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Commercial Radio Antennas - Please keep discussion related to professional, commercially used antennas and antenna systems for the two-way radio industry. Topics for the use of these antennas on amateur bands are accepted here.

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Old 08-25-2017, 7:43 PM
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Default DTR650 External Antenna

I posted this in another section 2 months ago but didn't get any replies. Figured it might get some more traffic here.

So...these little 650s are nothing short of amazing, as far as I'm concerned. I've put them through their paces and the dog gone things just won't fail. They have better coverage than most anything else I've tried in a handheld unit and are just incredible.

I live in the desert with dense urban city being several miles away. With one radio on my balcony (would put the radio roughly 15' high), and driving around, I have reached 13.8 miles coverage with the 1/2 wave rubber duck. I ran out of road, or I could have gone farther. The problem, if you want to call it that, is that on occassion, I would need to stick the radio out of my sunroof to establish a connection.

I thought I would remedy that, and perhaps extend the range even further, by purchasing an external antenna. I bought a Larsen 5db antenna, and a PCTel 5db antenna, with a magnet mount (rated to 3000 MHz), with 12' LMR200 coax. It's the same mount I have used for my dual band VHF/UHF antenna without any problem.

Using either antenna, the maximum range I can get is roughly 3 miles. I switch back to the 1/2 wave rubber duck and keep right on going. Short of pulling out an antenna analyzer, which I don't have access to at the moment, I can't for the life of me figure out what the issue is.

Is the magnet mount that bad on 900 MHz, or perhaps that length of coax run the issue? As I said, I know the mount and coax are not bad, because I use it on other bands (primarily UHF) regularly.

Any suggestions would be very much appreciated.

*** Update ***

I ordered another mount just in case, and exact same problem, exact same range. Is there something strange going on here? Even if the coax and antennas balance each other out (5dB), which I still in theory should get some gain from the antenna...they should be better than balanced, I would think just having the antenna outside of the car would make a huge difference. The maximum distances were accomplished with the antenna/radio in the car.

Attachments 1&2 are just some range tests I performed with the 7" 1/2 wave antennas.
Attachment 3 is 1/2 wave on one radio, and mobile antenna on the other in vehicle.
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Old 08-25-2017, 8:15 PM
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By 1/2 wave, I assume that it is the stock antenna on the radio? It might actually be a center fed coaxial 1/2 wave in that case.

When you attach the cable to the radio are you 100 % certain that the connector (SMA?) Is mating properly? You might have a mechanical problem in mating them up. Also any adaptors in line will reduce signal.

Failing that, it would appear that coaxial losses are not being overcome by the antenna gains claimed by the antenna manufacturer.

The azimuth of your third test is different than one and two. Have you tried to retrace the same route 1 or 2 entirely to see if the signal picks up at locations further out? I am suggesting local terrain differences.

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Old 08-25-2017, 9:45 PM
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Hi and thanks a bunch for the reply!

Yes, the 1/2 wave is the stock whip antenna. Motorla 8505241U04

From what I can tell, the connection (Yes, SMA) is working fine. Everything is tight. Double-checked the pins, etc. There are no adapters in line at all.

From my charts (which could be wrong or outdated), I'm showing about 1.2 dB loss for the 12' of coax. So even if there is another 3.8 dB loss somewhere else, I would think even unity gain outside the vehicle would perform. All my tests with the stock whip were from inside my car, with the very occassional sticking the radio out the sunroof as I got on the fringes.

To clarify on the tests, you are right, and I should have clarified. I put them together real fast just to show the line-of-sight distance. In all tests, I followed the same route (Avenue D if you can read it on the map) to the 14 freeway. The coverage is more or less a circle in that entire radius for all 3 tests, even around buildings. Terrain is more or less completely flat over the entire distance of my tests, with just very minor variations.

I'm just completely at a loss here. Thanks again!
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Old 08-25-2017, 9:49 PM
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Quote:
Originally Posted by alcahuete View Post
Hi and thanks a bunch for the reply!

Yes, the 1/2 wave is the stock whip antenna. Motorla 8505241U04

From what I can tell, the connection (Yes, SMA) is working fine. Everything is tight. Double-checked the pins, etc. There are no adapters in line at all.

From my charts (which could be wrong or outdated), I'm showing about 1.2 dB loss for the 12' of coax. So even if there is another 3.8 dB loss somewhere else, I would think even unity gain outside the vehicle would perform. All my tests with the stock whip were from inside my car, with the very occassional sticking the radio out the sunroof as I got on the fringes.

To clarify on the tests, you are right, and I should have clarified. I put them together real fast just to show the line-of-sight distance. In all tests, I followed the same route (Avenue D if you can read it on the map) to the 14 freeway. The coverage is more or less a circle in that entire radius for all 3 tests, even around buildings. Terrain is more or less completely flat over the entire distance of my tests, with just very minor variations.

I'm just completely at a loss here. Thanks again!
what antennas are you testing with?
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Old 08-25-2017, 9:56 PM
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Quote:
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what antennas are you testing with?

