DTR650 External Antenna

[JASII]

Good stuff. I have a pair of DTR550s. I don't use them very much, but they work pretty well. I keep hoping that they will make an updated version with Bluetooth some day, Then, they would be a great radio for ATVing with a wireless helmet system.

 

[RFI-EMI-GUY]

Good stuff. I have a pair of DTR550s. I don't use them very much, but they work pretty well. I keep hoping that they will make an updated version with Bluetooth some day, Then, they would be a great radio for ATVing with a wireless helmet system.

I am going to have to get a pair and reverse engineer the transmitter and T/R switch arrangement. I am thinking that putting them inside gutted 900 MHz SPECTRA chassis and using the SPECTRA 15 or 30 watt PA to boost them. I think that might be OK under amateur Part 97 since they are not encrypted. The trick will to be syncing the T/R switch of the PA with the DTR.

 

[RFI-EMI-GUY]

Hmm having second thoughts that the point where Motorola tapped RF for testing the radio at 50 ohms might be INSIDE the housing. The upper photo and left side of the board reveals a connector on the PC board, that may or may not be accessible through a plug in the housing.

This makes the antenna more of a mystery. Could the antenna impedance be something wildly different than 50 ohms? If so the DC continuity of the antenna might reveal an internal matching transformer. If the long antenna is an END FED 1/2 wave, there would be a very high impedance at the feed point requiring a high ratio step up transformer.. For a 1/4 wave end fed antenna, the feed point would be 1/2 of 72 ohms or ~ 36 ohms.

If one had a spare 1/4 wave antenna and peeled back the cover, that might reveal the secrets inside.

Putting the antennas on a network analyzer would be a non destructive way to determine feed point impedance. In this case a balun transformer from Minicircuits could be used to normalize to 50 ohms.


Otherwise it might be that the correct way to tap a 50 ohm connection is inside the radio and biasing that point to 4.0V to activate the proper PIN switch.

https://fccid.io/img.php?id=506305&img=bg1.png

 

[RFI-EMI-GUY]

See the photo of the rear housing at this link. There is a label or cover at the top, that when removed reveals an hole that appears to line up with the test connector, I think the XPR7550 MotoTRbo radio has a similar scheme.

https://www.radioparts.com/motorola-pmln4948a

 

[Chickenhawk56]

I think we might be overcomplicating the issue. I have tried multiple 900MHz antennas and found ALL of them had significantly shorter range than the stock 1/2-wave antenna. They were all making good connections, but at 900MHz, I concluded that the loss in the length of the cable easily exceeded the very minor gain from mounting it higher, on the roof of a vehicle. I made sure the cable run was exactly the right length for the frequency range, but they all behaved much worse.

Personally, I think the penetration capabilities of the 900MHz spectrum are just so good that an external antenna cannot perform better than the stock antenna.

While I would love to see a higher base station antenna that works well, I still see the loss in the cable length exceeds the gain from height.

But I am no means an expert here. I have just tried multiple external antennas for the DTRs that were all recommended for this application, and NONE of them work. If anyone finds one that works better, there are a lot of DTR enthusiasts who would love to know.

 

[alcahuete]

See the photo of the rear housing at this link. There is a label or cover at the top, that when removed reveals an hole that appears to line up with the test connector, I think the XPR7550 MotoTRbo radio has a similar scheme.

https://www.radioparts.com/motorola-pmln4948a

Yeah, I see that. Very interesting! I even tried searching for adapters specifically for the DTR and found nothing. At least the 7550 has the test adapter.

The funniest part is that I've seen tons of posts (even on this forum) of external antennas increasing range on these radios. I've only found one other post anywhere (not on this forum) that gets the same results I do.

 

[RFI-EMI-GUY]

I think we might be overcomplicating the issue. I have tried multiple 900MHz antennas and found ALL of them had significantly shorter range than the stock 1/2-wave antenna. They were all making good connections, but at 900MHz, I concluded that the loss in the length of the cable easily exceeded the very minor gain from mounting it higher, on the roof of a vehicle. I made sure the cable run was exactly the right length for the frequency range, but they all behaved much worse.

Personally, I think the penetration capabilities of the 900MHz spectrum are just so good that an external antenna cannot perform better than the stock antenna.

While I would love to see a higher base station antenna that works well, I still see the loss in the cable length exceeds the gain from height.

But I am no means an expert here. I have just tried multiple external antennas for the DTRs that were all recommended for this application, and NONE of them work. If anyone finds one that works better, there are a lot of DTR enthusiasts who would love to know.

I agree that overcoming excessive cable loss with antenna gain is a losing proposition but.,,

The OP is getting some pretty awful results. And using a mag mount external to the vehicle, he should have improvement over using the integral antenna inside a vehicle cabin.

