nevo

TX antenna combining

24 posts in this topic

A question re combining TX antenna .Ive read most of the relevant threads I can find but thought I'd double check.

I'm running 2 comtek transmitters off my cart.

The Bst75(216) is using a modded Sna600 which is for the director etc and the m216 opt7 with the whip is for boom.

I'd like to use a passive 2-1 splitter either shure or lectro to connect the m216 to the SNA

I know I'd have 3db loss with the split but im unsure about any intermod OR other issues.The transmitters are a distance from each other

From my research I think it should be fine with lectro ZSC24

Cheers Ant

 

 

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I am wary of Rf weirdness, Anthony.  So I go with tried and true. I use 2 BST75 TXs with the Phase Right antennas. My channels are "Bourgeousie" for Director and others who don't need to hear me communicating with boom ops, and "Proletariait" for my guys. No issues. All channels vetted for IM with NewEndian's App, freqfinder.  Splits, I dunno, seems like complication for no reason...

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There is method to my madness... in my quest for the smallest cart with most capacity one less antenna fits the model.....IF there is no technical reason for this not to work I'm keen to try.


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Understood, Ant.

I work with a cart. And it's always too heavy no matter what I do to alleviate that fact:) I think there may be a technical reason, but would love to be proven wrong.

 

Jim

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I think it's a valid question and would love to hear more discussion on this topic. 

 

There's already a good selection of purpose-made combiners for IEM transmitters available on the market. RF Venue makes one for example (http://www.rfvenue.com/4-channel-iem-combiner) and so does Sennheiser, Shure, etc. All of the models I've looked into are rack mount and only cover the UHF band 470-900 ish. 

 

I wish a small combiner existed for bag rigs so you could combine all the hops and IFB transmitters onto one piece of cable and run it to an antenna on your shoulder strap. I think it would result in cleaner scans on your receivers and better reception overall. 

 

On carts it seems like everyone is getting perfectly decent range both inbound and out using a standard antenna tree with all antennas mounted just a few feet from each other.  RF Venue has a good article on their site about the benefits of IEM combiners. 

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I was hoping if I was only using two Tx I'd be able to not use a combiner.... a mini mite would be a better solution then I guess


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Well for just two maybe get a passive splitter and just see how it works. Like the mini circuits one. 

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I'd be really wary of a passive combiner in this application.  You would basically be feeding a power output into another power output which is never a good idea!  An active combiner, designed for this purpose, should isolate outputs properly.

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8 hours ago, John Blankenship said:

I'd be really wary of a passive combiner in this application.  You would basically be feeding a power output into another power output which is never a good idea!  An active combiner, designed for this purpose, should isolate outputs properly.

Hi Ant and John,
There is about 30 to 35 dB of isolation between the input ports with a mini circuits (or Lectro) 2 way splitter-combiner. This is equivalent to having two separate antennas about 5 feet apart. There will be a small amount of 3rd order intermod (as with separate antennas) but rudimentary frequency co-ordination will prevent this from being a problem. You will also have this low level intermod with active combiners. The only downside is the 3 dB loss with a 2 way splitter-combiner but if you can live with 70% of normal range, then it is a viable option. Most times you won't even notice the slight range reduction.

Don't ever use a simple T adapter between two transmitters:  Intermod will be terrible, power losses will be more than 3 dB and the transmitters may very well go unstable, spewing random frequencies near and far.

Best Regards,
Larry F

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Thanks, Larry, for correcting me.  I didn't realize that there was that much isolation with a passive splitter.

Please be careful, folks, which you use though, as some splitters -- especially the cheaper ones -- are more of the simple T adapter design.  In choosing one, I would go with Larry's recommendations, especially the Lectro one.

 

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Thanks for clarifying Larry. Yes, I meant the mini-circuits type splitter. Definitely not a simple T-BNC adapter. 

 

Side question for Larry, if you were to try this setup with a 4-way splitter would the loss be the same if you only used 2 of the 4 splits? Does that make sense? Or is the loss the same regardless of how many splits you're using? Would using 50 ohm terminators help anything?