Using a Larsen NMO5T900B and PCTel MUF9035
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Old 08-25-2017, 10:06 PM
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Ok; I think, I have solved the mystery. With an ohm meter, measure the resistance across the SMA terminals on the antenna(s) supplied by Motorola. Also carefully measure any voltage across the antenna terminals of the radio. Tell me your results;

Also read the first paragraphs of the attached regarding a special test fixture.

https://fccid.io/AZ489FT5842/Test-Re...ort-506316.pdf
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Old 08-25-2017, 10:16 PM
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My theory.
In order to satisfy FCC requirements that the radio would not be fitted with an amplifier or a gain antenna, Motorola designed an internal switch that would be turned off whenever a non stock antenna was fitted. The manner in doing this was to install an RF choke and resistor to pull a voltage down to 4.0 volts when a proper antenna was fitted. Now, you say you have a 1/2 wave antenna, and if it is end fed,(not center fed) the impedance would be quite high requiring a network inside the radio to raise the impedance of the terminals. So if you have an end fed 1/4 wave antenna, whatever DC resistance you measure on that would be most correct for a 50 ohm mode.

It would take some research, but I suspect Motorola has patented this.
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Old 08-25-2017, 10:57 PM
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lookie here. the text alludes to a resistor at the antenna feed point.

Patent US20040087295 - Apparatus and method for antenna attachment - Google Patents
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Old 08-25-2017, 11:09 PM
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I'll be damned! Great theory! Let me grab a multimeter here and see what I can come up with. That is REALLY interesting.

Thanks again! Let me check this out.
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Old 08-25-2017, 11:11 PM
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Motorola iDen phones with Direct Talk have same method which impairs connecting an external antenna. However, I believe if you can make a "bias "T" with the appropriate resistance to ground, you should be able to connect an external antenna with negligible loss.
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Old 08-25-2017, 11:21 PM
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Also check the antennas to see if there is a third connection point for DC bias. They might be sneaky and have a tab that contacts the antenna base somehow. But I am guessing it is all within the SMA connection because it would be far more reliable. Apparently 4.0 volts is what you need to achieve across the terminals for 50 ohms.

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Old 08-25-2017, 11:31 PM
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The circuit you need to make is like this . But
The to LNA connection goes to the radio, and the SDR side goes to your antenna. Instead of a voltage, more likely a fixed resistor to ground, to set the voltage to 4.0 volts. This assuming the radio has a pull up resistor inside. If not it would be an external 4.0 volts applied from a regulator. More likely, there is a pull up resistor. Motorola has used this technique on radios like the SABER, where attaching an external Mike or adaptor controls the audio and RF ports via a voltage divide resistor or zener diode. An A/D convertor in the radio translates voltage to logic signals.

You don't have to make a PC board or anything fancy if you use very short leads for connections. In fact, you could use an SMA "T" connector, an SMA DC block an RF choke and capacitors. To do it right, Mini circuits probably has an inexpensive bias T module. I would bread board it, if it works OK, get a commercial one from them for lowest insertion loss.

http://lna4all.blogspot.com/2014/11/diy-bias-t.html

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Last edited by RFI-EMI-GUY; 08-25-2017 at 11:37 PM..
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Old 08-25-2017, 11:32 PM
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If this works, the mods should make a sticky!

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Old 08-25-2017, 11:42 PM
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Awesome! Thanks again for all the insight here. I'm not sure my multimeter is playing nice, because I'm not showing any resistance at all across the antenna. Go figure.

I'll look into the Bias T and see what I can find. I'd love it if this would work!
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Old 08-25-2017, 11:47 PM
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This might work. If you shop around, they might have similar with a DC feedthrough terminal instead of the SMA on the DC port.

https://www.minicircuits.com/WebStor...del=ZFBT-6G%2B

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Old 08-26-2017, 12:06 AM
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Quote:
Originally Posted by alcahuete View Post
Awesome! Thanks again for all the insight here. I'm not sure my multimeter is playing nice, because I'm not showing any resistance at all across the antenna. Go figure.

I'll look into the Bias T and see what I can find. I'd love it if this would work!
If the antenna is showing infinite resistance and has no third terminal. then the 4.0 V bias needs to be applied externally through a bias T. If the antenna has zero resistance, or close to it, then there is a choke across the terminals. But if what you are saying is that it shows infinite, then the 4.0 V bias must be applied externally which is sort of implied in the FCC filing.
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Old 08-26-2017, 12:19 AM
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Quote:
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If the antenna is showing infinite resistance and has no third terminal. then the 4.0 V bias needs to be applied externally through a bias T. If the antenna has zero resistance, or close to it, then there is a choke across the terminals. But if what you are saying is that it shows infinite, then the 4.0 V bias must be applied externally which is sort of implied in the FCC filing.

That's exactly what it's looking like. I'm showing zero resistance on several different antennas. No 3rd terminal that I can find.
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Old 08-26-2017, 12:55 AM
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That's exactly what it's looking like. I'm showing zero resistance on several different antennas. No 3rd terminal that I can find.
a quick test would be to use an RF choke across your external antenna.

But I wonder if the radio has DC voltage from the RF connector and what voltage it is at. The FCC filing was specific about 4.0 volts, but that could be the result of a voltage divider inside the radio itself also.
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Old 08-26-2017, 2:16 AM
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Quote:
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a quick test would be to use an RF choke across your external antenna.

But I wonder if the radio has DC voltage from the RF connector and what voltage it is at. The FCC filing was specific about 4.0 volts, but that could be the result of a voltage divider inside the radio itself also.

I don't have an RF choke laying around unfortunately.
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Old 08-26-2017, 2:24 AM
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Quote:
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I don't have an RF choke laying around unfortunately.
You can canniblize an old radio or other device. Or wind your own.

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