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Author Topic: RX & TX on Same Antenna Using Circulator?  (Read 7369 times)

Russell Ault

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RX & TX on Same Antenna Using Circulator?
« on: June 23, 2015, 05:24:00 PM »

Maybe I'm misunderstanding the principles involved, but are there any reasons (physics-related or otherwise) you couldn't use a properly-selected 3-port circulator (with any necessary filtering to remove the circulator's own harmonics) to share an antenna (and associated cabling) between a UHF IEM transmitter and one side of a diversity wireless microphone receiver (assuming, of course, enough frequency separation that your wireless mics aren't desensing your IEM receivers)? Is there something (like a properly-configured diplexer) that would work better?

Thanks!

-Russ
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Jeff Carter

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Re: RX & TX on Same Antenna Using Circulator?
« Reply #1 on: June 23, 2015, 08:43:26 PM »

In your proposed setup, the circulator would dump any reflected transmitter power (from impedance mismatch in the antenna or anywhere else) into the receiver. That's my physics-related objection, although I'm not sure how much of a problem it would be in practice.
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Mac Kerr

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Re: RX & TX on Same Antenna Using Circulator?
« Reply #2 on: June 23, 2015, 10:24:38 PM »

Maybe I'm misunderstanding the principles involved, but are there any reasons (physics-related or otherwise) you couldn't use a properly-selected 3-port circulator (with any necessary filtering to remove the circulator's own harmonics) to share an antenna (and associated cabling) between a UHF IEM transmitter and one side of a diversity wireless microphone receiver (assuming, of course, enough frequency separation that your wireless mics aren't desensing your IEM receivers)? Is there something (like a properly-configured diplexer) that would work better?

That's a lot of complexity and expense to avoid a 3rd antenna. It would also have to be tuned to the particular frequencies you want to use, somewhat limiting your flexibility. And any RF dumped into the receiver would desensitize it.

Mac
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Russell Ault

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Re: RX & TX on Same Antenna Using Circulator?
« Reply #3 on: June 23, 2015, 11:18:14 PM »

That's a lot of complexity and expense to avoid a 3rd antenna. It would also have to be tuned to the particular frequencies you want to use, somewhat limiting your flexibility. And any RF dumped into the receiver would desensitize it.

Mac

My thought is that this might be good for a venue small enough that getting decent RX/TX antenna separation would be difficult (or, at least, difficult within aesthetic minimums), like a 12' x 8' floor-level thrust performance area in a hotel ballroom with no backstage and a 6' banquet table (or smaller!) for FOH (which is not a made-up scenario...).

So far I have been able to find a relatively inexpensive (around the cost of an antenna + cabling) circulator that's pretty wideband (at least 20dB of isolation over 72MHz) designed to hand a couple orders of magnitude more power than we ever deal with, and that's only including the outfits that list their prices on their websites (which excludes a lot). From a cost standpoint alone, ditching the third antenna might be comparable (or even cheaper), not to mention a slightly reduced setup time (useful for a one-tech rig).

In your proposed setup, the circulator would dump any reflected transmitter power (from impedance mismatch in the antenna or anywhere else) into the receiver. That's my physics-related objection, although I'm not sure how much of a problem it would be in practice.

I feel like this is going to be the real problem: VSWR information seems scarce for many of the more common IEM TX antenna, and the ones I can find suggest that there may well be desense-level reflected signals coming back from the antenna. That's especially too bad, because it looks like scientists have recently discovered a way to make a non-magnetic, semiconductor-able circulator. Diplexer, anyone?

Thank you both for helping to lessen my ignorance!

-Russ
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Tim McCulloch

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Re: RX & TX on Same Antenna Using Circulator?
« Reply #4 on: June 24, 2015, 09:00:58 AM »

My thought is that this might be good for a venue small enough that getting decent RX/TX antenna separation would be difficult (or, at least, difficult within aesthetic minimums), like a 12' x 8' floor-level thrust performance area in a hotel ballroom with no backstage and a 6' banquet table (or smaller!) for FOH (which is not a made-up scenario...).

So far I have been able to find a relatively inexpensive (around the cost of an antenna + cabling) circulator that's pretty wideband (at least 20dB of isolation over 72MHz) designed to hand a couple orders of magnitude more power than we ever deal with, and that's only including the outfits that list their prices on their websites (which excludes a lot). From a cost standpoint alone, ditching the third antenna might be comparable (or even cheaper), not to mention a slightly reduced setup time (useful for a one-tech rig).

I feel like this is going to be the real problem: VSWR information seems scarce for many of the more common IEM TX antenna, and the ones I can find suggest that there may well be desense-level reflected signals coming back from the antenna. That's especially too bad, because it looks like scientists have recently discovered a way to make a non-magnetic, semiconductor-able circulator. Diplexer, anyone?

Thank you both for helping to lessen my ignorance!