 

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Hi Derek,
The 4 way splitter has a loss of 6 dB. If you only use 2 of the 4, the loss is still the same. The 50 Ohm terminators will make only a negligible improvement. With passive combiner-splitters you always lose. Active units just use amplifiers to make up for the losses.
We talk of dB of loss but another way to think about it, is that the loss is just the number of splits or combinations. Half power for a two way, 1/4 for a 4 way, 1/8 for an eight way, etc.
Best,
Larry F

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If you're using an M216, I would simply leave the antenna where it is. At 10mW, you can't afford to be losing 30% of your range (minimum). One little whip can't possibly be a weight or size issue. Just my opinion.

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Hi Ant and John,

There is about 30 to 35 dB of isolation between the input ports with a mini circuits (or Lectro) 2 way splitter-combiner. This is equivalent to having two separate antennas about 5 feet apart. There will be a small amount of 3rd order intermod (as with separate antennas) but rudimentary frequency co-ordination will prevent this from being a problem. You will also have this low level intermod with active combiners. The only downside is the 3 dB loss with a 2 way splitter-combiner but if you can live with 70% of normal range, then it is a viable option. Most times you won't even notice the slight range reduction.

Don't ever use a simple T adapter between two transmitters:  Intermod will be terrible, power losses will be more than 3 dB and the transmitters may very well go unstable, spewing random frequencies near and far.

Best Regards,

Larry F

Thank you Larry..... for your sage advice

 

 

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We did a great deal of work with combined Tx (often 2 sometimes 3) in the VHF hiband days by calculating exact cable lengths in the network and using passive T style connectors.  We got very acceptable range on car-to-car work with the TX feeding a single mag-mount antenna cut to length (in the VHF days) or two of the old Lectro omni antenna groundplanes (the "spider" style ones) with cut antennas in the early UHF days (like 195s, still fixed freq).  There was less RF around then but it all worked out very well numerous times, urban and rural.  The takeaway is to try things out for yourself, using the givens of your particular job, esp re range.  You won't win a Nobel for physics but you might get your job done cheaply and well.

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I've ordered a splitter and I'll do some experimenting.... thanks for everyone's help


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My interest in this topic stems from casual observation that I seem to get much better-behaved receiver performance when I'm not using hops transmitters in the bag or when using fewer transmitters like just an IFB. 

 

Thinking that it could be easier to remote a single transmitter whip on a shoulder or behind the back than implementing an entire receiver distro with two external antennas. 

 

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I don't doubt that using a single antenna on a harness would be a great benefit ergonomically. It seems the OP wants to make a mini cart. I just can't see that adding a cable from an M216 to a combiner (which has weight and size) offers any benefit over the rubber antenna attached to the transmitter itself. Believe me, I'm all about trying to reduce size and weight, but this seems to do neither. I'd love the OP to describe the scenario. I'm also all about learning new things :-)

 

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What length of antenna would you use on a combiner to cover 216 and uhf?

 

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43 minutes ago, r.paterson said:

What length of antenna would you use on a combiner to cover 216 and uhf?

 

We may well need a far better mind than mine, but...... I have only ever experienced antenna combining for antennas of similar frequency bands. I have no experience of combining something from 216 with something much higher, like UHF, 606 etc.....Could you explain what you are trying to achieve please? Also - you dont often mount the antennas directly on the combiner. Usually there is a cable between the antenna and the combiner. More details please ;-) sb

 

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I don't doubt that using a single antenna on a harness would be a great benefit ergonomically. It seems the OP wants to make a mini cart. I just can't see that adding a cable from an M216 to a combiner (which has weight and size) offers any benefit over the rubber antenna attached to the transmitter itself. Believe me, I'm all about trying to reduce size and weight, but this seems to do neither. I'd love the OP to describe the scenario. I'm also all about learning new things :-)
 

Hi Robert
I am trying to get better range from the M216.... whenever I have remoted the antenna it usually seems to help.
I have an antenna tree with a rf venue and a modded sna for the lectro venue and comtek bst75... was just thinking i might be able to combine the comtek antenna..... alternatively I could secure the m216 up on the tree and run a longer input cable.
I'll try the splitter and if it' doesn't have any benefit I'll rethink things


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I'm needing on occasion to add a couple of my SRB to my venue antenna so a splitter might come in handy for that


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I think your instinct to run the transmitter up with a longer input cable is good. Or try just hanging it on the side away from all the other RF bits. Or better yet, just run it out closer to where the receivers are. When I ran an M216, I put it on video village.

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The bst75 has sorted all my. Range issues... 300m plus.... I'm running a sk2000/G3 for boom but was getting some hits the other day.... the splitter was cheap and is worth a experiment....


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