM calculation and there certainly seems to be a big potential problem.

His example was as follows:

Repeater 1
TX1 460.000
RX1 465.000

Repeater 2
TX2 460.700
RX2 465.700

3rd order IM formula with 3 frequencies: F(im)=a+b-c

RX1 465.000 + TX2 460.7 - TX1 460.000 = RX2 465.700

I had decided on going 100KHz separation between R-R and T-T of both systems (and 5MHz between R-T), but now being aware of the above possibility, not sure really how high the real world chances would be of this happening. I even tried to play a bit modifying one of the repeater frequency pairs, but it quickly because obvious that would not help at all. Anyway, I am not sure if the interaction of the two TX's of the repeaters, which would be much stronger signals, would even make it possible for a much weaker signal in comparison to the other two still be able to interact enough to create a resulting IM product strong enough that would be an exact match of one of the the receivers.

Any thoughts on the above?

BTW equipment is starting to arrive, so will put up some additional information Shortly on that on my next post.

 

[prcguy]

There are lots of low pass filters from Isolators/circulators on Ebay for ok prices, search on mfr names like Telewave, DB Products, Wacom, EMR, Tx Rx systems, etc. I've seen them recently for about $40 which is too much unless you really need one bad.

Just about every isolator made for the two way radio industry is tunable and a single isolator should give about 35dB isolation. Sounds like your isloators are not tuned where you need them.

Well as mentioned earlier equipment finally started to arrive. I got the RD982i, and the two Sinclair Q3220E factory adjusted duplexer's, the 1/2 hardline and connectors, rack enclosure, plus some other stuff, among them all the 2-foot RF interconnecting jumpers made of RG142. Still waiting on the Telewave antenna as factory suddenly out of the blue pushed back delivery to late this month after the order had already been confirmed and processed.

Besides the Sinclair duplexer I still wanted to use at least a single stage circulator on each repeater so I stared looking around and came across a listing on ebay of someone that was offering a complete dual stage UHF IM Panel made by Decibel from around the 90's. It has two DB4325-B 450-512MHz circulators, one DB4332-B 450-470MHz LP Filter, one 100W and one 300W Decibel loads. All neatly mounted on an open aluminum rack, and of course only SS or aluminum hardware to avoid affecting the circulators. Obviously will have to take the pieces and remount individually on each repeater. Seller was asking $175, but after a week of no takers went down to $150. I then had a couple of back and forth chit chats and then politely offered $100 to which he agreed. My shipping was $75. I think the two loads alone are worth more than what I paid so I think that was not a bad deal even if I end up not being able to use it for the current project.

I put the new repeater on the bench with the duplexer and one circulator, plus the LP filter at the output of the circulator and everything seems to be working fine. The circulator has about -0.4dB forward insertion loss, and about 23.8dB isolation in reverse. These circulators don't seem to be adjustable, but worked just fine even though I doubt that they where operating anywhere near where I am which would have been a big coincidence. They seem to be rather broadband, hence the no adjustment? I thought all circulators had adjustments, but on these nothing under the labels, no adjustment holes at all. Also checked the LP filter between 400-960MHz and it is almost flat up to about 475MHz after which it starts to drop so looks good to suppress any 2F or above products.

So all in all on the TX path I got -0.8dB for the duplexer HP, -0.4dB for the circulator, and -0.3dB for the LPF. Total insertion loss -1.5dB. Given I am planning to set the repeater to 20W (-43dBm), excluding feed line loss that should give me about 14.2W (+41.5dBm) at the antenna. I figure this would be enough for the portables to operate within the designated 1 mile of the repeater. Will have to test and adjust if necessary, but off the bat want to keep TX power as low as possible.

Now I'm still short of one LPF, but could not find any of the same anywhere, not used or new. Contacted dbSpectra which is still carrying the same DB4332-B filter, but they said lead time would be 6-8 weeks. It seems everything nowadays is the same, and the "in stock" just died. There is also a similar filter made by EMR Corp but I have not yet contacted them, not sure how good they are, but the name strangely seems to ring a bell in my mind from back in the 70s or 80s and more affiliated to 27MHz CB products. But I'm probably wrong on that.

So took the small DB filter apart, its very easy to open with 4 screws the back cover comes off. Its housed in one of those Pomona metal utility boxes with one NM bulkhead connector on one end, and a NF connector on the other. Inside the box it actually says Pomona. So there are 4 Johanson brand trimmers (these have absolutely no markings but its easy to recognize the piston type), and 3 short coils. I disconnected one of the coils so I was able to check one of the trimmers, which turned out has an adjustment range from close to zero to 10pf. And the coils are made out of #18 enameled wire, one has 3.5 turns, and the other two have 2 turns. All 3 have a 7/64" diameter.

I figured that it should be quite easy to duplicate this filter. But Pomona does no longer use anything but BNC connectors on their boxes, and they are smaller in size. So went to look on ebay and found the same box as the original only with PL male/female connectors. Then also separately ordered one NM and one NF chassis mount to swap out the PL connectors. Next found several sellers offering almost identical looking Johanson trimmers of the value I needed, two sellers where even offering them in 4-packs. So went with the one that looked more similar in size and got the ones supposedly rated at 150V. There where others same value rated at 250V but they where clearly physically longer than the originals. Lastly got a small spool of #18 enameled wire. So now waiting for the stuff to arrive and see if I can duplicate the filter, which seems quite doable.

Any comments and suggestions welcomed. Thanks.

 

[tropiradio]

There are lots of low pass filters from Isolators/circulators on Ebay for ok prices, search on mfr names like Telewave, DB Products, Wacom, EMR, Tx Rx systems, etc. I've seen them recently for about $40 which is too much unless you really need one bad.

I have not been able to find any decent UHF LP filter on ebay that does not have the word "TVI" in the description and be from a reputable manufacturer but one used Telewave TLF-450, and the seller is asking $175 for it. When you say "need one bad" doesn't any circulator need one at the output? Its kind of mandatory to avoid interference and harmonic radiation that the duplexer might not be able to fully filter out.

Just about every isolator made for the two way radio industry is tunable and a single isolator should give about 35dB isolation. Sounds like your isloators are not tuned where you need them.

So how would these be field tuned? Anyone familiar with these DB Products DB4325-B circulator? I cannot even see how one would even be able to open it up, there are no visible seams or screws holding it together except for the ones on the in/out connectors. Its like a solid block of metal, looks like its sealed up completely. Is it possible these where factory custom tuned and no field retuning was ever intended?

I have not peeled back the front label but feeling with a finger all over it reveals no indentation or bump to an adjustment hole or a cap as one would expect. There is a small label on the back that dates the unit to 7-29-91. Nothing under that one either. On the back there are the only three small threaded holes one next to each connector, and I can see what appears brass showing at the very bottom, but certainly no adjustment. These holes are just for mounting the unit.

I am going to try measuring the reverse isolation at other frequencies up and down on the band, see if I can find a point where it increases to what you say should be closer to 35dB. But at least the 25dB I am getting now with only 0.4dB forward loss is certainly far from being optimum, but better than nothing I suppose.

 

[prcguy]

I found one but its way too expensive. There were a couple that were much more reasonable within the last few weeks and there are plenty available now at 800 and 900MHz ranges. Maybe search completed listings and if they didn't sell contact the seller. You could contact the seller of the expensive one and offer them $50 or whatever.

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