-Russ

You need to discuss this with a genuine RF guru, to show you a better way (apologies to Dr. Hook).  I suggest Henry Cohen:  info at productionradio dot com
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Henry Cohen

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Re: RX & TX on Same Antenna Using Circulator?
« Reply #5 on: June 27, 2015, 02:00:47 PM »

Using a circulator based method to reduce the number of antennas is perfectly viable, and can actually be a preferable means to achieve correct RX/TX isolation when sufficient spatial separation is not possible. It does however require a number of other support components to ensure proper performance. We actually use this very configuration for certain two-way radio applications. And today, there are a number of wideband offerings available with more than acceptable isolation, insertion loss and VSWR specs, some off the shelf, others built to order.

First, for this type of application, a 4 port circulator is a better option as it provides greater isolation between the IEM transmitter (on port 1) and the receiver (on port 3). Port 4 will be terminated. Second, a bandpass filter on port three, ahead of the receiver input, that provides at least 30dB of attenuation at the IEM frequency(ies), is required. Third, as circulators create a rather strong harmonic, a low pass or harmonic filter is required on port 2 (the antenna).

Finally, as the coax from the LP or harmonic filter to the antenna will now be carrying both strong TX signals along with weaker RX, a very high quality coax is required. Ideally, a corrugated shield is best (think Andrew Heliax), but eminently unwieldy and impractical for anything but a permanent installation. The next best choice is a double shielded coax with dual braid, such as RG214. The like shield material and the interlocking characteristic of two braids atop each other keeps potential RF noise (due to dissimilar metals rubbing against each other as with a typical braid over foil design) to a minimum. That said, we use LMR400 and 9913F7 (braid over foil designs) regularly on duplex applications; as long as the coax and connectors are in top condition, and not swaying in the wind, this coax works very well.

A diplexer is also a very good method for combining/separating disparate frequency ranges through a single coax/antenna, but is a custom made piece and depending on how the cavities are cut/machined, and without a high quality VNA and time, is not retunable by the end user.

Regarding the lack of VSWR or return loss numbers for the usual suspect antennas from wireless microphone manufactures, it's generally perceived (and not wrongly IMO) that most buyers have no clue as to what those numbers mean, how to interpret them, and just get confused by them, or start to concentrate on the wrong specification (think back to the days when audio power amplifier manufacturers had slew rate spec wars). In general, a good performing broadband antenna will have better than 10dB return loss (1.92:1), which is about 90% of the transmitted power going out the antenna. Not bad for a broadband device. Jason Glass actually tested a number of antennas for their RL and posted the results on the SFN about a year ago(?), but unfortunately one of the minor manufacturers who had a questionable antenna offering threatened legal action (rather than take the opportunity to actually improve their product) and SFN had to take the thread down (the irony is that antenna sells fairly well and users generally like the performance.) His findings pretty much agreed with the 10dB RL generalization.

One caveat: As with most things, you get what you pay for. RF passives seem especially vulnerable to this maxim. If you're not paying at least $250-300.00 for a 3 port circulator covering the UHF-TV band, I'd be very suspect. (Then again, if you have found something cheaper with good specs, please let me know; I'll be the first to buy a sample and test it ;) )
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Russell Ault

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Re: RX & TX on Same Antenna Using Circulator?
« Reply #6 on: June 30, 2015, 12:59:56 AM »

You need to discuss this with a genuine RF guru, to show you a better way (apologies to Dr. Hook).

Looks like I'll get the opportunity! (I'm sure Dr. Hook doesn't mind, although I'm not sure the March 1973 cover actually counted...)   :D

First, for this type of application, a 4 port circulator is a better option as it provides greater isolation between the IEM transmitter (on port 1) and the receiver (on port 3). Port 4 will be terminated. Second, a bandpass filter on port three, ahead of the receiver input, that provides at least 30dB of attenuation at the IEM frequency(ies), is required. Third, as circulators create a rather strong harmonic, a low pass or harmonic filter is required on port 2 (the antenna).

Finally, as the coax from the LP or harmonic filter to the antenna will now be carrying both strong TX signals along with weaker RX, a very high quality coax is required. Ideally, a corrugated shield is best (think Andrew Heliax), but eminently unwieldy and impractical for anything but a permanent installation. The next best choice is a double shielded coax with dual braid, such as RG214. The like shield material and the interlocking characteristic of two braids atop each other keeps potential RF noise (due to dissimilar metals rubbing against each other as with a typical braid over foil design) to a minimum. That said, we use LMR400 and 9913F7 (braid over foil designs) regularly on duplex applications; as long as the coax and connectors are in top condition, and not swaying in the wind, this coax works very well.

A diplexer is also a very good method for combining/separating disparate frequency ranges through a single coax/antenna, but is a custom made piece and depending on how the cavities are cut/machined, and without a high quality VNA and time, is not retunable by the end user.

From this description, it sounds a lot like a diplexer is a much better solution in situations like mine that don't benefit from broadband frequency-related flexibility (i.e. in situations where specific, bandwidth-limited wireless gear is always used together as a set, and sufficient whitespace between microphones and IEMs has already been allowed for), especially since you'd end up needing almost a diplexer's-worth of filtering to make a circulator work anyway (to say nothing of a circulator's cost and, from what I've been reading, potential fragility). Am I correct in this?