What is telling is that Motorola indicates in their FCC filing that a special DC/RF adaptor is required in order to test the radios at 50 ohms. I think Motorola is playing some game with feed point impedance tuning to prevent end users from installing an unauthorized gain antenna in contradiction to FCC rules.

So I think that a proper impedance match is the key to making this work.





Sent from my SM-T350 using Tapatalk

 

[alcahuete]

I think we might be overcomplicating the issue. I have tried multiple 900MHz antennas and found ALL of them had significantly shorter range than the stock 1/2-wave antenna. They were all making good connections, but at 900MHz, I concluded that the loss in the length of the cable easily exceeded the very minor gain from mounting it higher, on the roof of a vehicle. I made sure the cable run was exactly the right length for the frequency range, but they all behaved much worse.

I disagree. The loss isn't that much. Not sure what coax you were using, but the calculations for my LMR200 on my run were only 1.2dB. If you use cheap coax, for sure, the losses are going to add up extremely quickly in the 900Mhz band. If it really is a 5dB gain antenna (which I do have the tendency to believe Larsen, they make good products), then that still leaves 3.8dB just to make it a unity gain system overall. There is nothing else in the system that is going to account for an additional 3.8dB loss.

And even with a unity gain antenna, simply getting it up and outside of the vehicle should easily outperform the 1/2 wave antenna inside the vehicle. Even if for argument's sake Larsen lied completely and it's really a unity gain antenna, a 1.2dB loss should not account for a 10+ mile difference in range. There's no way. It's defies the laws of physics.

If we really want to get crazy, as soon as I get a chance, I will cut a 1" long section of LMR400 (darn near zero loss), attach it to the antenna and radio, and $10 says I still get the same results.

I do honestly believe that @RFI-EMI-GUY is onto something here with the impedence matching. It makes sense with what is going on here with my results.

 

[Chickenhawk56]

I may of course be completely wrong about cable loss. All I know is that I put three DTR550 radios in the front seat of my truck inside my garage: one with the 1/4-wave stock antenna, one with the 1/2-wave optional (now standard) antenna, and one with a cable to a mobile 3db antenna in the exact center of my roof. I then started walking away.

With the DTR, it is easy to call up individual radios, so I didn't even need a second person. I lost the mobile antenna signal within two urban blocks; the 1/4-wave antenna lasted 10 blocks and the 1/2-wave lasted 12 blocks.

I tried a 5dB gain mobile antenna with exactly the same results. Both antennas were from quality manufacturers, with the correct connector and were the ones recommended for these radios. Cables were all sized to the correct length for the frequency range and all connectors were factory-installed.

Neither one worked.

It wasn't like they were slightly less range; it was they were SIGNIFICANTLY less range, even when the mobile antenna was at least 4 feet higher than the ones inside my truck. (By the way, the DTR410 has a fixed stubby antenna about an inch long, and its range was within a block of the 1/4-wave removable antenna.)

 

[n1das]

7.1 RF Power -- Pursuant to 47 CFR 2.947(c)
Method of Conducted Output Power Measurement: Adaptation of TIA/EIA-603-A clause
2.2.1 for Pulsed Measurements
The RF output power is not adjustable by the user. The output power is controlled by the
radio software. To obtain RF output power reading, the radio was programmed to utilize
the maximum output power setting.
A special DC/RF test fixture was utilized to interface with the radio test RF connector
while simultaneously supplying the operating voltage of 4.0V. The radio RF connector is
utilized in all factory tuning and testing procedures, and provides a 50 ohm connection to
the transmitter path while disconnecting the radio antenna. All conducted measurements
were performed via this test port.
NOTE: This DC/RF test fixture in not offered for sale.

I've done this kind of transmitter testing in product development in my line of work (I'm an EMI/EMC Engineer).

The 4.0V supplied to the radio has nothing to do with the antenna. The 4.0V supplied to the radio is for powering the radio since the radio must transmit continuously during testing and the tests take some time. The 3.6V Li-ion battery pack would get depleted before all intentional radiator (transmitter) testing is complete. For example, transmitter frequency stability testing over extremes of ambient temperature (i.e, -20C to +40C) are done in a thermal chamber and this testing takes a few hours to complete. An unmodulated carrier is transmitted continuously for this test at the low, mid, and high channels and at full power. RF power measurements are done with modulation on the low, mid, and high channels in the band and done over the range of temperatures. All of this testing takes time to complete.

The stock antenna supplied with the DTR650 as it comes from the factory is a 1/4 wave 900MHz stubby duck. Many vendors switch it out for the longer 1/2 wave duck since it's a popular option. Since I my DTR650s were bought factory brand new, they came with the stock 1/4 wave antenna. The radios and batteries have a November 2016 build date and I got them in January 2017. I ordered the longer 1/2 wave antenna separately for them.

