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Radiax as passive antenna -- question about termination

This forum is for discussions regarding System Infrastructure and Related Equipment. This includes but is not limited to repeaters, base stations, consoles, voters, Voice over IP, system design and implementation, and other related topics.

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motorolo
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Radiax as passive antenna -- question about termination

Postby motorolo » Sat Aug 28, 2010 6:53 pm

I'm looking to improve transmit / reception in the basement of a building where currently, our portable units can sometimes hear the repeater, and rarely break squelch into it. I've been reading up on Radiax and its deployment in situations like this, and it looks like something that might work for our needs.

What I haven't been able to find, though, is how the line should be terminated at its in-building terminus. Of course, at the rooftop end, the cable would be connected to an antenna pointed toward the repeater. If the in-building end of the cable isn't intended to interface with a base station or other radio equipment, though, what are best practices for terminating the end? Any comments here would be greatly appreciated!

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Re: Radiax as passive antenna -- question about termination

Postby Jim202 » Sun Aug 29, 2010 9:07 am

motorolo wrote:I'm looking to improve transmit / reception in the basement of a building where currently, our portable units can sometimes hear the repeater, and rarely break squelch into it. I've been reading up on Radiax and its deployment in situations like this, and it looks like something that might work for our needs.

What I haven't been able to find, though, is how the line should be terminated at its in-building terminus. Of course, at the rooftop end, the cable would be connected to an antenna pointed toward the repeater. If the in-building end of the cable isn't intended to interface with a base station or other radio equipment, though, what are best practices for terminating the end? Any comments here would be greatly appreciated!


Either put an antenna on it or disconnect the cable at the last split where you have an antenna. Last resort
would be to terminate the cable with a dummy load. That seems like a real waste if your looking for coverage
to just terminate the cable. I would rather see an omni antenna hanging from the ceiling.

Jim

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Bill_G
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Re: Radiax as passive antenna -- question about termination

Postby Bill_G » Sun Aug 29, 2010 11:17 am

In my experience, passive antenna systems do not work well. If your intent is to fill a specific area, then you need as much gain as you can get in the donor antenna on the rooftop aimed towards the repeater, and a unity gain omni indoors centered in the area of interest with as low of loss in your line as you can afford between them. Try not to exceed 100ft. A BDA is your best option though it sounds like you will have poor separation between your donor and your indoor antennas. These are tough nuts to crack.

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Re: Radiax as passive antenna -- question about termination

Postby Jim202 » Mon Aug 30, 2010 2:56 am

As Bill has pointed out, there are a number of ways to approach this. You can add voter receivers to your
repeater to try and fill in the poor reception areas. This will require a voter box of some sort and several
receivers. The receiver with the best signal to noise ratio will feed the audio to the repeater. Uses normal
telephone line connections between the remote receiver and the voter box. The voter box can be mounted anyplace and then feed the repeater via another telephone line connection.

Another way as Bill mentioned is to use a bi directional amp system. This requires a good isolation between the
output antennas in the bowls of the building and the donner antenna. This also requires that coax be run from
the omni antennas inside the bowls of the building and the donner antenna. Multiple omni antennas can be used
to fill multiple areas with a signal.

The last way is where you were headed is to use leaky coax and run it directly from the repeater. You would
use normal coax to go from the repeater down to the bowls of the building and then transition to the leaky
coax. By using taps, splitters or power dividers, you can split or tap off to different runs where needed.

All of the above suggestions need to be done with good engineering. You can't just throw something up
and expect it to work. Do your homework to make sure the signal levels will be where they need to be to function.

Jim

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Bill_G
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Re: Radiax as passive antenna -- question about termination

Postby Bill_G » Mon Aug 30, 2010 4:56 am

Mornin Jim - We all run into this problem sooner or later. Poor coverage within a specific area within a specific building that is important to the customer, but resists a cost effective solution. It's almost better if there is absolutely no coverage - that the path is fully blocked both ways. Then the solution is obvious - put in a low power repeater (less than 1 watt) tied back to the main system by phone lines to a voter. It doesn't even have to be synchronized like simulcast. Just make a small hot spot to fill in that area, and amazingly the overlap is insignificant. The users transition from the fill site to the main system with very little trouble. I've done it many times for hospital security getting UHF coverage into XRay, or surgery, or the tunnel level between buildings where it was radio dark.