Regarding diplexers, is there a significant difference between a custom-built unit vs. a combination of off-the-shelf filters and a passive combiner/splitter (or, for that matter, a combiner/splitter and a filter on just the microphone RX feed that will drop the IEM frequencies by ~40dB)?

Regarding the lack of VSWR or return loss numbers for the usual suspect antennas from wireless microphone manufactures, it's generally perceived (and not wrongly IMO) that most buyers have no clue as to what those numbers mean, how to interpret them, and just get confused by them, or start to concentrate on the wrong specification (think back to the days when audio power amplifier manufacturers had slew rate spec wars). In general, a good performing broadband antenna will have better than 10dB return loss (1.92:1), which is about 90% of the transmitted power going out the antenna. Not bad for a broadband device. Jason Glass actually tested a number of antennas for their RL and posted the results on the SFN about a year ago(?), but unfortunately one of the minor manufacturers who had a questionable antenna offering threatened legal action (rather than take the opportunity to actually improve their product) and SFN had to take the thread down (the irony is that antenna sells fairly well and users generally like the performance.) His findings pretty much agreed with the 10dB RL generalization.

That makes a lot of sense. It's a shame that the threat of legal action is standing in the way of research publication. (It's also a pity that no one makes an antenna with a low enough VSWR to obviate circulator bandpass filtering, but that's probably just a pipe dream on my part, especially given the broadband nature of the stuff we're working with.)

One caveat: As with most things, you get what you pay for. RF passives seem especially vulnerable to this maxim. If you're not paying at least $250-300.00 for a 3 port circulator covering the UHF-TV band, I'd be very suspect. (Then again, if you have found something cheaper with good specs, please let me know; I'll be the first to buy a sample and test it ;) )

I think the chances of me finding something that you haven't already seen is basically nil (I'm assuming you've already heard about the new magnet-free circulator developments...).

Thanks!

-Russ
« Last Edit: June 30, 2015, 04:48:14 PM by Russell Ault »
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Henry Cohen

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Re: RX & TX on Same Antenna Using Circulator?
« Reply #7 on: July 03, 2015, 09:31:02 AM »

From this description, it sounds a lot like a diplexer is a much better solution in situations like mine that don't benefit from broadband frequency-related flexibility (i.e. in situations where specific, bandwidth-limited wireless gear is always used together as a set, and sufficient whitespace between microphones and IEMs has already been allowed for), especially since you'd end up needing almost a diplexer's-worth of filtering to make a circulator work anyway (to say nothing of a circulator's cost and, from what I've been reading, potential fragility). Am I correct in this?
If you in fact can always operate within the diplexer's operating parameters (bandpasses and separation) without retuning, then yes, it is the preferred method.


Quote
Regarding diplexers, is there a significant difference between a custom-built unit vs. a combination of off-the-shelf filters and a passive combiner/splitter (or, for that matter, a combiner/splitter and a filter on just the microphone RX feed that will drop the IEM frequencies by ~40dB)?
A custom built diplexer will, with rare exceptions, always provide less insertion loss (2-5dB depending on design), much higher Q and greater power handling than a 2-way splitter with two bandpass filters given the same volumetric space. It will however almost certainly cost more.


Quote
(I'm assuming you've already heard about the new magnet-free circulator developments...).
I have. Not ready for prime time.
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Henry Cohen

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Russell Ault

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Re: RX & TX on Same Antenna Using Circulator?
« Reply #8 on: July 09, 2015, 11:57:27 PM »

A custom built diplexer will, with rare exceptions, always provide less insertion loss (2-5dB depending on design), much higher Q and greater power handling than a 2-way splitter with two bandpass filters given the same volumetric space. It will however almost certainly cost more.

Just as a follow-up to this point (mainly, again, as a cost compromise), is a filter on the IEM TX send to the splitter/combiner absolutley necessary? What would be the downside to having the IEM transmitters feed directly into one leg of an off-the-shelf splitter/combiner, and then filtering those frequencies out from the microphone RX leg (using another off-the-shelf part)?

Thanks!

-Russ
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Henry Cohen

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Re: RX & TX on Same Antenna Using Circulator?
« Reply #9 on: July 10, 2015, 01:11:02 PM »

Just as a follow-up to this point (mainly, again, as a cost compromise), is a filter on the IEM TX send to the splitter/combiner absolutley necessary? What would be the downside to having the IEM transmitters feed directly into one leg of an off-the-shelf splitter/combiner, and then filtering those frequencies out from the microphone RX leg (using another off-the-shelf part)?

The reason for the filter on the TX side is two-fold:
1) Keeps any spurious emissions and side lobes from the transmitter attenuated;
2) Prevents other frequencies picked up by the antenna from getting into the TX's PA, mixing and getting transmitted back out.

Both of these can cause self interference and/or desense of the mic receiver.
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Henry Cohen

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Re: RX & TX on Same Antenna Using Circulator?
« Reply #9 on: July 10, 2015, 01:11:02 PM »


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