There is nothing special about the 1/4 wave and 1/2 wave antennas used on the DTR550 and DTR650. They are the exact same antennas used on a Motorola MTX9250 and other 900MHz Motorola radios.

Why the mobile antenna setups tried don't perform as well as the stock antenna on the DTR650 remains a mystery. It's probably so d@mn close to perfect that it's hard to beat and you're fighting losses every step of the way, including small mismatches in connectors and cabling.

Good luck.

 

[RFI-EMI-GUY]

I've done this kind of transmitter testing in product development in my line of work (I'm an EMI/EMC Engineer).

The 4.0V supplied to the radio has nothing to do with the antenna. The 4.0V supplied to the radio is for powering the radio since the radio must transmit continuously during testing and the tests take some time. The 3.6V Li-ion battery pack would get depleted before all intentional radiator (transmitter) testing is complete. For example, transmitter frequency stability testing over extremes of ambient temperature (i.e, -20C to +40C) are done in a thermal chamber and this testing takes a few hours to complete. An unmodulated carrier is transmitted continuously for this test at the low, mid, and high channels and at full power. RF power measurements are done with modulation on the low, mid, and high channels in the band and done over the range of temperatures. All of this testing takes time to complete.

The stock antenna supplied with the DTR650 as it comes from the factory is a 1/4 wave 900MHz stubby duck. Many vendors switch it out for the longer 1/2 wave duck since it's a popular option. Since I my DTR650s were bought factory brand new, they came with the stock 1/4 wave antenna. The radios and batteries have a November 2016 build date and I got them in January 2017. I ordered the longer 1/2 wave antenna separately for them.

There is nothing special about the 1/4 wave and 1/2 wave antenna used on the DTR550 and DTR650. They are the exact same antennas used on a Motorola MTX9250 and other 900MHz Motorola radios.

Why the mobile antenna setups tried don't perform as well as the stock antenna on the DTR650 remains a mystery. It's probably so d@mn close to perfect that it's hard to beat.

Good luck.

Hi David; Below is the text from the test report regarding conducted power measurements. My impression is that this 4.0 V is applied directly to the RF test connector and has an effect, directly or indirectly on a PIN diode in the output coupling of the radio. Note that Motorola does not offer this DC/RF fixture for sale. My interpretation is that Motorola does not want the end user to utilize this connector for normal operations or adding a non compliant antenna, and to use this connector, you must provide a bias T with a 4.0 Volt bias.

alcahuete notes that he measures a DC short across the terminals of the stock 1/2 wave antennas that he owns. I asked him to test for a DC bias voltage across the radio terminals, but I don't think he tested that yer.

Is it possible that the terminal impedance of the DTR radio is something far different than 50 ohms to prevent the user from substituting a non compliant 50 ohm antenna. I find it interesting that the 1/2 wave antenna is described as a "whip" in the literature and not a 1/2 wave center fed dipole. A whip, to me implies end fed, and the impedance would be quite high.

Could Motorola be installing a transformer inside the base of these antennas to transform, say a 12 ohm impedance to whatever compliant antenna is fitted?

Regarding your mention of the Motorola MTX9250. Does that radio provide a true 50 ohm test facility at the antenna terminals?

It is odd that the fitting of any external antennas other than the factory ones yields so poor performance. Also, if the DTR has a 50 ohm antenna terminal, why does Motorola go to the bother of a special internal RF connector?

-------
7.1
RF Power -- Pursuant to 47 CFR 2.947(c)
Method of Conducted Output Power Measurement
:
Adaptation of TIA/EIA-603-A clause
2.2.1 for Pulsed Measurements
The RF output power is not adjustable by the user. The output power is controlled by the
radio software. To obtain RF output power reading, the radio was programmed to utilize
the maximum output power setting.

A special DC/RF test fixture was utilized to interface with the radio test RF connector
while simultaneously supplying the operating voltage of 4.0V. The radio RF connector is
utilized in all factory tuning and testing procedures, and provides a 50 ohm connection to
the transmitter path while disconnecting the radio antenna. All conducted measurements
were performed via this test port.

NOTE: This DC/RF test fixture in not offered for sale

 

[RFI-EMI-GUY]

(EDIT The report seems to be referring to a calibration antenna for the phantom?)

David;
Is this the 1/2 wave antenna you speak of? Assuming it is compliant with DTR, it is a 50 ohm antenna, and it does have a DC short by design, though somehow it may be center fed, as they allude to "second arm" of the dipole.

https://fccid.io/ABZ99FT5011/Test-Report/Test-Report-2-1232559.pdf

 

[RFI-EMI-GUY]

Adding further confusion, I don't see the 1/2 wave antenna listed as a recommended accessory for the DTR650 (8505241U04 (NAF 5038)). Nor is it listed in the FCC certification AZ489FT5842.

