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Old 05-31-2018, 9:16 PM
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Default Questions about repeater and antenna setup

Hi, I help run a small event (about 3000 people) but we have to cover a pretty wide area (about 3 square miles) so we added a repeater setup a few years ago and haven't looked back. However, as more and more teams are spread out over the whole property we decided to move to a trunked repeater setup this year (Motorola Capacity Plus). It mostly worked really well until I had teams tying up the first repeater's 2 time slots. As the second repeater's antenna was 10' farther down the mast it didn't always pick up the signal from the portable radios. So I'm trying to figure out a solution to get the second repeater to work better.

Ideally I'd love to get one antenna for both repeaters, but from what I can tell that takes a ridiculous amount of equipment and introduces high amounts of signal loss.

So I was thinking about trying to consolidate the TX and RX antennas instead of having each repeater on it's own antenna like now. Put the RX antenna at the top of the mast (and maybe get a high gain antenna for the RX while I'm at it). Can I just get my existing duplexers retuned for this or do I need different equipment to consolidate the repeaters in this manner? Any other downsides I'm not thinking about?
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Old 06-01-2018, 6:20 PM
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You'd be better off using receive multi-coupler/pre-selector for both receivers running to a receive antenna on the very top of the mast. Then, use a transmit combiner for the two transmitters running to another antenna that is lower down the mast. Then, if you need to add another repeater to mix, you can easily do that without adding more antennas. Plan ahead and get a combiner that will handle more than your current 2 repeaters.

I did this at one of my sites where I needed 2 repeaters. Once I put a 5 channel combiner in, I added more repeaters and now have 5 repeaters on two antennas. I recommend always putting the receive antenna on top.
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Old 06-01-2018, 11:26 PM
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Looks like I can probably manage a multicoupler and a couple of preselectors on my budget, but man does that transmit combiner look like it's going to break the bank. Any advice on a good but not several times the cost of a repeater combiner?
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Old 06-02-2018, 4:31 AM
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Multicouplers and combiners are fine when needed, but why not a higher mast/location for the antenna?

If 10 feet makes a noticeable difference over 3 square miles, the antennas are probably pretty low.
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Old 06-02-2018, 8:03 AM
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It's a 50' mast. We only had it up about 35'-40' this time around due to the fact that in the past using more feedline caused us issues. Looking at the spec sheet about 100' of LMR-400 at UHF frequencies causes about 3dB of loss. So we're trying to avoid that problem.

So antenna at 40' worked great, the antenna at 30' was down in the trees still and didn't pick up from the further distances (it worked fine for people closer to the tower and people at the edges could hear the transmissions just fine too).
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Old 06-02-2018, 10:56 AM
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Use hardline instead of LMR400. Will cut your losses. Will hgelp on both TX and RX. Don't scrimp on smaller size hardline!

Frank KK4YTM
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Old 06-02-2018, 11:36 AM
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Could be that the second receiver is experiencing desensitization when the first repeater is transmitting.

Without knowing the frequencies in use, it's hard to know for sure. But using the right transmit combiner, proper frequency separation, maybe some filtering on the RX side, should address this.

LMR-400 might not be working in your favor, either. When used in repeater service, it can suffer some problems. Ruling out coaxial issues would be something I'd do in addition to the above. Using some 1/2" Heliax wouldn't hurt.
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Old 06-02-2018, 4:21 PM
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I may have been operating under a false understanding for a while. I thought you only used hardline when you were doing a semi-permanent or permanent install and that repeated bending to roll it back up after use would damage it. Did I get incorrect information along the way somewhere?
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Old 06-02-2018, 5:23 PM
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Quote:
Originally Posted by pboyd View Post
I may have been operating under a false understanding for a while. I thought you only used hardline when you were doing a semi-permanent or permanent install and that repeated bending to roll it back up after use would damage it. Did I get incorrect information along the way somewhere?
Yes, with the standard type of heliax that's correct. They do make a "superflex" version that's designed to better handle the unroll/roll up cycle better. If you've seen those TV remote trucks with the tower that is repeatedly raised and lowered every time that the truck is setup that's the type of coax used on their setups.
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Old 06-02-2018, 5:29 PM
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Any experience with the superflex stuff used about 3 times a year (pulled out of box, unrolled, put on tower, used, pulled off tower, rolled back up, put back in box all 3x)? I'd love lower feedline loss, but not at the cost of feedline reliability.
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Old 06-02-2018, 6:11 PM
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Superflex is often used for jumpers, either at the antenna end to transition from larger feedline, or in the equipment room to transition from the feedline to the equipment.
All my antennas at work use a short 1/2" superflex jumper.
I've got a few cell carriers on one of my towers, and they all use it, too.