It's the areas where the system almost works that are the hardest to improve. Then a fill site potentially adds interference in many unintended areas of overlap that you have no control over. Voters return the portable talk in to the system very well, but do nothing to improve the talk out. Simulcast might help, but I would expect to spend months finding all the multipath interference areas and minimizing them making an expensive solution even more expensive. JPS transmitter steering directs the energy to the area of interest, but it would also moves the areas of no coverage. There are no easy analog solutions.

OTOH, Mototrbo has been amazing with it's fill in abilities. Nothing changes in the physics of radio. No coverage remains no coverage. But, those noisy areas, those areas where you can almost call in, where you can almost hear dispatch, now work pretty good in Trbo digital mode. You recover 10db or more of signal quality making the audio usable and understandable. That lower parking area now has good coverage. The repeater covers the elevators now. You can talk from the whole receiving dock, not just one spot. I am hoping what comes out of the new digital stuff is a smoke detector sized low power appliance tied back to the main system that is quasi-synchronized that lets you put energy where it is needed. That would be a real problem solver.

motorolo
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Re: Radiax as passive antenna -- question about termination

Postby motorolo » Mon Aug 30, 2010 2:44 pm

Hey guys, thanks for all the replies so far. To address what's been discussed:

BDAs are definitely out, owing to budget constraints. I'd love to go that route, but it's just not realistic in this case. Voting is possible (and has been talked about for future work), but at this time is probably not something we could reasonably deploy.

One thing that I realize I neglected to mention earlier is that we really only need coverage in one (relatively small) area of the basement floor; covering the entire floor is not really a concern. In that regard, this really is just spot coverage as opposed to a widely-distributed deployment.

All that said, I'm beginning to think from your replies that it might just be best to string up some low-loss coax and throw an antenna on either end -- a donor on the roof, and one in the basement, in the area we need to cover. For spot coverage like this, can it be just that simple? How does antenna grounding at the in-building end work in that case?

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Bill_G
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Re: Radiax as passive antenna -- question about termination

Postby Bill_G » Tue Aug 31, 2010 6:11 am

Yes, a passive repeater is as simple as two antennas connected by a length of coax. However, I'd advise against it. It could easily be money not well spent especially if the physical path through the building is difficult, and there is low donor signal strength on the roof. Passive repeaters are the unicorn of radio - Lots of talk about them, but nobody has ever seen one. Yes, they are used in cars to improve cellular signal inside the vehicle, but the subscriber stays within the near field of the inside antenna. Yes, they have been used to improve portable coverage in dams and mines, but the again the users had to stay within a very small area to realize any benefit.

Several conditions should be met before you attempt this. First - you need line of sight from the repeater to the rooftop donor antenna. If you do not have a direct, unobstructed path between the two points, then stop there. Next, you need a signal strength of -80 or better on the rooftop. It has to be an excellent, strong signal, or very little will couple indoors. You need a high gain antenna on the roof. For UHF, a 10db yagi is probably the best you can buy. You need to use 100ft or less of high quality line like LMR400 or better to keep your losses minimized. Finally, you need a good quality antenna indoors. A unity or 3db suspended from the ceiling would be best.

If you have all that, then you may see some improvement in the room you are attempting to cover. Emphasis on may. On the bright side, if the project fails, you have a line already run to the roof for a base station on a desk in the room. That will definitely work.

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Re: Radiax as passive antenna -- question about termination

Postby Bill_G » Tue Aug 31, 2010 7:46 am

I'm going to amend this recommendation and state you need a -60db at your donor to make something like this work.