 

[n1das]

I don't have the info in front of me and my memory may be faulty but IIRC, the FCC ID for the DTR650 is AZ489FT5852. The other FCC ID (AZ489FT5842) is for the DTR410? The 410 is a Gen1 unit and the 550 and 650 are newer Gen2 units, IIRC.

 

[RFI-EMI-GUY]

I don't have the info in front of me but IIRC, the FCC ID for the DTR650 is AZ489FT5852. The other FCC ID (AZ489FT5842) is for the DTR410? The 410 is a Gen1 unit and the 550 and 650 are newer Gen2 units, IIRC.

If that is the case, the 1/2 wave antenna is a normal option and the RF measured from the antenna attachment should be 50 OHMs. The question now is the "non standard connector". Is the adapter used by the OP (and others) mating properly with the radio?

See figure 3-4 for the connector.

https://apps.fcc.gov/eas/GetApplicationAttachment.html?id=611172

https://fccid.io/AZ489FT5852/External-Photos/EXTERNAL-PHOTOS-611172

EXHIBIT 7: MEASUREMENT PROC
EDURES – Pursuant 47 CFR 2.947
7.1
RF Power -- Pursuant to 47 CFR 2.947(c)
Method of Conducted Output Power Measurement
:
Adaptation of TIA/EIA-603-A clause 2.2.1 for Pulsed Measurements

The RF output power is not adjustable by the user. The output power is controlled by the radio
software. To obtain RF output power reading, the radio was programmed to utilize the maximum output power setting.

A non-standard connector was used to interface with the radio at the antenna port to measure RF
Output power. All conducted measurements were performed via this test port.

 

[RFI-EMI-GUY]

I think Motorola made two DTR650 versions, changed outsourced vendors mid production:

AZ489FT5842
DTR650 External Photos:
https://apps.fcc.gov/eas/GetApplicationAttachment.html?id=506304

https://fccid.io/AZ489FT5842/External-Photos/Exhibit-3-External-Photos-506304

But the user manual calls it a RBR MotoTalk? (but I think it is a DTR410)


AZ489FT5852
DTR650 External PhotosSEE Label Inside Battery
https://apps.fcc.gov/eas/GetApplicationAttachment.html?id=611172
https://fccid.io/AZ489FT5852/External-Photos/EXTERNAL-PHOTOS-611172

 

[RFI-EMI-GUY]

So I will throw it back to the OP:

alcahuete Do your DTR's have FCC ID AZ489FT5852?

Does the antenna connector look like what is in this page?

https://fccid.io/AZ489FT5852/External-Photos/EXTERNAL-PHOTOS-611172

Can you tell us where you got your mating connector? Model type, Photo? Can you be absolutely sure it fully mates?

Also, I assume you don't have any PL259 connectors or adapters? Type N? BNC? all SMA? How are you hooked up?

 

[RFI-EMI-GUY]

alcahuete

One more question, do you have any sort of 1 GHz power meter and load?

 

[cmdrwill]

The antenna jack and antenna does look like Jedi style used by Motorola on a lot of handheld radios.
From what I have found on some so-call dual, tri band radios is there is matching circuits in the radio. Example Yaesu dual band radio uses a VHF antenna, and has a matcher in the radio that makes the VHF antenna work at UHF, bizar...

So some aftermarket antennas do not work.

 

[Chickenhawk56]

Keep in mind there are two to THREE generations of DTR radios (with possible firmware upgrades mid-generational change). The FCC photo shows the original gen 1 DTR650. It is recognizable by the white faceplate and the fixed stubby antenna. Gen 2 DTR650 radios have the 1/2-wave removable antenna (Motorola stock number 8505241U04), larger capacity battery and a faster charger.

Gen 1 DTR550 radios had the fixed stubby antenna. Gen 2 DTR550 radios had the 1/4-wave removable antenna (Motorola stock number 8505241U06). Gen 3 DTR550 radios had the 1/2-wave antenna, faster charger and upgraded firmware.

Gen 1 DTR410 radios had the fixed stubby antenna. Gen 2 DTR410 radios still have the fixed stubby but with the upgraded firmware and management functions of the DTR650.

Just for interest, the firmware on all three generations can be upgraded, and the larger capacity battery will fit all three models and generations. The 1/2-wave antenna can be easily purchased from dealers for any gen 2 DTR550 owners. (There is no practical way to upgrade the gen 1 fixed stubby models to gen 2 removable antennas - nor does there seem to be a need to do so.)

I know little about electronics but quite frankly, can not see why Motorola would purposely degrade the performance of any external antennas except their own. I am with n1das, who feels that it may simply because at those high frequencies, the stock antennas are so close to perfect that anything else with cable lengths and connectors just degrades the signal. Plus, the penetration ability of the 900MHz radios into the inside of a car is so superior that there is little need for an external antenna anyway. My informal tests have easily borne this out.