Reliability won't be an issue, as n5ims said, it's used in EFP/ENG remote trucks.

Just make sure you use heat shrink tubing on the connectors to help with strain relief. Other than that, it should last you decades if you look after it.
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Old 06-02-2018, 6:23 PM
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It looks like Superflex is pretty comparable to LMR at the same size/weight. LMR-400 is about 0.4" and weighs about .07 lbs/ft and attenuates at about 2.7dB/100ft (at about 450MHz which is close to where we operate) . Superflex FSJ2-50 is about 0.375" and weighs about .08lbs/ft and attenuates at about 2.6dB/100ft. I'm not seeing much difference here. Am I missing something? What would Superflex improve over my current LMR cable?
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Old 06-02-2018, 7:33 PM
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Yes, you can put two repeaters on one antenna.
BridgeCom has a good video detailing how to do it on youtube.
Or just give Ron at BridgeCom a call for details.
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Old 06-02-2018, 8:56 PM
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Quote:
Originally Posted by pboyd View Post
It looks like Superflex is pretty comparable to LMR at the same size/weight. LMR-400 is about 0.4" and weighs about .07 lbs/ft and attenuates at about 2.7dB/100ft (at about 450MHz which is close to where we operate) . Superflex FSJ2-50 is about 0.375" and weighs about .08lbs/ft and attenuates at about 2.6dB/100ft. I'm not seeing much difference here. Am I missing something? What would Superflex improve over my current LMR cable?


What you are missing is the construction. Superflex is a type of corrugated shield hardline where LMR is a braided shield coaxial cable. To get the isolation while still keeping the line low cost, Times Microwave wraps an aluminum foil shield around dielectric before the braid is on there. Due to dissimilar metals in LMR, you can have Passive Intermodulation (PIM) issues when in use in a full duplex environment. Super flex on the other hand is all copper.

Now to answer your question about combining your systems, RFI makes a hybrid combiner unit which offers the multi-coupler, preselector and combiner (2 or 4 channel) and gives you a single antenna output for both TX and RX. It’s a bit lossy (power output isn’t everything) but at least balances your transmit and receive coverage.


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Old 06-04-2018, 4:32 PM
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So looking at that video of the two repeaters on the same antenna it looks like they just added an extra band-pass/band-reject filter after the duplexers. Is there any reason I couldn't use a notch type duplexer in that role?

My repeater tx/rx pairs don't overlap and are actually a few MHz apart each (451.8/456.8 & 464.5/469.5). What is the downside to this kind of setup?
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Old 06-04-2018, 7:13 PM
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Has anyone actually ever checked your current set-up RF performance wise? What tickles my funny bone is the 10' of separation and the drastic coverage difference. All things equal, that small difference in height should be really minor, if even noticeable at all.

Are you by any chance using "mobile duplexers" because of the convenient size?
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Old 06-05-2018, 4:44 PM
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I haven't had anyone out to professionally check it no.

I've put a VSWR meter on it and got about 1:1.2 from that on both lines. So nothing jumped out there. Additionally, the RSSI when no repeaters are transmitting (according to RDAC from the repeaters) is about -120dB and while the other repeater is keyed both are at a little less than -110dB (sorry didn't write down the measurements).

Everything works fine during my test events on the same site. Everything works great during trainings (with about 200 people out there). Its when we get to the actual event with all the people, all the RVs, all the generators, all the extra stuff that we seem to lose range pretty quick.
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Old 06-05-2018, 9:56 PM
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Quote:
Originally Posted by pboyd View Post
I haven't had anyone out to professionally check it no.



Additionally, the RSSI when no repeaters are transmitting (according to RDAC from the repeaters) is about -120dB and while the other repeater is keyed both are at a little less than -110dB (sorry didn't write down the measurements).



That right there suggests you are having an issue with desense.



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Old 06-10-2018, 11:49 AM
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Is this event in an area that would be covered by a commercial system? It might be worth it to lease the radios, and TGs, from them.

Also, have you done an RF sweep, once the area is occupied? You could have someone else causing your desense issue.

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Old 06-12-2018, 10:34 PM
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Quote:
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Is this event in an area that would be covered by a commercial system? It might be worth it to lease the radios, and TGs, from them.

Also, have you done an RF sweep, once the area is occupied? You could have someone else causing your desense issue.
No, this is in the middle of nowhere there aren't any commercial systems that cover this area. I've run my SDR setup looking for anything around the frequencies in use and didn't see anything. Is there something specific I should be looking for besides that?
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