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Antenna for Motorola XTL-5000

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#1
Hi guys,

I have a Motorola XTL-5000, 800 MHz, that we use as an indoor base station for EMS. We've been having reception issues lately due to one of the city's nearby repeaters dying (and it isn't going to be replaced any time soon).

I'm trying to fix this with two approaches:

1) purchase a higher-gain antenna
2) mount it in a better location

Does anyone have any suggestions for a good high-gain omni antenna to use indoors? Price really isn't an issue at this point.

Also, I can put it in a location that has less material between it and the sky, but it would require mounting it about 120 feet away from the radio (which, I assume, would introduce far too much line loss). Any suggestions on that front?

Thanks!
 
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#2
You could experiment with other inside antennas with little if any improvement or you could do it correctly. The solution would be to run LDF 4-50a to a suitable outside location install a small Yagi antenna pointed toward your repeater. LDF 4-50a would only have a couple of dB loss at 800 MHz for a 120 ft. run.
 

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#3
I would avoid indoor control station antennas. I've done them in the past, but today we have more regulation over permissible exposure limits and emitting RF in proximity to people. The other thing is that the antenna may be moved or somehow obstructed. Plenums, closets, and equipment rooms tend to accumulate storage junk. Depending on composition, that junk might affect your reliability.

The best bet is to futureproof the system by putting the antenna outside. Rfradioconsult spec'ed LDF4-50A (1/2" Heliax or equivalent corrugated shield cable). At a 120 ft. run, that yields 2.4 dB attenuation. With a yagi pointed at your repeater, that's perfectly fine. In fact, you can save some money and run LMR-600, which is 1/2" coaxial cable with a foil and braid shield and still be under 3 dB attenuation (2.8 dB). Some folks might suggest hammie grade cables. Let's quickly go there. 120 feet of LMR400 is 4.4 dB of loss at 800 MHz. Belden 9913 is 4.7 dB loss at 800 MHz. Those figures would not be worth the work, for me at least.

Let's consider antennas now. If you are communicating through the repeater, it's best practice for you to use a directional antenna oriented toward the repeater. An omnidirectional antenna is not responsible radiation control and may contribute to interference in places where the frequency is reused. Don't buy garbage. There's a lot of quackery out there when it concerns antennas. Stick with commercial manufacturers. A PD or DB prefix series antenna is usually solid. Acceptable antennas are like DB492 8 dBd at 61 degrees horizontal beamwidth, or DB499 10 dBd at 60 degrees HBW. What that means is that your power is concentrated toward the repeater. The same works for receive. Your system can have a directional gain of 5.2 to 7.2 dB, given using those materials. Framing it in received signal terms, that can be like taking a 35 Watt repeater and cranking its power up to 116 W or 184 W, except you're doing that with your antenna system and you don't have to touch the repeater or play with frequency coordination.

Use the proper connectors, seal them up with weather-resistant materials, and ground the antenna mast, put a grounded lightning suppression device on your cable where it enters the building, and you should have a reliable control station for a good long time.
 
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#4
Thanks for all the help guys. It's turning out to be a bureaucratic nightmare to get the cable installed (it's a 4-story run up to the roof and we don't own the building), but it's the option I'm pushing hard for.

Does anyone have a suggestion of a compatible lightning arrestor that would work with these components? Also, is there a not-too-expensive tool and tutorial for aiming this antenna correctly?
 
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#5
A Yagi is a directional antenna with a fan shaped pattern toward the front of the antenna, not a rifle. Aiming the antenna in the general direction of your desired tower will be fine. A suitable protection device at the point where the cable enters the building such as a Polyphaser brand will be all you need. Generally you want to comply with your local building code regarding antennas.

This doesn't rank as rocket science, just good radio installation practices.
 
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#6
Google Earth is a good tool to aim your directional antenna, but as said above, it really isn't super critical.

If you are running cable up through a building, you may need to use plenum rated cable, not the standard Heliax. It'll cost more, but it might be a requirement.
As for PolyPhasers, make sure you get one that covers your frequency at the power levels you are running. Shouldn't be a big deal for 800MHz at 35 watts.
 
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#7
Since we are doing a professional installation here you should check your license to be sure you are licensed for a control station (this talks to a repeater and a base station if it talks directly to mobiles and portables) and that your ERP (effective radiated power) doesn't exceed the limits allowed on your license.
And as was said above most installations do not allow ANY kind of indoor antenna unless an RF study is done to determine the safe exposure limits.
 
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#9
Rfradioconsult spec'ed LDF4-50A (1/2" Heliax or equivalent corrugated shield cable). At a 120 ft. run, that yields 2.4 dB attenuation.
Now this may seem like a bizarre question, but does anyone know where I can find mini-UHF end connectors for this type of foam coaxial cables? I've only been able to find them for braided coax. TESSCO carries the cable, but no ends for it.
 
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#11
Now this may seem like a bizarre question, but does anyone know where I can find mini-UHF end connectors for this type of foam coaxial cables? I've only been able to find them for braided coax. TESSCO carries the cable, but no ends for it.

You may not want to hear this, but with your lack of understanding of this issue, it my be in your best interest to contact a local two way radio shop to work all this out for you. They will have the expertise and know how to get the antenna, grounding and license issues all taken care of. Doing a DO IT YOURSELF project like this is asking for problems and probably a poor finish in the long run.
 
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