isolation based on tx power

[brin831]

Guys this is more of a hypothetical than a real world application ... i come from an av background and we were talking about this the other day.

so most IEM (inner ear monitor) packs for performers run through tx combiners most are listed as 250mw max input basically because this is the max allowable power for fcc for these type devices ... most are simple passive combiners 4 to 1 or 2 to 1 some nicer ones have an amp that boosts combined signal back up to 250 mw after combiner loss.

250 mw is 24 dbm and most recommend something similar to the minicircuits zapd-21 for passive combining, which provides about 25 db of isolation and this is enough not to wreck anything.

so to my question is there a formula or way to know what is necessary isolation based on tx power ... for example 1 w or 30 dbm needs X isolation tx to tx to combine safely etc etc.

thanks for those who are smarter than i for explaining this !!

 

[jonwienke]

Are you trying to connect multiple transmitters to a single antenna?

 

[brin831]

jonwienke, not trying to do anything real world all of our radio repeaters are run through true Sinclair duplexers

this was more of a hey if this works for this application ... its rf, so what is the rule or formula when applying to a larger scale.

 

[jonwienke]

Nobody can answer that unless you answer the question. Your post is too incoherent to understand exactly what you are trying to do, or are asking about doing.

Are you talking about connecting multiple transmitters to a single broadcast antenna, multiple receiving antennas to a single receiver, or multiple receivers to a single antenna? The answer to your question will depend on which scenario you're contemplating.

 

[prcguy]

I can't answer your specific question but I do see more than one reason for achieving a specific amount of isolation between the transmitters. Having enough isolation to avoid damaging another transmitter in the system is a goal and depending on the transmitter, 6dB might be enough in a 2 channel system and for most any transmitter made, 10dB would surely suffice.

That would only be looking at what a transmitter sees as reflected power back into itself and having that extra power dissipate in the transmitter or possibly combining in phase with its own transmitter power and having that total exceed some components working voltage limit.

The other problem would be IMD or the creation of Intermodulation Distortion Products from signals mixing in the transmitters and producing new undesired signals that would be radiated by the antenna. There are formulas for that and the 25dB isolation provided by your combiner for your low power IEM system is probably enough based on the fact its a standard part used by that industry.

For the two way radio industry where transmitter powers are much higher and upwards of 100w, there are basically two kinds of transmitter combiners, cavity with isolator and hybrid with isolator and they can provide 70 to maybe 100dB or so of isolation.

The cavity type is used where the frequencies to be combined are far enough apart for a tuned resonant cavity filter to provide some usable isolation between frequencies, then the 35dB or so from a single isolater or up to 70dB from a dual stage isolator adds to that, providing the required total isolation.

A hybrid type combiner is used where frequencies are too close together for resonant cavity filters to be useful and a 3dB hybrid provides about 45dB isolation then a single or dual isolator is used to achieve the desired total isolation between transmitters. The cavity type combiner has minimal loss per channel and much less than your small passive combiner. A hybrid combiner has losses similar to your passive combiner, which is really not desirable in the 2-way radio industry where your 100w repeater transmitter might get 20w to the antenna from a 4 channel hybrid combiner system.

So you are lucky that the low power IEM systems can get by with just an inexpensive garden variety combiner because the other two I described can potentially cost more than your entire IEM system.
prcguy

Guys this is more of a hypothetical than a real world application ... i come from an av background and we were talking about this the other day.

so most IEM (inner ear monitor) packs for performers run through tx combiners most are listed as 250mw max input basically because this is the max allowable power for fcc for these type devices ... most are simple passive combiners 4 to 1 or 2 to 1 some nicer ones have an amp that boosts combined signal back up to 250 mw after combiner loss.

250 mw is 24 dbm and most recommend something similar to the minicircuits zapd-21 for passive combining, which provides about 25 db of isolation and this is enough not to wreck anything.

so to my question is there a formula or way to know what is necessary isolation based on tx power ... for example 1 w or 30 dbm needs X isolation tx to tx to combine safely etc etc.

thanks for those who are smarter than i for explaining this !!

 

[Mike_G_D]

Nobody can answer that unless you answer the question. Your post is too incoherent to understand exactly what you are trying to do, or are asking about doing.