For the guys in the shop - pop a mag mount on your service monitor generator on a UHF channel, vary the level from -80 to -60, and see how far you can walk around the building listening on a portable.

motorolo
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Re: Radiax as passive antenna -- question about termination

Postby motorolo » Tue Aug 31, 2010 4:04 pm

Thanks again for the followup, Bill. The updated info you posted is actually rather encouraging, as the building we're trying to do this fill-in coverage for is only about 1000 feet away from the repeater as the crow flies, and should have line of sight on the transmit antenna (I need to confirm this, though).

I don't know what the free-space attenuation of RF at UHF is off the top of my head, but we're about almost 50dBm ERP at the repeater. I would think with line of sight at that distance, we should be pretty well within budget at the donor antenna if the threshold is -80 to -60 as you mentioned. Then it'd be a run of LDF4-50 down to the basement, and an appropriate indoor UHF antenna. Like you said, even if that fails terribly, at least we have a line run for a base station.

On the subject, does anyone have a good source for indoor dome-style antennas for UHF? Something we could mount in the drop-ceiling paneling would be ideal. Thanks again for all the advice, this forum really has an incredible amount of expertise!

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Re: Radiax as passive antenna -- question about termination

Postby Bill_G » Tue Aug 31, 2010 7:26 pm

Glad to help. Good luck with the project. Let us know how well it works.

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Re: Radiax as passive antenna -- question about termination

Postby Jim202 » Wed Sep 01, 2010 3:27 am

The passive antenna system sort of works, but there are some big buts. You need to look at the coax cable
loss between the donner antenna and where your re radiated antenna will be. You will not be able to go very
far from the second antenna and still have the passive system work. You will also need a high gain antenna
at the donner to make up for the coax cable loss and add some gain.

Have tried this in several locations on both VHF and UHF. Seems the higher the band the better it works
due to the higher antenna gain you can place on the donner antenna. On the other hand, you face higher
losses in the coax cable as you go up in frequency. In your case with the repeater only 1000 feet away,
it might just work for you. Just don't expect to go very far from the inside antenna.

Jim

motorolo
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Re: Radiax as passive antenna -- question about termination

Postby motorolo » Wed Sep 01, 2010 7:00 am

Jim202 wrote:The passive antenna system sort of works, but there are some big buts. You need to look at the coax cable
loss between the donner antenna and where your re radiated antenna will be. You will not be able to go very
far from the second antenna and still have the passive system work. You will also need a high gain antenna
at the donner to make up for the coax cable loss and add some gain.

Have tried this in several locations on both VHF and UHF. Seems the higher the band the better it works
due to the higher antenna gain you can place on the donner antenna. On the other hand, you face higher
losses in the coax cable as you go up in frequency. In your case with the repeater only 1000 feet away,
it might just work for you. Just don't expect to go very far from the inside antenna.

Jim


Yeah, I'm cautiously optimistic about the endeavor. I did some back of the envelope calculations last night, and the free-space path loss from the repeater to the donor site is about 77dB at the frequency of interest. Assuming no obstructions, that puts us at -30dBm at the donor, ~1dB loss on the LDF run, and roughly equivalent loss on connectors. It looks like we should be well within budget. Here's hoping!

Any recommendations for UHF indoor antennas? I've found a couple, but nothing is blowing my socks off yet.

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Re: Radiax as passive antenna -- question about termination

Postby Tom in D.C. » Wed Sep 01, 2010 10:21 am

You could consider the Radiall-Larsen UHF radome-type bus antennas. They're vertically polarized and would probably work well on an inside ceiling. Only thing is you would have to feed the coax through the ceiling tile via a small hole.

And so far I don't think I've read a definitive answer to your original question about how one terminates a leaky coax antenna line. Plenty of diagrams in the Andrew literature etc. showing how they cut slits in the shield to allow the cable to radiate but nothing about the end of the line. Antennas or dummy loads have been mentioned, but what do the manufacturers say is the right way to do it?
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Re: Radiax as passive antenna -- question about termination

Postby Bill_G » Wed Sep 01, 2010 2:07 pm

Depends on what you want to do Tom. If, for example, you use radiax on a repeater, and your end of line is a rooftop, then a logical choice would be an antenna to give you outdoor coverage. But, if your application is a BDA in a tunnel, and you have sufficient signal level based on the engineering and measurements, then the best choice would be a load.

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Re: Radiax as passive antenna -- question about termination

Postby desperado » Tue Sep 07, 2010 9:43 pm

Termination can be handled two ways.
First is a dummy load with a sufficent rating. Figure out the loss in the line, subtract that from the output power of the transmitter and subtract any other losses like power splitters and size 20 % over that. Typically a 10 watt load will be fine if the runs are reasonably long and there are power dividers in the system.

The second way is a simple antenna.
There are ceiling mount antennas available for all bands.

Lastly, if you are working with a BDA system at 800Mhz, you might want to trap out the Sprint/Nextel channels as they are right at the 800Mhz public safety channels.
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Re: Radiax as passive antenna -- question about termination

Postby wavetar » Wed Sep 08, 2010 9:57 am

Bill_G wrote: Passive repeaters are the unicorn of radio - Lots of talk about them, but nobody has ever seen one.


Lol, best radio quote I've seen in a long time. I have been lucky enough to have seen one of these rare creatures in a working scenario. It was at 800MHz, with the repeater antenna within 600-feet or so (same building). High gain Yagi on the outside, approx 60 feet of LDF4, and unity gain SRL401 base antenna on the inside...so it basically followed the recipe given in this thread. I believe the outdoor dBm level at the Yagi was in the -40's, although I don't specifically remember. It covered a surprisingly large area in the far basement of this particular building, although many years later we replaced this with a powered bi-directional system as the formerly wide open area had been sectioned off & filled full of metal shelving, etc, and the passive just wasn't cutting it anymore.
No trees were harmed in the posting of this message...however an extraordinarily large number of electrons were horribly inconvenienced.

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Re: Radiax as passive antenna -- question about termination

Postby Bill_G » Wed Sep 08, 2010 6:37 pm

Basements are always a problem, and the passive repeater is always the first idea to get discussed. But, free space losses prevent them from functioning except in rare cases.

I have made uni-directional amplifiers out of a pair of mobiles to light up completely dark areas. One radio is on the roof with a standard repeater split. The other radio is in the basement with a reverse split. The two are tied together by in house phone line using cor to apply ptt to each other. (These days I would probably use a pair of NXU's and the in house ethernet to control the radios). As the users transition into the radio dark area, they will experience a zone of no coverage. When someone uses the repeater, it is received on the roof, relayed to the basement, and retransmitted. Calls from the basement are received by the local radio, relayed to the roof, and retransmitted. Works perfect if the squelch tail is kept very short (or turned off). Once users get used to how it operates - that they can't talk through the tail anymore - it works pretty good.

You can put as much power as you need to fill the area well as long as you keep it from overlapping with the main coverage to prevent beating since these are not synchronized in any way. In one application with a long telco run, I had to regenerate ptt with optoisolators on the cor line to stop a ground loop hum, and to stiffen the ptt. I've also done it through a 72M link to extend coverage to a small hamlet that wasn't served by the direct path. The users don't have to change channels on their radios. They just drive from the main coverage through the dead zone into the secondary coverage. It's a good plan for buildings or towns tucked into the hills if you have perfect isolation.

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Re: Radiax as passive antenna -- question about termination

Postby AEC » Mon Sep 20, 2010 3:12 pm

Here's a suggestion...

Hamtronics has commercial receiver/transmitter kits, or prebuilt.
I believe they are offered in both UHF and VHF.
Rock and synthesized...
Low cost alternative might be to set up a reverse repeater, but setting the reverse path TX signal as low as possible, but I think most of the Hamtronics transmitters are designed to be 2 watts or less.

If you are spending $ 1,000.00 to $ 3,000.00 on a passive system with questionable results, this may be the better(and cheaper) solution.

Plus you will have a real receiver to work with!

Rock bound is fine for stable climate areas, but I suggest a TCXO for reliability in temperature variable environments.

All things being equal(Occam's razor), this is a better alternative.


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