Are you talking about connecting multiple transmitters to a single broadcast antenna, multiple receiving antennas to a single receiver, or multiple receivers to a single antenna? The answer to your question will depend on which scenario you're contemplating.

As the OP did specify "tx" and did mention power levels ("250mW"), in my opinion, it was clearly meant to ask about combining transmitters or transceivers. prcguy did a great job of answering that question.

-Mike

 

[brin831]

prcguy

thanks for the reply ... so little confused about what you said: 6 to 10 db is probably enough for any transmitter ... i'm familiar with the various types of combining ie cavities and hybrid combiners and the pros and cons of each loss vs frequency separation ... so to relate to the conversation the av world seldom uses cavities because of their size and lack of flexibility or portability ... you won't want to re tune your cavity every time the rock show goes to a new city and you have to avoid the local tv station in the 600's. so you deal with the loss and flexibility to re tune.

that being said why is 70 to 100 db of isolation something needed to achieve in a 100w system where it may not be in a lower power system?? assuming this is where the reflected power comes into play ... so still trying to determine the root question.

the real question is there an equation that says this much power needs this much isolation ??

I've only brought up the low powered av devices as an example of where no isolators or cavities are used and someone has determined that enough isolation is provided to prevent damage at those transmitter power levels.

HAA trust me the cost of professional level iem's with appropriate antenna and combiners are no less expensive than radio equipment shure psm 1000's fairly industry standard tour level system retail over 5k a channel including body packs (for the tallent the rx end) add in 4 - 8 ch combiner and antenna and a 4 ch system can easily hit 25 to 30k. one could probably expect to pay 30k for a 2 repeater 4 ch dmr system running 40 watts into cavities to a single antenna and a hand held units haha

 

[prcguy]

As I indicated in my first response, I don't have the specific answer to your question or a formula for determining the needed isolation and I can only give some opinions and examples from other industries.

My comment on the 6 to 10dB of isolation possibly being enough for your use is based only on my opinion of a safe amount of power one transmitter would feed into another. 6dB of isolation in a 2 channel combined system would feed 62.5mw back into the other 250mw transmitter.

6dB isolation would be the same amount of power fed back to a transmitter if you were feeding a single transmitter to an antenna with about a 3:1 VSWR, which you probably do anyway. 10dB isolation would feed 25mw back to the neighboring 250mw transmitter and would be the same as feeding a single transmitter into antenna with about a 1.7:1 VSWR, which is not that bad.

There are formulas for calculating IMD in transmitters but I don't have any available, maybe a google search will turn some up. You would want one where you can enter various parameters of the amplifier in your transmitter like known IP1 or IP3 levels, your operating levels and isolation between amplifiers and not the common calculators that just produce a list of input vs possible output frequencies.

The 70 or 100dB of isolation in 2-way radio system combiner is due mostly from the power they can operate at. 100dB down from 100W is still -50dBm (did that on my fingers, hope its right) and a huge level into a sensitive receiver that will hear down to -120dBm.
prcguy

prcguy

thanks for the reply ... so little confused about what you said: 6 to 10 db is probably enough for any transmitter ... i'm familiar with the various types of combining ie cavities and hybrid combiners and the pros and cons of each loss vs frequency separation ... so to relate to the conversation the av world seldom uses cavities because of their size and lack of flexibility or portability ... you won't want to re tune your cavity every time the rock show goes to a new city and you have to avoid the local tv station in the 600's. so you deal with the loss and flexibility to re tune.

that being said why is 70 to 100 db of isolation something needed to achieve in a 100w system where it may not be in a lower power system?? assuming this is where the reflected power comes into play ... so still trying to determine the root question.

the real question is there an equation that says this much power needs this much isolation ??

I've only brought up the low powered av devices as an example of where no isolators or cavities are used and someone has determined that enough isolation is provided to prevent damage at those transmitter power levels.

HAA trust me the cost of professional level iem's with appropriate antenna and combiners are no less expensive than radio equipment shure psm 1000's fairly industry standard tour level system retail over 5k a channel including body packs (for the tallent the rx end) add in 4 - 8 ch combiner and antenna and a 4 ch system can easily hit 25 to 30k. one could probably expect to pay 30k for a 2 repeater 4 ch dmr system running 40 watts into cavities to a single antenna and a hand held units haha